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DWDM Principle
Contents
Section 1 DWDM Overview
1.1 DWDM Technology Background
1.2 DWDM Principles Overview
1.3 DWDM Equipment Operating Modes
1.3.1 Two-fiber bi-directional transmission
1.3.2 Single fiber bi-directional transmission
1.3.3 Add and drop of optical signals
1.4 Application Modes of DWDM
1.5 Advantages of DWDM
Section 2 DWDM Transmission Media
2.1 Optical Fiber Structures
2.2 Types of Optical Fiber
2.3 Basic Features of Optical Fiber
2.3.1 Physical Dimension (Mode field diameter)
2.3.2 Mode Field Concentricity Error
2.3.3 Bend Loss
2.3.4 Attenuation Constant
2.3.5 Dispersion Coefficient
2.3.6 Cutoff Wavelength
2.4 Types and Properties of Optical Fiber Cable
2.4.1 Types of Optical Fiber Cable
2.4.2 Properties of Optical Fiber Cable
Section 3 DWDM Key Technologies
3.1 Lasers
3.1.1 Laser Modulation Modes
3.1.2 Wavelength Stability and Control of Laser
3.2 Erbium-doped Optical Fiber Amplifier (EDFA)
3.2.1 EDFA Operating Principle
3.2.2 Applications of EDFA
3.2.3 Gain Control of EDFA
3.2.4 Limitations of EDFA
3.3 DWDM Components
3.3.1 Optical Grating Type DWDM Component
3.3.2 Dielectric Film Type DWDM Component
3.3.3 Fused Conical Type DWDM Component
3.3.4 Integrated Optical Waveguide Type DWDM Component
3.3.5 Performances of DWDM Components
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Section 4 DWDM Networking Design
4.1 Some Network Element Types of DWDM
4.1.1 Optical Terminal Equipment (OTM)
4.1.2 Optical Line Amplification Unit (OLA)
4.1.3 Optical Add/drop Multiplexing Unit (OADM)
4.1.4 Electrical Regeneration Unit (REG)
4.2 General Constitution of DWDM network
4.2.1 Point-to-point Networking
4.2.2 Chain Type Networking
4.2.3 Ring Type Networking
4.2.4 Network Management Information Channel Backup and
Interconnection Capability
4.3 Factors To Be Considered in DWDM Networking
4.3.1 Dispersion Limited Distance
4.3.2 Power
4.3.3 Optical Signal-to-Noise Ratio
4.3.4 Other Factors
4.4 DWDM Network Protection
4.4.1 Protection Based on single Wavelength
4.4.2 Optical Multiplex Section (OMSP) Protection
4.4.3 Applications in Ring Networks
4.5 Analysis to The Examples
4.5.1 Networking Diagram (Physical Network Stations)
4.5.2 Networking Diagram (considering the dispersion limited distance of
the lasers to divide the regenerator sections of the network)
4.5.3 Networking Diagram (considering the power of optical amplifiers to
divide the optical regenerator sections)
4.5.4 Networking Diagram (considering OSNR)
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DWDM Principle
Section 1 DWDM Overview
Section 1 DWDM Overview
Objectives:
To master the concepts of DWDM.
To know the background and technology characteristics of DWDM.
1.1 DWDM Technology Background
With the dramatic increase of voice services and emergence of various new
services, especially the quick change of IP technology, network capacity will
inevitably be faced with critical challenge. Traditional methods for transmission
network capacity expansion adopt space division multiplexing (SDM) or time
division multiplexing (TDM).
1. Space Division Multiplexing (SDM)
Space division multiplexing linearly expands the transmission capacity by adding
fibers, and the transmission equipment is also linearly added.
At present, fiber manufacture technology is quite mature. Ribbon optical fiber
cables with tens of cores are rather prevalent and advanced connection technique
for optical fiber simplifies cable construction. However, the increment of fibers
brings much inconvenience to the construction and circuit maintenance in the
future. Additionally, if the existing optical fiber cable lines have no sufficient
fibers and require to lay new fiber cables for capacity expansion, engineering cost
will increase in duplication. Moreover, this method doesn't sufficiently utilize the
transmission bandwidth of the optical fiber and wastes the bandwidth resources. It's
not always possible to lay new optical fibers to expand the capacity during the
construction of communication networks. Actually, in the initial stage of the
project, it is hard to predict the ever-growing service demands and to plan the
number of fibers to lay. Hence, SDM method for capacity expansion is quite
limited.
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Section 1 DWDM Overview
2. Time Division Multiplexing (TDM)
TDM is a commonly used method for capacity expansion, e.g. multiplexing of the
primary group to the fourth group of the traditional PDH, and STM-1, STM-4,
STM-16 and STM-64 of current SDH. TDM technology can enhance the capacity
of optical transmission information in duplication and greatly reduce the circuit
cost in equipment and line. Moreover, it is easy to extract specific digital signals
from the data stream via this multiplexing method. It is especially suitable for
networks requiring the protection strategy of self-healing rings.
However, TDM method has two disadvantages. Firstly, it affects services. An
overall upgrade to higher rate levels requires to replace the network interfaces and
equipment completely. Thus the equipment in operation must be interrupted during
the upgrade process. Secondly, rate upgrade lacks of flexibility. Let's take SDH as
an example, when a system with a line rate of 155Mbit/s is required to provide two
155Mbit/s channels, the only way is to upgrade the system to 622Mbit/s even
though two 155Mbit/s are unused.
For TDM equipment of higher rate, the cost is relatively high. Furthermore,
40Gbit/s TDM equipment has already reached the rate limitation of electronic
devices. Even the nonlinear effects of 10Gbit/s rate in different fiber types will set
various limitations to transmission.
Currently, TDM is a commonly used capacity expansion method. It can implement
capacity expansion via continuous system rate upgrade. When certain rate level is
reached, other solutions must be found because of characteristic limitations of
devices, lines, etc.
All the basic transmission networks, whether using SDM or TDM to expand the
capacity, adopt traditional PDH or SDH technology, i.e. utilizing optical signals on
a single wavelength for transmission. This transmission method is a great waste of
optical capacity because the bandwidth of optical fiber is almost infinite when
compared to the single wavelength channel we currently use. We are worrying
about the jam of networks, on the other hand huge network resources are being
wasted.
DWDM technology emerged under this background. It greatly increases the
network capacity, makes full use of the bandwidth resources of optical fibers and
cuts down the waste of network resources.
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Section 1 DWDM Overview
1.2 DWDM Theory Overview
DWDM technology utilizes the bandwidth and low attenuation characteristics of
single mode optical fiber, adopts multiple wavelengths as carriers and allows them
to transmit in the fiber simultaneously.
When compared to common single channel systems, dense-WDM (DWDM)
greatly increases the network capacity and makes full use of the bandwidth
resources of optical fibers. Moreover, DWDM has many advantages such as simple
capacity expansion and reliable performances. Especially, it can access various
types of services and this gives it a bright prospective of application.
In analog carrier communication systems, the frequency division multiplexing
(FDM) method is often adopted to make full use of the bandwidth resources of
cables and enhance the transmission capacity of the system, i.e. transmitting
several channels of signals simultaneously in a single cable and, at the receiver
end, utilizing band-pass filters to filter the signal on each channel according to the
frequency differences among the carriers.
Similarly, in optical fiber communication systems, optical frequency division
multiplexing method can also be used to enhance the transmission capacity of the
systems. In fact, this multiplexing method is very effective in optical
communication systems. Unlike the frequency division multiplexing in analog
carrier communication systems, optical fiber communication systems utilize optical
wavelengths as signal carriers, divide the low attenuation window of optical fibers
into several channels according to the frequency (or wavelength) difference of each
wavelength channel and implement multiplexing transmission of multi-hannel
optical signals in a single fiber.
Since some optical components (such as narrow-bandwidth optical filters and
coherent lasers) are currently not mature, it is difficult to implement ultra-dense
optical frequency division multiplexing (coherent optical communication
technology) of optical channels. However, alternate-channel optical frequency
division multiplexing can be implemented based on current component technical
level. Usually, multiplexing with a larger channel spacing (even in different
windows of optical fibers) is called optical wavelength division multiplexing
(WDM), and WDM in the same window with smaller channel spacing is called
dense wavelength division multiplexing (DWDM). With the progress of
technologies, nanometer level multiplexing can be implemented by using modern
technologies. Even sub-nanometer level multiplexing can be implemented but
merely with stricter component technical requirements. Hence, multiplexing of 8,
16, 32 or more wavelengths with smaller wavelength spacing is called DWDM.
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Section 1 DWDM Overview
The diagram of DWDM system structure and optical spectrum is shown in Figure
1-1. At the transmit end, optical transmitters output optical signals of different
wavelengths whose accuracy and stability meet certain requirements. These signals
are multiplexed via an optical wavelength multiplexer and sent to an erbium-doped
optical fiber power amplifier (it is mainly used to compensate the power loss
aroused by the multiplexer and enhance the launched power of the optical signals).
After amplification, this multi-channel optical signal is sent to the optical fiber for
transmitting. In the midway optical line amplifiers can be installed or not according
to practical conditions. At the receiver end, the signals are amplified by the optical
pre-amplifier (it is mainly used to enhance receiving sensitivity and prolong
transmission distance) and sent to the optical wavelength de-multiplexer which
separates them into the original multi-channel optical signals.
OTU
D
M
U
M
O TU
U
X
X
OTU
Optical
spectrum
Optical line
amplifier
Optical booster
amplifier
Single
channel
Optical line
amplifier
Optical pre-amplifier
Optical
spectrum
Wavelength
Figure 1-1 The diagram of DWDM system structure and spectrum
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Wavelength
DWDM Principle
Section 1 DWDM Overview
1.3 DWDM Equipment Operating Modes
1.3.1 Two-fiber Bi-directional Transmission
As shown in Figure 1-2, a single optical fiber implements only one directional
transmission of optical signals. Hence the same wavelengths can be reused in two
directions.
1
Detector
Optical source
1
WDM
N
Optical source
WDM
1
Detector
N
N
N
N
N+1
1
1
Detector
Optical source
1
N+1
1
2N
WDM
WDM
Detector
1
N
N
Optical source
N
Figure 1-2 Two-fiber bi-directional transmission DWDM system
This kind of DWDM system can effectively exploit the huge bandwidth resources
of optical fiber and expand the transmission capacity of a single optical fiber in
several or tens of times. In long-haul networks, capacity can be expanded by
adding wavelengths gradually according to the demands of practical traffic, which
is very flexible. This is, under the condition that the actual fiber dispersion isn't
known, also an approach to use multiple 2.5Gbit/s systems to implement ultra-large
capacity transmission, avoiding adopting ultrahigh speed optical systems.
1.3.2 Single fiber Bi-directional Transmission
As shown in Figure 1-3, a single fiber transmits optical signals of two directions
simultaneously, and the signals in the two different directions should be assigned
on different wavelengths.
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2N
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1
Section 1 DWDM Overview
Detector
Optical source
1
1
1
A single optical fiber
N
Detector
Optical source
N+1
WDM
Detector
WDM
1
N
Optical source
N+1
N+1
2N
N+1
N+1
2N
Detector
Optical source
2N
2N
Figure 1-3 The DWDM system which adopts single fiber bi-directional transmission
Single fiber bi-directional transmission allows a single fiber to carry full duplex
channels and, generally, saves one half of the fiber components of unidirectional
transmission. Since signals transmitted in the two directions do not interact and
create FWM (Four-Wave Mixing) products, its total FWM products are much less
than two-fiber unidirectional transmission. However, the disadvantage of this
system is that it requires a special measure to deal with the light reflection
(including discrete reflection resulted by optical connectors and Rayleigh backward
reflection of the fiber) to avoid multi-path interference. When the optical signal
needs to be amplified to elongate prolong transmission distance, components such
as bi-directional optical fiber amplifier and optical circulator must be adopted, but
their noise factor is a little worse.
1.3.3 Add and Drop of Optical Signals
1
N
N
N
N
OADM
Detector
OADM
Optical source
1
1
1-8
Detector
Optical source
2
2
2N
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Section 1 DWDM Overview
Figure 1-4 Optical add and drop transmission
By utilizing optical add/drop multiplexer (OADM), optical signals of the
wavelengths can be added and dropped in the intermediate stations, i.e.
implementing add/drop of optical paths. This method can be used to implement
ring type networking of DWDM systems. At present, OADM can only be made as
fixed wavelength add/drop device (as shown in Figure 1-4) and thus the flexibility
of this operating mode is limited.
1.4 Application Modes of DWDM
Generally, DWDM has two application modes:
⌧ Open DWDM
⌧ Integrated DWDM
The feature of open DWDM system is that it has no special requirements for
multiplex terminal optical interfaces as long as they meet the optical interface
standards defined in ITU-T G.957. The DWDM system adopts wavelength
conversion technology to convert the optical signal of multiplex terminal into
specific wavelength. Optical signals from different terminal equipment are
converted into different wavelengths meeting the ITU-T recommendation, then
multiplexed.
Integrated DWDM system, without adopting wavelength conversion technology,
requires that the optical signal wavelengths of the multiplex terminal meets
DWDM system specifications. Different multiplex terminal transmits different
wavelengths meeting the ITU-T recommendation. Thus, when connected to the
multiplexer, these wavelengths occupy different channels and multiplexing is
implemented.
Different application modes can be adopted according to the demands of
engineering. In practical applications, open DWDM and integrate DWDM can be
mixed.
1.5 Advantages of DWDM
The capacity of optical fiber is huge. However, traditional optical fiber
communication systems, with one optical signal in a single fiber, only exploited a
little part of the abundant bandwidth of optical fiber. To effectively use the huge
bandwidth resources of optical fiber and increase its transmission capacity, a new
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Section 1 DWDM Overview
generation optical fiber communication technology based on dense-WDM
(DWDM) has emerged.
DWDM technology has the following features:
1. Ultra-large capacity
The transmittable bandwidth of currently commonly used conventional fiber is very
wide, but the utilization ratio is still low. By using DWDM technology, the
transmission capacity of a single optical fiber is increased by several, tens of or
even hundreds of times when compared to the transmission capacity of single
wavelength systems. Recently, NEC Company, Japan, implemented a
132×20Gbit/s experimental DWDM system with a transmission distance of
120km. This system, with a total bandwidth of 35nm (1529nm~1564nm) and a
channel spacing of 33GHz, can transmit 40 million telephone calls.
2. Data rate "transparency"
DWDM systems conduct multiplexing and de-multiplexing in terms of optical
wavelength differences and are independent to signal rates and modulation modes,
i.e. transparent to the data. Hence, they can transmit signals with completely
different transmission characteristics and implement combination and separation of
various electrical signals, including digital signals and analog signals, and PDH
signals and SDH signals.
3. Utmost protection of the existing investment during system upgrade
During the expansion and development of the network, it is an ideal approach to
implement capacity expansion without the need to rebuild the optical fiber cables
and with the only requirement of replacing the optical transmitters and receivers.
This is also a convenient way to introduce broad-band services (such as CATV,
HDTV and B-ISDN). Furthermore, any new services or new capacity can be
introduced simply by adding an additional wavelength.
4. High flexibility, economy and reliability of networking
When compared to the traditional networks using electrical TDM networks, new
communication networks based on DWDM technology are greatly simplified in
architecture and have clear network layers. Dispatching of various services can be
implemented simply by adjusting the corresponding wavelengths of the optical
signals. Because of the simple network architecture, clear layers and convenient
service grooming, the flexibility, economy and reliability of networking are
obvious.
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Section 1 DWDM Overview
5. Compatibility with all optical switching
It is foreseeable that, in the realizable all optical networks in the future, processing
such as add/drop and connection of all telecommunication services is implemented
by changing and adjusting the optical signal wavelengths. So DWDM technology
is one of the key technologies to implement all optical networks. Moreover,
DWDM systems can be compatible with future all optical networks. It is possible
to implement transparent and highly survivable all optical networks based on the
existing DWDM system.
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Section 2 DWDM Transmission Media
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Section 2 DWDM Transmission Media
Objectives:
To master basic structures and types of optical fibers.
To know basic characteristics of optical fibers.
2.1 Optical Fiber Structures
The kernel of optical fiber used in communication systems consists of a cylindrical
glass core and a glass cladding. The outermost layer is a plastic wear-resisting
coating. The whole fiber is cylindrical. The typical structure of optical fiber is
shown in Figure 2-1.
Coating
Cladding
n2
Figure 2-1 The typical structure of optical fiber
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n1
Core
Section 2 DWDM Transmission Media
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n2
n1
2b
2a
DWDM Principle
2b
2a
n2
n1
2b
2a
n2
n(r)
Figure 2-2 Three typical types of optical fibers
Thickness of the core and refractive indexes of the core material and cladding
material are critical to the properties of the fiber. Figure 2-2 shows three typical
optical fibers. As can be seen from this figure, there are two typical refractive
index distributions in the fiber core-cladding cross-section. One is that the
refractive index radial distributions of the core and the cladding are uniform, and
the change of refractive index at the core-cladding boundary is a step function.
This fiber is called step-index fiber. The other one is that the refractive index of the
core is not a constant. It gradually decreases as the radial coordinate of the core
increases until it equals to the index of the cladding. Hence this fiber is called
graded-index fiber. The common feature of this two fiber cross-section is that the
refractive index of the core n1 is larger than that of the cladding n2. This is also a
necessary condition for the optical signal to transmit in the fiber. For a step-index
fiber, total internal reflection can occur at the core-cladding boundary and the light
wave can propagate along the core. For a graded-index fiber, the continuous
refraction occurs to the light wave in the core, forming a light ray similar to the
sine-wave through the fiber axis and guiding the light wave to propagate along the
core. The tracks of the two light rays are shown in Figure 2-2. With the difference
of the diameter size of the core of step-index and graded-index fibers, the number
of modes transmitted in the fiber is different. Hence, step-index fiber or gradedindex fiber can be classified into single mode fiber and multimode fiber according
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Section 2 DWDM Transmission Media
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to the number of transmission modes. This is also a classification method of optical
fiber. The core diameter of a single mode fiber is very small and, generally, less
than 10µm, and the core diameter of a multimode fiber is relatively large and often
equal to 50µm. However, there is little difference between the profiles of these two
types of fiber. The diameters of fibers with a plastic jacket are less than 1mm.
2.2 Types of Optical Fiber
Since the single-mode optical fiber has advantages of low internal attenuation,
large bandwidth, easy upgrade and capacity expansion and low cost, it is
internationally agreed that DWDM systems will only utilize single mode fiber as
transmission media. At present, ITU-T has defined four types of single mode
optical fiber with different design in Recommendations G.652, G.653, G.654 and
G.655.
G.652 fiber is currently a single mode fiber for extensive use, called 1310nm
property optimal single mode fiber and also called dispersion unshifted fiber.
According to the refractive index cross section of the core, it can also be divided
into two categories: matched cladding fiber and depressed cladding fiber. They
have similar properties. The former is simple in manufacturing but has relatively
larger macrobend loss and microbend loss while the later has larger connection
loss.
G.653 fiber is called dispersion shifted fiber or 1550nm property optimal fiber. By
designing the refractive index cross section, the zero dispersion point of this kind
of fiber is shifted to the 1550nm window to match the minimum attenuation
window. This makes it possible to implement ultrahigh speed and ultra long
distance optical transmission.
G.654 fiber is cut-off wavelength shifted single mode fiber. This kind of fiber is
mainly designed to reduce the attenuation at 1550nm. Its zero dispersion point is
still near 1310nm. The dispersion at 1550nm is relatively high, up to
18ps/(nm.km). So single longitudinal mode laser must be used to eliminate the
affect of the dispersion. G.654 fiber is mainly used for submarine optical fiber
communication with very long regenerator section distance.
G.655 fiber, a nonzero dispersion shifted single mode optical fiber, is similar to
G.653 fiber and preserves certain dispersion near 1550nm to avoid four-wave
mixing phenomenon in DWDM transmission. It is suitable for DWDM system
applications.
Except for the above-mentioned four types of standardized fiber, a large effective
area fiber suitable for higher capacity and longer distance has emerged. Its zero
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Section 2 DWDM Transmission Media
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dispersion point is near 1510µm and its effective area is up to 72 square
µm. Therefore, it can effectively overcome the nonlinear affects and is especially
suitable for DWDM system applications based on 10Gbit/s.
Thinking:
Which type of optical fiber is widely laid at present?
2.3
Basic Features of Optical Fiber
2.3.1 Physical Dimension (Mode Field Diameter)
The fiber core diameter of a single mode fiber is 8~9µm in the same magnitude as
the operating wavelength 1.3~1.6µm. Because of the optical diffraction effect, it is
not easy to measure the exact value of the fiber cord diameter. In addition, since
the field intensity distribution of the fundamental mode LP01 isn't confined within
the fiber core, the concept of single mode fiber core diameter is physically
meaningless and should be replaced with the concept of mode field diameter.
Mode field diameter measures the concentrate level of the fundamental mode field
spatial intensity distribution within the fiber.
The nominal mode filed diameter of G.652 fiber at 1310nm wavelength area
should be 8.6~9.5µm with a deviation of less than 10%, and the nominal mode
filed diameter of G.655 fiber at 1550nm wavelength area should be 8~11µm with a
deviation of less than 10%.
The cladding diameter of both types of above-mentioned single mode optical fibers
is 125µm.
2.3.2 Mode Field Concentricity Error
Mode field concentricity error refers to the distance between the mode field center
and the cladding of the interconnected fibers. Fiber connector loss is in proportion
to the square of the mode field concentricity error. So reducing mode field
concentricity error is one of the key factors to reduce the fiber connection loss and
should be strictly controlled in process. The mode field concentricity error of the
two types of single mode optical fibers G.652 and G.655 shouldn't be greater than
1. Generally, it should be less than 0.5.
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Section 2 DWDM Transmission Media
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2.3.3 Bend Loss
Bend of the optical fiber will cause radiation loss. Actually, bend arises to an
optical fiber in two cases. One is that the curvature radius of the bend is much
larger than the diameter of the fiber (e.g. this kind of bend may occur when the
fiber cable is laid). The other case is microbend. There are many causes for
microbend. Microbend, limited to process conditions, may be caused during the
production process of the fiber and the cable. Microbends of different curvature
radiuses are randomly distributed along the fiber. The bent fiber with larger
curvature radius can transmit fewer modes than the straight fiber, and a part of
modes are radiated out from the fiber to cause loss. The randomly distributed fiber
microbend will result in mode coupling in the fiber and cause energy radiation loss.
Bend loss of the fiber is inevitable because it can't be guaranteed that no bend in
any form will occur to the fiber and the cable during production or utilization
process.
Bend loss is related to the mode field diameter. The bend loss of G.652 fiber
shouldn't be larger than 1dB at 1550nm wavelength area, and the bend loss of
G.655 fiber shouldn't be larger than 0.5dB at 1550nm area.
2.3.4 Attenuation Constant
Attenuation in optical fiber is mainly determined by three types of loss: absorption
loss, scattering loss and bend loss. Bend loss, as described above, has no great
effect on the attenuation constant in fiber. So, it is absorption loss and scattering
loss that mainly determine the attenuation constant in fiber.
Absorption loss is caused by the fiber material where excessive metal impurity and
OH- ion absorb the light to result in loss.
Scattering loss is often caused in the case that a part of optical power is scattered
outside the fiber when uneven refractive index distribution local area emerges
within the fiber and causes light scattering because of the micro-change in fiber
material density and uneven density of compositions such as SiO2, GeO2 and P2O5.
Or, scattering loss can be aroused if some defect occurs or some bubbles and gas
scabs are remained at the core-cladding boundary. The physical dimension of these
structural defects is much larger than the lightwave, causing wavelength
independent scattering loss and upward shifting the whole curve of fiber loss
spectrum. However, this kind of scattering loss is much less than the former one.
Combining the above losses, the attenuation constant of single mode fiber at
1310nm and 1550nm wavelength areas is 0.3~0.4dB/km (1310nm) and
0.17~0.25dB/km (1550nm), respectively. As defined in ITU-T Recommendation
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Section 2 DWDM Transmission Media
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G.652, the attenuation constant at 1310nm and 1550nm should be less than
0.5dB/km and 0.4dB/km, respectively.
2.3.5 Dispersion Coefficient
Dispersion in optical fiber refers to a physical phenomenon of signal distortion
caused when various modes carrying signal energy or different frequencies of the
signal have different group velocity and disperse from each other during
propagation. Generally, three kinds of dispersion exist in optical fiber.
1) Modal dispersion: This is caused when the fiber carries multiple modes of the
same frequency signal energy and different mode has different time delay during
transmission.
2) Material dispersion: Because the refractive index of the fiber core material is a
function of the frequency, signal components of different frequency propagate at
different velocities along the fiber. This causes dispersion.
3) Waveguide dispersion: In the fiber, for a signal carrying different frequencies in
the same mode, dispersion is caused because of different group velocities during
propagation.
These three types of dispersion are called chromatic dispersion. ITU-T G.652
defines a zero dispersion wavelength range of 1300nm~1324nm and a maximum
dispersion slope of 0.093ps/(nm2.km). In the wavelength range of 1525~1575nm,
the dispersion coefficient is approximately 20ps/(nm.km). ITU-T G.653 defines a
zero dispersion wavelength 1550nm and a dispersion slope of 0.085ps/(nm2.km) in
the wavelength range of 1525~1575nm where the maximum dispersion coefficient
is 3.5ps/(nm.km). The absolute value of the dispersion coefficient of G.655 fiber
should be within 0.1~6.0 ps/(nm2.km) in the range of 1530~1565nm.
Technical details:
The following figure shows the dispersion characteristics of several types of fiber.
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Section 2 DWDM Transmission Media
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G.655 fiber with positive
dispersion coefficient
G.653 fiber
Dispersion coefficient
(ps/nm km)
17
G.652 fiber
G.655 fiber with negative
dispersion coefficient
1310
1550
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Wavelength
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Section 2 DWDM Transmission Media
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2.3.6 Cutoff Wavelength
To avoid modal noise and dispersion penalty, the cutoff wavelength of the shortest
optical fiber cable in the system should be less than the shortest operating
wavelength of the system. The cutoff wavelength condition can guarantee single
mode transmission in the shortest cable and suppress the occurrence of higher
order modes or reduce the power penalty of the generated higher order mode noise
to an negligible degree. At present, ITU-T has defined three types of cutoff
wavelengths.
1) Cutoff wavelength of primary coating fiber in jumper cable shorter than 2m.
2) Cutoff wavelength of 22m cable optical fiber.
3) Cutoff wavelength of 2~20m jumper cable.
For G.652 fiber, the cutoff wavelength is
2~20m jumper cable, and
1260nm in 22m cable,
1260nm in
1250nm in jumper cable shorter than 2m. For G.655
fiber, the cutoff wavelength is
1480nm in 22m cable,
coating fiber of jumper cable shorter than 2m, and
1470nm in primary
1480nm in 2~20m jumper
cable.
2.4
Types and Properties of Optical Fiber Cable
2.4.1 Types of Optical Fiber Cable
In terms of the structure, optical fiber cable can be classified into four types: loose
jacket twist type, skeleton type, central beam nominal type and ribbon optical fiber
cable.
According to the laying methods, optical fiber cable can be classified into plow-in
optical cable, optical fiber cable for installation in duct, aerial optical cable,
submarine optical cable and office optical cable, etc.
According to application situation, traffic demands and capacity expansion
demands, the core number of optical fiber cable is classified into 4, 6, 8, 10, 12, 14,
16, 18, 20, 22, 24, 26, 28, 30, 32, 34 and 36, and can be increased in even number.
2-19
DWDM Principle
Section 2 DWDM Transmission Media
y
2.4.2 Properties of Optical Fiber Cable
1. Mechanical property: An optical fiber cable should possess certain mechanical
property that makes it withstand items including tension, bruise, impulsion,
repeated bending, twisting, flexure, hook hang, kink, reeling, etc.
2. Protective property: Optical fiber cable should possess property of moisture
proof and water proof. Additionally, it should meet some requirements including
protection of termite, rat and insect gnawing, corrosion, lightning, etc.
2-20
DWDM Principle
Section 3 DWDM Key Technologies y
Section 3 DWDM Key Technologies
Objectives:
To understand the requirements and solutions of DWDM optical resources.
To understand DWDM optical amplification technology.
To understand DWDM multiplexing and de-multiplexing technology.
3.1 Lasers
Laser, whose function is to generate laser, is an important component of DWDM
system. At present, lasers used in DWDM system are semiconductor laser LD
(Laser diode).
The operating wavelengths of DWDM systems are relatively dense. Generally, the
wavelength spacing is from several nanometer to sub-nanometer. Hence, the laser
diode is required to operate in a standard wavelength and possess good stability.
On the other hand, the non-electrical regeneration distance of DWDM systems is
increased from 50~60km of single SDH system transmission to 500~600km. lasers
of the DWDM system are required to adopt lasers more advanced in technology
and excellent in performance in order to elongate the dispersion limited distance of
the transmission system and overcome fiber nonlinear effects {such as stimulated
Brillouin scattering (SBS), stimulated Raman scattering (SRS), self-phase
modulation (SPM), cross-phase modulation (XPM), modulation instability and
four-wave mixing (FWM)}.
In summary, lasers of DWDM system have two major features:
1. Relatively large dispersion tolerance;
2. Standard and stable wavelength.
3-21
DWDM Principle
Section 3 DWDM Key Technologies y
3.1.1 Laser Modulation Modes
At present, optical fiber communication systems for extensive use employing
intensity modulation — direct detection system. There are two types of intensity
modulation for lasers, i.e. direct modulation and indirect modulation.
1. Direct modulation
Direct modulation: It is also called internal modulation, i.e. directly modulating the
laser and changing the launched lightwave intensity by controlling the injection
current. LED or LD sources used in traditional PDH and SDH systems of 2.5Gbit/s
or below employ this modulation method.
One character of direct modulation is that the launched power is in proportion to
the modulation current. It has advantages of simple structure, low loss and low
cost. Since it changes the length of the laser resonant cavity, the variation of
modulation current will cause a linear variation of the emitting laser wavelength
corresponding to the current. This variation, called modulation chirp, is actually a
kind of wavelength (frequency) jitter inevitable for direct modulation sources. The
chirp broadens the bandwidth of the emitting spectrum of the laser, deteriorates its
spectrum characteristics and limits the transmission rate and distance of the system.
Generally, for conventional G.652, the transmission distance is
transmission rate
100km and the
2.5Gbit/s.
For DWDM system without optical line amplifier, direct modulated lasers can be
considered to save the cost.
2. Indirect modulation
Indirect modulation: This modulation method is also called external modulation,
i.e. modulating the laser indirectly and adding an external modulator in its output
path to modulate the lightwave. In fact, this modulator works as a switch, as shown
in Figure 3-1.
3-22
DWDM Principle
Section 3 DWDM Key Technologies y
Constant
light
source
Optical
modulator
Optical signal
output
Electric modulation
signal input
Figure 3-1 The structure of external modulated laser
The constant laser is a highly stable source continuously emitting lightwave with
fixed wavelength and power. It isn't affected by the electric modulation signal
during emitting, so no modulating frequency chirp occurs and the line breadth of
its optical spectrum keeps at minimum. According to the electric modulation
signal, the optical modulator processes the highly stable light from the constant
laser light in a way of either passing through or blocking. During the modulation
process, the spectrum characteristics of the lightwave won't be affected. This
guarantees the quality of the spectrum.
Lasers adopting indirect modulation are relatively complex with high loss and cost,
but its modulating frequency chirp is very low. It can be used in systems whose
transmission rate is
2.5Gbit/s and transmission distance longer than 300km.
Hence, in DWDM systems with optical line amplifiers, the lasers at the transmit
end are generally indirectly modulated.
Commonly used external modulators are photoelectric modulator, acoustooptic
modulator and waveguide modulator.
The basic operating principle of photoelectric modulator is crystal linear
photoelectric effect. Photoelectric effect refers to the phenomenon that electric
field causes the variation of the refractive index of a crystal. A crystal that is able
to generate the photoelectric effect is called photoelectric crystal.
Acoustooptic modulator is made by utilizing the acoustooptic effect of the
dielectric. Acoustooptic effect refers to the phenomenon that the dielectric changes
under the pressure of an acoustic wave when it propagates through the dielectric.
This change causes the variation of the refractive index of the dielectric and affects
the transmission characteristics of the lightwave.
3-23
DWDM Principle
Section 3 DWDM Key Technologies y
Waveguide modulator is manufactured from titanium (Ti) diffused LiNbO2
substrate material on which waveguide is made via photoetching method. It has
many advantages such as small in dimension, light in weight and facile for optical
integration.
According to the integration and separation conditions of the laser and the external
modulator, external modulated lasers can be classified into two categories:
integrated external modulated laser and separated external modulated laser.
As a maturing technology, integrated external modulation becomes the
development trend of DWDM lasers. The commonly used modulator is
electroabsorption modulator which, small and compact and integrated with the
laser, meets most application requirements in performances.
Electroabsorption modulator, a kind of loss modulator, operates at the boundary
wavelength of the material absorption region. When the modulator isn't biased, the
wavelength from the laser is out of the absorption range of the modulator material.
Thus the launched power of this wavelength is maximum and the modulator is
turned on. When the modulator is biased, the boundary wavelength of the material
absorption region shifts and the wavelength from the laser is within this region.
Thus the launched power is minimum and the modulator is turned off, as shown in
Figure 3-2.
Biased
Unbiased
Absorption
region
Absorption
region
1
0
0
is the absorption side wavelength of unbiased modulator
is the absorption side wavelength of biased modulator
0 is the operating wavelength of the constant light source
1
2
Figure 3-2 Variation of the absorption wavelength of an electroabsorption modulator
Electroabsorption modulator can be manufactured by utilizing the same technical
process as semiconductor laser. Therefore, it is easy to integrate the laser and the
modulator, suitable for batch production. So its development speed is high. For
example, InGaAsP optoelectronic integrated circuit monolithically integrates a
3-24
2
DWDM Principle
Section 3 DWDM Key Technologies y
laser and an electroabsorption modulator on a single chip that is put on a
thermoelectric cooler (TEC). This typical optoelectronic integrated circuit is called
electroabsorption modulated laser (EML). It can support transmission of 2.5Gbit/s
signal over 600km, far exceeding the transmission distance of directly modulated
lasers. Its reliability is similar to that of standard DFB lasers with an average life
span of 20 years.
Separated external modulated laser generally uses constant output laser (CW) +
LiNbO3 Mach-Zehnder external modulator. This modulator separates the light
input into two equal signals that, respectively, enter the two branches. These two
branches employ electrooptic material whose refractive index changes with the
magnitude of the external electrical signal applied to it. Change of the refractive
index of the optical branches will result in variation of the signal phases. Hence,
when the signals from the two branches recombine at the output end, the combined
optical signal is an interfering signal with varying intensity. Via this method, the
information of the electrical signal is transferred onto the optical signal. Thus
optical intensity modulation is implemented. The frequency chirp of separated
external modulated laser can be zero. Moreover, its cost is relatively low when
compared to electroabsorption modulated external laser.
3.1.2 Wavelength Stability and Control of Laser
In DWDM system, wavelength stability of lasers is a critical problem. According
to ITU-T G.692, deviation of the central wavelengths shouldn't be greater than one
fifth (±1/10) of optical channel spacing, i.e. the deviation of the central
wavelengths shouldn't be greater than ±20GHz in a system with a channel spacing
of 0.8nm.
Because the optical channel spacing is very small (can be as low as 0.8nm),
DWDM system has strict requirements to the wavelength stability of the lasers. For
example, a 0.5nm variation of wavelength can shift an optical channel to another
one. In practical systems, the variation should be controlled within 0.2nm. The
specific requirement is determined according to the wavelength spacing, i.e. the
smaller the spacing, the higher the requirement. So the lasers should adopt strict
wavelength stabilization technology.
Fine tuning of the wavelength of integrated electroabsorption modulated laser is
mainly implemented by adjusting the temperature. The temperature sensitivity of
the wavelength is 0.008nm/
. The normal operating temperature is 25
adjusting the chip temperature from 15
to 35
. By
, the EML can be set up to a
specific wavelength with an adjustable range of 1.6nm. The chip temperature is
adjusted by changing the drive current of the cooler and using a thermal resistance
as feedback. Thus the chip temperature is stabilized and stays at a constant value.
3-25
DWDM Principle
Section 3 DWDM Key Technologies y
According to the correspondent characteristics of wavelength and chip
temperature, distributed feedback laser (DFB) controls its wavelength by
controlling the temperature of the laser chip to achieve wavelength stability. For
DFB laser, the wavelength-temperature coefficient is about 0.02nm/
central wavelength meets the requirement within the range of 15
-35
and its
. This
temperature feedback control method completely depends on the chip temperature
of the DFB laser. At present, MWQ-DFB laser technical process can guarantee that
the wavelength deviation meets the requirements of DWDM system in the life span
(20 years) of the laser.
Except for the temperature, laser drive current can also affect the wavelength. The
sensitivity is 0.008nm/mA, smaller than the affect of the temperature in one order.
In some cases, its effect is negligible. Additionally, package temperature may also
affect the device wavelength (e.g. temperature conduction brought by wires from
the package to laser platform and inward radiation from the package shell will also
affect the device wavelength). In a well-designed package, its effect can be
controlled to minimum.
The above methods can effectively solve the problem of short-term wavelength
stability. However, they are incapable of dealing with long-term wavelength
variation caused by factors such as laser aging. It is ideal to directly utilize a
wavelength sensitive component for wavelength feedback control of the laser. The
theory is shown in Figure 3-3. Standard wavelength control of this type of scheme
and reference frequency disturbance wavelength control are promising and being
developed.
Optical output
LD
LD control circuit
Wavelength sensitive
component
For wavelength control
Signal
processing
Figure 3-3 Theory for wavelength control
3-26
For wavelength monitoring
DWDM Principle
Section 3 DWDM Key Technologies y
Thinking:
Why does the DWDM system set strict requirements to the wavelength
stability?
3.2 Erbium-doped Optical Fiber Amplifier (EDFA)
As a key component of new generation optical communication systems, erbium
doped fiber amplifier (EDFA) has many advantages such as high gain, large output
power, wide operating optical bandwidth, polarization independence, low noise
factor and amplifying characteristic independent to system bit rate and data format.
It is an indispensable key component of high capacity DWDM systems.
3.2.1 EDFA Operating Theory
To amplify optical power, some passive optical components, pump source and
erbium-doped fiber are combined together according to specific optical structure.
Then EDFA optical amplifier is formed. Figure 3-4 shows a typical optical
structure of dual-pumping source erbium-doped optical fiber amplifier.
Optical splitter ISO
Signal input
WDM
WDM Optical coupler
TAP
EDF
Optical isolator
Pumping laser
PD
Pumping laser
ISO
Signal output
TAP
PD Optical detector
Figure 3-4 Typical internal light path of EDFA
As shown in Figure 3-4, signal light and pump light from the pumping laser are
combined via a DWDM multiplexer, then they are sent to the erbium-doped fiber
3-27
EDF
DWDM Principle
Section 3 DWDM Key Technologies y
(EDF). The two pumping lasers form a two-stage pump. Excited by the pumping
light, the EDF yields the amplification function. Therefore, the function of
amplifying the optical signal is implemented.
1.Erbium-doped optical fiber (EDF)
Erbium-doped optical fiber (EDF), doped with Er3+ of a given density, is the kernel
of the optical fiber amplifier. To illustrate its amplification principle, we need to
begin with the energy level diagram of Er3+. The outer-shell electrons of Er3+ have
three-level structure (E1, E2 and E3 in Figure 3-5), where E1 is ground state, E2 is
metastable state and E3 is high level, as shown in Figure 3-5.
E3 excited state
Decay
Pump
light
1550nm
signal light
E2 metastable state
1550nm
signal light
E1 ground state
Figure 3-5 EDFA energy level diagram
When high energy pumping lasers are used to excite the EDF, lots of bound
electrons of the erbium ions are excited from the ground state to the high level E3.
However, the high level is not stable and erbium ions are soon dropped to the
metastable state E2 via a radiationless decay process (i.e. no photon is released).
E2 level is an metastable energy band on which particles' survival span is relatively
long. Particles excited by the pumping light gather on this level via nonradiative
transition. Thus, population inversion distribution is implemented. When an optical
signal of wavelength 1550nm passes through this erbium-doped fiber, particles in
the metastable state are transited to the ground state via stimulated radiation and
generate photons identical to the photons of the incident signal light. This greatly
increases the quantity of the photons in the signal light, i.e. implementing the
function of continuous amplifying the signal light transmitted in the EDF.
2. Optical coupler (WDM)
3-28
DWDM Principle
Section 3 DWDM Key Technologies y
Optical coupler, as its name implies, has function of coupling. It couples the signal
light and the pumping light and sends them into the erbium-doped fiber. It, also
called optical multiplexer, usually employs optical fiber fusible cone multiplexer.
3. Optical isolator (ISO)
Optical isolator (ISO), a kind of component utilizing Faraday magnetooptical
effect, allows only unidirectional light transmission. Along the light path, the
functions of the two isolators are as follows: The input isolator can block the
backward ASE in the EDF, keep it from interfering the transmitters of the system
and from generating larger noise when it is reflected at the input end and reenter
the EDF. The output isolator prevents the amplified optical signal, when reflected
at the output end, from reentering the EDF, consuming particles and affecting the
amplification characteristics of the EDF.
4. Pumping laser (PUMP)
Pumping laser, the energy source of EDFA, provides energy for amplifying the
optical signal. Generally, it is a semiconductor laser with output wavelength of
980nm or 1480nm. When passing through the EDF, the pumping light pumps the
erbium ions from low level to high level. Thus population inversion is formed.
When the signal light passes through, the energy will be transferred to it. Hence,
optical amplification is implemented.
5. Optical splitter (TAP)
The optical splitter used in the EDFA is a one by two component. Its function is to
tap off a small part of the optical signal for monitoring the optical power of the
main channel.
6. Optical detector (PD)
The PD is an optical power detector. Its function is to convert the received optical
power into photocurrent via photoelectric conversion. Hence, it monitors the input
and output optical power of the EDFA module.
3.2.2 Applications of EDFA
According to its location in the DWDM optical transmission network, EDFA can
be classified into booster amplifier (BA), line amplifier (LA) and preamplifier
(PA).
1. Booster amplifier (BA)
Booster amplifier is installed behind the transmitters of terminal equipment or
regeneration equipment, as shown in Figure 3-6. The major function of the booster
3-29
DWDM Principle
Section 3 DWDM Key Technologies y
amplifier is to boost the launched power and elongate transmission distance by
enhancing the optical power injected into the fiber (generally above 10dBm). So in
some documents, it is also named as power booster amplifier. Here, its noise
characteristic requirement is not high. The major requirement is linear power
amplification characteristic. Generally, booster amplifier works in the saturation
range of gain or input power in order to enhance the conversion efficiency from
pumping source power to optical signal power.
Repeating section
D
W
D
M
D
W
D
M
BA
e q u ip
equip
ment
m e n t
Optical fiber connector
Figure 3-6 Location of the amplifier in the regenerator section
2. Line amplifier (LA)
Line amplifier is located in the middle of the whole regenerator section, as shown
in Figure 3-7. This is an application form to insert the EDFA into the optical fiber
transmission link and amplify the signal directly. A regenerator section can be
configured with multiple line amplifiers according to the demands. Line amplifier
is mainly applied in long-haul communication or CATV distribution networks.
Here, the EDFA is required to have high small-signal gain and low noise factor.
Repeating section
D
W
D
M
LA
equip
ment
D
W
D
M
equip
ment
Optical fiber connector
3-30
DWDM Principle
Section 3 DWDM Key Technologies y
Figure 3-7 Location of the line amplifier in the regenerator section
3. Pre-amplifier (PA)
Pre-amplifier is located at the end of the regenerator section but in front of the
optical receiving equipment, as shown in Figure 3-8. The main function of this
amplifier is to amplify the small signal attenuated along the link and enhance the
receiving sensitivity of the optical receiver. Here the main problem is noise. The
main noise in EDFA is amplified spontaneous emission (ASE). This noise makes
the optoelectronic detector output three noise components, i.e. extra shot noise due
to the increase of optical power, signal-ASE beat noise and ASE-ASE beat noise.
By using a narrow-band optical filter (1nm bandwidth), most ASE-ASE beat noise
can be filtered and extra shot noise can be reduced. But the signal-ASE beat noise
can't be filtered. Despite of this, the noise characteristic of EDFA is greatly
improved by adopting the optical filter. The pre-amplifier greatly improves the
sensitivity of receivers employing direct detection. For example, the sensitivity of
an EDFA receiver of 2.5Gbit/s can be up to -43.3dBm. An improvement of about
10dB is achieved when compared to the receivers employing direct detection
without EDFA.
R e p e a t in g s e c tio n
D
W
D
M
PA
e q u ip
m ent
O p tic a l f ib e r c o n n e c t o r
Figure 3-8 Location of the pre-amplifier in the regenerator section
Β
Tricks:
3-31
DWDM Principle
Section 3 DWDM Key Technologies y
BA, PA and LA differ from each other in that their locations in the DWDM
network are different. However, the most important difference lies in their input
optical power and gain:
BA: relatively high input optical power and low gain;
PA: relatively low optical power and low gain, similar to BA;
LA: relatively low input optical power, similar to PA, but its gain larger than BA.
3.2.3 Gain Control of EDFA
1. EDFA gain flatness control
In DWDM systems, the more the optical channels multiplexed, the more the optical
amplifiers needed in cascading. This requires that a single amplifier occupies a
wider and wider bandwidth.
However, EDFA based on ordinary pure silicon optical fiber has a very narrow flat
gain range between 1549 and 1561nm, a range of approximately 12nm. And the
gain fluctuation between 1530 and 1542nm is very large, up to about 8dB. When
the channel arrangement of the DWDM system exceeds the flat gain range,
channels near 1540nm will suffer severe signal-to-noise degradation and normal
signal output can't be guaranteed.
To solve the above-mentioned problem and adapt to the development of DWDM
systems, a gain flattened EDFA based on aluminum-doped silicon optical fiber is
developed. It greatly improves the operating wavelength bandwidth of the EDFA
and suppresses gain fluctuation. The up-to-date mature technology can achieve
1dB gain flattened range which almost expands to the whole erbium pass-band
(1525nm~1560nm). Basically, it has solved the problem of gain unflatness of
ordinary EDFA. Figure 3-9 compares the gain curves of non-aluminum-doped
EDFA and aluminum-doped EDFA.
3-32
DWDM Principle
Section 3 DWDM Key Technologies y
1525nm-1565nm
non-aluminum-doped EDFA
Gain
1525nm-1565nm
aluminum-doped EDFA
Gain
Figure 3-9 Improvement of EDFA gain curve flatness
Technically, the range of 1525nm~1540nm in EDFA gain curve is called blue band
area and the range of 1540nm ~1565nm is called red band area. Generally, red
band area is preferential when the transmission capacity is less than 40Gbit/s.
3-33
DWDM Principle
Section 3 DWDM Key Technologies y
Technical details:
Performance comparison of EDFA gain unflatness and flatness is given in Figure
3-10.
Cascading amplification of amplifier gain unflatness
Cascading amplification of amplifier gain flatness
Figure 3-10 Diagram of EDFA gain flatness
2. EDFA gain-locking
EDFA gain-locking is an important problem because the WDM system is a multiwavelength working system. When certain wavelengths are dropped, their energy
will be transferred to those undropped signals due to gain competition. Thus the
power of other wavelengths increases. At the receive end, abrupt increment of the
electrical level is possible to cause error. In limiting case, if seven wavelengths of
eight wavelengths are dropped, all the energy will concentrate to the one
wavelength left and its power may be up to about 17dBm. This will result in strong
nonlinear effects or receiving power overload of the receiver, and this will also
cause lots of errors.
There are many gain-locking technologies for EDFA. One typical method is to
control the gain of pumping laser. The internal monitoring electric circuit of the
EDFA controls the output of the pumping source by monitoring the input-output
power ratio. When some signals of the input wavelengths are dropped, the input
power will decrease and the output-input power ratio will increase. Via the
3-34
DWDM Principle
Section 3 DWDM Key Technologies y
feedback circuit, the output power of the pumping source will be reduced in order
to keep the gain (output/input) of the EDFA. Hence, the total output power of the
EDFA is reduced and the output signal power is kept stable. The process is shown
in Figure 3-11.
INPUT
OUTPUT
TAP
PUMP
PIN
TAP
PIN
Non-linear
control
Figure 3-11 Gain-locking technology of controlling the pumping laser
Another method is saturation wavelength. At the transmit end, except for the eight
operating wavelengths, system sends another wavelength as saturation wavelength.
In normal cases, the output power of this wavelength is very small. When some
line signals are dropped, the output power of the saturation wavelength will
automatically increase in order to compensate the energy of the lost wavelengths
and maintain the output power and gain of the EDFA. When the multi-wavelength
line signals are restored, the output power of the saturation wavelength will
correspondingly decrease. This method directly controls the output of the
saturation wavelength laser, so its speed is faster than controlling the pumping
source.
3-35
DWDM Principle
Section 3 DWDM Key Technologies y
Technical details:
falling
wavelength
>1dB
adding
wavelength
>1dB
Figure 3-12 NO Gain-locking when EDFA falling wavelength and adding wavelength
Falling
wavelength
adding
wavelength
<0.5dB
<0.5dB
Figure 3-13 Having Gain-locking when EDFA falling wavelength and adding wavelength
3.2.4 Limitations of EDFA
EDFA solves the problem of line attenuation in DWDM systems. However it also
brings some new problems.
3-36
DWDM Principle
Section 3 DWDM Key Technologies y
1. Non-linearity problem
Although enhanced by adopting EDFA, the optical power is not the higher the
better. When it reaches a certain level, the optical fiber will generate nonlinear
effects (including Raman scattering and Brillouin scattering). Especially, EDFA
has greater affect to stimulated Brillouin scattering (SBS). Nonlinear effects greatly
limits the amplification performance of the EDFA and the implementation of long
distance repeaterless transmission.
2. Optical surge problem
EDFA can enhance the input optical power rapidly. However, since its dynamic
gain variation is slow, optical surge will occur at the moment when the input signal
power jumps, i.e. a peak occurs to the output optical power. The optical surge
phenomenon is especially obvious in the case of EDFA cascading. The peak power
can be up to a few watts and is possible to damage the O/E converter and the end
surface of the optical connector.
3. Dispersion problem
Although the problem of attenuation limited repeaterless long haul transmission is
solved after adopting EDFA, the total dispersion increases as the distance becomes
longer. Thus the former attenuation limited system turns into dispersion limited
system.
3.3 DWDM Components
In a DWDM system, DWDM components are classified into two types: multiplexer
and de-multiplexer, as shown in Figure 3-14. The main function of the multiplexer
is to combine multiple signal wavelengths into a single optical fiber for
transmission. The main function of the de-multiplexer is to separate the multiple
signal wavelengths transmitted in a single optical fiber. The key to the performance
of a DWDM system is DWDM component whose requirements are enough
multiplexing channels, low insertion loss, large crosstalk attenuation, wide passband, etc. Multiplexer and de-multiplexer are the same in principle and only need
to change the input and output directions. The performances of the DWDM
components used in DWDM systems should meet the requirements defined in ITUT G.671 and other related recommendations.
3-37
DWDM Principle
Section 3 DWDM Key Technologies y
1
2
WDM
1
2
n
(a) Multiplexer
1
1
2
WDM
n
2
n
(b) De-multiplexer
Figure 3-14 DWDM components
There are many methods to manufacture DWDM components each of which has its
own features. At present, there are four types of widespread commercial DWDM
components: interference light filter type, optical fiber coupler type, optical grating
type and arrayed waveguide grating (AWG) type.
3.3.1 Optical Grating Type DWDM Component
Optical grating type DWDM component, a kind of angular dispersion type
component, employs the angular dispersion component to separate and combine
optical signals of different wavelengths. The most prevalent diffraction grating is
made by depositing epoxy resin on a glass substrate and then fabricating grating
lines on the epoxy resin to form a so-called reflective-type blazed diffraction
grating. When the incident light reaches the optical grating, the optical signals with
different wavelengths are reflected in different angles due to the angular dispersion
function of the grating. Then these signals are converged to different output optical
fibers via lenses in order to implement wavelength selection function. The inverse
process is also right, as shown in Figure 3-15. The advantage of the blazed
diffraction grating is high-resolution wavelength selection function which can
separate most energy of specific wavelength from other wavelengths in centralized
directions.
3-38
n
DWDM Principle
Section 3 DWDM Key Technologies y
1
2
3
4
5
n
output (in)
Diffraction
grating
1
2
n input (out)
Figure 3-15 Principle of blazed optical grating type DWDM component
The blazed grating type filter has excellent wavelength selectivity and can reduce
the wavelength spacing to about 0.5nm. Moreover, the grating type component is
parallel operated and its insertion loss doesn't increase with the number of
wavelengths multiplexed. Hence large number of multiplexing channels can be
achieved. At present, multiplexing of 131 wavelengths with a spacing of 0.5nm is
implemented and the isolation is good. For a wavelength spacing of 1nm, the
isolation is up to 5dB. The disadvantage of blazed grating is relatively large
insertion loss, generally 3~8dB. Moreover, it is very sensitive to polarization and
its optical channel bandwidth-to-spacing ratio isn't ideal. So the optical spectrum
utilization ratio isn't high enough. And the wavelength fault-tolerance requirement
for the laser and DWDM component is relatively high. Additionally, its
temperature drift varies with the thermal expansion coefficient and refractive index
of the material. Typically, the component temperature shift is relatively high,
approximately 0.012nm /
. If temperature control measures are adopted, the
temperature shift can be reduced to 0.0004nm /
. So temperature control
measures are feasible and necessary.
This optical grating requires high manufacturing accuracy and is not suitable for
mass production. Hence, it is generally applied in experimental scientific research.
Except for the above-mentioned optical fiber component, the manufacturing
technology for optical fiber Bragg grating filter is gradually maturing. It is
manufactured employing the interference of high power ultraviolet light beams to
form periodic variation of refractive index at the optical fiber core. The accuracy
can be up to 10000 lines per centimeter, as shown in Figure 3-16. Fiber Bragg
grating can be feasibly designed and manufactured with low cost. It has very low
3-39
DWDM Principle
Section 3 DWDM Key Technologies y
insertion loss and stable temperature characteristic. Its intraband filtering
characteristic is flat and out-of-band is very steep ( rolling slope is better than
150dB/nm and out-of-band suppress ratio is up to 50dB). This component can be
directly melted with the optical fiber of the system. So it can be fabricated into
band-pass or band-stop filter with small channel spacing. At present, it is
extensively applied in DWDM system. However this kind of optical fiber grating
has relatively narrow wavelength range, only applicable to single wavelength. The
benefit it brings in is that the filters can be added or removed according to the
number of wavelength used. So the application is flexible.
Ultraviole light interference
1
2
3
2
Periodic variation of the refractive index (grating)
Figure 3-16 Optical fiber Bragg grating filter
3.3.2 Dielectric Film Type DWDM Component
Dielectric film filter type DWDM component is a kind of interactive DWDM
component consists of dielectric films (DTF). DTF interference filter is composed
of tens of dielectric films of different material, different refractive index and
different thickness combined according to design requirements. Each layer is 1/4
wavelength in thickness. Layers of high refractive index and low refractive index
are alternatively overlapped. When the light incidents on the high refractive layer,
the reflected light has no phase shift. However, when the light incidents on the low
refractive layer, the reflected light undergoes a 1800 phase shift. Since the layer
thickness is 1/4 wavelength (900), the light reflected by the low refractive layer
undergoes a 3600 phase shift and in-phase superposes with the light reflected by
the high refractive layer. Thus, reflected lights of the layers superpose near the
central wavelengths and form intensive reflected light at the front-end surface of
the filter. In the highly backward reflecting area, the reflected light suddenly
decreases and most light becomes transmitted light. Accordingly, the film
3-40
DWDM Principle
Section 3 DWDM Key Technologies y
interference type filter can be made to band pass certain wavelength range and
band stop the other wavelength range, forming the required filter characteristics.
The structural principle of the film interference type filter is shown in Figure 3-17.
The main features of dielectric film filter DWDM component are as follows:
miniaturization and structural stability of the component can be implemented via
design, the signal pass-band is flat and polarization-independent, and its insertion
loss is low and channel isolation is good. The disadvantage is that the number of
channels can't be large. The specific characteristics are related to its structure. For
instance, if the film filter type DWDM component utilizes soft material, its
wavelength may be changed under the environmental influence because the filter
can easily absorb moisture. When employing hard dielectric film material, the
temperature stability is better than 0.0005nm/
. Additionally, this kind of
component has relatively long design and manufacturing process and low volume
of production. And if epoxy resin is used along the light path, it is not easy to
achieve high isolation and narrow bandwidth.
In DWDM systems, when only 4 to 16 wavelengths are involved, this type of
DWDM component is relatively ideal.
1-4
1 filter
Self-focusing lens
1
3 filter
2
3
4
Glass
Figure 3-17 Principle of film interference filter type de-multiplexer
3.3.3 Fused Conical Type DWDM Component
There are two types of optical fiber coupler. The extensively used one is fused
biconical tapered coupler, i.e. drawing multiple fibers under hot-melt condition to
3-41
DWDM Principle
Section 3 DWDM Key Technologies y
form a cone and slightly twisting them to fuse them together. Because the cores of
different fibers are extremely close, the required coupling power can be obtained
via evanescent wave coupling on the conical region. The second type of coupler
employs grinding and polishing methods to remove part of cladding of the optical
fiber so that only a thin cladding layer is left. Then two optical fibers processed via
the same method are butt jointed and coated a layer of index matched solution
between them. Thus the two fibers can couple via the evanescent wave in the
cladding and obtain the demanded coupling power. Fused conical type DWDM
component is simple to manufacture and is extensively applied.
3.3.4 Integrated Optical Waveguide Type DWDM Component
Integrated optical waveguide type DWDM component is a plane waveguide
component based on optical integration technology. The typical manufacturing
process is to deposit a thin layer of silica glass on the silicon substrate, form the
demanded pattern by utilizing photetch and etch. This component supports
integration manufacture and has great application prospective in future access
networks. Moreover, except for DWDM component, it can be fabricated into
matrix structure to add/drop optical signal channels (OADM). This is a preferred
scheme for implementing optical switching in future optical transport networks.
A typical component which uses integrated optical waveguide DWDM is arrayed
waveguide grating (AWG) optical multiplexer/de-multiplexer manufactured by
NTT Company, Japan. It has many advantages, including small wavelength
spacing, large number of channels and flat pass-band. So it is especially suitable
for ultrahigh-speed and large capacity DWDM systems. Its structural diagram is
shown in Figure 3-18.
Waveguide
grating
1
2
Free space
Fan-like
waveguide
3-42
Fan-like
waveguide
DWDM Principle
Section 3 DWDM Key Technologies y
Figure 3-18 Principle of AWG DWDM component
3.3.5 Performances of DWDM Components
Table 3-1 Comparison of various DWDM components
Componen
t type
Mechanis
m
Mass
producti
on
Channel
spacing
(nm)
Number
of
channels
Crosstalk (dB)
Insertion
loss (dB)
Main
disadvant
ages
Refractive
grating
type
Angular
diffraction
Average
0.5~10
131
-30
3~6
Temperat
ure
sensitive
Dielectric
film type
Interferenc
e/absorptio
n
Average
1~100
2~32
-25
2~6
Small
number of
channels
Fusible
cone type
Wavelengt
h
dependent
Relativel
y easy
10~100
2~6
-
0.2~1.5
Small
number of
channels
Integrated
optical
waveguide
type
Plane
waveguide
Easy
1~5
4~32
-25
6~11
Large
insertion
loss
3-43
10~45)
DWDM Networking
Section 4 DWDM Networking Design
Section 4 DWDM Networking Design
Objectives:
To understand the basic concepts of DWDM networking.
To master the configuration of different network elements of DWDM.
To master some factors to consider during DWDM network design.
To know general protection mechanisms of DWDM networks.
4.1 Some Network Element Types of DWDM
In terms of usage, DWDM equipment is generally classified into four types: optical
terminal equipment, optical line amplifier equipment, optical add/drop
multiplexing equipment and electrical regeneration equipment. Now we take the 16
32
-wavelength equipment of DWDM system as an example to explain the
functions of these network element types in the network.
4.1.1 Optical Terminal unit (OTM)
At the transmitter end, OTM multiplexes the STM-16 signals of 16 wavelengths
λ1~λ16 (λ32)into a DWDM main optical channel via the multiplexer, amplifies the
optical power of the main channel, and then adds an optical supervisory channel
雜.
At the receiver end, firstly OTM extracts the optical supervisory channel 雜. Then
the DWDM main optical channel is amplified and de-multiplexed into STM-16
signals of the 16/32 different wavelengths.
The signal flow of OTM is shown in Figure 4-1.
4-44
DWDM Networking
Section 4 DWDM Networking Design
A
RI
S
W
P
A
RO
C
RM
M
M
R
W
C
D
16
D
32
S
D
S
C
1
TM
H
A
TO
A
A
TI
M
T
W
C
M
W
B
A
16
M
M
32
Figure 4-1 OTM signal flow
4.1.2 Optical Line Amplifier Unit (OLA)
OptiX BWS 320G optical regeneration equipment is configured with an optical
line amplifier in each transmission direction. OLA of each direction firstly extracts
the optical supervisory channel (OSC) and processes it, then amplifies the main
optical channel signals, multiplexes them with the optical supervisory channel and
sends them onto the fiber. The signal flow of OLA is shown in Figure 4-2.
ROA
RI
S
W
P
A
A
TO
TI
W
B
A
M
S
M
C
RM1
C
2
TM1
TO
A
W
B
A
M
West
4-45
C
RM2
A
TI
M
TM2
S
W
P
A
East
A
RO
A
RI
DWDM Networking
Section 4 DWDM Networking Design
Figure 4-2 OLA signal flow
The whole equipment is installed in a subrack. In the figure, each direction
employs a pair of WPA+WBA to conduct optical line amplification. It can also use
single WLA or WBA to conduct unidirectional optical line amplification.
4.1.3 Optical Add/Drop Multiplexing Unit (OADM)
The optical add/drop multiplexing unit (OADM) of DWDM system operates in
two modes, i.e. a board uses static OADM to add/drop wavelengths or two OTMs
adopts back-to-back mode to form an OADM equipment which can add/drop
wavelengths.
1. Static add/drop multiplexing equipment of DWDM system
In HUAWEI OptiX BWS 320G DWDM system, optical add/drop multiplexer
equipment can use a board to implement static add/drop of wavelengths. Each
OADM equipment is capable of adding/dropping 1 to 8 wavelengths in order to
meet the practical demands of various projects.
After receiving the line optical signal, the OADM equipment firstly extracts the
optical supervisory channel and then uses a WPA to pre-amplify the main channel.
Via the ADD/DROP unit, a given number of signals are dropped from the optical
signal with 16/32 STM-16 according to wavelengths. The other wavelengths are
directly inserted into the main channel via the ADD/DROP unit. After power
amplification, the main channel is combined with the local optical supervisory
channel and sent to the remote end. The main channel between ADD/DROP units
is configured with a variable attenuator to adjust optical power equalization
between pass-through channels and ADD channels. Channels dropped at the local
station are required connect to the SDH equipment via RWC, and those added at
the local station are required to connect to the SDH equipment via TWC.
For example of an OADM (add/drop four wavelengths), its signal flow is shown in
Figure 4-3.
4-46
DWDM Networking
Section 4 DWDM Networking Design
RM1
TM2
SC2
TM1
RI
TO
S
C
A
SCC
R R
W W
C C
λ1 λ2
WPA
RO
RM2
OHP
T T
W W
C C
λ1 λ2
MR2
TO
WBA
TI
MR2
TI
S
C
A
RI
RO
WBA
λ1 λ2
T T
W W
C C
WPA
λ1 λ 2
R R
W W
C C
Figure 4-3 Static OADM signal flow
2. OADM equipment consists of two back-to-back OTMs
Two back-to-back OTMs are used to form an OADM equipment which can
add/drop wavelengths. This mode is more flexible when compared to the static
OADM which uses a board to conduct wavelength conversion. It can add/drop any
of wavelengths from 1 to 16/32, more feasible for networking. If a signal channel
isn't added/dropped at this station, it can directly access the TWC of the same
wavelength via the D16/D32 output port and then enter the M16/M32 board in the
other direction.
The signal flow of the OADM consisting of two back-to-back OTMs is shown in
Figure 4-4.
ROA
RI
S
C
RM
TM
TO
A
W
P
A
S
C
2
/1
A
D
32
TM
S
S
D
D
H
RM
T
W
C
M
16
M
32
A
TO
TI
M
W
B
A
S
M
TM
C
RM
H
A
A
TI
M
M
M
D
16
R
W
C
M
16
W
B
A
M
4-47
M
32
T
W
C
west
R
W
C
east
W
D
P
16 M A
D
32
M
A
A
RO
RI
DWDM Networking
Section 4 DWDM Networking Design
Figure 4-4 The signal flow of the OADM consisting of two back-to-back OTMs
4.1.4 Electrical Regeneration Unit (REG)
For projects adopting regenerator section cascading, electric regenerator (REG) is
required. The electric regeneration equipment has no services to add/drop and is
merely used to elongate dispersion limited transmission distance. The signal flow
of the electrical regeneration equipment is shown in Figure 4-5.
A
ROA
RI
S
C
RM
TM
TO
A
W
P
A
S
C
2
/1
A
D
32
M
W
B
A
S
M
32
M
TM
RM
RM
A
M
16
W
B
A
M
M
32
W
D
P
16 M A
T
W
C
west
east
Figure 4-5 The signal flow of the electrical regeneration equipment REG
Note:
4-48
D
32
TO
TI
M
16
TM
A
TI
M
M
M
D
16
T
W
C
M
C
A
A
RO
RI
DWDM Networking
Section 4 DWDM Networking Design
Other basic network unit types of DWDM equipment are included in these types,
with similar functions and same position in the network. They only differ in names.
The following contents related to the network element or board are described by
using OptiX BWS 320G equipment configuration and board.
4.2 General Constitution of DWDM Network
Basic network modes of DWDM system are point-to-point , chain and ring . Other
complex network forms can be combined by using these three modes. When
application together with STM-16 equipment, they can form very complex optical
transmission network.
4.2.1 Point-to-point Networking
SDH
OTM
SDH
OTM
Figure 4-6 Schematic diagram of WDM point-to-point networking
4-49
DWDM Networking
Section 4 DWDM Networking Design
4.2.2 Chain Networking
SDH
OADM
OTM
SDH
SDH
OTM
Figure 4-7 Schematic diagram of WDM chain type networking
4.2.3 Ring Networking
In local area network especially metropolitan network applications, DWDM optical
add/drop multiplexers can be used to form ring networks according to user
demands. Generally in ring networks, path protection ring and multiplex section
protection are provided by SDH equipment itself, so it is not necessary for the
DWDM equipment to provide other protection methods. But wavelength protection
can be provided according to user requirements. The ring networking is shown in
Figure 4-8.
4-50
DWDM Networking
Section 4 DWDM Networking Design
1~8
OADM
1~8
OADM
OADM
1~8
OADM
1~8
Figure 4-8 Schematic diagram of DWDM ring network
4.2.4 Network Management Information Channel Backup and Interconnection
Capability
High reliability is required by optical transmission networks adopting DWDM. In a
transmission network, network management information is transmitted via a
supervisory channel which generally uses the same physical channel as the main
channel. Thus, the supervisory channel will also fail when the main channel fails.
So network management information backup channel is required.
In ring networking, when certain section fails (e.g. fiber cable damage), network
management information can be automatically switched to the supervisory channel
in the other direction of the ring. So the management of the whole network won't
be affected. Figure 4-9 illustrates an automatic backup approach of the network
management information for ring networking.
4-51
DWDM Networking
Section 4 DWDM Networking Design
NMS
NE
NE
Management information
GNE
Management information
NE
NE
Figure 4-9 Network management information channel backup in ring networking (when certain
transmission section fails)
However, when both of the two ends of certain office in a fiber section fail or
certain transmission section in point-to-point and chain networking fails, network
management information channel will fail. Consequently, network management
administrators won't be able to get the supervisory information of failed office and
do some operation with it.. To avoid this circumstance, network management
information should use the backup channel. The SDH network elements can
provide backup network management information channel by using data
communication network.
Between two network elements in need of protection, a network management
information backup channel can be established by accessing the data
communication network via routers. When the network is normal, network
management information is transmitted by the main supervisory channel, as shown
in Figure 4-10.
4-52
DWDM Networking
Section 4 DWDM Networking Design
NMS
Main channel/Supervisory channel
GNE
NE
Management information
Router
Router
DCN
Backup supervisory channel
Figure 4-10 Network management information channel backup (in normal case)
When the main channel fails, network elements automatically switch the
management information to the backup channel to guarantee that the network
management system can supervise and operate the entire network. The whole
switching process is conducted automatically, not requiring manual intervention. A
network management information channel backup is illustrated in Figure 4-11.
NMS
Main channel/Supervisory channel
GNE
NE
Management information
Router
DCN
Backup supervisory channel
4-53
Router
DWDM Networking
Section 4 DWDM Networking Design
Figure 4-11 Network management information channel backup (in case of main channel failure)
Attention: In network planning, different routes should be selected respectively for
backup supervisory channel and main channel. Otherwise backup function won't be
implemented.
HUAWEI OptiX BWS 320G equipment provides various data interfaces (e.g. RS232 and Ethernet interface) for management information channel interconnection
among different DWDM networks and between DWDM and SDH. This
implements unified network management for different transmission equipment.
Figure 4-12 illustrates management information channel interconnection among
different transmission equipment.
Management
Information
Channel
Management
Information
Channel
Management
Information
Channel
NMS
Figure 4-12 Network interconnection among different transmission equipment
4-54
DWDM Networking
Section 4 DWDM Networking Design
4.3 Factors To Be Considered in DWDM Networking
4.3.1 Dispersion Limited Distance
1. Dispersion effect description
Dspersion, caused by transmitter optical spectrum characteristic and optical fiber
dispersion, is a dominant factor which limits the transmission capacity.
Generally, adding optical amplifier into a system won't remarkably change the total
dispersion. As the active gain media in the EDFA, rare-earth-doped optical fiber
will cause a little dispersion. The length of this fiber is only in order of several
tens or hundreds. The dispersion of rare-earth-doped fiber has little difference with
that of fibers defined in ITU-T Recommendations G.652, G.653 and G.655. For a
system of tens or hundreds of kilometers, the effect of this dispersion is negligible.
2. Transmission limitation
As the transmission rate in optical fiber communication systems continuously
grows and because optical amplifier greatly elongates the no-electrical-regenerator
optical transmission distance, the total dispersion and the corresponding dispersion
penalty of the whole transmission link may become very large and must be
seriously dealt with. Dispersion limitation has currently become the determinant of
the regenerator section length. In single mode optical fiber, major dispersion
includes material dispersion and waveguide dispersion. It results in different time
delay for different frequency components when arriving at the optical receivers
after transmitted via the optical fiber. In time domain, it causes broadening of
optical pulses, crosstalk among them and degradation of eye patterns, and finally
results in the degradation of system error performance.
Different frequency components in the signal are originated from the laser source
optical spectrum characteristics, including wavelength, spectrum width, laser chirp,
etc. At present, the -20dB optical spectrum width of SLM lasers at 1550nm region
can be up to 0.05nm. In this case, laser chirp is the determinant limiting the
regenerative length.
3. Method of reducing the effect
Since the presence of optical amplifiers doesn't affect the dispersion effects in the
system, it is not required to regulate specific methods of reducing these effects to
minimum. However, EDFA, which makes possible long distance no-regenerative-
4-55
DWDM Networking
Section 4 DWDM Networking Design
regenerator system, will aggravate the impairment caused by the dispersion in the
system.
In some subsystems of optical amplifier, a type of passive dispersion compensation
device can be assembled with the optical amplifier to form an amplifier subsystem.
This subsystem will add limited dispersion to the system. And the dispersion
coefficient, inverse to the optical fibers of the system, will reduce the system
dispersion. This device can be installed together with an EDFA to compensate the
loss related to the passive dispersion compensation function. Additionally,
adopting G.655 optical fiber and G.653 optical fiber is favorable for dispersion
reduction. If nonlinear impairments are considered thoroughly, G.655 optical fiber
has optimal over-all properties in long haul transmission.
4. Consideration in network design
In DWDM network design, firstly the whole network is divided into several
regenerator sections, letting the length of each section less than the dispersion
limited distance of the laser. Hence, the performance of the whole network can
tolerate the effect of dispersion.
Β
Tricks:
When we calculate dispersion during DWDM network design, the typical
dispersion coefficient at 1550nm window is 17ps/nm.km because optical fibers
employed in the world are primarily G.652 fiber. But in engineering design,
20ps/nm.km is adopted for budget.
4.3.2 Power
Long distance transmission of optical signal requires that the signal power is
enough to compensate the attenuation of the optical fiber. Generally, the
attenuation coefficient of G.652 optical fiber at 1550nm window is about
0.25dB/km. When factors such as optical connectors and optical fiber redundancy
are taken into consideration, the combined optical fiber attenuation coefficient is
generally less than 0.275dB/km.
During practical calculation, power budget is only conducted for two pieces of
adjacent equipment in the transmission network instead of conducting unified
power budget for the whole network. The distance (attenuation) between two
4-56
DWDM Networking
Section 4 DWDM Networking Design
pieces of adjacent equipment in the transmission network is called regeneration
distance (attenuation).
S
R
Pout
P in
Station A
L
Station B
Figure 4-13 Schematic diagram of regeneration attenuation
As shown in above figure, S is the transmit reference point of station A, R is the
receive reference point of station B and L is the transmission distance between
point S and point R. Then:
regeneration distance =
Pout - Pin) /a
Pout: channel output power of point S (in dBm). The optical power of point S is
related to the configuration of point A.
Pin: permissive channel minimum input power of point R (in dBm).
a: optical fiber cable attenuation per kilometer (dB/km) (using 0.275dB/km
according to ITU-T recommendations. It contains the effect of various factors,
including connectors and redundancy).
4.3.3 Optical Signal-to-Noise Ratio
1. Generation principle of noise
Optical amplifier creates light around the signal wavelength, i.e. amplified
spontaneous emission (ASE). In a transmission system with several cascading
EDFAs, ASE noise of optical amplifiers will regenerate a periodic attenuation and
amplification process. Because in each optical amplifier the ASE noise input is
amplified and superimposed to the ASE generated by that optical amplifier. Hence
the total ASE noise power will increase with the number of amplifiers in
approximate proportion and the signal power will decrease. The noise power may
exceed the signal power.
ASE noise frequency spectrum distribution is expanded with the system length.
When the ASE noise from the first optical amplifier is sent to the second one, the
gain distribution of the second optical amplifier will change due to the ASE noise
4-57
DWDM Networking
Section 4 DWDM Networking Design
caused by gain saturation effect. Similarly, the effective gain distribution of the
third optical amplifier will also change. This effect will be transmitted downstream
to the next optical amplifier. Even though narrow band filter is used in each optical
amplifier, ASE noise will also accumulate. This is because the noise exists in the
signal frequency band.
Optical signal-to-noise ratio (OSNR) is defined as:
OSNR = channel optical signal power / channel noise optical power
2. Transmission limitation
ASE noise accumulation affects the system SNR because SNR degradation of the
received signal is mainly caused by ASE related beat noise. This type of beat noise
linearly increases with the number of optical amplifiers. Hence, error rate degrades
as the number of optical amplifiers increases. Besides, the noise is accumulated in
exponential form to the gain amplitude of the amplifier.
As the result of optical amplifier gain, the ASE noise frequency spectrum will have
a wavelength peak after accumulating in many optical amplifiers. To specially
point out, when adopting closed all-optical ring, the ASE noise will infinitely
accumulate if cascading infinite number of amplifiers. Although in systems with
filters ASE accumulation is remarkably decreased due to the filters, intraband ASE
will still increase as the number of optical amplifiers increases. Hence, SNR will
degrade with the increase of amplifiers.
3. Methods of ASE reduction
ASE noise accumulation may decrease as the interval of optical amplifiers reduces
(when the total gain is equal to the total transmission channel attenuation) because
ASE accumulates in exponential form with the increase of the gain amplitude of
the amplifier. One of the following filter technologies can further reduce
unexpected ASE noise, i.e. adopting ASE noise filter or utilizing self-filtering
effect (self-filtering method).
Self-filtering method is suitable for systems with tens of or more optical amplifiers.
This method adjusts the signal wavelength to the self-filtering wavelength in order
to reduce the ASE noise received by the detector, similar to using a narrow band
filter. This is most effective when the approach of reducing optical amplifier
interval and employing low gain optical amplifier is used to reduce the initial ASE
noise.
If all-optical DWDM closed ring network is adopted, the self-filtering method isn't
suitable. In fact, the peak formed in the whole gain frequency spectrum of the
optical amplifier may severely affect system performance. In this case, utilizing
4-58
DWDM Networking
Section 4 DWDM Networking Design
ASE filtering method can utmost reduce ASE noise accumulation. This is achieved
via the approach of filtering the DWDM channels which are not sent to the
network node before being switched out the node.
For systems with a few optical amplifiers, self-filtering method is not as effective
as ASE filtering method. ASE filtering method can flexibly select signal
wavelengths and has other advantages. Characteristics of the filter must be
carefully selected because cascading filter has narrower pass-band than the signal
filter (unless it has a rectangular frequency band).
4. Considerations for OSNR in DWDM network design (Note: If you feel
that the contents of this part is a bit abstruse, it is OK to skip it and read
next chapter)
For different network applications, OSNR requirements are almost the same, with
slight differences as shown in Table 4-1.
4-59
DWDM Networking
Section 4 DWDM Networking Design
Table 4-1 OSNR comparison
Amplifier cascading type
Minimum OSNR (dB)
32-channel 8×22dB system (8*80km)
22
32-channel 5×30dB system (5*100km)
20
32-channel 3×33dB system (3*120km)
20
OSNR is one of the most important factors that affect the DWDM
system error performance. For a DWDM system with multiple cascading
optical line amplifiers, the noise power is dominated by amplified
spontaneous emission (ASE) noise.
1)ASE noise accumulation of cascading optical line amplifier
The mathematical model of the ASE noise accumulation in multiple
cascading optical line amplifiers is illustrated in Figure 4-14.
G1
G2
L1
EDFA1
GN-1
L2
EDFA2
GN
LN-1
EDFAN-1
EDFAN
Figure 4-14 The mathematical model of the ASE noise accumulation
In Figure 4-14, GN is the gain of EDFAN (in linear unit); LN is the optical fiber
cable attenuation of the regenerator section N (in linear unit).
The total ASE noise power = The ASE noise power generated by EDFAN
+ (The ASE noise power generated by EDFAN-1LN-1 GN)
+ …+ (The ASE noise power generated by EDFA2L2 G3 匞N-1 LN-1 GN)
+ (The ASE noise power generated by EDFA1L1 G2 匞N-1 LN-1 GN)
( Equation 4-1)
2)Noise of a single EDFA
4-60
DWDM Networking
Section 4 DWDM Networking Design
The ASE noise power per unit frequency band generated by a single optical
amplifier, PASE is
PASE=2NSP
G-1) hν
(4-2)
where NSP is the spontaneous noise coefficient of the EDFA;
G is the internal gain of the EDFA;
h is the Planck constant;
v is the optical frequency.
The external noise coefficient of the amplifier, NF is
NF=10Log[2NSP
2NSP
1) /G]
ηin
(4-3)
ηIN is the insertion loss of the amplifier (in dB).
3?¨º?Simplified calculation of network OSNR in case of uniform attenuation of the
regenerator sections.
Assume that all EDFAs have the same properties and all regenerator sections have
uniform attenuation, the total power (including the accumulated ASE power) of
each amplifier is the same, and G>>1, where G=L. According to (4-1) and (4-2),
(4-3) undergoes a series of processing. Then OSNR is given by:
OSNR=POUT
L
NF
10LogN
10Log[h ν
ν0]
(4-4)
where POUT is the channel output power (in dBm);
L is the attenuation between amplifiers (in dB);
NF is the external noise coefficient (in dB);
N is the number of intervals along the link;
ν 0 is the optical bandwidth;
10Log[h ν
ν0]=-58 dBm (1.55μm band zone with 0.1nm bandwidth).
This calculation method can meet the requirements of the general engineering
design. However, the following conditions must also be satisfied except for the
above-mentioned assumptions.
⌧ The optical de-multiplexers have no periodic characteristic;
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DWDM Networking
Section 4 DWDM Networking Design
⌧ The optical transmitters have enough extinctivity.
In practical DWDM systems, EDFA gain inequality may cause difference in
channel output power and EDFA noise coefficient. So during design, the OSNR of
the worst channel should meet the requirements and have sufficient redundancy.
4.3.4 Other Factors
1. Stimulated Brillouin Scattering (SBS)
1) Principle
In the intensity-modulated system employing narrow spectrum line breadth laser,
the strong forward transmission signal will convert to backward transmission once
the signal optical power exceeds the stimulated Brillouin scattering (SBS)
threshold. In SBS, the forward transmission light is scattered in the form of
photons. Only the backward scattered light is in single mode optical fiber. The
scattered light is shifted from 1550nm by about 11GHz.
SBS effect has a minimum threshold power. However, research indicates that
different types of optical fibers and even different optical fibers of the same type
have different SBS threshold power. For external modulation systems adopting
narrow spectrum line lasers, the typical SBS threshold power is on the order of
20~30mw. Since the effective core area of G.653 fiber is relatively small, the SBS
threshold power of systems adopting G.653 fiber is lightly lower than that of
systems adopting G.652 fiber. This is true for all the nonlinear effects. SBS
threshold power is sensitive to the spectrum line breadth of the laser and the power
level.
2) Transmission limitation
SBS greatly limits the optical power transmittable in the fiber. Figure 4-15
describes this effect for narrow band lasers, where all the signal power falls into
the Brillouin bandwidth. Then the forward transmission power gradually saturates
and the backward scattering power rapidly increases.
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Section 4 DWDM Networking Design
15
15
10
5
5
-5
0
-15
-5
-25
-10
-15
-35
-5
0
5
10
15
20
25
Input power
Figure 4-15 SBS threshold of the narrow band laser
3) Methods of reducing the effect
In a system whose laser line breadth is apparently larger than the Brillouin
bandwidth or whose signal power smaller than the threshold power, SBS
impairment won't occur.
2. Stimulated Raman Scattering (SRS)
1) Principle
SRS is a broadband effect related to the interaction of light with silicon atom
vibration modes. SRS makes the signal wavelength works as a Raman pump of the
channels of longer wavelength or the Raman-shifted light of spontaneous
scattering. In any circumstance, the signals of shorter wavelength will always be
weakened by this process. At the same time, the signals of longer wavelength will
be enhanced.
2) Transmission limitation
SRS may occur in both single wavelength systems and multi-wavelength systems.
In systems with a single wavelength and no line amplifier, the signals may be
impaired by this effect when its power is greater than 1W. However, in multiwavelength systems of relatively wider channel spacing, the channels of shorter
wavelength will lose a portion of power to the higher-wavelength channels due to
4-63
Output power
Scattered power
25
DWDM Networking
Section 4 DWDM Networking Design
the effect of SRS, leading to a degradation of the signal-to-noise ratio performance.
This may limit the total capacity of systems with fixed total number of channels,
channel spacing, mean launched optical power and total system length. The SRS
dispersion threshold of systems adopting G.653 optical fiber is slightly lower than
that of systems employing G.652 fiber because G.653 fiber has smaller equivalent
core area. SRS won't cause practical degradation effect for single wavelength
systems. However, it may limit the capacity of DWDM systems.
3) Method of reducing the effect
In single wavelength systems, optical filters can be used to filter the unwanted
frequency components. However, up to now there are no practical technologies for
multi-wavelength systems to eliminate the effect of SRS. The effect of SRS effects
can also be released by reducing the signal power. Nevertheless, no apparent SRS
limitation has appeared in the carefully-designed DWDM systems implemented at
the present time.
3. Self-phase Modulation (SPM)
1) Principle
Because of the Kerr effect, instantaneous variations in the power of an optical
signal result in self modulation. This effect is called self-phase modulation. In
single wavelength systems, SPM effect will broaden the signal's spectrum when
changes in the signal's intensity result in variations in its phase, as shown in Figure
4-16. In the normal dispersion zone of optical fiber, signals propagating along the
fiber will experience a longer instantaneous widen once the frequency spectral
broadening is caused by SPM due to the dispersion. In the abnormal dispersion
zone, the dispersion of optical fiber and SPM may compensate with each other.
Thus the signal broadening will be smaller.
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Section 4 DWDM Networking Design
Figure 4-16 Compression and spectrum broadening of the transmission pulse caused by selfphase modulation
2) Transmission limitation
Generally, SPM is relatively apparent only in systems with high accumulated
dispersion or ultra long length. Dispersion limited systems may be unable to
tolerate SPM effects. In multi-wavelength systems of narrow channel spacing,
spectral broadening caused by SPM may lead to interference between adjacent
channels.
In G.652 optical fiber, SPM of the low chirp intensity modulated signal leads to
compression of the pulse. For G.655 optical fiber of abnormal dispersion
characteristic, the SPM effect of the signal is a function of the transmitter power.
Pulse compression can suppress the dispersion and provide certain dispersion
compensation. However, the maximal dispersion limitation and the corresponding
transmission distance limitation still exist.
Figure 4-16 illustrates compression of the transmission pulse caused by SPM of the
low chirp intensity modulated signal in G.652 optical fiber, also it can be regarded
as spectral broadening.
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Section 4 DWDM Networking Design
3) Method of reducing the effect
To adopt G.653 optical fiber and configure the signal channels near the zerodispersion zone will benefit the reduction of SPM effects. For systems employing
G.652 optical fiber and less than 100km in length, the effects of SPM can be
controlled by using dispersion compensation in appropriate intervals. The SPM
effects can also be weakened by reducing the input optical power or configuring
the system operating wavelengths over the zero dispersion wavelength of G.655
fiber.
4. Cross-phase Modulation (XPM)
1) Principle
In multi-wavelength systems, when variations in light intensity lead to a phase
shift, cross-phase modulation will generally broaden the signal spectrum due to the
interaction between adjacent channels. The spectral broadening caused by XPM is
related to the channel spacing. This is because the dispersion caused by difference
in group velocities may lead to the interaction among the pulses which should
separately propagate along the optical fiber. In case that XPM results in spectral
broadening, the signals will suffer a relatively large instantaneous spectral
broadening due to the dispersion effect when propagating along the optical fiber.
2) Transmission limitations
Impairment caused by XPM in G.652 fiber-optic systems is more obvious than that
in G.653 and G.655 fiber-optic systems. The broadening, caused by XPM, leads to
interference between adjacent channels in multi-wavelength systems.
3) Methods of reducing the effect
XPM can be controlled by selecting appropriate channel spacing. Study shows that
the signal distortion caused by XPM in multi-wavelength systems only occurs
between adjacent channels. In a 3-channel system, the signal-to-noise ratio (SNR)
of the central channel is nearly equal to that of single channel systems. This is
because the channel spacing has increased. Hence, the effect of XPM is negligible
if signal channels have appropriate spacing. In simulation experiments for systems
with a channel power consumption of 5mw, it is approved that a channel spacing
of 100GHz is enough for reducing the effects of XPM. The dispersion penalty
caused by XPM can be controlled by adopting dispersion compensation in proper
intervals along the system.
5. Four-wave Mixing (FWM)
1) Principle
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Section 4 DWDM Networking Design
Four-wave mixing (FWM), also called four-phonon mixing, occurs in the case that
two or three lightwaves with different wavelength interact and cause new
lightwaves at other wavelengths. These extra wavelengths are so called mixing
products or sidebands. This interaction may occur among signals in multiwavelength systems and EDFA ASE noises and between main modes and side
modes.
In case of 3 signals, the mixing products are shown in Figure 4-17.
f113
F132,312
f221
F123,213
f112 f223
F1
F2
F231,321
f332 f331
F3
Figure 4-17 The mixing products caused by 3-wave interaction
When channel spacing is equal, these products will right enter the adjacent signal
channels. If the phase matched condition is reached between the sideband and the
initial signal, these two lightwaves propagating along the optical fiber will generate
highly efficient FWM.
2) Transmission limitation
Occurrence of FWM sidebands may cause remarkable reduction to the signal
power. Even more severely, residual interference occurs when the mixing products
directly enter the signal channels. This kind of interference is determined by the
interaction between the phases of the signals and the sidebands and indicated by
the increase and decrease of the signal pulse amplitude.
Residual loss leads to closure of the eye pattern of the receiver and causes bit-error
rate (BER) performance degradation. The effect of FWM can be reduced by the
breakdown function of frequency spacing and dispersion and the phase matching
among lightwaves. G.652 fiber-optic systems suffer less impairment of FWM than
those adopting G.653. On the contrary, if a signal is right located at or near the
zero-dispersion point, FWM may surge in a relatively short fiber length (i.e. tens of
kilometer). Moreover, FWM is sensitive to the channel spacing.
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Section 4 DWDM Networking Design
Four-wave mixing may cause severe damage to multi-wavelength systems adopting
ITU-T G.652 optical fiber because signals can merely tolerate a very small
dispersion. In single channel systems, FWM interaction may occur between signals
and ASE noises, as well as the main modes and sidemodes of the transmitters. The
ASE phase noises accumulated by optical Kerr effect are superimposed to the
signal carriers and cause the broadening of the rear part of the signal spectrum.
3) Method of reducing the effect
As mentioned above, FWM band can be suppressed by utilizing the fiber
dispersion such as G.655. FWM damage can be released by arranging uneven
channel spacing. To lower the power level of G.653 fiber-optic systems can permit
multi-wavelength operation, but this will weaken the advantages of the optical
amplifiers.
To properly suppress the generation of mixing products, a scheme has been
proposed (an existing recommendation or new recommendation for future study) to
adopt the optical fiber with a minimum permissible dispersion (non-zero
dispersion) in the amplification bandwidth of EDFA. It is also a possible scheme to
use the non-zero dispersion optical fiber of inverse dispersion characteristic as
replacement section. However, this replacement may encounter difficulties during
installation, operation and maintenance because of the introduction of another kind
of fiber. Some similar methods are discovered to adopt long fiber sections of
limited dispersion and short fiber sections of inverse but relatively greater
dispersion (for compensation).
A scheme has been proposed to adopt uneven and relatively large channel spacing
to reduce the nonlinear effects and allow to arrange DWDM systems in G.653 fiber
to reduce the effect of FWM. To use uneven channel spacing can guarantee that the
mixing products caused by three or more channels won't fall into the wavelengths
of other channels. However, the power transfer from the signals to the mixing
products (i.e. power loss of the signals) keep fixed due to the configuration of
uneven channel spacing, and will still lead to remarkable closure phenomenon of
the eye pattern. Increase of the channel spacing can also reduce the effect of FWM.
This kind of remission technology may be restricted since the gain spectrum will
be narrowed due to the cascading of optical amplifiers and the amplification
spectrum will be narrowed due to the access of optical amplifiers.
Thinking:
At 1550nm window and among three types of optical fiber: G.652, G.655 and
G.653, which fiber has the most severe FWM effect? why?
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Section 4 DWDM Networking Design
6. Polarization Mode Dispersion (PMD)
1) Principle
As we know, the fundamental mode in a circular symmetric dielectric waveguide is
dual-degenerate. In a physical optical fiber, this degeneration is separated by
birefringence. For polarization-maintaining fibers, birefringence is deliberately
introduced. However, for general communication optical fibers, birefringence is an
unexpected product which is randomly introduced due to the stress perturbation the
fiber suffers.
For birefringent optical fibers, the first term generates a group delay time called
polarization dispersion. This kind of polarization dispersion leads to a group delay
difference between the orthogonal polarization states, as shown in Figure 4-18.
Slow in propagation
Differential group delay
Fast in propagation
Figure 4-18 Occurrence of group delay between the orthogonal polarization states
Although PMD effect randomly changes the polarization state of pulses
propagating in optical fiber, a pair of orthogonal states or primary states can be
determined, i.e. the signal incident to the fiber at the input end keeps its
polarization state at the output end. For the first term, these states are independent
to the wavelength. However, in some cases the occurrence of the primary states
may be related to the wavelength. This, together with the dispersion of the optical
fiber, will lead to further degradation.
Birefringence of optical fiber is randomly introduced due to factors such as stress,
bending, twisting and temperature. Random birefringence mechanism redetermines
the local birefringence axis along the optical fiber and leads to the coupling
between polarization modes. The fiber length between this change is called
coupling length. The coupling length of an optical fiber refers to the sum of the
average value of total local coupling length.
2) Transmission limitation
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Section 4 DWDM Networking Design
In digital transmission systems, PMD leads to intersymbol interference. When the
total dispersion is equal to 0.4T (T is the bit period), an optical power penalty of
about 1dB is introduced. Current study shows that, optical fibers or optical fiber
cables are apt to be standardized according to mean PMD, as well as digital
transmission systems. It is predicted via computer simulation that the probability
for the optical power penalty of a system to exceed 1dB is less than 10-9 if the
mean PMD isn't greater than 0.1T.
In a long distance amplified systems employing polarization scrambler (a
component which deliberately modulates the polarization state of the laser and
makes it work in an unpolarized state), PMD leads to the increase of signal
polarization. The interaction between polarization dependent loss and polarization
hole-burning causes the degradation of system performances. When additional
polarization dependent loss occurs in the system, greater secondary loss will be
aroused.
The secondary effect may generate coupling between PMD and dispersion and
increase the statistical component of the dispersion. This field is under study.
3) Method of reducing the effect
Since the problem is caused by birefringence, all the efforts for reducing the effects
of PMD are related to reducing the birefringence introduced during optical fiber
cable manufacturing, such as optimizing optical fiber manufacturing, guaranteeing
the concentration of optical fiber, reducing the residual of fiber core and employing
accurate cable structure. Typical mean PMD of optical fiber cables is in the
following range:
0 < () < 0.5ps/ km
Another method is to add polarization controllers at the input end and the output
end. A polarization splitter is connected after the output polarization controller and
used to generate an error signal. The output polarization controller searches this
error signal and readjusts the polarization controller to minimize the error signal.
At the zero-error signal point, the input polarization state is the primary state of the
system. This technology has been used to compensate a 5Gbit/s system. Coherent
frequency division multiplexing systems also adopt similar technology.
7. Polarization Dependent Loss (PDL)
1) Principle
Polarization dependent loss is caused by dichromatism of optical passive
components such as isolator and coupler. When a signal passes through a
dichromatic component, its electric field part parallel to the loss axis will suffer
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Section 4 DWDM Networking Design
certain attenuation. Like PMD, the axis direction which determines the PDL is
randomly changed.
2) Transmission limitation
In amplified systems, the amplifiers operate in the power conservative mode. The
signal and the noise are affected by PDL. However, the signal and the noise suffer
different effects because the noise is unpolarized. The noise can be divided into a
component parallel to the signal and another one orthogonal to the signal. Optical
amplification may increase the component orthogonal to the signal. Additionally,
variations of the signal polarization lead to mode dispersion. Thus, magnitude of
the orthogonal component of the noise is time-varied. This will reduce the signalto-noise ratio at the receiver end and cause impairment to the system.
3) Methods of reducing the effects
For PMD, it is important to reduce the polarization mode dependent loss of the
components. To be pointed out, the effect of polarization mode dependent loss
increases with the number of the amplifiers. For example, this requirement is
extremely strict in long distance submarine systems. In short distance systems with
only several amplifiers, the effect of polarization mode dependent loss is for further
study.
8. Polarization Hole-burning (PHB)
1) Principle
Polarization hole-burning (PHB) is the result of the anisotropic saturation caused
by the polarization saturated signal light incident in the erbium-doped optical fiber.
This will reduce the options of stimulated states utilizing the polarization field to
locate. Hence, the available gain in the orthogonal direction is relatively large.
Although erbium ions are randomly distributed in the glass fiber rod material,
dipoles related to the erbium ions are anisotropic in the micro level. When the
linear polarization saturated signal is equidirectional to the primary axis of the
dipoles, the polarization hole-burning has the greatest effect. However, when the
polarization state of the saturated signal is elliptical or circular, its effect decreases.
Because the total differential gain is the vector sum of these two effects, both the
signal laser and the pumping laser will affect the total effect. The degree of holeburning is in direct proportion to the polarization. Unpolarized saturated signals
have no hole-burning problem. This case, as a whole, is similar to the case of
circular polarization signal.
2) Transmission limitation
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Section 4 DWDM Networking Design
Because it makes the noise formed along the link larger than the noise budget
calculated according to the simple linear theory, PHB will affect the performances
of the system. The effects are that the signal-to-noise ratio decreases due to the
PHB and that the ultimately measured Q value fluctuates under PMD and PDL.
Since there are two factors affecting PHB, there are two ways to affect the system
performances. The total effect is in direct proportion to the gain saturation and
increases with the saturation.
Firstly, we consider the effect of the polarized pumping laser. To reach the purpose
of the discussion, the pumping polarization can be assumed as fixed. Pumping
causes differential gain in the orthogonal polarization axis direction. The noise
orthogonal to the pump is greater than the noise equidirectional to the pump.
However, the polarization axes of the pumping lasers of the amplifiers along the
link are incoherent to each other. The accumulation effect shall be similar to a
random walk. The pump which results in PHB can be regarded as a related factor
to the PDL of an amplifier. Hence, the noise obtained by averaging the number of
the amplifiers should be linear, the same as the budget calculated by the simple
linear theory.
Signal lasers which cause PHB are slightly different. The lasers are used to
propagate signals, so the polarization noise equidirectional to a signal laser obtains
the same gain as the signal. However, the noise orthogonal to the signal laser is
always orthogonal to the polarization axis of the signal. Hence, the signal increases
in a nonlinear mode along the amplified link.
The total differential gain caused by PHB will change with the variations of the
signal polarization state along the amplified link (caused by PMD). It changes
because the hole-burning effect of the signal is related to the pump effect. When
staying in their corresponding polarization states, the signal laser and the pumping
laser will change the amplitude of the differential gain variation. Hence, although
this makes the total noise increase in a nonlinear form, the noise may be timevaried. As mentioned above, the signal-to-noise ratio will decrease and be timevaried.
3) Method of reducing the effect
There are several methods for reducing the effect of PHB. It is a feasible method to
amplify in the small-signal area, but it is not always possible. In many cases, it can
meet the demands. Actually, the simplest method is to adopt unpolarized signals
which can be generated via many approaches. The most common approach is to
adopt polarization scramble to generate signal. If a phase modulator is used, the
polarization state will change between the two orthogonal states at all time. Thus,
the signal seems to have no polarization.
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Section 4 DWDM Networking Design
This indicates that it's better to arrange the polarization modulation according to a
double bit rate because the PDL in the amplifier will be converted from
polarization modulation to amplitude modulation. By adopting double bit rate
polarization modulation, the amplitude fluctuation stays at the rate above the
bandwidth of the detector and is not sensible to the receiver. If this technology is
used, the performances of very long distance systems will be improved and reach
the expected purpose of high reliability. Polarization modulation has become the
standard implementation method for overseas large systems.
However, in long distance amplified systems, PMD will result in secondary
polarization and cause PHB which leads to the performance degradation of the
systems. This effect improves the complex properties of the interaction of
polarization effects in amplified links.
4.4 DWDM Network Protection
Since the load of DWDM systems is large, reliability is especially important.
There are two major protection modes for point-to-point line protection. One,
based on single wavelength, is 1+1 or 1:N protection implemented on the SDH
layer. The other is based on optical multiplex section protection and protects the
multiplexed signals simultaneously in the optical path. This kind of protection is
also called optical multiplex section protection (OMSP). In addition, there are
other protections based on ring networks.
4.4.1 Protection Based on single Wavelength
1. 1+1 protection based on single wavelength and implemented on SDH
layer
Tx1w
Tx1p
MUX
LA
DMUX
WDM system
working system
Tx2w
Tx2p
MUX
LA
Rx2w
Rx2p
DMUX
WDM system
working system
Txnw
Txnp
Note: Here SDH equipment is
ADM
4-73
W: Working
channel
Rx1w
Rx1p
Rxnw
Rxnp
P: Protection
channel
DWDM Networking
Section 4 DWDM Networking Design
Figure 4-19 1+1 protection based single wavelength and implemented on the SDH layer
This protection system mechanism is similar to the 1+1 MSP of SDH system. All
the system equipment needs backup, such as SDH terminal, multiplexer/demultiplexer, optical line amplifier and optical fiber cable line. The SDH signals
are permanently bridged to the working system and the protection system. At the
receiver end, the status of the SDH signals received by the two DWDM systems
are monitored and the more appropriate signal is chosen. This method has high
reliability, but its cost is relatively high.
In a DWDM system, switching of each SDH channel isn't related to other channels,
i.e. when Tx1 of DWDM system 1 fails and switches to DWDM system 2, Tx2 can
still operate in DWDM system 1. Once a switching condition is detected, the
protection switching must be completed within 50ms.
2. 1:n protection based on single wavelength and implemented on SDH
layer
DWDM system can implement 1:N protection based on single wavelength and
implemented on SDH layer. As shown in Figure 4-20, Tx11, Tx21 and Txn1 share a
protection section and, together with Txp1, form a 1: n protection relationship. Tx12,
Tx22 and Txn2 share a protection section and, together with Txp2, form a 1: n
protection relationship. And so on, Tx1m, Tx2m and Txnm share a protection section
and, together with Txpm, form a 1: n protection relationship. The SDH multiplex
section protection (MSP) monitors and measures the status of received signals and
conducts bridge and selection for the appropriate SDH signals from the protection
section.
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Section 4 DWDM Networking Design
Tx11
Tx12
Tx1m
MUX
Tx21
Tx22
Tx2m
MUX
LA
W D M
MUX
Rx 11
Rx 12
Rx 1m
DMUX
Rx 21
Rx 22
Rx 2m
DMUX
Rx n1
Rx n2
Rx nm
DMUX
Rx p1
Rx p2
Rx pm
w o r k in g
s y s te m
Txn1
Txn2
Txnm
2
LA
WDM working
system n
Txp1
Txp2
Txpm
N o te : H e re
LA
WDM working
system 1
DMUX
MUX
th e
e q u ip m e n t is
LA
WDM working
system p
S D H
A D M
Figure 4-20 1:n protection based on single wavelength and implemented on SDH layer
In a DWDM system, switching of each SDH channel isn't related to other channels,
i.e. when Tx11 of DWDM system 1 fails and switches to DWDM protection system
1, Tx12, Tx13 匱 x1m can still operate in DWDM working system 1. Once a
switching condition is detected, the protection switching must be completed within
50ms.
3. 1:n protection based on single wavelength within the same DWDM
system
Consider a DWDM line which can carry multiple SDH channels. The idle
wavelengths in the same DWDM system can function as protection channels.
W orking system
Rx1
W orking system
Rx2
W orking system
Tx1
W orking system
Tx2
Txn
W orking system
MUX
LA
W DM working
system
W orking system
Txp1
DMUX
W orking system
Rxn
W orking system
Rxp1
Note: Here the SDH equipment
is ADM
Figure 4-21 1:n protection based on single wavelength within the same DWDM system
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Figure 4-21 shows a DWDM system of n+1 wavelength channels with n working
wavelengths and 1 protective wavelength as protection system. However, in
practical systems, the reliability of optical fibers and optical fiber cables is worse
than that of the equipment. So practically, it does not mean much to provide
protection only for the system instead of the line.
Once a switching condition is detected, the protection switching must be completed
within 50ms.
4.4.2 Optical Multiplex Section Protection (OMSP)
This technology provides 1+1 protection only in the optical path instead of the
terminal line. At the transmit end and the receive end, 1×2 optical splitter and
switch are used, respectively. Or other approaches are adopted (such as glowing
status which refers to a case that the optical amplifier stays at low bias current and
the pumping source at low output and that the launched signal is small and can
merely detected for monitoring to determine whether it is in normal operating
state). At the transmit end, the multiplexed optical signals are separated while at
the receive end they are routed. The features of the optical switch are low insertion
loss, transparent to the wavelength amplification region of optical fiber, fast in
speed. Moreover, it can be highly integrated or miniaturized.
W orking system
Tx1
Tx 2
MUX
1:2
optical
splitter
Tx n
LA
W DM working
line 1
W DM working
line 2
Rx 1
1:2
optical
switch
DMUX
LA
Protection
system
Figure 4-22 Optical multiplex section protection (OMSP)
Figure 4-22 shows an optical multiplex section protection scheme employing
optical splitter and optical switch. In this protection system, only the optical fiber
cables and DWDM line systems are backed up, and the SDH terminals and
multiplexers in the DWDM system terminal stations aren't backed up. In practical
systems, an N: 2 coupler can be used to replace the multiplexer and the 1: 2
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Rx 2
Rxn
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Section 4 DWDM Networking Design
splitter. When compared to 1+1 protection, this reduces the cost. OMSP has
practical meaning only for two independent optical fiber cables.
4.4.3 Applications in Ring Networks
DWDM systems can also be used to form ring networks. One application is to
connect the point-to-point DWDM systems based on single wavelength to form a
ring, as shown in Figure 4-23. On the SDH layer, 1: n protection is implemented.
The SDH system must adopt ADM equipment.
WDM terminal
WDM terminal
WDM terminal
WDM terminal
Figure 4-23 A ring formed by point-to-point DWDM systems
In the protection system shown in Figure 4-24, path protection ring and MSP
protection ring of the SDH system can be implemented. The DWDM system only
provides "virtual" optical fibers. The protection for each wavelength on SDH layer
is independent to the protection mode of other wavelengths. This ring can be twofiber or 4-fiber.
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OADM
OADM
OADM
OADM
Figure 4-24 A ring formed by OADMs
To employ OADMs with the add/drop multiplexing capability to form rings is
another application mode of DWDM technology in ring networks. At present, ring
networks formed by OADMs can be classified into two modes.
One is wavelength path protection based on single wavelength protection, i.e. 1+1
protection of single wavelength which is similar to the path protection of SDH
system.
The other is line protection ring which protects the signals of multiplexed
wavelengths. When a fiber is cut off, the "loop back" function can be implemented
in the two nodes near the fiber cutoff. Thus all the services are protected. This is
similar to the MSP of SDH. From the aspect of specific forms, the line protection
ring can be divided into two-fiber bi-directional ring and two-fiber unidirectional
ring, and four-fiber bi-directional ring can also be formed. In a two-fiber bidirectional ring, half of wavelengths operate as working wavelength and others as
protection wavelength.
4.5 Analysis to the Examples
4.5.1 Networking Diagram (Physical Network)
In a practical network shown in the figure below, there are 14 stations: A, B, C, D,
E, F, G, H, I, J, K, L, M and N, where A, E and N add/drop services while other
stations don't. Distances between the stations are shown in Figure 4-25.
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Section 4 DWDM Networking Design
54KM
97KM
125KM
110KM
86KM
131KM
G
B
A
E
D
C
F
138KM
H
98KM
128KM
N
75KM
M
110KM
L
60KM
K
176KM
J
I
Figure 4-25 The physical networking diagram of a network
4.5.2 Networking Diagram (considering the dispersion limited distance of the
laserlaser to divide the regenerator sections of the network)
The dispersion limited distance of a laser depends on its modulation mode.
Generally, the maximum dispersion limited distance of an EA laser can be up to
640km and that of an M-Z external modulated laser can reach 1000km (The line
width of M-Z laser is too narrow, unfavorable for overcoming the nonlinear effects
of optical fiber). Here we divide the regenerator sections of the network in terms of
EA laser which are most commonly used in practical engineering.
We analyze the distance between two adjacent stations which have services to
add/drop, as shown in Figure 4-26. The distance between A and E is 386km,
meeting the dispersion limited distance requirement of the EA laser. The distance
between E and N is 1002km. It doesn't meet the EA laser dispersion limited
distance requirement of less than or equal to 640km. So it is necessary to divide the
distance between E and N. This distance can be divided into two or three shorter
regenerator sections. Here we change the optical amplifier station I between E and
N into an electrical regeneration station and divide the regenerator section between
E and N into two regenerator sections: E---I and I---N, where the distance of
regenerator section E---I is 453km and I---N is 549km. After division, the distances
meet the EA laser dispersion limited distance requirement of less than or equal to
640km. The networking is shown in Figure 4-26.
4-79
DWDM Networking
Section 4 DWDM Networking Design
A
B
C
D
E
F
G
386KM
453KM
H
N
M
L
K
J
I
549KM
Figure 4-26 Networking diagram which considers the dispersion limited distance of the lasers
4.5.3 Networking Diagram (considering the power of optical amplifiers to divide
the optical regenerator sections)
According to the related ITU-T recommendations, the regenerating distances
between adjacent DWDM stations can be specified as four types: 80km, 100km,
120km and 160km. The 160km standard is adopted only when no line amplifier
(optical regenerating station) is used. In the cases that line amplifiers are
employed,, the recommended distance is generally less than 120km (33dB).
However, networking modes greater than 120km (33dB) can still be adopted as
long as the specification such as power of optical amplifiers and OSNR meet the
requirements. In applications with line amplifiers, the total launched power of the
optical amplifiers is generally not greater than +20dBm (for an 16-wavelength
system, the power is +8dBm in each channel), the received power of the
preamplifiers isn't less than -30dBm in each channel and the distance between two
adjacent stations (for an 16-wavelength system) shouldn't less than 139km
(38dB/0.275DB/km).
In Figure 4-26, the distance between stations I and J is 176km and exceeds the
requirement of the optical amplifier. The distances between other adjacent stations
almost meet the requirement. So it is necessary to add an optical
regeneratingregeneration station between I and J, dividing I----J into two
regenerator sections. As shown in Figure 4-27, the requirement of the amplifier is
met after station X is added.
4-80
DWDM Networking
Section 4 DWDM Networking Design
A
B
C
D
E
F
G
386KM
453KM
H
N
M
L
K
J
X
84KM
92KM
I
549KM
Figure 4-27 Network diagram considering the power of the optical amplifiers
4.5.4 Networking Diagram (considering OSNR)
Networking modes are related to the OSNR requirement of networks. Via
calculation, the OSNR values at the receive ends of the regenerator sections after
networking according to Figure 4-26 are, in the direction of A---E-----I-----N, as
follows
Station E: OSNR=21.8dB
Station I: OSNR=20.4dB
Station N: OSNR=20.7dB
They meet the OSNR requirement of the network. So the network structure keeps
unchanged, as shown in Figure 4-28.
(When the OSNR doesn't meet the network requirement, the regenerator sections
should be redivided referring to the dispersion limited distance of the lasers and
eventually make the network to meet the requirements of dispersion limited
distance, OSNR and optical power budget. Although the nonlinearity of optical
fiber is also one of the factors which limit the optical transmission, this limitation
can be completely negligible under the application conditions meeting the
recommended specifications)
4-81
DWDM Networking
Section 4 DWDM Networking Design
A
B
C
D
E
F
G
386KM
453KM
H
N
M
L
K
X
J
84KM
92KM
I
549KM
4-82
DWDM Networking
Section 4 DWDM Networking Design
Figure 4-28 Networking diagram considering OSNR
4-83