Download P5: EDFA and Measurement Techniques Chair for Communications Prof. Dr.-Ing. Werner Rosenkranz

P5:
EDFA and Measurement Techniques
Chair for Communications
Prof. Dr.-Ing. Werner Rosenkranz
1 Introduction
3 2 EDFA Background and Theory
3 3 2.1 History............................................................................................................................... 3 2.2 Background and Theory .................................................................................................... 5 2.3 Stimulated Emission and Absorption ................................................................................ 6 2.4 Spontaneous Emission ...................................................................................................... 7 2.5 Optical Gain of EDFA ...................................................................................................... 7 2.6 EDFA Architecture ......................................................................................................... 10 2.7 Amplifier Noise............................................................................................................... 12 Lab Instructions
13 3.1 Optical Fiber ................................................................................................................... 13 3.2 Fiber Optic Connectors and Their Maintenance ............................................................. 14 3.2.1 Adapters ............................................................................................................. 17 3.3 Cleaning Procedures ....................................................................................................... 17 3.3.1 General Cleaning Process .................................................................................. 19 3.4 Eye safety ........................................................................................................................ 19 3.4.1 Eye Safety Regulations! ..................................................................................... 20 3.4.2 Laser Classification ............................................................................................ 20 4 Measurement Instruments
4.1 22 Optical Spectrum Analyzer ............................................................................................. 22 4.1.1 OSNR Measurement with OSA ......................................................................... 23 4.2 Optical Power Meter ....................................................................................................... 26 5 Questions
27 6 EDFA Experiment
28 6.1 Experiment Set Up .......................................................................................................... 28 6.1.1 Construction of an amplified spontaneous emission (ASE) broadband light source
and measure its emission spectrum ................................................................................. 29 6.1.2 Modification of the ASE source into a broadband multi-wavelength erbium doped
fiber amplifier (EDFA) ................................................................................................... 30 6.1.3 Input spectrum .................................................................................................... 30 6.1.4 Output spectrum ................................................................................................. 31 6.1.5 Measurements of the gain over the output ......................................................... 32 6.1.6 Characterization of the output spectrum ............................................................ 33 6.1.7 Output spectrum vs. input power ....................................................................... 34 6.1.8 Determination of the noise figure (NF) .............................................................. 35 7 Bibliography
37 8 Appendix
38 I
Introduction
2.1
II
1 Introduction
In this lab instruction manual the Erbium doped fiber amplifiers (EDFAs) will be presented, from the basic theory behind the process of amplification to the equipment used in
the experiment. Students will understand the importance of the EDFA in optical communication systems, become familiar with the measurement techniques and learn about the
EDFA’s building blocks.
2 EDFA Background and Theory
2.1
History
After a certain distance, the cumulative loss of signal power causes the signal to become
too weak to be detected. Therefore, the signal power has to be restored. Prior to the advent
of the optical amplifiers, the only option was to regenerate the signal, that is, to detect the
signal and retransmit it. This process is accomplished by a regenerator, that converts the
optical signal to the electrical domain, reconstructs the signal and converts it back into an
3
EDFA Background and Theory
optical signal for onward transmission (optical-electrical-optical (OEO) conversion)[1]. Figure 1 shows a block diagram of such a regenerator.
Optical regenerations are classified into 3 categories by the 3 R's scheme:

1R: reamplification of the data pulse.

2R: pulse reshaping is carried out

3R: retiming of data pulse is done (clock recovery and clock jitter cleaning).
o/e
electrical
receiver
electrical
transmitter
e/o
Figure 1: Regenerator [2]
OEO conversion is complex and expensive. The device which is not transparent and
therefore not useful for parallel transmission of different data formats on many different
wavelengths (wavelength division multiplexing1). Using all-optical regenerators can avoid
OEO conversion of the signal using non-linear optical effects for regeneration. The key advantage over old regenerators is power efficiency provided by the device, and simple integration into an optical network.
There are several advantages that optical amplifiers offer over regenerators. Optical amplifiers (OA) are insensitive to the bit rates or the signal format and thus transparent. Due to
the transparency, a system using OA can be more easily upgraded without having to replace
the amplifiers. Furthermore, thanks to the large bandwidth a single OA can simultaneously
amplify several WDM signals (Optical regenerators need a regenerator for each wavelength). OAs have become essential components in high-performance optical communication systems and have largely replaced regenerators (which are much more expensive and
difficult to construct). One of the most commonly used OA is the erbium doped fiber amplifier (EDFA).
1
Parallel transmission of data on many different wavelength. Comparable to FDM, parallel transmission on different
frequencies.
4
EDFA Background and Theory
EDFAs for amplification of signals in the around the conventional band (C-Band around
µm) were simultaneously developed in 1987 at the University of Southampton and
AT&T Bell Laboratories. A key advance was the recognition that the Erbium ions, with its
transition at
µm was ideally suited as an amplifying medium for modern fiber-optic
transmission systems in the 3rd optical window. The high signal gains obtained with these
erbium-doped fibers immediately attracted worldwide attention. Starting from 1989, EDFAs
were the catalyst for an entirely new generation of high-capacity undersea and terrestrial
fiber optic links and networks [3].
2.2
Background and Theory
In dense wavelength division multiplexed (DWDM) optical networks, it would be very
expensive to separate all the channels for regeneration and to recombine all the channels
afterwards. Optical amplifiers are capable of amplifying the power levels of all the channels
simultaneously in the optical domain in the manner that is transparent to the modulation
format and providing a wide gain bandwidth. Amplifiers, depending on the role in the optical
networks, are generally classified into the following categories:

Booster amplifiers: Tunable lasers, for example, are designed to provide low optical
output power and they are immediately followed by an optical amplifier which boosts
the power level.

In-line amplifiers: allow the signal to be amplified within the transmission system
Good gain flatness is usually required (because of the fact that many amplifiers may
be cascaded).

Preamplifiers: used to amplify weak signals before the receivers [4].
There are various optical amplifiers, including semiconductor optical amplifiers (SOAs)
and fiber amplifiers.
The SOA is constructed much like a semiconductor laser without the resonator. As an
interpretable optical device it can be useful when trying to achieve a small footprint. The
drawbacks are a higher noise figure and commonly polarization dependency, which can lead
to crosstalk especially in WDM applications.
5
EDFA Background and Theory
Fiber amplifiers make use of rare-earth elements as a gain medium by doping the fiber
core during the manufacturing process. Amplifier properties such as the operating wavelength and the gain bandwidth are determined by the dopants rather than by the silica fiber,
which plays the role of a host medium. Many different rare-earth elements, such as erbium,
praseodymium, holmium, neodymium, samarium, thulium, and ytterbium, can be used to
realize fiber amplifiers operating at different wavelengths in the range
μm. Er-
bium-doped fiber amplifiers (EDFAs) have attracted the most attention because they operate
μm (3rd optical window). Their deployment in WDM
in the wavelength region near
systems after 1995 revolutionized the field of fiber-optic communications and led to lightwave systems with capacities exceeding 1 Tb/s. The advantages of EDFAs over SOAs are:

High amplification (30 dB)

High output power (10-25 dBm)

Polarization independent

Low crosstalk

Low noise figure 5-8 dB
The following section focuses on the main characteristics of EDFAs [5].
2.3
Stimulated Emission and Absorption
The key physical phenomenon behind signal amplification in EDFA is stimulated emission of radiation by atoms in the presence of an electromagnetic field. Consider an atom and
two of its energy levels,
satisfies
and
, with
. An electromagnetic field, whose frequency
, induces transitions of atoms between the energy levels
( is Planck’s constant). Both kinds of transitions,
and
and
, occur.
transitions are accompanied by absorption of photons from the incident electromagnetic
field.
transitions are accompanied by the stimulated emission of photons of energy
, the same energy as that of the incident photons. Thus if stimulated emission dominates
over absorption, a net increase in the number of photons of energy
leads to an amplifica-
tion of the signal (otherwise, the signal will be attenuated). At thermal equilibrium, there
exists only absorption of the input signal. In order for amplification to occur, the relationship
between the populations of levels
and
must be inverted, that prevails under thermal
equilibrium. Population inversion can be achieved by supplying additional energy to pump
6
EDFA Background and Theory
the electrons to the higher energy level, i.e. by an optical pump (laser). Figure 2 gives a
description of the physical phenomenon of absorption, stimulated and spontaneous emission
(which will be presented in the next paragraph).
Laser level
Ground level
Figure 2: Absorption, spontaneous emission and stimulated emission [1].
2.4
Spontaneous Emission
Consider again the atomic system with the two energy levels discussed earlier. Independent of any external radiation, atoms in energy level
transit to the lower energy level
thereby emitting a photon. Although the emitted photons have the same energy
,
as the
incident optical signal, they are emitted in random directions, polarizations and phase. This
is unlike the stimulated emission process, where the emitted photons not only have the same
energy as the incident photons, but also the same direction of propagation, phase, and polarization (stimulated emission process is coherent, whereas the spontaneous emission process
is incoherent). The amplifier treats spontaneous emission radiation as another electromagnetic field at the frequency
and the spontaneous emission is amplified, as well in addition
to the incident optical signal. This amplified spontaneous emission (ASE) appears as noise
at the output of the amplifier [1].
2.5
Optical Gain of EDFA
The main task of an OA is the optical gain, realized when the amplifier is pumped to
achieve population inversion.
7
EDFA Background and Theory
Figure 3: EDFA energy levels [2].
Figure 3 shows energy levels of
ions in silica glasses. Due to the energy of the pump
laser which operates at 980 nm, erbium ions move from
carriers have a short lifetime of several µs. Transition to
is thereby non radiating. At level
to
by absorption. At level
occurs by emission of heat and
there is an inversion state with high carrier density,
where carriers have a long lifetime of several ms (meta-stable state). Therefore, signal light
at
nm is amplified by stimulated emission (
). In reality pump energy levels
are not discrete but divided in many sub levels ("Stark-splitting"). Therefore, we have a
broad gain bandwidth in the range of
nm with wavelength-dependent gain as
shown in Figure 4.
Popt
/ in dBm
Wavelength
1530
1560
/ in nm
Figure 4: Typical gain spectrum of EDFA [2]
The optical gain, in general, depends not only on the frequency of the incident signal, but
also on the local beam intensity at any point inside the amplifier. Let us consider the case in
8
EDFA Background and Theory
which the gain medium is modeled as a homogeneously broadened two-level system. The
local gain coefficient of such a medium can then be written as [5]:
೚
బ
(1.1)
మ మ ು
మ ುೞ
where g 0 is the peak value of the gain,  is the optical frequency of the incident signal, 0
is the atomic transition frequency, and P is the optical power of the signal being amplified.
The saturation power
depends on the gain-medium. The parameter
as the dipole relaxation time, is typically quite small (
in Eq. (1.1), known
ps).
The concept of amplifier bandwidth is commonly used in place of the gain bandwidth.
The difference becomes clear when one considers the amplifier gain G, known as the amplification factor and defined as:
೚ೠ೟
(1.2)
೔೙
By noting that
,
and using
the amplification fac, where the frequency depend-
tor for an amplifier of length L is given by [5]:
ence of both G and g is shown explicitly. Both the amplifier gain G(ω) and the gain coefficient
are maximal when
and decrease with the signal detuning. However,
G(ω) decreases much faster than
.
. Since
The origin of gain saturation lies in the power dependence of
when
becomes comparable to
, the amplification factor
is reduced
decreases with an increase
in the signal power. This phenomenon is called gain saturation. Consider the case in which
incident signal frequency is exactly tuned to the gain peak
). The detuning effects
can be incorporated in a straightforward manner. The following implicit relation for the
large-signal amplifier gain [5] can be achieved by using the relation
బ
ೞ
:
೚ೠ೟
(1.3)
ೞ
which shows that the amplification factor
becomes comparable to
decreases from its unsaturated value
.
9
when
EDFA Background and Theory
50
gain [dB]
40
30
20
10
0
-30
-25 -20
-15
-5
-10
0
5
10
15
20
signal input power [dBm]
Figure 5: Gain saturation [2]
2.6
EDFA Architecture
ls
ER-fiber
opt. isolator
opt. isolator
BP filter
ls
lp+ ls
lp
Figure 6: EDFA block diagram [2]
A WDM Coupler combines the laser signal and (modulated) input signal at the input of
the Erbium-doped fiber, which is several meters (e.g.
m) long. The purpose of the
optical isolator is to reduce oscillations due to reflections from connectors, components, etc.
The pump laser provides the optical energy needed for the amplification and works at either
nm or 1480 nm with a pump power of
mW. The optical filter for noise
reduction is optional. Other arrangements for the optical pump such as reverse pumping, bidirectional pumping, remote pumping (undersea systems) are also possible. A block diagram
of an EDFA is shown in Figure 6. The gain of the EDFA depends on a number of device
parameters: erbium ion concentration, amplifier length, core radius and pump power [6]. The
three-level rate-equation model commonly used for lasers [7] can be adapted for EDFAs.
The model assumes that the top level of the three-level system remains nearly empty because
10
EDFA Background and Theory
of a rapid transfer of the pumped population to the excited state. It is, however, important to
take into account the different emission and absorption cross sections for the pump and signal fields. The population densities of the two states,
and
, satisfy the following two
rate equations [3]:
మ
మ
(1.4)
భ
భ
మ
(1.5)
భ
where
and
are the absorption and emission cross sections at the frequency
( stands for pump and
stand for signal),
is the spontaneous lifetime of the
excited state (about 10 ms for an EDFA), the quantities
is the transition cross section at the frequency
and
ೕ
for the pump and signal waves, defined such that
ೕ
, and
with
ೕ
represent the photon flux
where
is the optical power,
is the cross-sectional area of the
fiber mode for. The pump and signal powers vary along the amplifier length because of
absorption, stimulated emission, and spontaneous emission. If the contribution of spontaneous emission (3rd term in the equations) is neglected,
and
satisfy the equations:
ೞ
(1.6)
೛
(1.7)
where α and α’ take into account fiber losses at the signal and pump wavelengths, respectively. The confinement factors
and
account for the fact that the doped region within
the core provides the gain for the entire fiber mode. The parameter
the direction of pump propagation;
depending on
in the case of a backward propagating pump.
Equations (1.6) – (1.7) can be solved analytically, in spite of their complexity, and after some
approximations [5] and it can be derived that total amplifier gain G for an EDFA of length
L has the form:
(1.8)
11
EDFA Background and Theory
2.7
Amplifier Noise
The amplifier noise is the ultimate limiting factor for system applications. EDFA noise
originates from ASE noise, which is unpolarized and therefore occurs in both polarizations.
The total ASE noise power is summed over all the spatial modes that the optical fiber supports in an optical bandwidth and equals [2]:
(1.9)
where
is the spontaneous emission factor,
is the optical bandwidth,
stands for optical power gain of the EDFA,
is the carrier frequency and
impact of the ASE noise is quantified through the noise figure
is the Planck’s constant. The
given by [8]:
ಲೄಶ
(1.10)
೎ ೚೛೟
The spontaneous emission factor
depends on the relative populations N1 and N2 of the
మ
ground and excited states as
మ
భ
. Since EDFAs operate on the basis of a three-
level pumping scheme,
and
length L and the pump power
, just as the amplifier gain does. Noise figures close to 3 dB
. The noise figure depends both on the amplifier
can be obtained for a high-gain amplifier pumped such that
12
[9].
Lab Instructions
3 Lab Instructions
In this part, the reader will find important information about fiber-optic components and
equipment used in the experiment: optical connectors, optical spectrum analyzers, power
meters and attenuators. During the lab exercise, the students will become familiar with different types of connectors, as well as connector cleaning and eye safety procedures.
3.1
Optical Fiber
The light is guided through the core of the fiber by an optical cladding with a lower
refractive index that traps light in the core through total internal reflection. The cladding is
coated by a buffer that protects it from moisture and physical damage. The coating protects
the very delicate strands of glass fiber - about the size of a human hair - and allow it to
survive the rigors of manufacturing, proof testing, cabling and installation. In Figure 7 there
is a cross section of an optical fiber. Fibers can be very flexible, but traditional fiber's loss
increases significantly if the fiber is bent with a radius smaller than around
Figure 7: Cross section of an optical fiber (Bob Mellish)
13
mm.
Lab Instructions
3.2
Fiber Optic Connectors and Their Maintenance
Optical fibers may be connected to each other by connectors or by splicing, that is,
joining two fibers together to form a continuous optical waveguide. The purpose of each
connector is to hold the two cores of the glass fiber together. Table 1 shows some of the
standard connector types.
Insertion Loss
Description and Applications
10log10(Pin/Pout)
Fiber Optic Connectors
Name
Average:
MM/SM
Snap-in connector with 2.5 mm
ferrule.
Used in single-mode systems.
Key and screw connector with 2.5
mm ferrule.
Typically used in single-mode
systems.
SC
0.2 dB
0.25 dB
FC
0.2 dB
0.25 dB
FC/A
PC
0.1 dB
Standard FC connector with angle
polish
0.1 dB
2.5mm ferrule, a good connector
to use where the ruggedness of a
metal screw on connector is required.
DIN
Table 1: Optical fiber connections
Although sizes, shapes and interlocking mechanisms may vary, one element is shared
by all connectors: the ferrule. The ferrule (made of metal or ceramic) is the central part of
the male connector. It is designed to align and protect the fiber core during connection, see
Figure 8. Inside , protected by and concentric to the ferrule is the glass fiber.
Since connector performance and compatibility depends on which ferrule polish is
used, it is important to understand the differences between types of polish. The ferrule tip is
polished to ensure a smooth finish on the fiber end. Polish can also minimize connector loss
or back reflection, depending on the angle used.
14
Lab Instructions
Figure 8: Optical fiber connectors with a ferrule, body, cap and strain relief boot [10]
FC (fiber connection) connectors are made with a flat end face. A small air gap causes airto glass interface and high reflections.
The PC (physical contact) connector is polished such that the whole end face has a slight
convex shape. This insures a glass-to-glass connection. The FC/PC, ST, SC, APC, and DIN
are all physical contact connectors using the same
mm diameter ferrule. They differ in
the mechanical holding assembly that holds the ferrule. In Figure 9, four different types of
geometries are shown.
Figure 9: Geometry of optical fiber connectors [8]
Higher grades of polish give less insertion loss and lower back reflection. and to achieve
better return loss, angled fiber ends were introduced. With an angled interface, the light may
have a large reflection at the surface, but the angle causes the reflected light to go into the
cladding and dissipate before the reflected light gets back to the source. Angle-polished con-
15
Lab Instructions
nections are distinguished visibly by the use of a green strain relief boot, or a green connector body. The parts are typically identified by adding "/APC" (angled physical contact)
to the name. For example, an angled FC connector may be named FC/APC, or merely FCA.
Non-angled versions may be denoted FC/PC. Due to their angled design, APC connectors
are not compatible with PC, SPC or UPC types. Joining an APC connector to a different
connector type must be avoided, since this will cause more insertion and return loss than
either connector would normally produce on its own and can in addition damage the connectors. PC, SPC and UPC connectors are all compatible and any combination between them
will typically generate an insertion loss of
dB. Figure 10 shows a clean connector end
face, a dirty end face from a poor cleaning is shown on Figure 11 and Figure 12 shows
damaged connectors.
Figure 10: Clean connector [8]
Figure 11: Dirty connector [8]
Figure 12: A damaged connector from abuse and using an unkeyed ferrule to mate with this
cable. The top left image is 50X. The black center is the glass fiber and the ring around it is
the centering "stake". The bottom two are at 200X and the top right in the glass fiber end at
400X showing the ringed damage on the glass. [8]
16
Lab Instructions
3.2.1
Adapters
Fiber optic adapter connect the same or different fiber optical connectors in the fiber
optical link. Some examples of optical fiber adapters are shown in Figure 13.
Figure 13: Optical fiber adapters [8]
3.3
Cleaning Procedures
Optical fiber connectors are susceptible to damage that is not immediately obvious to the
naked eye. The damage can have significant effects on the results of measurements being
made. Figure 10 is a close-up of a clean PC cable end face. It is important that every fiber
connector will be inspected and cleaned prior to connecting. Clean optical fiber components
are a requirement for connections between optical fiber equipment. A particle that partially
or completely blocks the core generates strong back reflections, which can cause instability
of the laser system. Dust particles trapped between two fiber faces can scratch the glass
surfaces. Moreover, even if a particle is only situated on the cladding or the edge of the end
face, it can cause an air gap or misalignment between the fiber cores, which significantly
attenuates the optical signal.


A 1-micrometer dust particle on a single-mode core can block up to 1% of the light
(a
dB loss).
A 9-micrometer speck, still too small to be seen without a microscope, can completely block the fiber core. These contaminants can be more difficult to remove than
dust particles.
In addition to dust, other types of contamination must also be removed off the end face.
Such materials include oils, film residues and powdery coatings
17
Lab Instructions
These contaminants are more difficult to remove than dust particles and can also cause
damage to the equipment if not removed. With the high powered lasers any contaminant can
be burned into the fiber end face if it blocks the core while the laser is turned on. This burn
in might damage the optical surface enough so that it cannot be cleaned and is permanently
damaged. Remember that inspection, cleaning and re-inspection are critical steps,
which must be done before making any fiber-optic connection! [8]
Reminders While Cleaning






Always turn off any laser sources before inspecting fiber connectors, optical components
Always inspect and clean the connectors before making a connection.
Always use the connector housing to plug or unplug a fiber.
Always keep a protective cap on unplugged fiber connectors.
Always store unused protective caps in a container in order to prevent the possibility
of the transfer of dust to the fiber
Discard used tissues properly.[11]
Warnings







Never look into a fiber while the system lasers are on.
Never touch any equipment without being properly grounded.
Never connect a fiber to a fiberscope while the system lasers are on.
Never touch the end face of the fiber connectors.
Never twist or pull forcefully the fiber cable.
Never reuse or touch any tissue, swab or cleaning cassette reel.
Never touch the dispensing tip of the alcohol bottle.
Connector Inspection Technique
This inspection technique is done using a fiberscope, seen in Figure 14, in order to view
the end face. A fiberscope is a customized microscope used to inspect optical fiber components. It should provide at least a
x total magnification.
Figure 14: Fiberscope [10]
18
Lab Instructions
3.3.1
General Cleaning Process
Use a pure grade of isopropyl alcohol on a clean cotton swab to wipe off the end-face and
ferrule. Figure 15 shows the cleaning process. When reinserting the cable into the connector,
insert it gently, in a straight line (inserting with the angle can scrape off the material from
the inner part of the connector).
Figure 15: Connector cleaning procedure
Complete these steps to clean the fiber:
1. Inspect the fiber connector, component, or bulkhead with the fiberscope.
2. If the connector is dirty, clean it with a wet cleaning technique followed immediately
with a dry clean in order to ensure no residue is left on the end face.
3. Inspect the connector again.
4. If the contaminate still cannot be removed, repeat the cleaning procedure until the
end face is clean or you are sure the end face is damaged.
3.4
Eye safety
Exposure to the large majority of installed systems, will unlikely result in any health impact, since power levels are usually infrared and below 1 mW, e.g. Class 1. An additional
factor with these systems, is that light around the 1550 nm wavelength band is regarded as
relatively low risk, since the eye does not absorb it very much. Nevertheless, there are a few
significant exceptions. For example, high power optical amplifiers are used in long distance
transmission systems. They use internal pump lasers with power levels up to a few watts.
However, these power levels are contained within the amplifier module.
19
Lab Instructions
3.4.1
Eye Safety Regulations!
Optical microscopes and magnifying devices also present unique safety challenges. If any
optical power is present and a simple magnifying device is used to examine the fiber end,
the user is no longer protected by beam divergence, since the entire beam may be focused
onto the eye.

Always turn off any laser sources before inspecting fiber connectors, optical components, or bulkheads.

Always wear the appropriate safety glasses when required.

Any accident should be immediately reported to the responsible medical authority. If
there is an accidental exposure to the eye, the services of an ophthalmologist should be
sought.
3.4.2
Laser Classification
Class 1 / 1M Maximum power output is a few microwatts. Visible and non-visible
spectrum output (302.5-4000 nm)
Considered incapable of producing hazardous eye exposure unless viewed with collecting
optics (1M). This does not apply to open Class 1 enclosures containing higher-class lasers.
Class / M Maximum power output is
mW. Visible spectrum output (
nm).
Considered incapable of producing hazardous eye/skin exposure within the time period of
human eye aversion response (
s), unless viewed with collecting optics (2M).
Class R Maximum power output is mW
mW. Visible and non-visible spectrum.
Potentially hazardous under some direct and specular reflection viewing condition if the eye
is appropriately focused and stable or if viewed with collecting optics.
Class 3B Maximum power output is mW
mW. Visible and non-visible spectrum.
Present a potential eye hazard for intrabeam (direct) or specular (mirror-like) conditions.
Present a significant skin hazard by long-term diffuse (scatter) exposure if operated at high
powers and in
nm ranges.
Wavelength
nm
(Ultraviolet UV-B, UVC)
nm
Area of Damage
Cornea; Deep-ultraviolet light causes
accumulating damage, even at very low
power
Cornea and Lens
(Ultraviolet UV-A)
20
Lab Instructions
nm
Retina; Visible light is focused on the
retina
nm
Retina; Near IR light is not absorbed
by iris and is focused on the retina
(Visible)
(Visible)
Cornea and Lens; IR light is absorbed
by transparent parts of eye before
reaching the retina
Cornea
nm
(Infrared)
nm
(Far Infrared)
Table 1: Eye Damage – Wavelengths [12]
Figure 16: a) The retinal hazard b) ultraviolet light c) infra-red region [13]
Lasers used in our EDFA experiment:
of the optical spectrum (
nm which is in visible and Infrared-A part
nm) and
nm).
21
nm which is in Infrared-B (
Measurement Instruments
4 Measurement Instruments
In this section, the used instruments will be explained in detail.
4.1
Optical Spectrum Analyzer
Figure 17: Optical spectrum analyzer
Optical spectrum analysis is the measurement of optical power as a function of wavelength. A typical optical spectrum analyzer (OSA) is shown in Figure 17. WDM (wavelength
division multiplexing) has made optical spectrum analysis a key measurement capability that
must be embedded inside telecommunication network elements.
Figure 18: OSA block diagram Measurement of optical power within the resolution
bandwidth Bm (RBW) of optical band-pass filter for various band-pass filter center
frequencies (frequency sweep) [8]
22
Measurement Instruments
A simplified OSA block diagram is shown in Figure 18. The incoming light passes
through a wavelength-tunable optical filter, which extracts the individual spectral components. The photodetector converts the incident optical power to an electrical current. The
current is then converted to a voltage by the transimpedance amplifier and is then digitized.
A ramp generator determines the horizontal location of the trace as it sweeps from left to
right. The ramp also tunes the optical filter so that its center wavelength is proportional to
the horizontal position. The wavelength resolution is determined by the bandwidth of the
optical filter in the OSA whereby the term resolution bandwidth describes the width of this
optical filter. Typically an OSA has a selectable filter ranging from
nm down to
nm.
The OSA sweeps across a spectral band making measurements at discretely spaced wavelength points. This spacing depends on the bandwidth resolution of the instrument (tracepoint spacing). OSA can be used for measurement of the amplifier gain, as well as for obtaining ASE spectral shape, source characteristics and optical signal to noise ratio (OSNR).
4.1.1
OSNR Measurement with OSA
Key parameters providing information about BER such as Q factor and eye analysis cannot be measured directly on a multichannel system (spectral demultiplexing is required), but
optical signal to noise ratio (OSNR) for each individual channel can be derived from an
optical spectrum measurement and it is the most useful parameter available from the measured spectrum. The OSNR is used to characterize a system, much like the SNR electrical
transmission systems. This is often plotted vs. bit error ratio BER.
Other parameters like peak wavelength and peak power are secondary parameters. It is
the OSNR that is usually adjusted at the commissioning of a system to optimize the performance of the system on all channels.
The IEC2 standard defines optical signal-to-noise ratio as the ratio of the signal power at
the peak of a channel to the noise power interpolated at the position of the peak and is described by the following equation [2]:
2
೔
೘
೔
ೝ
International Electrotechnical Commission
23
(1.11)
Measurement Instruments
where
is the optical signal power in watts at the ith. channel;
width of the measurement;
is the resolution band-
is the interpolated value of noise power in watts measured in
the resolution bandwidth of the measurement (
mid-channel spacing point;
) derived from the noise measured at the
is the reference optical bandwidth, typically chosen to be
nm. The second term of the equation is used to provide an OSNR value that is independent
of the instrument’s resolution bandwidth (
) for the measurement (so that results obtained
with different instruments can be compared).
The standard also identifies key OSA characteristics that are required to perform an adequate OSNR measurement [14]:



The wavelength measurement range of the OSA must be wide enough to encompass all channels plus one-half of a grid spacing at each end.
The sensitivity, defined as the lowest level at which spectral power can be measured with a specified accuracy, must be below the minimum expected channel
peak power by the desired OSNR value with a margin (see optical rejection ratio
below) to achieve the specified accuracy.
The resolution bandwidth (RBW) must be wide enough to encompass the entire
signal power spectrum of each modulated channel, and must be accurately calibrated as it has a direct impact on the accuracy of the noise measurement and that
of the modulated signal when its power spectrum is larger than the RBW.
In order to obtain the best possible OSNR measurement with a non-ideal filter-response
OSA, the standard suggests using a two-pass measurement for broad modulated signals; one
with a narrow RBW setting to measure noise accurately close to the peak (at mid-channel
spacing) and one with a broader RBW to accurately measure the signal power.
The optical rejection ratio (ORR) performance of an OSA determines its ability to measure low-level signals close to a peak. It is defined as the ratio, in dB, of the power at a given
distance from the peak (
) to the power at the peak of the OSA filter response for a given
narrow input (delta function equivalent source with linewidth
manufacturers used to specify the ORR of their instrument at
and now specify it at
mid-channel spacing of
nm,
nm and
GHz,
RBW of the OSA). OSA
nm and nm from the peak
nm, which are more relevant values, given the
GHz and
GHz WDM transmission systems. In
order to adequately measure the OSNR with a desired accuracy, the IEC
suggests a required ORR at the noise measurement position (mid-channel) of at least 10 dB
below the measured OSNR. For example, to measure an OSNR of 25 dB at
24
nm (
nm
Measurement Instruments
around
nm is
GHz and would be the mid-channel spacing for a channel set with a
GHz spacing), the ORR of the instrument at
uncertainty below
nm must be at least
dB to ensure an
dB on the OSNR measurement. Figure 19 shows a configuration for
OSNR measurement.
Figure 19: OSNR measurement setup [14]
InFigure 20, there is an output of two different OSAs. Clearly to be noticed is the
difference in ORR for the two devices in the same measurement.
Figure 20: OSNR measurement error introduced by OSA ORR for same input signal (taken
from [14]).
25
Measurement Instruments
4.2
Optical Power Meter
Two main groups of optical power meters (OPMs) can be identified: power meters with
thermal detectors, in which the temperature rise caused by optical radiation is measured, and
photodetectors, in which the incident photons generate electron-hole pairs. Photodetectortype power meters have a great sensitivity and are used more often. Power meters are calibrated using a traceable calibration standard such as a NIST standard. Important elements
are the antireflective coating on the connector adapter, the pinhole and the angled position
of the detector (all to avoid multiple reflections). Thermoelectric cooler ensures the temperature stabilization. Figure 21 shows an OPM. The most important contributions to accurate
power measurements are:
Individual correction of wavelength dependence, temperature stabilization, wide power
range with good linearity, good spatial homogeneity, low polarization dependence, low reflections, compatibility with different types of fiber.
Figure 21: Optical Power Meter (OPM) [15]
Figure 22: OPM with attenuator
OPM with attenuator, depicted in Figure 22, combine the power control capability of a
variable attenuator with the measurement capability of an inline power monitor. You can
vary power levels and simultaneously see the absolute optical power being delivered to your
lightwave system. They are optically passive, featuring low insertion loss, low polarization
dependant loss, and high return loss.
26
Questions
5 Questions
1. What are the advantages of EDFA over classical regenerators?
2. Explain the difference between stimulated and spontaneous emission?
3. Why do we have to use pumping lasers for EDFA and on which wavelengths do they
operate?
4. In what optical range is the working wavelength of erbium laser and why is it important for optical fiber communications?
5. What is the origin of ASE noise?
6. What is a ferrule?
7. What is an APC?
8. Explain, in short, the cleaning procedure of optical connectors?
9. What is the difference between laser classes 1 and 1M?
10. What is an optical spectrum analyzer and what can be measured with it? Explain the
principle and give an example.
11. What is dBm? How can it be converted to watts? Give 0 dBm in watts.
12. What is an attenuation of 3 dB in linear scale?
27
EDFA Experiment
6 EDFA Experiment
The aim of the experiment is to enable students to: (i) construct an ASE broadband incoherent light source and EDFA, (ii) characterize EDFA performance; (iii) establish deep understanding of the concepts of optical amplifiers; (iii) obtain practical experience of operating advanced optical components and instruments.
6.1
Experimental Set Up
The schematic diagrams of ASE broadband light source and EDFA setups are shown in
Figure 23 and Figure 25 respectively. The wavelength we use is:
nm. The
output of DFB laser is connected to the input of a Variable Optical Attenuator (VOA). The
output from the VOA is considered as the input signal of EDFA. The EDFA consists of a
WDM component, two optical isolators, an erbium doped fiber strand with approximately
20 meters in length, a pump laser, and a laser diode driver. The input signal transmits through
the optical isolator to reach the 1480/1550 WDM. The pump laser (
nm) which is
mounted on the laser diode mount and driven by the laser current source is connected to the
WDM.
Figure 23: ASE broadband light source [16]
Before setting up the EDFA, a closer look is taken at the pump laser diode and the
use of the OSA and power meter is explained.
Discuss what is needed for a correct good sketch of a measurement.
28
EDFA Experiment
6.1.1
Construct an amplified spontaneous emission (ASE)
broadband light source and measure its emission spectrum
An ASE broadband light source is built by using a
nm laser diode pumping an
erbium-doped fiber as shown in Figure 23. Connect the output of the erbium-doped fiber to
the Optical Spectrum Analyzer (OSA), gradually increasing pump laser diode power. The
optical spectra of the amplified spontaneous emission (ASE) at different pump powers are
shown in Figure 24. The figure shows that ASE power spectrum covers a broadband wave-
spectral density in dBm/nm
length range and total ASE power increases with the pump power.
-5
pump power 80 mW
pump power 40 mW
pump power 20 mW
-10
-15
-20
-25
-30
Wavelength
1510
1530
1560
/ in nm
Figure 24: ASE Power Spectrum for different pump powers.
29
EDFA Experiment
6.1.2
Modification of the ASE source into a broadband multiwavelength erbium doped fiber amplifier (EDFA)
Figure 25: EDFA setup that uses backward pumping.
6.1.3
Input spectrum
Measure and sketch the input optical spectrum. Use the test setup according to Figure 26.
Following parameters should be used:
dBm, and OSA ResBW
Figure 26: Optical spectrum setup
30
nm.
EDFA Experiment
Figure 27: Sketch the optical spectrum
Measure the signal input power (peak) with the help of the OSA Marker.
6.1.4
Output spectrum
Measure and sketch the output optical spectrum. Use the test setup according to Figure
dBm and pump power
play (on OSA):
nm
, i.e.
nm.
31
mA; spectrum dis-
EDFA Experiment
Figure 28: Setup for measuring the output spectrum
Figure 29: Sketch the output optical spectrum.
6.1.5
Measurements of the gain over the output
Determine the gain as a function of the input power. To do so, vary the input power
from -30 dBm to 0 dBm in 2 dB steps using VOA. Read the power on OSA by using the
marker (ResBW = 0.2 nm!).
32
EDFA Experiment
Psig ,in
in dBm
Psig ,out
in dBm
G
in dB
Psig ,in
in dBm
Psig ,out
in dBm
G
in dB
Psig ,in
in dBm
Psig ,out
in dBm
G
in dB
0
-2
-4
-6
-8
-10
-12
-14
-16
-18
-20
-22
-24
-26
-28
-30
Figure 30: Sketch the gain saturation
6.1.6
Characterization of the output spectrum
Get the output spectrum of the EDFA using the OSA.
33
EDFA Experiment
Output spectrum vs. pumping
Sketch the pure noise output spectrum of the EDFA using the OSA, i.e only ASE with no
input signal (Pin = 0 mW). Outline the various spectra, if you vary the pump power. For a
pump power Ppump levels of: 10 mW (135 mA), 30 mW (305 mA), 45 mW (433 mA). Note:
Use the trace function of the OSA. This allows three curves to be simultaneously plotted on
the display.
Figure 31: Sketch the output spectrum
6.1.7
Output spectrum vs. input power
Set the output spectrum of the EDFA using the OSA. Outline the various spectra, if you vary
the input power. The input power values are:
power is constant (
dBm,
mA).
34
dBm and
dBm. The pump
EDFA Experiment
Figure 32: Sketch the output spectrum
6.1.8
Determination of the noise figure (NF)
The noise figure of an amplifier is a measure of the degradation of the signal to noise ratio
for a signal passing through the amplifier. The noise figure (NF) is defined as the ratio of the
signal to noise at the input of the EDFA to that at the output of the EDFA ( NF =
SNRin/SNRout ). It can be expressed as:
య
ಲೄಶ
ಲೄಶ
మ
where
is the optical wavelength, h is Plank’s constant, c is the speed of light,
optical bandwidth, PASE is the ASE power (linear units) measured in the bandwidth
(1.12)
is the
, G is
the gain of optical amplifier (linear units). When use optical spectrum analyzer (OSA) to do
measurement, the ASE power PASE, optical bandwidth
, peak wavelength , and gain G
can be measured by the Analysis Function of the OSA. When the pump power is as low as
mW, the NF is as high as
dB. A high noise figure indicates that the signal to noise ratio
has been impaired by the amplification process. The NF decreases dramatically as the pump
power increases until the pump saturation occurs. All amplifiers degrade the signal to noise
ratio (SNR) of the amplified signal because of spontaneous emission that adds noise to the
35
EDFA Experiment
signal during the amplification. The degraded NF is one of the major limits for the applications of EDFA in optical communication links and systems.
For NF measurement, it is best to interpolate ASE noise. The ASE power is measured at
wavelengths below and above the signal’s wavelength and then interpolated. Both the input
signal (
ASE,in)
and the output signal (
ASE,out)
are considered here because of the noise com-
ponent of the source must also be taken into acount. The measurement of the ASE power
always takes place between the main mode and the first secondary mode.
The interpolated ASE power is then calculated as:
Measurement
Designation
Signal input power
Psig ,in
Signal output power
Psig ,out
Gain
G
lower
PASE
,in
ASE input power
ASE output power
Measurement
[in dBm]
upper
PASE
,in
lower
PASE
, out
upper
PASE
, out
Other experimental parameters:
dBm, pump laser(EDFA),
mA
36
Measurement
[in mW]
Bibliography
7 Bibliography
[1]
R. Ramaswami, Optical Networks, Elsevier, 2010.
[2]
W. Rosenkranz, Online Optical Communication Lectures.
[3]
P.C. Becker, N.A. Olsson, J.R. Simpson, Erbium-Doped Fiber Amplifiers
Fundamentals and Technology, Academic Press, 1999.
[4]
A. Yariv, Optical Electronics in Modern Communications, Oxford University
Press, 1997.
[5]
G. P. Agrawal, Fiber-Optic Communication Systems, New York: John Wiley
& Sons, Inc., 1997.
[6]
C. R. Giles and E. Desurvire, J. Lightwave Technol., vol. 9, p. 271, 1991.
[7]
A. E. Siegman, "Lasers," University Science Books, Mill Valley, CA, 1986.
[8]
D. Derickson, Fiber Optic Test and Measurement, Prentice Hall PTR, 1998.
[9]
S. B. Alexander, J. Lightwave Technol. , vol. 5, p. 523, 1987.
[10] B. W. Andrew Oliviero, Cabling, The Complete Guide to Copper and FiberOptic Networking, Wiley.
[11] "http://www.microscopyu.com/articles/fluorescence/lasersafety.html,"
[Online].
[12] "www.fiberoptics4sale.com/wordpress/how-to-take-care-of-fiber-opticconnectors," [Online].
[13] UCLA Laser safety manual.
[14] I. Gillett, "Introduction to Laser safety," Imperial College.
[15] Daniel Gariepy, Gang He, "Measuring OSNR in WDM Systems- Effects of
Resolution Bandwidth and Optical Rejection Ratio".
[16] Wen Zhu, Li Quan, Amr S. Helmy, "An Advanced Photonics Lab".
[17] Praktikum Nachrichten- und Informationstechnik: Der optische Verstärker.
[18] M. Schwartz, Information Transmission, Modulation, and Noise, 4th ed., New
York: McGraw-Hill, 1990.
[19] H. Ghafouri-Shiraz, Y. H. Heng, and T. Aruga, Microwave Opt. Tech. Lett.,
vol. 11, p. 14, 1996.
37
Appendix
8 Appendix
To be completed by the student
I have received information on the hazards associated with operating lasers. I have read
eye safety instructions manual and I agree to follow the safe work practices which are outlined in the script, as well as the standard operating procedures for the laser systems I will
be working with.
Laser User Registrant Signature:_________________________
Date:_________
Laser Supervisor Signature:_____________________________
Date:_________
38