Download Chapter 8: Optical Fibers and Components

Chapter 8:
Optical Fibers and Components
TOPICS
–
–
–
–
–
WDM optical networks
Light transmitted through an optical fiber
Types of optical fibers
Impairments
Components: Lasers, optical amplifiers, couplers,
OXCs
Connection-Oriented Networks
1
WDM optical networks
1
1
Tx
Power
amplifier
optical
fiber
In-line
amplification
optical
fiber
…
Tx
W
Rx
Preamplifier
Wavelength
multiplexer
W
Rx
Wavelength
demultiplexer
A point-to-point connection
Connection-Oriented Networks
2
An example of an optical network
Mesh network
Ring 4
Ring 1
Ring 2
Connection-Oriented Networks
Ring 3
3
How light is transmitted through an
optical fiber
Wave
Electric
field
Source
Waves and electrical fields
Connection-Oriented Networks
4
An optical fiber
Cladding
Core
Cladding
Core and cladding
Cladding
Cladding
Core
Core
n1
n1
n2
Radial distance
a) Step-index fiber
Connection-Oriented Networks
n2
Radial distance
b) graded-index fiber
5
Refraction and reflection of a light ray
f
Refracted ray
n2
n1

Incident ray
Connection-Oriented Networks
r
Reflected ray
6
Angle of launching a ray into the fiber
Cladding
Cladding
Core
l

Core
r
Cladding
Cladding
Cladding
Optical
transmitter
Core
Cladding
Connection-Oriented Networks
7
Multi-mode and single-mode fibers
• Core/diameter of a multi-mode fiber:
– 50/125 m,
– 62.5/125 m,
– 100/140 m
• Core/diameter of single-mode fiber
– 9 or 10 / 125 m
Connection-Oriented Networks
8
Electric fields
A
2
Cladding
Core
1
B
Connection-Oriented Networks
Cladding
9
Electric field amplitudes for
various fiber modes
Cladding
Core
Cladding
m=0
Connection-Oriented Networks
m=1
m=2
10
Propagation of modes
Cladding
Cladding
a) step-index fiber
Cladding
Cladding
b) Graded-index fiber
Connection-Oriented Networks
11
Single-mode fiber
Cladding
Cladding
Connection-Oriented Networks
12
Impairments
• The transmission of light through an
optical fiber is subjected to optical
effects, known as impairments.
• There are:
– linear impairments, and
– non-linear impairments.
Connection-Oriented Networks
13
Linear impairments
• These impairments are called linear because
their effect is proportional to the length of
the fiber.
• Attenuation:
– Attenuation is the decrease of the optical power
along the length of the fiber.
• Dispersion
– Dispersion is the distortion of the shape of a
pulse.
Connection-Oriented Networks
14
Attenuation
2.5
Attenuation, dB
2.0
1.5
1.0
0.5
800
1000
Connection-Oriented Networks
1200
1400
Wavelength, nm
1600
1800
15
Attenuation in Fiber
• Attenuation
– P(L) = 10-AL/10P(0)
• Where P(0) optical power at transmitter,
• P(L) power at distance L Km, and
• A = attenuation constant of the fiber
• Received Power must be greater or equal to
– receiver sensitivity Pr
– Lmax = 10/A log10(P(0)/P(r))
Connection-Oriented Networks
16
Dispersion
• Dispersion is due to a number of
reasons, such as
– modal dispersion,
– chromatic dispersion,
– polarization mode dispersion.
Connection-Oriented Networks
17
Modal dispersion
Power
Power
Power
Time
Time
Time
• In multi-mode fibers some modes travel a longer
distance to get to the end of the fiber than others
• In view of this, the modes have different delays,
which causes a spreading of the output pulse
Connection-Oriented Networks
18
Chromatic dispersion
• It is due to the fact that the refractive index
of silica is frequency dependent. In view of
this, different frequencies travel at different
speeds, and as a result they experience
different delays.
• These delays cause spreading in the
duration of the output pulse.
Connection-Oriented Networks
19
• Chromatic dispersion can be corrected using
a dispersion compensating fiber. The length
of this fiber is proportional to the dispersion
of the transmission fiber. Approximately, a
spool of 15 km of dispersion compensating
fiber is placed for every 80 km of
transmission fiber.
• Dispersion compensating fiber introduces
attenuation of about 0.5 dB/km.
Connection-Oriented Networks
20
Polarization mode dispersion (PMD)
• It is due to the fact that the core of the fiber is not
perfectly round.
• In an ideal circularly symmetric fiber the light gets
polarized and it travels along two polarization
planes which have the same speed.
• When the core of the fiber is not round, the light
traveling along the two planes may travel at
different speeds.
• This difference in speed will cause the pulse to
break.
Connection-Oriented Networks
21
Non-linear impairments
• They are due to the dependency of the
refractive index on the intensity of the
applied electrical field. The most important
non-linear effects in this category are: selfphase modulation and four-wave mixing.
• Another category of non-linear impairments
includes the stimulated Raman scattering and
stimulated Brillouin scattering.
Connection-Oriented Networks
22
Types of fibers
• Multi-mode fibers: They are used in LANs
and more recently in 1 Gigabit Ethernet and
10 Gigabit Ethernet.
• Single-mode fiber is used for long-distance
telephony, CATV, and packet-switched
networks.
• Plastic optical fibers (POF)
Connection-Oriented Networks
23
Single-mode fibers:
• Standard single-mode fiber (SSMF): Most
of the installed fiber falls in this category. It
was designed to support early long-haul
transmission systems, and it has zero
dispersion at 1310 nm.
• Non-zero dispersion fiber (NZDF): This
fiber has zero dispersion near 1450 nm.
Connection-Oriented Networks
24
• Negative dispersion fiber (NDF): This type
of fiber has a negative dispersion in the
region 1300 to 1600 nm.
• Low water peak fiber (LWPF): The peak in
the attenuation curve at 1385 nm is known
as the water peak. With this new type of
fiber this peak is eliminated, which allows
the use of this region.
Connection-Oriented Networks
25
Plastic optical fibers (POF)
• Single-mode and multi-mode fibers have a high
cost and they require a skilled technician to install
them.
• POFs on the other hand, are very low-cost and they
can be easily installed by an untrained person.
• The core has a very large diameter, and it is about
96% of the diameter of the cladding.
• Plastic optic fibers find use in digital home
appliance interfaces, home networks, and cars
Connection-Oriented Networks
26
Components
•
•
•
•
•
Lasers
Photo-detectors and optical receivers
Optical amplifiers
The 2x2 coupler
Optical cross connects (OXC)
Connection-Oriented Networks
27
Light amplification by stimulated
emission of radiation (Laser)
• A laser is a device that produces a very strong and
concentrated beam.
• It consists of an energy source which is applied to
a lasing material, a substance that emits light in all
directions and it can be of gas, solid, or
semiconducting material.
• The light produced by the lasing material is
enhanced using a device such as the Fabry-Perot
resonator cavity.
Connection-Oriented Networks
28
Fabry-Perot resonator cavity.
It consists of two partially reflecting parallel flat
mirrors, known as facets, which create an optical
feedback that causes the cavity to oscillate.
Light hits the right facet and part of it leaves the
cavity through the right facet and part of it is
reflected.
Left facet
Connection-Oriented Networks
Right facet
29
• Since there are many resonant wavelengths,
the resulting output consists of many
wavelengths spread over a few nm, with a
gap between two adjacent wavelengths of
100 to 200 GHz.
• A single wavelength can be selected by
using a filtering mechanism that selects the
desired wavelength and provides loss to the
other wavelengths.
Connection-Oriented Networks
30
Tunable lasers
• Tunable lasers are important to optical
networks
• Also, it is more convenient to manufacture
and stock tunable lasers, than make
different lasers for specific wavelengths.
• Several different types of tunable lasers
exist, varying from slow tunability to fast
tunability.
Connection-Oriented Networks
31
Modulation
• Modulation is the addition of information
on a light stream
• This can be realized using the on-off keying
(OOK) scheme, whereby the light stream is
turned on or off depending whether we want
to modulate a 1 or a 0.
Connection-Oriented Networks
32
WDM and dense WDM (DWDM)
• WDM or dense WDM (DWDM) are terms used
interchangeably.
• DWDM refers to the wavelength spacing proposed
in the ITU-T G.692 standard in the 1550 nm
window (which has the smallest amount of
attenuation and it also lies in the band where the
Erbium-doped fiber amplifier operates.)
• The ITU-T grid is not always followed, since there
are many proprietary solutions.
Connection-Oriented Networks
33
The ITU-T DWDM grid
Channel
code
18
 (nm)
 (nm)
1563.05
Channel
code
30
 (nm)
1553.33
Channel
code
42
 (nm)
1543.73
Channel
code
54
19
1562.23
31
1552.53
43
1542.94
55
1533.47
20
1561.42
32
1551.72
44
1542.14
56
1532.68
21
1560.61
33
1590.12
45
1541.35
57
1531.90
22
1559.80
34
1550.12
46
1540.56
58
1531.12
23
1558.98
35
1549.32
47
1539.77
59
1530.33
24
1558.17
36
1548.52
48
1538.98
60
1529.55
25
1557.36
37
1547.72
49
1538.19
61
1528.77
26
1556.56
38
1546.92
50
1537.40
62
1527.99
27
1555.75
39
1546.12
51
1536.61
28
1554.94
40
1545.32
52
1535.82
29
1554.13
41
1544.53
53
1535.04
Connection-Oriented Networks
1534.25
34
Photo-detectors and optical receivers
• The WDM optical signal is demultiplexed
into the W different wavelengths, and each
wavelength is directed to a receiver.
• Each receiver consists of a
– photodetector,
– an amplifier, and
– signal-processing circuit.
Connection-Oriented Networks
35
Optical amplifiers
• The optical signal looses its power as it
propagates through an optical fiber, and
after some distance it becomes too weak to
be detected.
• Optical amplification is used to restore the
strength of the signal
Connection-Oriented Networks
36
1
1
Tx
…
W
Tx
Power
amplifier
optical
fiber
Wavelength
multiplexer
In-line
amplification
optical
fiber
Rx
…
Preamplifier
W
Rx
Wavelength
demultiplexer
Amplifiers:
power amplifiers,
in-line amplifiers,
pre-amplifiers
Connection-Oriented Networks
37
1R, 2R, 3R
• Prior to optical amplifiers, the optical signal
was regenerated by first converting it into
an electrical signal, then apply
– 1R (re-amplification), or
– 2R (re-amplification and re-shaping) or
– 3R (re-amplification, re-shaping, and re-timing)
and then converting the regenerated signal
back into the optical domain.
Connection-Oriented Networks
38
Amplification and Regeneration
Connection-Oriented Networks
39
The Erbium-doped fiber amplifier
(EDFA)
Coupler
Signal to be amplified
1550 nm
Erbium-doped
fiber
Isolator
Isolator
Laser
850 nm
Connection-Oriented Networks
40
Two-stage EDFA
Coupler
Coupler
Signal to be
amplified
1550 nm
Erbium-doped
fiber
Isolator
Laser
850 nm
Connection-Oriented Networks
Isolator
Laser
850 nm
41
The 2x2
coupler
Fiber 1
Input 1
Output 1
Input 2
Output 2
Fiber 2
Tapered
region
Coupling
region
Tapered
region
The 2x2 coupler is a basic device in optical networks, and
it can be constructed in variety of different ways. A
common construction is the fused-fiber coupler.
Connection-Oriented Networks
42
3-dB coupler
A 2x2 coupler is called a 3-dB coupler when the
optical power of an input light applied to, say
input 1 of fiber 1, is evenly divided between
output 1 and output 2.
Connection-Oriented Networks
43
• If we only launch a light to the one of the
two inputs of a 3-dB coupler, say input 1,
then the coupler acts as a splitter.
• If we launch a light to input 1 and a light to
input 2 of a 3-dB coupler, then the two
lights will be coupled together and the
resulting light will be evenly divided
between outputs 1 and 2.
• In the above case, if we ignore output 2, the
3-dB coupler acts as a combiner.
Connection-Oriented Networks
44
A banyan network of 3-dB couplers
1
128
2
128
3
128
4
128
5
128
6
128
7
128
8
128
Connection-Oriented Networks
45
Optical cross connects (OXCs)
Input
fibers
CPU
Output
fibers
1
1
W
W
Fiber 1
Fiber 1
1
1
W
W
Fiber N
Connection-Oriented Networks
Switch fabric
Fiber N
46
OXC (cont’d)
• Optical cross-connects
Wavelength
Router
OXC
WDM link
To & from
other nodes
GMPLS
Plane
To & from
other nodes
UNI
Access
Station
Tx
Connection-Oriented Networks
IP router
Local Add
Rx
Local Drop
47
OXC: switching fabric
• Switching fabric
MEMS: one mirror
per output
Input WL λ1
to output 1
OXC
Output
1
Connection-Oriented Networks
2
3
4
48
OXC: switching fabric (cont’d)
• Switching fabric
MEMS: one mirror
per output
Input WL λ1
to output 4
OXC
Output
1
Connection-Oriented Networks
2
3
4
49
OXC functionality
• It switches optically all the incoming
wavelengths of the input fibers to the
outgoing wavelengths of the output fibers.
• For instance, it can switch the optical signal
on incoming wavelength i of input fiber k
to the outgoing wavelength i of output
fiber m.
Connection-Oriented Networks
50
• Converters:
If it is equipped with converters, it can switch the
optical signal of the incoming wavelength i of
input fiber k to another outgoing wavelength j of
the output fiber m.
This happens when the wavelength i of the output
fiber m is in use.
Converters typically have a limited range within
they can convert a wavelength.
Connection-Oriented Networks
51
• Optical add/drop multiplexer (OADM):
An OXC can also be used as an OADM.
That is, it can terminate the optical signal of a
number of incoming wavelengths and insert new
optical signals on the same wavelengths in an
output port.
The remaining incoming wavelengths are
switched through as described above.
Connection-Oriented Networks
52
Transparent and Opaque Switches
Transparent switch:
The incoming wavelengths are switched to the
output fibers optically, without having to convert
them to the electrical domain.
Opaque switch:
The input optical signals are converted to
electrical signals, from where the packets are
extracted. Packets are switched using a packet
switch, and then they are transmitted out of the
switch in the optical domain.
Connection-Oriented Networks
53
Switch technologies
Several different technologies exist:
–
–
–
–
–
micro electronic mechanical systems (MEMS)
semiconductor optical amplifiers (SOA)
micro-bubbles
holograms
Also, 2x2 directional coupler , such as the
electro-optic switch, the thermo-optic switch,
and the Mach-Zehnder interferometer, can be
used to construct large OXC switch fabrics
Connection-Oriented Networks
54
2D MEMS switching fabric
Input
ports
i
…
…
…
…
Up
Down
Actuator
…
…
Mirro
r
Output
ports
j
Connection-Oriented Networks
55
A 2D MEMS OADM
Drop wavelengths
12W
Add
wavelengths
Terminate
wavelengths
Logical design
Connection-Oriented Networks
i
…
12W
…
12W
…
…
…
…
…
…
12W
… …
Add
wavelengths
2D MEMS implementation
56
3D MEMS switching fabric
Output wavelengths
y axis
MEMS
array
Inside
ring
Input wavelengths
Mirro
r
x
axis
MEMS
array
Connection-Oriented Networks
57
Semiconductor optical amplifier (SOA)
• A SOA is a pn-junction that acts as an
amplifier and also as an on-off switch
Current
p-type
n-type
Optical signal
Connection-Oriented Networks
58
A2x2 SOA switch
• Wavelength 1 is split into two optical signals, and each
signal is directed to a different SOA. One SOA amplifies
the optical signal and permits it to go through, and the
other one stops it. As a result 1 may leave from either the
upper or the lower output port.
• Switching time is currently about 100 psec.
Polymer
waveguides
SOAs
Polymer
waveguides
1
2
Connection-Oriented Networks
59