Chapter8

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 - Harry Perros

1

WDM optical networks
λ1

λ1

Tx

…
λW
Tx

Power
amplifier

optical
fiber

In-line
amplification

optical
fiber

Rx

…
Preamplifier

Wavelength
multiplexer

λW
Rx
Wavelength
demultiplexer

A point-to-point connection


2

1

An example of an optical network
Mesh network

Ring 4

Ring 1

Ring 2

Ring 3


3

How light is transmitted through an
optical fiber
Wave
Electric
field
Source

Waves and electrical fields


4

2

An optical fiber
Cladding
Core
Cladding

Core and cladding

Cladding

Cladding

Core

n1

Refractive index

Refractive index

Core

n2

Radial distance

n1

n2

Radial distance

a) Step-index fiber

b) graded-index fiber


5

Refraction and reflection of a light ray
θf

Refracted ray

n2

n1

θι
Incident ray


θr
Reflected ray

6

3

Angle of launching a ray into the fiber
Cladding
Cladding
Core

θl

θι

Core

θr

Cladding
Cladding

Cladding
Optical
transmitter

Core
Cladding


7

Multi-mode and single-mode fibers
• Core/diameter of a multi-mode fiber:
– 50/125 µm,
– 62.5/125 µm,
– 100/140 µm

• Core/diameter of single-mode fiber
– 9 or 10 / 125 µm


8

4

Electric fields
Cladding
Core

A
2

1
Cladding

B


9

Electric field amplitudes for
various fiber modes
Cladding

Core

Cladding
m=0

m=1


m=2

10

5

Propagation of modes
Cladding

Cladding

a) step-index fiber
Cladding

Cladding

b) Graded-index fiber

11

Single-mode fiber
Cladding

Cladding


12

6

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.

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.

14

7

Attenuation
2.5

Attenuation, dB

2.0

1.5
1.0
0.5

800

1000

1200
1400
Wavelength, nm

1600

1800


15

Dispersion
• Dispersion is due to a number of
reasons, such as
– modal dispersion,
– chromatic dispersion,
– polarization mode dispersion.


16

8

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

17

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.

18

9

• 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.

19

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.

20

10

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.

21

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)

22

11

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.

23

• 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.


24

12

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

25

Components
•
•
•
•
•

Lasers
Photo-detectors and optical receivers
Optical amplifiers
The 2x2 coupler
Optical cross connects (OXC)


26

13

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.

27

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


Right facet

28

14

• 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.

29

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.

30

15

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.


31

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.

32

16

The ITU-T DWDM grid
Channel
code
18

λ (nm)

λ (nm)

1563.05

Channel
code
30

λ (nm)

1553.33

Channel
code
42

λ (nm)

1543.73

Channel
code
54

19

1562.23

31

1552.53

43

1542.94

55

20

1561.42

32

1551.72

1533.47

44

1542.14

56

21

1560.61

33

1532.68

1590.12

45

1541.35

57

22

1559.80

1531.90

34

1550.12

46

1540.56

58

23

1531.12

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

1534.25


33

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.


34

17

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


35

λ1

λ1

Tx

…
λW
Tx

Power
amplifier

optical
fiber

In-line
amplification

optical
fiber

Rx

…
Preamplifier

λW
Rx

Wavelength
multiplexer

Wavelength
demultiplexer

Amplifiers:
power amplifiers,
in-line amplifiers,
pre-amplifiers


36

18

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.

37

The Erbium-doped fiber amplifier
(EDFA)
Coupler

Signal to be amplified
1550 nm

Erbium-doped
fiber

Isolator

Isolator

Laser
850 nm


38

19

Two-stage EDFA
Coupler

Coupler

Signal to be
amplified
1550 nm

Erbium-doped
fiber

Isolator

Laser
850 nm

Isolator

Laser
850 nm


39

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.

40

20

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.


41

• 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.

42

21

A banyan network of 3-dB couplers
λ1,λ2..,λ8

λ
1

λ1,λ2..,λ8

λ
2

λ

λ1,λ2..,λ8

3

λ

λ1,λ2..,λ8

4

λ

λ1,λ2..,λ8

5

λ

λ1,λ2..,λ8

6

λ

λ1,λ2..,λ8

7

λ1,λ2..,λ8

λ
8


43

Optical cross connects (OXCs)
CPU

Input
fibers

Output
fibers
λ1

λ1

…

…
λW

λW

Fiber 1

...

...

Fiber 1

λ1

λ1

…

…
λW

Switch fabric

Fiber N

λW
Fiber N

A logical diagram of an OXC


44

22

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 λi of input fiber k
to the outgoing wavelength λi of output
fiber m.

45

• Converters:
If it is equipped with converters, it can switch the
optical signal of the incoming wavelength λi of
input fiber k to another outgoing wavelength λj of
the output fiber m.
This happens when the wavelength λi of the
output fiber m is in use.
Converters typically have a limited range within
they can convert a wavelength.

46

23

• 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.


47

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.

48

24

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


49

2D MEMS switching fabric
Input
ports

…

Up

…

Down

…

…

…

…
…

i

…

…

…
…

…

…

…

Actuator

Mirro
r

Output
ports

j


50

25

A 2D MEMS OADM
Drop wavelengths

Add
wavelengths

i

…

…

…

λ1,λ2..,λW

…

…

…

…

…

…

… …

…
λ1,λ2..,λW

…

λ1,λ2..,λW

λ1,λ2..,λW

… …

Terminate
wavelengths

Add
wavelengths

Logical design

2D MEMS implementation


51

3D MEMS switching fabric
Output wavelengths
y axis
MEMS
array

Inside
ring
Input wavelengths

Mirro
r
x
axis

MEMS
array


52

26

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


53

Α 2x2 SOA switch
• Wavelength λ1 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 λ1 may leave from either the
upper or the lower output port.
• Switching time is currently about 100 psec.
Polymer
waveguides

SOAs

Polymer
waveguides

λ1

λ2


54

27

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