1

Chapter 3 Components

Couplers, Isolators and Circulators, Multiplexers and

Filters, Optical Amplifiers, Transmitters, Detectors switches, Wavelength

converters.

2

3.1 Couplers [ wavelength independent, wavelength

selective for 1.31/1.55 multiplexing]

α ： coupling ratio3dB couple α= 1/2α = 0.95 (for monitoring)

1 α

1-α

3

Def: excess loss: the loss of the device above the fundamental loss introduced by the coupling ratio α Example: A 3dB coupler may have 0.2dB excess loss

For multiplexing 1310nm

1550nm

1310nm1550nm

For EDFA 1550nm 1550nm

980nmor 1480nm

980nmor 1480nm

4

3.1.1 Principle of Operation

E: electrical fieldS-parametersFor lossless couplers

12

21

01 1

02 2

1 11 1

2 22 2

( ) ( )cos( ) sin( )(3.1)

( ) sin( ) cos( ) ( )ij

i

E f E fk i ke

E f i k k E f

b s s a

b s s a

1

:

: ,

, ...

the coupling length

k coupling coefficient depending on width

shape of waveguides n distance

a1→

a2→

→ b1

→ b2

5

The power transfer function2

02

211

212

( )

: , :

( ) cos ( )(3.2)

( ) sin ( )

jij

ii

ET f

E

i input j output

T f kf

T f kf

11 12

2 2

3

1( ) ( )

21

sin ( ) cos ( )2

(2 1) 04

For a dB coupler

T f T f

kf kf

k n n

6

3.1.2 Conservation of Energy (S-parameter)

11 1201 1

02 221 22

(3.3)i

i

s sE E

E Es s

11 12

21 22

0 01 02

1 2

0

2 20 0 1 2

:

( )

( )

ij

T

Ti i i

i

T

i i

The scattering matrix is

s sS s complex

s s

Denote E E E

E E E

E SE

The sum of input power is proportional to

E E E E

complex conjugate

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7

Similarly the sum of output power is proportional to

If it is lossless

This relation holds for arbitrary

Eq(3.4) can be extended to any number of ports

2 20 0 01 02

TE E E E

����������������������������

0 0 ( ) ( )

( )

T Ti i

T Ti i

Ti i

E E SE SE

E S S E

E E

��������������������������������������������������������

����������������������������

����������������������������

iE��������������

(3.4)

:

TS S I

I identity matrix

8

For a 2 x 2 symmetrical coupler21 12 22 11

2 21 (3.5)

0 (3.6)

T

s s a s s b

S S I

a b

ab ba

( ) ( )

cos( ) sin( ) (3.7)

cos( ) , sin( )

0

cos( )sin( ) 0

cos( ) 0

2 1

2

a b

a b b a

i i

i i

a b

a b

a x b x

let a x e b x e

ab ba

x x e e

k

lossless combination is impossible

9

3.2 Isolators and Circulators (nonreciprocal devices)

Isolators are for transmitter, circulators are for add and drop or others.

The insertion loss should be small ~ 1dBA circulator is similar to an isolator except it has

multiple ports.

10

3.2.1 Principle of Operation of an Isolator

SOP= state of Polarization

11

A spatial walk-off polarized splits the signal into

two orthogonally polarized components.

12

3.3 Multiplexer and FiltersMultiplexers and filters are for WDM, add/drop. WXC,

13

Dynamic WXCs use optical switches and mux/demux.

14

The desired characteristics of filters

1. Low insertion loss

2. Polarization-independent loss

3. Low temperature coefficient

4. Reasonable broad passbands

5. Sharp passband skirts

6. Low cost

a. integrated-optic (may be polarization dependent)

b. all-fiber devices

passband skirt

15

16

3.3.1 GratingsAny device whose operation involves interference among

multiple optical signals originating from the same source but with different relative phase shifts. An exception is a device where the multiple optical signals are generated by repeated traversals of a single cavity (etalons).

F-P

17

18

Principle of OperationThe pitch of the grating (distance between adjacent slits)=a

Assuming plane wave is incident at angle

: diffraction angle

The slits are small compared to λ,

phase changes across a slit is negligible

id

19

For construction interference at λ occurs at the image plane if

sin sin

sin sin

i d

i d

AB CD a a

a

sin sin

:

i da m

m the order of the grating

20

21

The energy at a single λ is distributed over all the discrete angles that satisfy (3.9).

For WDM only light of a certain order m will be collected, the remaining energy is lost.

m=0 has most energy θi= θd

The wavelengths are not separated.blazed reflection grating maximize the light e

nergy at α

22

3.3.2 Diffraction PatternRelax the constrain a <<λ, the phase change

across the slit is not negligible, consider a slit of length from

The relative phase shift of the diffracted light from y at an angle θ compared to that from y=0 is given by

,2 2w wy to w

（

2w

2w

y

sin( ) 2

yy

23

The amplitude A(θ) at θ (Ref: Optics, page401)

Fourier Transform of rectangular slit.

For any diffracting aperture f(y)

2

2

2

2

0exp ( )

0exp 2 (sin )

sin( sin )0(3.10)

sin

w

w

w

w

AA i y dy

w

A yi dyw

wA

w

0 ( )exp 2 (sin ) (3.11)yA A f y i dy

2where A intensity distribution

24

1= w

( ) 1

( )0

( )

( ) 0.5 2 2

0exp 2 (sin )2 22

0exp 2 (sin ) exp 2 (s22

If f y dy normalized

For a rectangular slit

y wf y

otherwise

For a pair of narrow slits infinite long with spacing d

d df y y y

A yd dA y y i dy

A di i

=

in ) 2

0 cos( sin )

d

dA

25

3.3.3 Bragg Gratings (BGs)

BGs are widely used in WDM

BGs: any periodic perturbation in the

propagating medium. (periodic

variation of n)

(Fiber BGs are written by UV)

BGs can also be formed by acoustic waves.

26

Principle of OperationConsider two waves with β0 and β1 propagating in opposit

e directions.

If the Bragg phase-matching condition is satisfied

when Λ= the period of the grating

Consider β1 wave propagating from left to right,

Then the energy from this wave is coupled onto a scattered wave traveling from right to left at the same wavelength provided.

0 0 0

0 0

0

0 0

2( ) 2

2 /

:

:

2

eff

eff

eff

let n

wavelength of the incident wave

n effective refractive index

n Bragg wavelength

0 1

2

27

These reflections add in phase, when the path length in λ0 each period is equal to half the incident wavelength λ0

02effn Bragg condition

28

29

Δλ: detuning from λ0

Δ is inversely proportional to the length of the grating

Apodized grating: the refractive index change is made small toward the edges of the grating

=> increasing the main lobe width

The index distribution over the length of BG is analogous to the grating aperture in sect3.3.2.

The side lobes arise due to the abrupt start and end of the grating, which result in a sinc(.) behavior for the side lobes.

Apodization is similar to pulse shaping to reduce the side lobes of signal spectrum.

30

3.3.4 Fiber Gratings (FGs)A. Useful for filter, add/drop compensating dispersionB. Advantages: a. low loss (0.1dB) b. ease of coupling c. polarization insensitivity d. low temperature coefficient e. simple packaging f. extremely low costC. Made from photosensitive fiber (Ge-doped) UV intensity ↑ n↑] change of n ~ 10-4

D. Two kind of FGs a. short period (Bragg Grating Λ~ 0.5μm) b. long period (Λ~ 100+μm – 1000+μm)

31

Fiber Bragg Gratings (FBG)

A. extremely low loss ~ 0.1dB

B. high wavelength accuracy (±0.05nm)

C. high crosstalk suppression (Fig 3.8) (40dB)

D. flat tops

E. typical temperature coefficient ~1.25x10-

2nm/℃ For passive temperature-compensated ~

0.07x10-2nm/℃

32

33

Long-Period Fiber Grating (a few intermeters)

Useful for EDFA gain (equalization)

They may be cascaded to obtain the desired profile.

34

Principle of OperationThe propagating mode in core couples onto the

modes in the cladding => induce loss

For a given λ

coupling occurs depending on Λ

β= propagation constant of the core mode

: propagation constant of the path order cladding mode

The phase matching condition

1Pc

2Pc

PcBecause is very small

long a few hundred m

35

Let and be the refractive indices of the core and the path-order cladding modes

effn peffn

2

22

( )

eff

P Pc eff neff

Peff neff

n

n n

n n

, ,

.

Peff neffGiven n n obtain

It is a wavelength dependent loss element

core cladding

36

3.3.5 Fabry-Perot Filters

This filter is called Fabry-Perot interferometer or etalon.

Principle of Operation The wavelengths for which the cavity length is an integral

multiple of half the wavelength in the cavity are called resonant wavelengths.

37

A round trip through the cavity is an integral multiple of the wavelength.

The light waves add in phase.

Assume r1=r2 t1=t2

The reflectance R=r1r2

A: absorption loss of mirror

T=t1t2=transmission:

:

nOne way delay en reflective index

r1 r3t1

t2

iE

(1 )iiE e A R

3 (1 )iiE e R A R

5 2(1 )iiE e R A R

l

38

2 2 40 (1 ) 1 ...i i i

iE E A R e Re R e

=E

E

2

2

1(1 )

1

(1 )

1

ii i

ii

i

A R eRe

A R e

Re

2

2

2

=+ -2

+ -2

- +4

2 2

02

2

2

2 2 2

2

2

2

2

2

2

(1 )( )

1

1

1 cos2 sin2

(1 )

(1 cos2 ) sin

(1 )cos2 1 2sin2

1 cos2

(1 )

1 4 sin

(1 )

(1 ) sin ( )

11

21

1

i

FP ii

E A R eT f

E Re

A R

R iR

A R

R R

A R

R R

A R

R R R

A R

R R

AR

R

2

2

2

2sin

2: ,

11

( ) (3.12)2

1 sin(2 )1

( ) sin2 0 / 2

FP

FP

nR

nnone way delay c

AR

T fR

fR

For maximam T f f f k

39

A=0, R=0.75, 0.9 and 0.99TFP (f) is periodic function with period FSRWhere FSR: free spectral range = The spectral range between two successive passband = 1/2τ

1( ' ) , 1 '

2 2

kf f k FSR f f

40

1: (3.12) 0

2sin2 ' 1, 3

11

sin2 '2

1 1 sin2 ' 2 ' , '

1' , 2 '

4

1 1(3.13)

2 12

Define finesse

F

FSRFWHM

R

Rproof Assume A

Rf for dB point

RR

fR

R f f f

Rf FWHM f

R

FSR RRF

FWHM RR

is the smallest value satisfied the condition

41

Tunability

1. change cavity length

2. change refractive index n

Recall

The wave with frequency will be selected.

1. mechanical tuning

2. piezoelectric tuning

=> thermal instability, hysteresis

0 :2

kf k positive integer

0f

nc

42

3.3.6 Multilayer Dielectric Thin-Film FiltersA thin-film resonant multicavity filter (TFMF) consist of

two or more cavitied.

Advantages: flat top, sharp skirt, low loss, insensitive to the polarization

43

44

45

3.3.7 Mach-Zehnder Interferometers (MZI)

Usage: filter, MUX/DEMUX, modulator, switch

Problems:

a. wavelength drift caused by aging or temperature variation

b. not exact 50:50

c. not flat top passbands

Change temperature (or refractive index) of one arm=> tuning

46

Principle of Operation

01 1

02 2

2

01

02

( ) ( )cos( ) sin( )(3.1)

( ) sin( ) cos( ) ( )

( ) 0

( ) cos( )

( ) sin( )

:

ii

i

i

i

i

Recall

E f E fk i ke

E f i k k E f

let E f for DEMUX

E f e k

E f e i k phase lag due to i

let L length difference in lower arm

L

another phase lag

47

At the upper output .

The signal all through the upper arm as reference.

The signal through the lower arm and the upper output has phase lag

At the lower output the phase difference

2 2L L

2 2L L

through low arm由第一個 3dB coupler產生 delayπ/2

through upper arm由第二個 coupler 到第二個 output產生 delayπ/2 所以互相 cancel

48

(2 1)

(2 1) 2( 2)

, (2 1) (2 1)

( )

(2 )

,

If L k k is odd k n

n n in phase

The signals at the upper arm add in phase at upper arm

At the lower output the phase difference is n L n

out of phase no signal

If L n

At the upper arm output the

2

11

212

(2 ) (2 1)

(2 )

sin( ) 2(3.14)

( ) cos 2

1: cos( ) sin( )

phase difference is

n n out of phase no signal

At the lower arm output

L n signals add in phase

The transfer function of MZI is

LT f

T f L

hint k k

1 01

2 02

2

' '

' '

2

( ) ( )1 0

0( ) ( )

( ) 0 (3.14)

i

i

i

i L

i L

and

The input and output relation of the middle section is

E f E fe

eE f E f

let E f and Multiply three matrices

49

consider K MZI interconnected

The path length difference for the kth MZI is assumed to be 12kL

50

51

MZI can be used as a 1x2 demultiplexer or multiplexer

λ1 λ2 chosen to be coincide with the peaks or troughs of the transfer function

If , and mi is odd, say mi=1 output 1 has signal, output 2 has no signal,

If and mi is even, output 1 has no signal.

λ1 λ2 λ1

λ2

MZI

2 2

2

2, :

2 2

sin sin2 2

eff

effi i i ii i

eff i eff

i

n

nm mlet L L m m integer

n n

mL

1

2 eff

i

Ln

m

2

2 eff

i

Ln

m

52

3.3.8 Array wavelength Grating (AWG)

Usage: a. nx1 multiplexer b. 1xn demultiplexer c. crossconnect (wavelengths and FSR must be chosen)Advantages: low loss, flat passband, ease to realized on a

integrated-optic substrate (silicon), the waveguides are silica. Ge-doped silica, or SiO2-Ta2O5

Because the temperature coefficient = 0.01nm/ is large℃Temperature control may be needed.

目前除了用 Rowland circle 之外尚可用multimode interference (MMI) 做 coupler

53

54

Principle of OperationLet number of inputs and outputs be n, and the numbe

rs of inputs and outputs of the couplers be nxm and mxn

ΔL=length difference between two adjacent waveguides. = difference in distance between input i and array waveguide k =difference in distance between array waveguide k and output j

inikdoutk jd

n

ik

m m

n

55

The relative phase

(3.15)1 2 1

2 in outijk ik kjn d n k L n d

input outputthrough k

k= 1. 2. …m

= +

1 2 1 1 1

11 2 1

2

2 2(3.16)

in in inik i i

out out outkj j j

in out in outijk i j i j

in out in outi j i j

If we design that

d d k

d d k

Then n d n k L n d n k n k

n kd d n n L n

56

Rowland circle constructiongrating circle

Rowland

57

3.16)

1 2 1

1 2 1

1

,

,

22

(

( 1) '

'

j

in outi j j

jj

j

j

in outi j j

j

ini

If appears at input i and

n n L n p p is integer

kp kp

will add in phase at output j prob

will be present at output j

If n n L n p

will be also present at output j

let n

2 1

2

( 1)

'

'

outj

pc p cn L n p

f f

cFSR f f

n L

58

3.3.9 Acoustic-Optic tunable Filter (AOTF)

polarization-dependent, polarization-independent.

59

Principle of OperationAs Fig 3.27 AOTF is constructed from a birefringent m

aterial and only supporting the lowest-order TE and TM modes.

If an acoustic wave is launched, the n varies to form gratings.

The Bragg condition is satisfied

TE mode is converted to TM mode.

For LiNbO3, |nTE-nTM|=0.07=Δn. at 1.55μm λ=ΛΔn (3.18)At 170MHz Λ=22μm, acoustic wavelength

1(3.17)TM TZn n

60

The transfer function is

where Δλ=λ-λ0 λ0 satisfies (3.17) Δ=λ0

2/lΔn l : the length of acoustic-optic interaction FWHM bandwidth=0.8Δ

2 2

2

sin 1 (2 )2

( )1 (2 )

T

-10dB down is not enough => cross talk

61

62

Disadvantages: high loss, large crosstalk, bulky

wide passband> 100GHz

dynamic crossconnect

response time ~ millisecond

63

3.3.10 High Channel Count Multiplexer Architectures

A. Serial (only for small number of ports)

不同 channel 有不同 insertion loss

64

B. Single stage (AWG)

最好的選擇

65

C. Multistage banding

66

D. Multistage Interleaving

67

3. 4 Optical Amplifiers

Advantages: transparent to bit rate, pulse format, large bandwidth, high gain

Disadvantages: noise accumulates

A. Erbium-doped fiber amplifiers (EDFA)

B. Raman amplifiers (RA)

C. Semiconductor optical amplifiers (SOA)

68

3.4.1 Stimulated Emission (EDFA or SOA)

69

Two energy levels E2>E1

hfc= E2-E1, h: Planck's constant= 6.63x10-34JS

(absorption)

E1→E2 excitation (by photons or population inversion)

E2→E1 emission photons

a. stimulated emission

b. spontaneous emission

If emission > absorption => amplification

N1: Population (number of atoms) at E1

N2: population at E2

If N2 > N1, population inversion occurs.

1 2

2 1

rate of E Er

rate of E E

70

3.4.2 Spontaneous Emission

If ASE is very large

=> Saturate the amplifier

E2

E1

noncoherent

hf=E2-E1

amplified spontaneous emission (ASE)(noise)

71

3.4.3 EDAF

Erbium fiber = Er3+ doped silica fiber

Pumping wavelength = 980nm or 1480nmAdvantages(1) Availability of high power pump lasers(2) All fiber device, polarization independent, ease to

couple, reliable(3) Simple(4) Less crosstalk

72

Principle of Operation

73

Stark splitting : an isolated ion of erbium is split into multiple energy levels.

Each stark splitting level is spread into a band.

Thermalization : the erbium ions are distributed in the various levels within the band.

Capable of amplifying several wavelengths simultaneously.

page 39, c-band from 1530~1565nm

2 1

32 3 2

21 2 1

2

1 sec

10 sec

, 980

1520 1570 50c

hfc E E

E E

E E m

atoms stay at E longer nm pump is usable

nm f nm BW nm

74

When 980nm pump is used

τ32≈ 1μsec << τ21

We have population inverse between E2 and E1

We can amplify 1530-1570nm signalsWhen 1480nmpump is used the absorption from

the bottom of E1 to the top of E2

1480nm pump is less efficient Less population inversionHigher noise figure980nm for low noise EDFAHigh power 1480nm pump is available => High output power and pump can be located

remotely

75

76

77

78

Gain Flatness

79

80

81

82

Multistage Designs

The first stage: high gain, low noise The second stage: high output powerTwo-stage amplifier is more reliable (pump failure)The inserted loss element can be gain compensation,

add/drop or dispersion compensation,L-band EDFA needs high pumping power and produces

high ASE

83

84

3.4.4 Raman Amplifiers (RA)

RA can provide gain about 100nm band (13THz) above the pumping wave λp<λs (Signal Wavelength)

A. RA is a distributed device and can provide gain in different bands

B. No special fibers are neededC. Required high pump power~1wD. Pump power fluctuations induce noise (propagating in same

direction), propagating in opposite directions will have lower noise

E. Crosstalk (modulated signals will deplete the pump power => fluctuation => noise) so, pumping opposite direction will lower the noise. (average out)

F. Another noise is due to Rayleigh scattering of the pumping signal

For example

1550~1600nmsignal

1460~1480nm pump

85

86

87

3.4.5 Semiconductor Optical Amplifiers (SOAs)

Amplifier, Switches, wavelength converters

88

89

90

91

92

93

94

95

96

97

98

99

100

101

102

103

3. 5 Transmitters

A transmitter includes a driving circuit and a light source.

The light source can be laser or LED. For WDM systems, a laser needs to have the following important characteristics:

a. Reasonably high power 0~10dBm, low threshold current, high slop efficiency

b. Narrow spectral widthc. Wavelength stability (low aging effect)d. Small chirping (direct modulation)

104

105

106

Lasers Semiconductor lasers, fiber lasers, gas lasers, solid state lasers (Ruby lasers), free electron laser,

107

Principle of Operation (semiconductor laser)

Reference: John Gowar “Optical Communication Systems” PP262~323

108

109

110

111

112

113

114

115

116

117

118

119

120

121

122

123

124

125

Longitudinal ModesMultiple-longitudinal mode (MLM) lasers have large spectra

l widths~10nm (Fabry-Perot lasers) =>cause chromatic dispersion

Single—longitudinal mode (SLM) lasers have very narrow spectral widths

Side-mode suppression ratio is an Important parameter for SLM lasers. (~30dB)

126

Distributed-Feedback Lasers (DFB Lasers) Distributed Bragg reflector (DBR) Lasers

The temperature coefficient ~ 0.1nm/ at 1550nm.℃

127

External Cavity Lasers

Grating External Cavity Lasers

128

3.5.3 Tunable lasersTunable lasers are useful to reduce the inventory, (spare p

arts), to reconfigure the network, to be used for optical packet switched networks and for laboratory testing.

Tuning mechanismsa. Injecting current (change n) tuning range ~10~15 nm a

t 1550nmb. Temperature tuning 0.1nm/℃c. Mechanical tuning (wide range but bulky)Desirable propertiesa. Short tuning timeb. Wide tuning range (100nm)c. Stable over its lifetimed. Easily controllable and manufacturable

129

Two-and Three-Section DBR Lasers

Problemsa. Agingb. Temperature changesc. Current recalibrationd. Mode hopping

130

Vertical grating-assisted coupler filter (VGF) LasersThe coupling condition (3.17)

λ=ΛB(n1-n2)

ΛB: The period of the Bragg grating

n1 and n2 are refractive indices of two waveguides.

If n1 changes to n1+Δn11 1 2

1

1

1

1 2

' ( )B

B

B

n n n

n

n

n

n n

131

Sample Grating and Super-Structure Grating DBR lasers

132

Grating Coupled sampled Reflection lasers

133

3.5.4 Direct and External ModulationDirect modulation

Advantage: SimpleDisadvantage: induce chirping

Biasing above the threshold will reduce chirping but decrease the extinction ratio.

134

External Modulationa. Lithium niobate modulator, b. electro-absorption modula

tor

135

: coupling coefficient depending on width of the waveguide, refractive indices, distance of two waveguides

211

212

( ) cos ( )(3.2)

( ) sin ( )

T f k

T f k

k

136

211

212

( ) sin ( 2)(3.14)

( ) cos ( 2)

T f L

T f L

MZI can achieve high extinction ratio ~15 ~20dB with almost on chirping. Polarization control is needed.

137

3.6 Detectors

138

3.6.1 PhotodetectorsPhotons incident on a semiconductor are absorbed by

electrons in the valence band. These are excited into the conduction band and leave holes in the valence band. When a reversed bias voltage is applied, these electron –hole pairs produce photo current.

(3.19)

1.24( . )

cc g

c

g g

fhf eE

fm ev

eE E

139

140

4

(1 ) (3.20)

:

:

:

1 (3.21)

10 , 10 , 0.99

Labs in

in

Labs

in

P e P

P incident power

absorption coefficient

L Thickness of the semiconductor

The efficiency

Pe

P

Example L mcm

141

AW

AW

A= W1.24The quantum efficiency

(=

( / sec) /

/ ( / sec)

)/

P

in

inP

c

c

ph

in ph

ph

in ph

ph

in

The responsivity

IR

P

PI e hf

eR

hf

e

hc

I e electrons

P E photons

I eP E

I hc hcR

P e e

142

143

PIN Photodiodesa. A very lightly doped intrinsic

semiconductor between the p-type and n-type Layers can improve the efficiency. The depletion region extends across the intrinsic layer.

b. If the p-type or n-type layer is transparent the efficiency can be further improved.

144

145

146

147

Avalanche Photodiodes (APD)

When the generated election in a very high electric field, it can generate more secondary electron-hole pairs. This process is called avalanche multiplication.

Gm: multiplicative gain

M: multiplication factor (Gm: M-1)

Large Gm will induce large noise.

If Gm→∞, avalanche breakdown occurs.

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3.6.2 Front-End Amplifiersa. High-impedance amplifier

b. Transimpedance amplifier

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3.7 Switches

Important parametersa. Number of portsb. Switching timec. The insertion lossd. The crosstalke. Polarization-dependent lossf. Latching (maintaining its switch state)g. Monitoring capabilityh. Reliability

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3.7.1 Large Optical SwitchedThe main considerationsa. Number of switch elements requiredb. Loss uniformityc. Number of crossoversd. Blocking characteristics blocking and nonblocking (strict sense, wide sens

e, rearrargeable)e. Synchronous or asynchronous

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Crossbar

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Spanke

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3.7.2 Optical Switch Technologies

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MEMS Switches

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Bubble-Based Waveguide Switch

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Liquid Crystal Switches

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A. Thermal-Optic Switches (MZI)

B. Semiconductor Optical Amplifier Switches

C. Large Electronic Switched

a) Single stage

b) Multistage

c) Line rate

d) Total capacity (line rate x number of ports)

e) Circuit switching V.S. packet switching

f) Cross bar V.S. shared memory

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3.8 Wavelength Converters

a. A device converters data from one incoming wavelength to another outgoing wavelength.

b. Used in WDM networks i. input wavelength is not suitable for the networks ii. Improving the wavelength utilization in WDM

networks iii. Converting to suitable outgoing wavelengthsc. Types i. fixed-input, fixed-output ii. Variable-input, fixed-output iii. Fixed-input, variable-output iv. Variable-input, variable-output

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d. Other important characteristics

i. convertion range

ii. Transparent to data rate or modulation format

iii. Loss (efficiency)

iv. Noise, crosstalk

e. Mechanism to achieve wavelength convertion

i. optoelectronic (commercial available)

ii. Optical gating

iii. Interferomatric

iv. Wave mixing

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3.8.1 Optoelectronic Approach (O/E, E/O)

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3.8.2 Optical GratingUsing the principle of cross-gain modulation

in a SOA. (For high input signal power, the carrier will be depleted => less gain for the probe wavelength)

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Disadvantages

i. small extinction ratio

ii. High input signal power to deplete the carriers (simultaneously changes n)

iii. Requiring to filter this high-powered signal

iv. Changing refractive index inducing pulse distortion

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3.8.3 Interferometric Techniques

1

1

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Principle of Operation (cross phase modulation CPM)

When λs presents, the carrier densities (or n) change to induce different phase changes of λp. At the port A, the intensity of λp will be modulated.

i. digital signal only

ii. Higher extinction ratio

iii. Providing reamplification and reshaping

iv. Low input power

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Stage1 samples the dataStage2 reshapes and retimes the data (inverse)Stage3 reamplifies

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3.8.4 Wave Mixing