Download Chapter 2

Chapter 3
Components
Couplers, Isolators and Circulators,
Multiplexers and Filters, Optical Amplifiers,
Transmitters, Detectors switches,
Wavelength converters.
1
3.1 Couplers
[ wavelength independent, wavelength
selective for 1.31/1.55 multiplexing]
1
α
1-α
α：coupling ratio
3dB couple α= 1/2
α = 0.95 (for monitoring)
2
For multiplexing
1310nm
1310nm
1550nm
1550nm
For EDFA
1550nm
1550nm
980nm
or 1480nm
980nm
or 1480nm
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
3
a1→
3.1.1 Principle of Operation
→ b1
→ b2
a2→
E: electrical field
S-parameters
For lossless couplers
 E01( f ) 
 j   cos( k ) i sin( k

e

E
(
f
)
 i sin( k ) cos( k
 02

)   Ei1( f ) 


)   Ei 2 ( f ) 
(3.1)
 b1   s11 s12  a1 
 
 
b
s
s
 2   21 22  a2 
: the coupling length
k : coupling coefficient depending on width,
shape of waveguides, n1 distance...
4
The power transfer function
Tij ( f ) 
E0 j
2
Eii
2
i : input, j : output
 T11( f )   cos2 ( kf ) 


  
2
T
(
f
)
 12
  sin ( kf ) 
(3.2)
For a 3dB coupler
1
T11( f )  T12 ( f ) 
2
1
sin ( kf )  cos ( kf ) 
2
2
k  (2n  1) 
2
4
n0
5
3.1.2 Conservation of Energy (S-parameter)
 E01   s11 s12   Ei1 
 

  

 E02   s21 s22   Ei 2 
(3.3)
The scattering matrix is
 s11 s12 
S  

 s21 s22 
sij : complex
Denote E 0  ( E01 E02 )T
E i  ( Ei1 Ei 2 )T
E0  S Ei
The sum of input power is proportional to
T

E 0 E 0  Ei1
2
 Ei 2
 complex conjugate
2
6
Similarly the sum of output power is proportional to

0
T
0
2
E E  E01  E02
2
If it is lossless
T

E 0 E 0  ( S E i )T ( S E i )
T
T
T


 E i ( S S )E i
 Ei Ei

This relation holds for arbitraryE i
 ST S  I
(3.4)
I : identity matrix
Eq(3.4) can be extended to any number of ports
7
For a 2 x 2 symmetrical coupler
s21  s12  a
s22  s11  b
ST S  I
a
2
 b
2
1
ab  ba  0
a  cos( x )
b  sin( x )
(3.5)
(3.6)
(3.7)
let a  cos( x )eia , b  sin( x )eib
ab  ba  0
cos( x ) sin( x ) ei (a b )  ei (b a )   0
cos( a  b )  0
 2k  1 
 a  b  


 2 
 lossless combination is impossible
8
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 ~ 1dB
A circulator is similar to an isolator except it has
multiple ports.
9
3.2.1 Principle of Operation of an Isolator
SOP= state of Polarization
10
A spatial walk-off polarized splits the signal into
two orthogonally polarized components.
11
3.3 Multiplexer and Filters
Multiplexers and filters are for WDM, add/drop. WXC,
12
Dynamic WXCs use optical switches and mux/demux.
13
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
passband skirt
6. Low cost
a. integrated-optic (may be polarization
dependent)
b. all-fiber devices
14
15
3.3.1 Gratings
Any 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
16
17
Principle of Operation
The pitch of the grating (distance between adjacent slits)=a
Assuming plane wave is incident at angle  i
 d : diffraction angle
The slits are small compared to λ,
phase changes across a slit is negligible
18
AB  CD  a sin i   a sin  d 
 a  sin i   sin  d  
For construction interference at λ occurs at
the image plane if
a sin i   sin  d    m
m : the order of the grating
19
20
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
energy at α
21
3.3.2 Diffraction Pattern
Relax the constrain a <<λ, the phase change
across the slit is not negligible, consider a slit of
length from y   w 2 to w 2 , w  
w
2
y

w
（
2
The relative phase shift of the diffracted light from y at
an angle θ compared to that from y=0 is given by
 ( y )  2
y sin

22
The amplitude A(θ) at θ
A   


A 0
w
A 0
w

w
2
w
2

w
2
w
2
(Ref: Optics, page401)
exp  i ( y ) dy

exp i 2 (sin ) y

 dy
A  0  sin( w sin  )
(3.10)
 w sin 
Fourier Transform of rectangular slit.
For any diffracting aperture f(y)

y
A A 0
f ( y )exp 2 i(sin )
dy
(3.11)
 

  


where A    intensity distribution
2
23
If



f ( y )dy  1 normalized
For a rectangular slit
1
y w

f ( y) = w

otherwise
0
For a pair of narrow slits (infinite long ) with spacing d

2    y  d 2 
y dy
d
d

y



y

exp
2

i
(sin

)
  2   2  

f ( y )  0.5  y  d
A   
A 0

2 
A 0 

d
d

exp
2

i
(sin

)

exp

2

i
(s
in

)
2
2 
2 
= A  0  cos(  sin  d )





24
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.
25
Principle of Operation
Consider two waves with β0 and β1 propagating in
opposite directions.
If the Bragg phase-matching condition is satisfied
0  1 
2

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. 2
0  (  0 )  20 
let 0  2 neff / 0

0 : wavelength of the incident wave
neff : effective refractive index
0  2neff  0  Bragg wavelength
26
These reflections add in phase, when the
path length in λ0 each period is equal to
half the incident wavelength λ0
neff  
0
2
 Bragg condition
27
28
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.
29
3.3.4 Fiber Gratings (FGs)
A.
B.
Useful for filter, add/drop compensating dispersion
Advantages:
a. low loss (0.1dB)
b. ease of coupling
c. polarization insensitivity
d. low temperature coefficient
e. simple packaging
f. extremely low cost
C. 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)
30
Fiber Bragg Gratings (FBG)
A.
B.
C.
D.
E.
extremely low loss ~ 0.1dB
high wavelength accuracy (±0.05nm)
high crosstalk suppression (Fig 3.8) (40dB)
flat tops
typical temperature coefficient ~1.25x102nm/℃
For passive temperature-compensated ~
0.07x10-2nm/℃
31
32
Long-Period Fiber Grating (a few intermeters)
Useful for EDFA gain (equalization)
They may be cascaded to obtain the desired profile.
33
Principle of Operation
The 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
 cP1: propagation constant of the path order
cladding mode
The phase matching condition
  cP  2 
Because    cP is very small
  long
a few hundred  m
34
p
n
Let eff and neff be the refractive indices of the
core and the path-order cladding modes
  2 neff 
2
P
  
neff  nneff
 2 

P
  (neff  nneff
)
P
c
core


cladding
P
Given neff , nneff
,   obtain 
It is a wavelength dependent loss element.
35
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.
36
A round trip through the cavity is an integral
multiple of the wavelength.
The light waves add in phase.
r1
r3
t1
t2
Ei
E i e  i  (1  A  R )
Ei ei 3  R(1  A  R )
Ei ei 5  R 2 (1  A  R )
l
Assume r1=r2 t1=t2
The reflectance R=r1r2
A: absorption loss of mirror
T=t1t2=transmission
 : One way delay  n e
n : reflective index
37
E0  Ei (1  A  R )e i 1  Rei 2   R 2ei 4   ...
=Ei (1  A  R )e
i
1
1  Rei 2 
Ei (1  A  R )ei

1  Rei 2 
TFP ( f ) 
E0
Ei
2
2
(1  A  R )e  i 

1  Re  i 2 
2

1 A  R
1  R cos 2   iR sin 2 

(1  A  R )2
(1  R cos 2  )2  R 2 sin2 
=
(1  A  R )2
1+ R 2 -2 R cos 2 
cos 2 

(1  A  R )2
1+ R -2 R  4 R sin2 

(1  A  R )2
(1- R )2 +4 R sin2 ( 

 1  2 sin 2 
2
A 

1 1 R 


)
2
2 R
2 n
1 
sin
 1 R






2
2 n
 : one way delay  n c ,  

TFP ( f ) 
A 

1 1 R 


2
2
2 R

1 
sin(2

f

)

 1 R



For maximam TFP ( f ) sin 2 f   0
(3.12)
38
f  k / 2
A=0, R=0.75, 0.9 and 0.99
TFP (f) is periodic function with period FSR
Where FSR: free spectral range
= The spectral range between two successive passband
= 1/2τ
39
( f ' f ) 
k
, k 1
2
FSR  f ' f

1
2
Define finesse
F
 FSR
FWHM

 R
1 R
proof : (3.12)
Assume A  0
2 R
sin 2 f '  1, for  3 dB point
1 R
1 R
sin 2 f ' 
2 R
1 R
1
sin 2 f '  2 f ' , f ' is the smallest value
satisfied the condition
1 R
f '
,
FWHM  2 f '
4 R
FSR
1
F 

2
FWHM
 R
1 R

1 R
2 R
(3.13)
40
Tunability
1. change cavity length
2. change refractive index n
k
k : positive integer
Recall f0 
2
The wave with frequency f 0 will be selected.
 n
c
1. mechanical tuning
2. piezoelectric tuning
=> thermal instability, hysteresis
41
3.3.6 Multilayer Dielectric Thin-Film Filters
A thin-film resonant multicavity filter (TFMF) consist of
two or more cavitied.
Advantages: flat top, sharp skirt, low loss, insensitive
to the polarization
42
43
44
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
45
Recall
Principle of Operation
 E01( f ) 
 i   cos( k ) i sin( k

e

 i sin( k ) cos( k
 E02 ( f ) 
let Ei 2 ( f )  0 for DEMUX
)   Ei1( f ) 


)   Ei 2 ( f ) 
(3.1)
E01( f )  e  i cos( k )
E02 ( f )  e  i i sin( k )  phase lag due to i
let L  length difference in lower arm
 L : another phase lag
46
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

2
 L  
2
   L
At the lower output the phase difference
  L    L
2
2
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 47
If L  k k is odd k  (2n  1)
  (2n  1)  2( n  2)  in phase
The signals at the upper arm add in phase at upper arm
At the lower output, the phase difference is (2n  1) L  (2n  1)
 out of phase ( no signal )
If L  (2n )
At the upper arm output, the phase difference is
  (2n)  (2n  1) out of phase  no signal
At the lower arm output
L  (2n)  signals add in phase
The transfer function of MZI is




 2 L

 T11 ( f )   sin
2 
(3.14)

  

T
(
f
)
 12
  cos2 L 
2 

1
hint : cos( k )  sin( k ) 
and
2
The input and output relation of the middle section is
'
 E i'1 ( f ) 
1 0
  E 01 ( f ) 
 i L 

e



 i L 
'
 E' ( f )

0
e

  E 02 ( f ) 
 i2

let E i 2 ( f )  0 and Multiply three matrices  (3.14)
48
consider K MZI interconnected
The path length difference for the kth MZI is assumed to be L
49
2k 1
50
MZI can be used as a 1x2 demultiplexer or multiplexer
λ1 λ 2
λ1
MZI
λ2
λ1 λ2 chosen to be coincide with the peaks or troughs of the
transfer function

2 neff
let L 

2neff
sin2 L
If
1 

mi i

2 neff mi i
, L 
 mi
i 2neff
 sin2 
2

2Lneff
mi
mi : integer

2 
, and mi is odd, say mi=1 output 1 has
signal, output 2 has no signal,
2Ln


If
and mi is even, output 1 has no signal.
m
mi
eff
2
i
51
3.3.8 Array wavelength Grating (AWG)
Usage: a. nx1 multiplexer
目前除了用Rowland circle之外尚可用
b. 1xn demultiplexer multimode interference (MMI) 做coupler
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
52
Temperature control may be needed.
53
Principle of Operation
Let number of inputs and outputs be n, and the
numbers of inputs and outputs of the couplers be
nxm and mxn
i
n
k
m
n
m
ΔL=length difference between two adjacent waveguides.
d i ink = difference in distance between input i and array waveguide k
=difference in distance between array waveguide k and output
dk out
54 j
j
The relative phase
2
in
out
ijk 
n
d

n
k

L

n
d

1 ik
2
1 kj 

(3.15)
k= 1. 2. …m
input output
through k
If we design that
dikin  diin  k  iin
out
d kjout  d out

k

j
j
2
in
out
Then ijk 
n1diin  n2 k L  n1d out

n
k


n
k

j
1
i
1
j

2 n1 in
2 k
out

di  d j 
n1 iin  n2 L  n1 out
j


= + 






(3.16)
55
Rowland circle construction
grating circle
Rowland
56
If  j appears at input i, and
n1 iin  n2 L  n1 out
 p j ,
j
 
2 k
j
p is integer
p j  2 kp
 j will add in phase at output j ( prob 3.16)
 j will be present at output j
If n1 iin  n2 L  n1 out
 ( p  1) j '
j
 j ' will be also present at output j
let n 
in
1 i
 n2 L  n1
c
FSR  f ' f 
n2 L
out
j
pc ( p  1)c
 p 

f
f'
57
3.3.9 Acoustic-Optic tunable Filter (AOTF)
polarization-dependent, polarization-independent.
58
Principle of Operation
As Fig 3.27 AOTF is constructed from a birefringent
material 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
nTM nTZ 1


(3.17)

 
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
59
The transfer function is


sin2 
1  (2  )2 
 
2

T ( ) 
1  (2  )2

where Δλ=λ-λ0
λ0 satisfies (3.17)
Δ=λ02/lΔn
l : the length of acoustic-optic interaction
FWHM bandwidth=0.8Δ
-10dB down is not enough
=> cross talk
60
61
Disadvantages: high loss, large crosstalk, bulky
wide passband> 100GHz
dynamic crossconnect
response time ~ millisecond
62
3.3.10 High Channel Count Multiplexer
Architectures
A. Serial (only for small number of ports)
不同channel有不同insertion loss
63
B. Single stage (AWG)
最好的選擇
64
C. Multistage banding
65
D. Multistage Interleaving
66
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)
67
3.4.1 Stimulated Emission (EDFA or SOA)
68
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
rate of E1  E2
r
rate of E2  E1
If N2 > N1, population inversion occurs.
69
3.4.2 Spontaneous Emission
E2
hf=E2-E1
E1
noncoherent
amplified spontaneous emission (ASE)
(noise)
If ASE is very large
=> Saturate the amplifier
70
3.4.3 EDAF
Erbium fiber = Er3+ doped silica fiber
Pumping wavelength = 980nm or 1480nm
Advantages
(1) Availability of high power pump lasers
(2) All fiber device, polarization independent, ease to
couple, reliable
(3) Simple
(4) Less crosstalk
71
Principle of Operation
72
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.
hfc  E2  E1
 32  E3  E2  1  sec
 21  E2  E1  10m sec
atoms stay at E2 longer, 980nm pump is usable
1520nm  f c  1570nm BW  50nm
page 39, c-band from 1530~1565nm
73
When 980nm pump is used
τ32≈ 1μsec << τ21
We have population inverse between E2 and E1
We can amplify 1530-1570nm signals
When 1480nmpump is used the absorption from
the bottom of E1 to the top of E2
1480nm pump is less efficient
Less population inversion
Higher noise figure
980nm for low noise EDFA
High power 1480nm pump is available
=> High output power and pump can be located
remotely
74
75
76
77
Gain Flatness
78
79
80
81
Multistage Designs
The first stage: high gain, low noise
The second stage: high output power
Two-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
82
83
3.4.4 Raman Amplifiers (RA)
For example
1550~1600nm
signal
1460~1480nm pump
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 needed
C. Required high pump power~1w
D. 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 pumping84
signal
85
86
3.4.5 Semiconductor Optical Amplifiers (SOAs)
Amplifier, Switches, wavelength converters
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
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 width
c. Wavelength stability (low aging effect)
d. Small chirping (direct modulation)
103
104
105
Lasers
Semiconductor lasers, fiber lasers, gas
lasers, solid state lasers (Ruby lasers), free
electron laser,
106
Principle of Operation (semiconductor laser)
107
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
Longitudinal Modes
Multiple-longitudinal mode (MLM) lasers have large
spectral 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)
125
Distributed-Feedback Lasers (DFB Lasers)
Distributed Bragg reflector (DBR) Lasers
The temperature coefficient ~ 0.1nm/℃ at 1550nm.
126
External Cavity Lasers
Grating External Cavity Lasers
127
3.5.3 Tunable lasers
Tunable lasers are useful to reduce the inventory, (spare
parts), to reconfigure the network, to be used for
optical packet switched networks and for laboratory
testing.
Tuning mechanisms
a. Injecting current (change n) tuning range ~10~15 nm
at 1550nm
b. Temperature tuning 0.1nm/℃
c. Mechanical tuning (wide range but bulky)
Desirable properties
a. Short tuning time
b. Wide tuning range (100nm)
c. Stable over its lifetime
d. Easily controllable and manufacturable
128
Two-and Three-Section DBR Lasers
Problems
a. Aging
b. Temperature changes
c. Current recalibration
d. Mode hopping
129
Vertical grating-assisted coupler filter (VGF) Lasers
The 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+Δn1
 '   B ( n1  n1  n2 )
    B n1
   B n1
n1



n1  n2
130
Sample Grating and Super-Structure
Grating DBR lasers
131
Grating Coupled sampled Reflection lasers
132
3.5.4 Direct and External Modulation
Direct modulation
Advantage: Simple
Disadvantage: induce chirping
Biasing above the threshold will reduce chirping but decrease the
extinction ratio.
133
External Modulation
a. Lithium niobate modulator, b. electro-absorption
modulator
134
 T11( f )   cos2 ( k ) 


   2
 T12 ( f )   sin ( k ) 
(3.2)
k : coupling coefficient depending on
width of the waveguide, refractive
indices, distance of two waveguides
135
 T11( f )   sin2 ( L 2) 


  
2
 T12 ( f )   cos ( L 2) 
(3.14)
MZI can achieve high extinction ratio ~15 ~20dB with almost on
chirping. Polarization control is needed.
136
3.6 Detectors
137
3.6.1 Photodetectors
Photons 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.
hf c 

fc

 eEg
(3.19)
fc
1.24

(  m.ev )
eEg
Eg
138
139
Pabs  (1  e  L )Pin
(3.20)
Pin : incident power
 : absorption coefficient
L : Thickness of the semiconductor
The efficiency
Pabs
 L

 1 e
Pin
Example
  10
4
cm
(3.21)
, L  10  m,   0.99
140
The responsivity
R
IP A
W
Pin
IP  e 
R

=
Pin
hf c

e A
W
hf c
e
hc
 A
1.24 W
The quantum efficiency I ph e ( electrons / sec) /
Pin / E ph ( photons / sec)
=
( I ph e )
Pin / E ph
I ph hc
hc

R
Pin e
e
141
142
PIN Photodiodes
a. 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.
143
144
145
146
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.
147
148
149
150
151
152
153
154
155
156
157
3.6.2 Front-End Amplifiers
a. High-impedance amplifier
b. Transimpedance amplifier
158
159
160
3.7 Switches
Important parameters
a. Number of ports
b. Switching time
c. The insertion loss
d. The crosstalk
e. Polarization-dependent loss
f. Latching (maintaining its switch state)
g. Monitoring capability
h. Reliability
161
3.7.1 Large Optical Switched
The main considerations
a. Number of switch elements required
b. Loss uniformity
c. Number of crossovers
d. Blocking characteristics
blocking and nonblocking (strict sense, wide
sense, rearrargeable)
e. Synchronous or asynchronous
162
Crossbar
163
Spanke
164
3.7.2 Optical Switch Technologies
165
MEMS Switches
166
167
Bubble-Based Waveguide Switch
168
Liquid Crystal Switches
169
A. Thermal-Optic Switches (MZI)
B. Semiconductor Optical Amplifier Switches
C. Large Electronic Switched
a)
b)
c)
d)
e)
f)
Single stage
Multistage
Line rate
Total capacity (line rate x number of ports)
Circuit switching V.S. packet switching
Cross bar V.S. shared memory
170
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 wavelengths
c. Types
i. fixed-input, fixed-output
ii. Variable-input, fixed-output
iii. Fixed-input, variable-output
iv. Variable-input, variable-output
171
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
172
3.8.1 Optoelectronic Approach (O/E, E/O)
173
3.8.2 Optical Grating
Using 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)
174
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
175
3.8.3 Interferometric Techniques
1 
1 
176
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
177
Stage1 samples the data
Stage2 reshapes and retimes the data (inverse)
Stage3 reamplifies
178
3.8.4 Wave Mixing
179