Chapter 3 Components

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Chapter 3 Components
Couplers, Isolators and Circulators, Multiplexers
and Filters, Optical Amplifiers, Transmitters,
Detectors switches, Wavelength converters.
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3.1 Couplers wavelength independent,
wavelength selective for 1.31/1.55 multiplexing
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a
1-a
acoupling ratio 3dB couple a 1/2 a 0.95 (for
monitoring)
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• Def excess loss the loss of the device above
  the
• fundamental loss introduced by the
  coupling ratio a
• Example A 3dB coupler may have 0.2dB
  excess loss

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3.1.1 Principle of Operation

• E electrical field
• S-parameters
• For lossless couplers

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• The power transfer function

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3.1.2 Conservation of Energy (S-parameter)
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• 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

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• For a 2 x 2 symmetrical coupler

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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.

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3.2.1 Principle of Operation of an Isolator
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A spatial walk-off polarized splits the signal
into two orthogonally polarized components.
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3.3 Multiplexer and Filters
Multiplexers and filters are for WDM, add/drop.
WXC,
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Dynamic WXCs use optical switches and mux/demux.
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• The desired characteristics of filters
• Low insertion loss
• Polarization-independent loss
• Low temperature coefficient
• Reasonable broad passbands
• Sharp passband skirts
• Low cost
• a. integrated-optic (may be polarization
  dependent)
• b. all-fiber devices

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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).

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Principle of Operation

• The pitch of the grating (distance between
  adjacent slits)a
• Assuming plane wave is incident at angle
• diffraction angle
• The slits are small compared to ?,
• phase changes across a slit is negligible

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• For construction interference at ? occurs at the
  image plane if

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• The energy at a single ? 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.
• m0 has most energy ?i ?d
• The wavelengths are not separated.
• blazed reflection grating maximize the light
  energy at a

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3.3.2 Diffraction Pattern

• Relax the constrain a ltlt?, 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 ? compared to that from y0 is
given by
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• The amplitude A(?) at ? (Ref Optics, page401)
• Fourier Transform of rectangular
  slit.
• For any diffracting aperture f(y)

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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.

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Principle of Operation

• Consider two waves with ß0 and ß1 propagating in
  opposite directions.
• If the Bragg phase-matching condition is
  satisfied
• when ? the period of the grating
• Consider ß1 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.

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• These reflections add in phase, when the path
  length in ?0 each period is equal to half the
  incident wavelength ?0

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• ?? detuning from ?0
• ? is inversely proportional to the length of the
  grating
• Apodized grating the refractive index change is
  made small toward the edges of the grating
• gt 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.

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3.3.4 Fiber Gratings (FGs)

• 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
• Made from photosensitive fiber (Ge-doped)
• UV intensity ? n?
• change of n 10-4
• Two kind of FGs
• a. short period (Bragg Grating ? 0.5µm)
• b. long period (? 100µm 1000µm)

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Fiber Bragg Gratings (FBG)

• extremely low loss 0.1dB
• high wavelength accuracy (0.05nm)
• high crosstalk suppression (Fig 3.8) (40dB)
• flat tops
• typical temperature coefficient 1.25x10-2nm/?
• For passive temperature-compensated
  0.07x10-2nm/?

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Long-Period Fiber Grating (a few intermeters)

• Useful for EDFA gain (equalization)
• They may be cascaded to obtain the desired
  profile.

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Principle of Operation

• The propagating mode in core couples onto the
  modes in the cladding gt induce loss
• For a given ?
• coupling occurs depending on ?
• ß propagation constant of the core mode
• propagation constant of the path order
  cladding mode
• The phase matching condition

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

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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.

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• A round trip through the cavity is an integral
  multiple of the wavelength.
• The light waves add in phase.
• Assume r1r2 t1t2
• The reflectance Rr1r2
• A absorption loss of mirror
• Tt1t2transmission

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A0, R0.75, 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 1/2t
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is the smallest value satisfied the condition
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• Tunability
• 1. change cavity length
• 2. change refractive index n
• Recall
• The wave with frequency will be
  selected.
• mechanical tuning
• piezoelectric tuning
• gt thermal instability, hysteresis

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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

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3.3.7 Mach-Zehnder Interferometers (MZI)

• Usage filter, MUX/DEMUX, modulator, switch
• Problems
• wavelength drift caused by aging or temperature
  variation
• not exact 5050
• not flat top passbands
• Change temperature (or refractive index) of one
  armgt tuning

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Principle of Operation
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• 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

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• consider K MZI interconnected
• The path length difference for the kth MZI is
  assumed to be

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• MZI can be used as a 1x2 demultiplexer or
  multiplexer
• ?1 ?2 chosen to be coincide with the peaks or
  troughs of the transfer function
• If , and mi is odd, say
  mi1 output 1 has signal, output 2 has no signal,
• If and mi is even, output
  1 has no signal.

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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.

?????Rowland circle????? multimode interference
(MMI) ?coupler
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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

?Llength 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
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• The relative phase

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Rowland circle construction
grating circle
Rowland
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3.3.9 Acoustic-Optic tunable Filter (AOTF)

• polarization-dependent, polarization-independent.

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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
• TE mode is converted to TM mode.
• For LiNbO3, nTE-nTM0.07?n. at 1.55µm
• ???n
  (3.18)
• At 170MHz ?22µm, acoustic wavelength

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• The transfer function is
• where ???-?0 ?0 satisfies
  (3.17)
• ??02/l?n
• l the length of acoustic-optic
  interaction
• FWHM bandwidth0.8?

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• Disadvantages high loss, large crosstalk, bulky
• wide passbandgt 100GHz

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3.3.10 High Channel Count Multiplexer
Architectures

• A. Serial (only for small number of ports)

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B. Single stage (AWG)
?????
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C. Multistage banding
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D. Multistage Interleaving
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3. 4 Optical Amplifiers

• Advantages transparent to bit rate, pulse
  format, large bandwidth, high gain
• Disadvantages noise accumulates
• Erbium-doped fiber amplifiers (EDFA)
• Raman amplifiers (RA)
• Semiconductor optical amplifiers (SOA)

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3.4.1 Stimulated Emission (EDFA or SOA)
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• Two energy levels E2gtE1
• 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 gt absorption gt amplification
• N1 Population (number of atoms) at E1
• N2 population at E2
• If N2 gt N1, population inversion occurs.

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3.4.2 Spontaneous Emission

• If ASE is very large
• gt Saturate the amplifier

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3.4.3 EDAF

• Erbium fiber Er3 doped silica fiber
• Pumping wavelength 980nm or 1480nm
• Advantages
• Availability of high power pump lasers
• All fiber device, polarization independent, ease
  to couple, reliable
• Simple
• Less crosstalk

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Principle of Operation
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• 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 15301565nm

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• When 980nm pump is used
• t32 1µsec ltlt t21
• 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
• gt High output power and pump can be located
  remotely

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Gain Flatness
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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

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3.4.4 Raman Amplifiers (RA)

• RA can provide gain about 100nm band (13THz)
  above the pumping wave ?plt?s (Signal Wavelength)
• RA is a distributed device and can provide gain
  in different bands
• No special fibers are needed
• Required high pump power1w
• Pump power fluctuations induce noise (propagating
  in same direction), propagating in opposite
  directions will have lower noise
• Crosstalk (modulated signals will deplete the
  pump power gt fluctuation gt noise) so, pumping
  opposite direction will lower the noise. (average
  out)
• Another noise is due to Rayleigh scattering of
  the pumping signal

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3.4.5 Semiconductor Optical Amplifiers (SOAs)

• Amplifier, Switches, wavelength converters

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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
• Reasonably high power 010dBm, low threshold
  current, high slop efficiency
• Narrow spectral width
• Wavelength stability (low aging effect)
• Small chirping (direct modulation)

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

• Multiple-longitudinal mode (MLM) lasers have
  large spectral widths10nm (Fabry-Perot lasers)
  gtcause chromatic dispersion
• Singlelongitudinal mode (SLM) lasers have very
  narrow spectral widths
• Side-mode suppression ratio is an Important
  parameter for SLM lasers. (30dB)

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

• The temperature coefficient 0.1nm/? at 1550nm.

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External Cavity Lasers

• Grating External Cavity Lasers

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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
• Injecting current (change n) tuning range 1015
  nm at 1550nm
• Temperature tuning 0.1nm/?
• Mechanical tuning (wide range but bulky)
• Desirable properties
• Short tuning time
• Wide tuning range (100nm)
• Stable over its lifetime
• Easily controllable and manufacturable

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Two-and Three-Section DBR Lasers

• Problems
• Aging
• Temperature changes
• Current recalibration
• Mode hopping

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Vertical grating-assisted coupler filter (VGF)
Lasers

• The coupling condition (3.17)
• ??B(n1-n2)
• ?B The period of the Bragg grating
• n1 and n2 are refractive indices of two
  waveguides.
• If n1 changes to n1?n1

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Sample Grating and Super-Structure Grating DBR
lasers
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Grating Coupled sampled Reflection lasers
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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.
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• External Modulation
• a. Lithium niobate modulator, b.
  electro-absorption modulator

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coupling coefficient depending on width of
the waveguide, refractive indices, distance of
two waveguides
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MZI can achieve high extinction ratio 15 20dB
with almost on chirping. Polarization control is
needed.
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3.6 Detectors
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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.

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

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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?8, avalanche breakdown occurs.

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3.6.2 Front-End Amplifiers

1. High-impedance amplifier
2. Transimpedance amplifier

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3.7 Switches

• Important parameters
• Number of ports
• Switching time
• The insertion loss
• The crosstalk
• Polarization-dependent loss
• Latching (maintaining its switch state)
• Monitoring capability
• Reliability

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3.7.1 Large Optical Switched

• The main considerations
• Number of switch elements required
• Loss uniformity
• Number of crossovers
• Blocking characteristics
• blocking and nonblocking (strict sense,
  wide sense, rearrargeable)
• 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
• 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

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3.8 Wavelength Converters

• A device converters data from one incoming
  wavelength to another outgoing wavelength.
• 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
• 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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• Other important characteristics
• i. convertion range
• ii. Transparent to data rate or modulation
  format
• iii. Loss (efficiency)
• iv. Noise, crosstalk
• 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 Grating

• Using the principle of cross-gain modulation in a
  SOA. (For high input signal power, the carrier
  will be depleted gt 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
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Principle of Operation (cross phase modulation
CPM)

• When ?s presents, the carrier densities (or n)
  change to induce different phase changes of ?p.
  At the port A, the intensity of ?p 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 data Stage2 reshapes and
retimes the data (inverse) Stage3 reamplifies
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3.8.4 Wave Mixing