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Chapter 3 Components

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Chapter 3 Components Couplers, Isolators and Circulators, Multiplexers and Filters, Optical Amplifiers, Transmitters, Detectors switches, Wavelength converters. – PowerPoint PPT presentation

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Title: Chapter 3 Components


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
1
a
1-a
acoupling ratio 3dB couple a 1/2 a 0.95 (for
monitoring)
3
  • 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

4
3.1.1 Principle of Operation
  • E electrical field
  • S-parameters
  • For lossless couplers

5
  • The power transfer function

6
3.1.2 Conservation of Energy (S-parameter)
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

8
  • For a 2 x 2 symmetrical coupler

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 1dB
  • A circulator is similar to an isolator except it
    has multiple ports.

10
3.2.1 Principle of Operation of an Isolator
11
A spatial walk-off polarized splits the signal
into two orthogonally polarized components.
12
3.3 Multiplexer and Filters
Multiplexers and filters are for WDM, add/drop.
WXC,
13
Dynamic WXCs use optical switches and mux/demux.
14
  • 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

22
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
23
  • 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.

30
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

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.

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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
40
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
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

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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
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 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
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