Useful components

Apr. 19, 2018

Wavelength converters (sec 3.8): enable relocation and reuse of optical channels, adds to flexibility and efficiency.
Cross-gain modulation Cross-gain modulation - gain saturation in OA can lead to cross talk cross modulation, i.e. transfer modulation fromλ1 toλ2 which is originally without modulation.
FWM - XPM causes cross talk among channels, e.g.ω2=ω1+ω3ω4 whereω1 carries signal,ω3 andω4 are pumps without modulation.
Extractω2 with a band pass filter.
Similar idea for optical freq shifter, except we use a dispersion-shifted fiber as the nonlinear medium.
Optical phase locked loop (OPLL) (sec 4.4.8): To lock on the phase of 2 lasers, i.e. syn. two clocks or local oscillators. Important for coherent detection.
E.g. Freq stabilization -- Use a hollow cathode lamp as calibrator for DFB lasers. When DFB laser lases at 1.296μm , a voltage is induced at the cathode lamp. This voltage is then feedbacked to lock freq of a master DFB laser. To lock freq of other lasers, a FP interferometer (FPI) is used at each node. The FPI biased by a feedback voltage so that it lets 1.296μm from the master DFB pass through to a photodetector (PD) that generates a biasing voltage for a slave DFB.
Optical directional couplers as external modulator (Fig. 3.58): when a sectionLo of fibers becomes very close with a separation ofd , light can transfer from 1 fiber to another through evanescent property of lightwave.
When applied voltage V=0, power is completely transferred to adjacent channels.
WhenV=Vs , power stays in the same waveguide.

Directional coupler has 4 ports with relationsP1out/P1in=cos2(κLo) andP2out/P1in=sin2(κLo) whereκ is the coupling coef. and length parameterLc=π/(2κ)
Couple coef. decreases as d increases.
By setting coupling lengthLo to be a fraction ofLc , we can make attenuator, splitter.
Power loss is about 3dB orΓ with the coupler calledΓ1/2 coupler.
Ring resonators: 1Γ power coupled into the ring.
Ring has circumference of L and attenuation coefα
Light returns to coupler with modified amplitude (exp(αL) ) and phase (exp(jβL) ) factors which dep. onL andλ
Freq. difference between max. and min.Δf=c/(2neffL) whereneff is the weighted average index of waveguide and the substrate.
Ring resonator is a notch filter.
Optical equalizers (Sec 5.5.2): maintain equal gain for all channels. Output at each wavelength is measured and gain is adjusted, based on the measurement -- preferably, a dynamic wavelength equalizer (DWE).

Complete article on DWE fabricated by Silicon Light Machine

• Optical isolators (Sec 3.2): It is based on a set of cross polarizers with a magneto-optical material between them. It allows transmission of light in one direction.
Parameters - insertion lossL=PinPT(dB) and isolationI=PinPR(dB)

Last Modified: April 11, 2018
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