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 - gain saturation in OA can lead to cross talk [equation] cross modulation, i.e. transfer modulation from [equation] to [equation] which is originally without modulation.
FWM - XPM causes cross talk among channels, e.g. [equation] where [equation] carries signal, [equation] and [equation] are pumps without modulation.
Extract [equation] 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 1.296 [equation], 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 1.296 [equation] 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 section [equation] of fibers becomes very close with a separation of [equation], 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.
When [equation], power stays in the same waveguide.

Directional coupler has 4 ports with relations [equation] and [equation] where [equation] is the coupling coef. and length parameter [equation].
Couple coef. decreases as d increases.
By setting coupling length [equation] to be a fraction of [equation], we can make attenuator, splitter.
Power loss is about 3dB or [equation] with the coupler called [equation] coupler.

+ Ring resonators: [equation] power coupled into the ring.
Ring has circumference of L and attenuation coef [equation].
Light returns to coupler with modified amplitude ([equation]) and phase ([equation]) factors which dep. on [equation] and [equation]
Freq. difference between max. and min. [equation] where [equation] 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 loss [equation] and isolation [equation].

Last Modified: April 11, 2018
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