More on sources

Apr. 3, 2018

• Edge emitting laser: conventional configuration with radiation from the facet of a laser chip.

`->` Highly elliptic beam
• Surface emitting laser: vertical cavity surface emitting laser (VCSEL).
Advantages - easy to integrate and manufacture, have parallel architecture and circular beam.
Disadvantages - require multilayer mirror (distributed Bragg reflector, DBR) to increase power reflectivity, require special structure to laterally confine the radiation area and require high current density.
• Single (longitudinal) mode laser: further narrow the emission bandwidth by DBR and distributed feedback (DFB).
Both DBR and DFB provide the narrow bandwidth of a grating.
• Tunable lasers: two classes
Single freq lasers - source can be tuned to emit one freq at a time, e.g. laser with tunable grating or tunable DBR.
Example of the performance of a tunable VCSEL.

Multifreq lasers - e.g. integrated cavity lasers with the integrated cavity functioning as a filter as well as a multiplexer.
e.g. arrayed lasers with an array of tuned freq lasers. By turning on the current of one more laser, we get one more freq.
• Multifreq cavity lasers (MFL): `N times N` waveguide grating router (WGR) (see Sec. 3.3.8) integrated with a source with several multiplexed wavelengths at each input ports.
WGR is a generalized MZ interferometer.
The optical path difference `Delta L` between waveguides in adjacent ports causes a wavelength dep. phase shift between ports. Only light of particular wavelength will couple to a certain output port.
Channel spacing `CS = {Delta f_{FSR})/N` where `N` is number of channels (ports), `Delta f _{FSR} = c/{n_g Delta L}` where `n_g` is the group refractive index of WGR.
Advantages - accurate CS, low loss, simultaneous operation of all wavelengths, fast tuning, scalable, CS indep. temp.
Disadvantages - not fiber based, large size, electrical cross talk, performance degrade as `N` increases.
• DFB arrays: it has indep. filter determining freq of each DFB laser and is integrated with amplifier and elecroabsorption modulator.
Advantage - small size, DFB independently controlled with high speed, all DFB lasers have the same temp.
Disadvantage - CS is not maintained, freq drift from channel to channel, lossy, require a fine period grating, electrical cross talk.
• Modulation: Direct modulation by modulating the biasing current.
Disadvantage - slow < 40Gb/s.
External modulation modifies the phase, freq., amplitude, polarization of optical carrier outside the laser cavity.
Advantage - fast, avoid freq. chirp in output spectrum
Disadvantage - larger size, integration problem.
MZ modulator - use interference to convert phase shift into intensity modulation (on or off). The phase shift is controlled by modulating signal, e.g. microwave message.
Electroabsorption modulator - light absorption controlled by a microwave voltage. Result in intensity modulation.
Electrorefraction modulator - refractive index controlled by a microwave voltage. Result in phase modulation.
• Supercontinuum sources: A broadband source that can be filtered to generate multiple channel freq. E.g. a CW source `->` SPM `->` spectrum broadening to cover C band `->` demultiplexing with a grating `->` individual freq is modulated `->` multiplexed and coupled into a single fiber.

Last Modified: March 27, 2018
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