What is the difference between a Fabry-Perot (FP), a gain-guided and a Distributed Feedback (DFB) laser diode?
I think Fabry-Perot is the default configuration, gain-guided has something to do with a strip electrode (but I'm not clear on how it works) and DFB uses a Bragg-grating to limit the output of the laser to a single wavelength. However, I'm still a little blurry on the details. Help clearing up the differences would be appreciated.
Fabry-Perot laser diodes are lasers whose mirrors are simply the flat cleaved surfaces at the ends of the laser chip.
Distributed feedback (DFB) laser diodes are lasers that have a grating structure in the cavity that produces multiple reflections throughout the cavity. This leads to narrower linewidths than are produced by FP lasers.
(Image source: Laser Focus World)
Another type you didn't ask about is the Distributed Bragg reflector (DBR) laser. A DBR typically has two separate grating regions, on either side of the gain region. They're usually distinguished from the DFB because the grating doesn't overlap the gain region, although there is some grey area and the terms are not always used consistently.
(Image source: US Patent #6638773)
Where FP, DFB, and DBR types describe how the longitudinal reflections that provide laser feedback are produced, the designation gain-guided describes how the mode is confined in the transverse dimensions. In a gain-guided laser there is no waveguide patterned between the reflective structures (end facets or DBR regions). Instead, we rely on a narrow region where current is injected to maintain the transverse mode.
This works mainly because any transvese modes that don't overlap the injection region will see much lower gain and so won't lase.
The Fabry-Perot laser is the default configuration for a laser (the usual population inversion, stimulated emission and positive feedback), except of mirrors at either end for creating positive feedback are replaces with cleaved surfaces.
The gain-guided laser diode, as described here, uses a strip to direct current to the center of the active region to reduce the chance of edge emissions. This is usually combined with index-guiding, which is bordering the active region of a laser diode with materials with a high refractive index, this also reduces edge emissions.
The DFB as you said, has a Bragg-grating in the active region to limit the emission to a single wavelength, since a laser actually emits multiple wavelengths called longitudinal modes.
A Fabry-Perot laser is a resonator that lases at wavelength selected by the gain bandwidth of the lasing medium. These are called longitudinal modes. It will usually lase with multiple longitudinal modes that fall within the gain bandwidth. The cavity modes are determined by the optical dimensions of the resonator. Depending on the dimensions of the cavity, the laser may also oscillate on multiple spatial modes.
To select a single spatial mode, a laser diode is designed with a single mode waveguide, to confine the spatial mode. To select a single longitudinal mode, the laser diode is fabricated with a Bragg grating. A distributed feedback laser, or DFB, is designed with the grating distributed along the entire length of the resonator. A distributed Bragg reflector, or DBR laser is designed with a grating outside of the gain region. DBRs generally have lower linewidth and higher power than DFBs and are used for metrology and spectroscopy. DFBs generally have a wider tuning range and are used in telecom applications.
A more detailed discussion about the differences between DFBs and DBRs can be found here (http://photodigm.com/difference-between-dbr-and-dfb-lasers/).
DFB lasers require two epitaxy steps and e-beam grating step, which makes process much more expensive than FP laser:
1st "Planar" epitaxy growth where usually: n-type cladding side of device is grown + MQW region (multiple quantum wells), spacer, GRATING LAYER, and cap layer.
e-beam processing ( hardmask deposition (SiNx), photoresist spin, lithography with e-beam, development, etching, resist strip, cleaning).
2nd epitaxy ("Regrowth/Infill"), where grating is "infilled" (with InP in InGaAsP/AlInGaAs lasers) and p-type cladding is grown, and structure is terminated with contact layers.
Then processing of final wafer.
Cleaving wafer to bars
Coating the bars
Dicing bars to dies
Packaging dies to TO-can or Butterfly package
In InP lasers Gain-Guided lasers are those which GRATING LAYER is composed of compound which partially absorb laser wavelength. Index-Guided are lasers, where Grating layer have 0 absorption(imaginary refractive index part), and rely on just real refractive index for getting proper wavelength filter which form laser feedback and lasing mechanism.
