FP Laser vs DFB Laser, DFB Laser vs DBR Laser
Difference Between FP Laser and DFB Laser
A Fabry–Pérot laser diode (FP laser diode) is the most common type of laser diode, having a laser resonator which is a Fabry–Pérot interferometer. This means that substantial light reflections occur at both ends, but not within the gain medium.
A distributed-feedback laser (DFB laser) is a laser where the whole resonator consists of a periodic structure in the laser gain medium, which acts as a distributed Bragg reflector in the wavelength range of laser action. Typically, the periodic structure is made with a phase shift in its middle. Essentially, one has a direct concatenation of two Bragg gratings with optical gain within the gratings.
FP Fabry-perot, that is to say, there is a Fabry–Pérot cavity in laser diode. F-P cavity is a kind of multi-longitudinal mode Fabry-perot cavity.
The main difference between DFB LD and FP LD is that DFB LD does not have a cavity mirror for convergency reflex, and its reflection is provided by Bragg Grating on the waveguide in active area.
DFB laser is based on FP laser using a grating filter to make the device have only one longitudinal mode output. The features with large optical power output, small divergence angle, narrow spectrum and high modulation rate are suitable for long-distance communication. DFB laser mainly works at 1550nm wavelength and its date rate is above 2.5G.
DFB Laser Performance Parameters
Working Wavelength: Center wavelength of laser output spectrum
Side-Mode Suppression Ratio: The relation of power between center peak longitudinal mode with the nearest higher order mode.
-20dB Spectrum Width: Reduce the spectral width at 20dB from the highest point of the laser output spectrum
Threshold Current: When the operating current of device exceeds the threshold current, the laser will emit a laser with a good coherence. Threshold currents lie typically in the range of 25 – 250 mA and the output powers are typically in the range of 1 – 10 mW.
Output Optical Power: Optical power emitted by the output port of laser
FP laser is a semiconductor light-emitting device that uses the FP cavity as a resonant cavity to emit multi-longitudinal mode coherent light. The features with large output optical power, small divergence angle, narrow spectrum and high modulation rate are suitable for long-distance communication.
FP Laser Performance Parameters
Working Wavelength: Center wavelength of laser output spectrum.
Spectrum Width: Root mean square spectral width of multi longitudinal mode lasers.
Threshold Current: When the operating current of device exceeds the threshold current, the laser will emit a laser with a good coherence.
Output Optical Power: Optical power emitted by the output port of laser.
Difference Between DFB and DBR
A distributed Bragg reflector laser is a laser, where the laser resonator is made with at least one distributed Bragg reflector (DBR) outside the gain medium (the active region). A DBR is a Bragg mirror, i.e., a light-reflecting device (a mirror) based on Bragg reflection at a periodic structure. In most cases, the Bragg mirror is more specifically a quarter-wave mirror, providing the maximum amount of reflection for the given number of layers.
The differences between DBR lasers and DFB lasers. The distinguishing difference between the two architectures is the location of the grating within the epitaxial structure. The DBR uses a high index contrast, high reflectivity surface grating. The DFB uses a low index contrast, low reflectivity buried grating.
The built-in grating of DFB laser is used to stabilize the wavelength, and the entire resonant cavity is composed of periodic structures.
The built-in grating of DBR laser is used to stabilize the wavelength, it’s a multi-segment laser with gain, phase and DFB cross-sections.
DFB laser and DBR laser have similar characteristics at narrow line width and high side mode suppression.
DFB lasers have a larger tuning range without mode hopping, more consistent tuning coefficients, and better uniformity of spectral characteristics.
DBR laser can realize larger current input because the grating is not distributed in the entire active area, and the output power will be higher than that of DFB laser. But it will cause larger power, current noise and power jitter.