DFB: Distributed Feedback Laser.
In the optical fiber communication network, the most commonly used laser is the distributed feedback (DFB) laser, which is a single longitudinal mode or single frequency semiconductor laser. The single frequency laser refers to a laser with only one longitudinal mode (spectral line) spectrum characteristic of the semiconductor laser. It can work in the third generation optical fiber system with the minimum loss window (1.55 um).
In ordinary LD, only the active region provides the necessary optical feedback at its interface.
But in DFB lasers, the feedback of light is distributed not only on the interface, but also on the whole length of the cavity, as implied by the name of DFB. This is achieved by forming a diffraction grating with periodically varying refractive index in the cavity.
In the DFB laser, besides the active region, a guided wave region is added to it and adjacent to it. The structure of this area is a corrugated dielectric grating, whose function is to partly reflect the light radiated into the area from the active region.
The radiation from the active region into the guided wave region is in the whole length of the cavity, so it can be considered that the corrugated medium also has gain, so part of the reflected wave has gain.
The following figure shows the structure and typical output spectrum of DFB laser.
