This longitudinal diffraction grating has periodic changes in refractive index that cause reflection back into the cavity.
The strongest grating operates in the first order, where the periodicity is one-half wave, and the light is reflected backwards.
Semiconductor DFB lasers in the lowest loss window of optical fibers at about 1.55 μm wavelength, amplified by erbium-doped fiber amplifiers (EDFAs), dominate the long-distance communication market, while DFB lasers in the lowest dispersion window at 1.3 μm are used at shorter distances.
If the temperature of a semiconductor Fabry–Perot laser changes, the wavelengths that are amplified by the lasing medium vary rapidly.
If one or both of these end mirrors are replaced with a diffraction grating, the structure is then known as a DBR laser (distributed Bragg reflector).
Furthermore, creating an exact quarter-wave shift can be technologically difficult to achieve, and often requires directly written electron-beam lithography.
An alternate way of breaking this degeneracy is by coating the back end of the laser to a high reflectivity (HR).
At other times, however, the phase shift between the grating and the high-reflector back mirror is not optimal, and one ends up with a two-moded lasers again.
To encode data on a DFB laser for fiber-optic communications, generally the electric drive current is varied to modulate the intensity of the light.
In very high-performance coherent optical communication systems, the DFB laser is run continuously and is followed by a phase modulator.
These DFB fibre lasers are often used in sensing applications where extreme narrow line width is required.