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.
These frequency shifts, together with dispersion in the fiber, cause the signal to degrade after some distance, limiting the bandwidth and the range.
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.