In a Distributed Feedback Laser, a grating is etched into the active region to force the operation of the laser at very specific wavelength determined by the gratingperiodicity. As a result, the laser emits on a single spectral setting which may be adjusted slightly by changing the temperature of the active region with a tuning rate of 1/n dn/dT = 6x10-5K-1. 


SEM image of a DFB grating. The periodicity of the grating selects the wavelength emitted.