Distributed Feedback lasers and Distributed Feedback Gratings can also be modeled by simply creating an arbitrary index profile within the cavity, like it is showed on the picture below on the left. The spatial oscillation represents the offset of the Bragg wavevector with respect to the semiconductor band-gap taken here as a reference. Here the oscillation is purely harmonic signaling the absence of chirp within the grating. One can see the influence of the index grating on the right figure as it acts as a distributed reflector for the field amplitudes.
By tuning the Bragg wavelength, i.e. the spatial frequency of the grating, one can achieve control of the laser emission, as depicted by the tuning curve obtained below.
Here, the final state of one time trace is used as an initial condition for the next one tuning from the red to the blue side of the spectrum. In each case, the dynamics is monomode with more than 15 dB of side more suppression. The oscillation at the beginning of each time trace signal the onset of a modal switching following the newly established Bragg resonance.