Figure 8b is the spectral output of a DFB laser. Immediately it is obvious that a DFB laser’s output is spectrally
narrower than its FP counterpart. Periodic changes in the laser’s refractive index makes it highly reflective at certain
wavelengths. Reflections provide feedback to the laser creating high losses at all but one wavelength so that the
laser oscillates at one frequency only, making the DFB essentially a monochromatic source.