Measurement of relaxation resonance, damping, and nonlinear gain coefficient from the sidebands in the field spectrum of a 1.3 μm InGaAsP distributed feedback laser

We calculate an expression for the fine structure in the field fluctuation spectrum of a semiconductor laser in terms of accessible device parameters. We measure the spectrum of an InGaAsP DFB laser and determine the linewidth, relaxation resonance, and damping versus current. From the relation between damping and current, we determine the nonlinear gain coefficient.