CHARACTERIZATION OF THE DYNAMICS OF SEMICONDUCTOR-LASERS USING OPTICAL MODULATION

An optical modulation technique for measuring the intrinsic frequency response of semiconductor lasers is described. This technique, which uses an RF-modulated pump laser to create an optical modulation signal to inject into a dc-biased probed laser, offers significant advantages over previous methods. Most importantly, it is unaffected by electrical parasitics of either the laser to be characterized or the photodetector. In addition, it allows extremely accurate measurements of many important dynamic parameters. Notable examples are the nonlinear gain coefficients, the amount of spontaneous emission into the guided modes, and the differential carrier lifetime at lasing threshold. The technique also revealed for bulk lasers that K, the ratio of the damping rate to the resonance frequency squared, is not a universal constant, as previously thought, but depends on the doping level. Furthermore, this dependence was shown to result from differences in the differential gain, not the nonlinear gain coefficient, with doping level. Contrary to the prediction of current theories of nonlinear gain, this result implies that the nonlinear gain coefficient does not scale linearly with differential gain.