Subpicosecond gain and index nonlinearities in InGaAsP diode lasers

Abstract

Studies of active waveguides with short optical pulses provide information about fundamental carrier dynamics and nonlinear processes relevant to high speed modulation, amplification, and switching. We describe recent investigations of the gain and index nonlinearities in InGaAsP optical amplifiers operating in the 1.5 μm region of the spectrum. We have measured the group velocity dispersion, -λd 2 n eff/dλ 2, for these diodes using time domain reflectometry. We have used a novel heterodyne pump- probe technique to study the gain and refractive index nonlinearities; and we have performed short pulse saturation measurements that confirm predictions derived from these time-domain studies. Nonlinear gain compression is found to be due mainly to carrier heating and two-photon absorption. A small portion of the total response has been attributed to spectral hole burning. The index nonlinearities are dominated by carrier heating and by the instantaneous optical Kerr effect. We discuss how these time domain results, for both the real and imaginary parts of χ (3), can be related to other experimental determinations of diode nonlinearities. In addition, we include results that go beyond the perturbational limit and show large rapid gain and index nonlinearities for high pump powers. These results will be discussed in the context of all-optical switching in active semiconductor waveguides.