SUBPICOSECOND GAIN AND INDEX NONLINEARITIES IN INGAASP DIODE-LASERS

Studies of active waveguides with short optical pulses provide informa tion about fundamental carrier dynamics and nonlinear processes releva nt 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 mum region of the spectrum. W e have measured the group velocity dispersion, - lambdad2n(eff)/dlambd a2, for these diodes using time domain reflectometry. We have used a n ovel heterodyne pump-probe technique to study the gain and refractive index nonlinearities; and we have performed short pulse saturation mea surements that confirm predictions derived from these time-domain stud ies. Nonlinear gain compression is found to be due mainly to carrier h eating and two-photon absorption. A small portion of the total respons e has been attributed to spectral hole burning. The index nonlineariti es are dominated by carrier heating and by the instantaneous optical K err effect. We discuss how these time domain results, for both the rea l and imaginary parts of chi(3), can be related to other experimental determinations of diode nonlinearities. In addition, we include result s that go beyond the perturbational limit and show large rapid gain an d index nonlinearities for high pump powers. These results will be dis cussed in the context of all-optical switching in active semiconductor waveguides.