Abstract

We report pump–probe measurements of sub-picosecond gain dynamics in InGaAsP bulk and quantum well laser amplifiers. Pulses with a width of 120 fs and tunable from 1485 to 1540 nm are generated by using a color center laser with additive pulse mode locking. The probe signal, measured as a function of pump–probe delay for different wavelengths and different bias currents, shows that the gain undergoes very complicated dynamics with several time constants involved. By comparison with theoretical calculations based on the density matrix approach and including pulse propagation effects we conclude that the dynamics reflect the combined effect of two-photon absorption (TPA), spectral hole-burning (SHB), and carrier heating (CH). TPA leads to a gain decrease for all wavelengths and follows the pump pulse instantaneously, whereas SHB (associated with a ~50 fs intraband relaxation time) changes sign at the transparency point. Carrier heating, with a temperature relaxation time of 700 fs, leads to a gain decrease and is seen in almost the entire wavelength region. Carrier cooling is observed deep into the absorption region. These results have implications for fast modulation of laser diodes and amplification of ultrashort pulses and could find application in laser diode mode locking.

© 1992 Optical Society of America