Femtosecond nonlinearities in AlGaAs diode laser amplifers

As optical communication systems continue to advance in speed and complexity, the demand grows for high-speed devices to perform wavelength conversion, multiplexing, and switching operations. Semiconductor diode laser amplifiers exhibit large and ultrafast optical nonlinearities that make them promising candidates as the basic building blocks for such all-optical devices. The primary goal of this thesis is the measurement and characterization of the ultrafast optical nonlinearities exhibited by an AlGaAs diode laser amplifier. The results lead to a better understanding of fundamental processes in semiconductors and enable the evaluation of active semiconductor waveguides for use in all-optical devices. This thesis presents a detailed study of ultrafast gain and refractive index dynamics in a bulk AlGaAs diode laser amplifier. Pump-probe measurements are performed with -100 femtosecond optical pulses generated by a modelocked titanium:sapphire laser system. The refractive index measurements employ a time-division interferometer that allows the measurement of phase shifts as small as r/1000 radians. Measurements of gain and refractive index dynamics are performed at a series of carrier densities and. over a broad range of aboveand below-band wavelengths from 810 to 905 nanometers. The phenomena encountered in this work include two-photon absorption, carrier heating through free-carrier absorption, spectral hole burning, carrier cooling through stimulated transitions, and an instantaneous refractive index dynamic attributed to the optical Stark effect. The carrier heating dynamic recovers with a 1.1 picosecond time constant, and below-band refractive index measurements show that there is a delay of -120 femtoseconds in the onset of carrier heating. Values for the nonlinear refractive index coefficients associated with the carrier heating index nonlinearity and the instantaneous index nonlinearity are determined as a function of wavelength and carrier density. For the instantaneous index dynamic, a maximum n21 of -5 x 1 0 12 cm 2 /W is measured. Thesis Advisor: Erich P. Ippen Title: Elihu Thomson Professor of Electrical Engineering