Ultrafast Characteristics of Indium Gallium Phosphide/indium Gallium Aluminum Phosphide Lasers and Amplifiers

Semiconductor lasers and amplifiers offer an attractive source of ultrafast optical pulses for both scientific and industrial applications. This dissertation examines the dynamics of visible wavelength, InGaP/InGaAlP semiconductor lasers pertinent to short pulse generation and amplification. Using ultrafast optical pulses as a probe, the gain, differential gain, saturation energy, and probe pulse spectral dynamics are measured as a function of bias current, probe wavelength, and probe pulsewidth. We find that both the gain and differential gain agree with a bulk material model. The differential gain is used to calculate the differential index, and the linewidth enhancement factor, using the Kramers-Kronig relations. The saturation energy is on the order of 10pJ, and we find significant spectral distortion of the pulse in the amplifier. The ramifications of these measured material parameters on short pulse generation and amplification is discussed. Using a pump and probe technique, the ultrafast (t <= 1ps) gain dynamics of the semiconductor laser amplifier are studied. This technique provides time resolution limited only by the optical probe pulsewidth, which for our case is 200fs. Two photon absorption and free carrier absorption, leading to ultrafast carrier heating, are found to be the dominant gain compression mechanisms. The highly excited carriers rapidly thermalize by carrier-carrier scattering, depleting the gain, and subsequently relax to thermal equilibrium with the lattice via phonon interaction. This relaxation is on the order of 500 femtoseconds, and is nearly independent of the operating parameters. These ultrafast gain transients may ultimately limit the performance and utility of semiconductor lasers in ultrafast optical applications. The results presented here are in qualitative agreement with those reported for AlGaAs and InGaAsP amplifiers, but we report measurably different quantitative changes.