Nonlinearities and all-optical switching in semiconductor laser amplifiers

The nonlinear properties of semiconductor laser amplifiers have been investigated, particularly at the transparency bias point, in the InGaAsP/InP and GaAs/AlGaAs material systems. An all-solid-state F-centre laser system has been developed. It consisted of a diode-pumped Nd:YLF laser mode-locked at 77 MHz repetition rate, producing pulse durations of 6-20 ps with an average power of 2.2 W. The Nd:YLF laser synchronously pumps the F-centre laser producing pulse widths as short as 4ps with average powers of 200 mW. The available wavelength tuning range was from 1.45 #mu#m to 1.55 #mu#m. The Nd:YLF pump laser, although not designed to do so, Kerr lensed mode-locked. The reason for this unexpected behaviour was investigated and was shown to be due to the unusual laser cavity design. Thermal lensing is shown have a significant impact on the cavity stability and is believed to degrade the laser performance. The nonlinear coefficient, n_2, has been measured by self-phase-modulation, for pulses of picosecond duration in a 5QW InGaAsP laser amplifier at a wavelength of 1.5 #mu#m. Subsequently a pump-probe study, showed the likely origin of this nonlinearity was carrier heating. The pump-probe studies have also shown that this nonlinearity saturates for mW power levels at the pulse durations used. The gain saturation caused by carrier heating is modelled and the calculated saturation powers relate well to the pump-probe data. The mechanism that causes the effective saturation of the nonlinearity is therefore attributed to carrier heating induced gain saturation. For the first time an integrated active asymmetric Mach-Zehnder interferometer laser amplifier has been fabricated in the AlGaAs/GaAs material system. This structure has been characterised as a laser and tested as an all-optical switch at the transparency bias point. The Mach-Zehnder was switched with pulses of 100 fs duration corresponding to a pulse energy of 3pJ. The device shows inferior performance to passive waveguide and fibre all-optical switches, in the area of switching contrast. Similar active amplifier switches in the nonlinear directional coupler configuration also show better characteristics. The poor performance of the Mach-Zehnder used here is believed to be due to arm asymmetries caused by gain saturation or fabrication variations. A detailed model of the switching characteristics is presented. However it was not possible to use the model to deduce key material parameters because the model provided a good fit to the experimental data across a wide matrix of parameters, up to 100% variation in n_2, #alpha# and #beta#. (author)SIGLEAvailable from British Library Document Supply Centre-DSC:DXN038370 / BLDSC - British Library Document Supply CentreGBUnited Kingdo