A Study Of Ultrafast All-optical Polarization Switch Based On Spin Relaxation In InGaAs/InP MQWs

Although performance improvement is being made continuously in optoelectronic and electronic switching devices, their operation at a bit rate higher than 100 Gb s - 1 seems to be still difficult due to various limiting response times including the carrier recombination lifetime, carrier transit time in the device, and also the external RC time constant. Whereas all-optical switches have attracted more and more interests because of their favorable optical characteristics and much faster switching speed.Therefore in this thesis, all-optical switch based on multiple quantum wells (MQWs) was investigated. A further study of carrier transport, ultrafast spin dynamics, absorption saturation, bleaching and spin related characteristics of excitions, optical nonlinearities, dichroism and other optical characteristics in InGaAs/InP MQWs were made. A novel all-optical polarization switch based on 120 period InGaAs/InP MQWs was presented, its working mechanism and relevant theories were consummated, and a femtosecond pump-probe experimental platform was set up. This optical switch has ultrasfast switching speed of several picoseconds, operating wavelength compatible of 1.55μm window, maximum rotation angle of 60°under 0.1nJ pump drive, low switching energy and large rotation angle. It is applicable in Gb s^-1 bit rate optical transmission systems.Then a preliminary design was given, focusing on the parameters of MQW materials and structures of Fabry-Perot (FP) etalon, with two modes presented: one transmission mode and one Asymmetrical Fabry-Perot (AFP) reflection mode.A double three-energy-level model of carrier transition was set up upon condition that the pump beam was circularly polarized and the probe beam was linearly polarized. The in-well carrier population as function of delay time was calculated through the rate equations. Changes in absorption were analyzed and calculated based on the three mechanisms of exction absorptive saturation. Connections between transient rotation angle of probe beam and in-well carrier density, dynamic shift of the absorption resonance peak and in-well carrier density were obtained for the first time.Some recommends about improvements for the all-optical switch were given, consists of novel all-optical switches such as low-temperature-grown optical switch (LOTOS) and inter-subband transition (ISBT) switch, in order to solve the problem that the repetition rate was limited by the carrier lifetime in our polarization switch.At last conclusions about the performance characteristics of all-optical polarization switch was made in two aspects: transient rotation angle and dynamic shift. It is expected that our theory and conclusions are helpful as a reference in the designs and experiments of future practical optical switches.