| Summary: | 博士 === 國立臺灣大學 === 電機工程學研究所 === 88 === Abstract
In this dissertation, we report the results of the fabrication and characterization of a miniature GaAs/AlGaAs all-semiconductor-optical-amplifier nonlinear optical loop mirror (NOLM). We observed efficient nonlinear switching effects with cw- and pulsed-signals. The miniaturized device, designed to imitate a conventional fiber-based NOLM, had a latency of 31.8 psec that is several tens thousand times smaller than that of a conventional NOLM (about 500 nsec). Curved single-lateral-mode ridge waveguide was used to form the loop structure. A multimode-interference-waveguide-amplifier (MMIWA) was used for closing the loop. In such a device, four separate electrodes were prepared for providing different gain constants in different areas. There were two electrodes on the loop to break the symmetry. The fabrication process of the device was identical to that of a semiconductor optical amplifier or laser except the use of the UV laser-assisted cryo-etching technique was used to provide good etching features in forming the curved ridge waveguide.
In device design, because it used the MMIWA for replacing a coupler in a conventional NOLM, the operation principle was quite different from what previously reported. In cw-signal operation, the nonlinear switching behaviors resulted from the combined effect of nonlinear coupling in the MMIWA and the amplification and lateral field redistribution of signal through the loop structure. Experimentally, efficient self- and cross-switching were observed. Numerical simulations showed consistent trends in varying device parameters. In cw-signal operation, a contrast ratio 3.19 between the maximum and minimum output power levels was achieved. The switching power, defined as the input power corresponding to the maximum output power, was 13.3 mW.
In pulsed-signal operation, the gain asymmetry in the loop played a crucial role for the nonlinear switching effects. Compared to cw-signal operation, the required switching power with pulsed signals could be two orders of magnitudes smaller. Meanwhile, the pulsed signals could receive several dB gain (5 dB typically) at the device output end. In pulsed-signal operation, a contrast ratio 2.35 and a switching power 0.6 mW (corresponding to a switching energy of 79 fJ) were obtained. Pump-probe experiments were conducted for studying the response speed limitation of the device. We found that although the device response speed was limited by the recovery time from gain saturation (the same order as the carrier lifetime), GHz operation was feasible with an appropriate control of pump and probe parameters.
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