200-Gb/s waveform analysis of ultrafast all-optical semiconductor gates towards low-power consumption operation

This thesis is aimed at a fundamental understanding of all-optical signal processing gates based on semiconductor optical amplifiers (SOAs), for their low-power consumption operation in ultrafast frequency region. In particular, modeling of a frequency-dependent electric power consumption in the SOA and suppression of several kinds of output waveform degradations in the ultrafast wavelength conversion are discussed in detail. After an introduction in Chapter 1, we present a power consumption model of an SOA gate in Chapter 2, to clarify the power consumption dependence on the operation frequency B and other fundamental parameters. The carrier density dynamics in the SOA was modeled using a rate equation including three different carrierconversion efficiencies of the SOA, which we introduced as the dominant factors. The validity of this rate equation model was tested with measured characteristics of custom-designed SOA samples with different structures. Using the model, we predicted that the power consumption increase with B in the high frequency limit. This B-dependence was experimentally demonstrated for two different SOA samples in the frequency range of 20 ∼ 100 GHz. We expect that this initial model will contribute to the designing of SOA power consumption or to the future outlook of SOA gates. In Chapter 3, we discuss the generation of small sub-pulses from a delayed interference signal-wavelength converter (DISC). This sub-pulse generation is regarded as one potential issue which prevents the practical operation of the DISC. We experimentally revealed the sub-pulse generation in a relatively low frequency region (10 ∼ 25 GHz) through cross-correlation measurements, and verified our sub-pulse generation model with high accuracy (1 ∼ 2 dB). We demonstrated the increase of sub-pulse intensity up to −10 dB with the carrier recovery rate, which will lead to the trade-off between the sub-pulse intensity and the pattern-induced intensity fluctuation. The sub-pulse model verified in this work will be valuable for future solution of this issue. In Chapter 4, we report on our wavelength conversion experiment for 200 Gb/s, 4992-bit data signal using a DISC gate. For this purpose we developed an original low jitter, low repetition rate (40 MHz), ultrafast (600 ∼ 700 fs) and synchronized sampling pulse generator for the cross-correlator input and succeeded in precise waveform-monitoring of the ultrafast long-pattern signal. The pattern-induced intensity fluctuation (PIF) of the output waveform was systematically measured, and the impact of the holding-beam injection and bandpass filter detuning on its suppression were compared. We systematically exhibited that the filter detuning efficiently suppressed PIF even for 200 Gb/s, long-pattern data signal and it saved the required holding-beam power by ∼ 3 dB. We also discovered a new method using the nonlinear polarization rotation in the SOA, and obtained a guideline for use of this method thorough systematic measurements. These results will provide indispensable backgrounds for the practical, ultrafast, low-power operation of the DISC. In conclusion, we developed the understanding of the SOA gate (DISC) operated in the frequency range of 10 ∼ 200 Gb/s, and considerably facilitated its low-power, small signal-degradation operation and the system design.