A CW signal experiences a time-varying gain when it co-propagates with pulsed pump in a semiconductor optical amplifier (SOA) due to cross gain modulation, and this can be used to experimentally measure gain recovery time. In this study, we establish a linear relation between the measured gain recovery time and the carrier lifetime of SOAs through detailed numerical simulations of a cross-gain modulated system. The small signal gain and the saturation power are determined experimentally. We also study the dependence of gain recovery time on the pump pulse widths which are smaller than the carrier lifetime and on the CW power. These studies help in an accurate estimation of carrier lifetime of a packaged SOA device.
A scheme for detecting the phase of an incoming optical signal is systematically optimized and demonstrated experimentally. It makes use of four-wave mixing in a highly nonlinear fiber but differs from a phase-sensitive amplifier by its focus on phase detection, rather than on phase-dependent noise reduction. This experimental setup combines the signal with its phase-conjugated idler at the same frequency, before sending the combination to a photodetector. Experimental results show that the ratio of maximum and minimum currents can exceed 1000 even at a relatively low pump power of 10 mW. The proposed scheme may be useful for detecting the signal phase in coherent communication systems without the need of heterodyne detection.
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