A Nonsymmetrical Mach-Zehnder Interferometer for Suppressing Pattern Effect in SOAs

摘要

The semiconductor optical amplifier (SOA) is very attractive for its wide gain spectrum, capability of integration with other devices, and potential low cost. A number of experiments have been performed to show its capability to be cascaded and used as in-line amplifier in WDM transmission systems. However, its performance is limited owing to the pattern effect caused by the gain saturation, which results in distortion of the pulse shape and cross-talk between symbols and channels. Quite a few methods, including electronic feedback, compensation by saturable absorber, reduction of the carrier life time by the injection of CW light, and clamping of the gain by laser oscillation inside the device, have been put forward to suppress the pattern effect. However, these methods have not given a satisfactory solution to this problem, mainly because of their speed limitation. When a SOA reaches a saturated state, a decrease in carrier density will result in simultaneous variation of the gain and the index of refraction. So, a better way to compensate for the gain variation, which is the main source of the pattern effect, is to use the phase variation of the output signal of SOA. This can be achieved with a nonsymmetrical Mach-Zehnder interferometer (MZI) structure. Since this compensation method involves two phenomena that always accompany each other, it will not suffer any speed limitation and is bit-rate and waveform transparent as long as only interband processes are considered.