We explain the generation of four wave mixing (FWM) components at the front and back facets of semiconductor optical amplifiers (SOAs) based on the Bragg-scattering from the propagating gratings in the SOAs. We propose a counter-propagating cross-polarized degenerate pumping scheme for the polarization-insensitive conjugate generation, simultaneously in both input and output ports of the SOA for the first time. The corresponding Bragg scattering processes along with the phase matching conditions are described and the detuning performance of the generated conjugate in either port are experimentally validated. Polarization-insensitive phase conjugate generation at both input and output ports of the SOA through Bragg scattering FWM is further demonstrated.

Owing to advances in fabrication technology and device design, semiconductor optical amplifiers (SOAs) are evolving as a promising candidate for future optical coherent communication links. This review article focuses on the fundamentals and broad applications of SOAs, specifically for optical channels with advanced modulation formats, as an integrable broadband amplifier in commercial transponders and as a nonlinear medium for optical signal processing. We discuss the basic functioning of an SOA and distortions of coherent signals when SOAs are used as amplifiers. We first focus on the techniques used for low-distortion amplification of phase-modulated signals using SOAs. Then we discuss optical signal processing techniques enabled by SOAs with an emphasis on all-optical wavelength conversion, optical phase conjugation, and phase quantization of coherent optical signals.

We experimentally demonstrate the simultaneous compensation of both dispersion and nonlinear effects in a 100 km optical fiber link using optical phase conjugation of a 21 GB aud QPSK and 16QAM signal with nonlinear SOAs. Error-free performance is recorded for a launched power of up to 12 dBm, without any digital signal processing to compensate for distortions due to chromatic dispersion and nonlinear effects in fiber. The performance is verified for operation across the C-band.

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