Semiconductor optical amplifiers (SOAs) have become promising components for optical communication systems. Owing to the quick development of their fabrication technologies, the performance of SOAs has been greatly improved and their cost are reduced. Commercialized SOAs are available now that can provide gain as high as 25 dB (fiber-to-fiber), saturation power of more than 10 dBm and a polarization sensitivity of less than 1 dB over a bandwidth of about 50 nm. Although fiber amplifiers are still preferable for inline amplification in current optical networks, SOAs are finding new applications as a result of their nonlinear properties. Compared with optical fiber, SOAs have the advantages of compact size and large nonlinear coefficients, and offer the possibility of monolithic integration with other optical devices. SOAs can be deployed as building blocks in optical switching, wavelength conversion, signal regeneration, clock recovery, dispersion compensation, etc. In section 7.1, we discuss SOA structures and the fabrication technologies which are adopted to manufacture high-performance SOAs. Section 7.2 concentrates on static SOA characteristics, gating type SOA space switches and combined wavelength/space switching using SOA-based wavelength conversion. Section 7.3 gives an overview of the recently developed SOA applications for ultrafast switching using nonlinear interferometers. Their working principles will be explained in detail. In section 7.4, we briefly discuss integration of SOA-based optical switches for space, wavelength and ultrafast switches.
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