Optical nonlinearity in optical fibers and semiconductor optical amplifiers

Optical nonlinearity is being intensively studied for advanced optical functions, such as all-optical switching and wavelength conversion, in future lightwave communications. For transparent operation independent of signal format and data rate, especially usable is partially degenerate four-wave mixing (FWM) in which a new wavelength light (FWM light) is generated from a pump and a signal light. Simultaneous wavelength conversion and phase conjugated wave generation are possible by FWM. In order to obtain high nonlinear efficiency, a waveguide structure is preferable because intense light power is confined in a small area along the length. For this reason, optical fibers and semiconductor optical amplifiers (SOAs) have been mainly utilized as nonlinear devices for lightwave communications. This paper overviews optical nonlinearity (especially FWM) in optical fibers and SOAs. Both fibers and SOAs have advantages and disadvantages. (1) The pump wavelength is automatically determined in a fiber in order to satisfy phase-matching. On the other hand, it can be arbitrarily chosen in SOAs because the device length is short and thus phase-matching does not matter. (2) Fibers have better noise performance because ASE is generated in SOAs. (3) Fibers have flat efficiency over a wavelength range, while SOAs strongly depend on the wavelength separation. This feature is important for simultaneous wavelength conversion. (4) SOAs are preferable from the viewpoint of compactness.