Fiber-Based Nonlinear Processing of Optical Signals

Optical processing in fiber has enabled recent advances such as large, tuneable signal delays, the shifting of signals to mid-IR wavelengths, and the creation of specific quantum states useful in sensing, imaging, and communication applications. Introduction The invention of the optical maser and subsequent explosion in laser research, discovery of new laser lines, and commercial development of lasers provided unprecedented access to high-power, narrowspectrum light. This enabled observation and use of weak optical nonlinearities in materials. Among the early uses of optical nonlinearity1 was frequency doubling or sum-frequency generation in crystals such as potassium dihydrogen phosphate (KDP). Such crystals facilitated the attainment of nonlinearity and phase matching, two conditions necessary to achieve usable optical nonlinearity. The nonlinearity generally arises from intense optical fields driving electrons into significantly anharmonic regions of their potential. Nonlinear response of the nuclei also contributes to the nonlinearity. For some optical nonlinearity, phase matching is required to ensure that light generated throughout the interaction region adds coherently. In nonlinear crystals, phase matching is often obtained through exploitation of the crystal birefringence. The amount of light generated by nonlinear interaction can be increased by focusing the available power to a narrow beam waist. However this shortens the confocal parameter and hence shortens the interaction length.