Photorefractive beam coupling-a new approach to the measurement of the nonlinear refractive index of short (<25 m) lengths of silica and erbium-doped fibers

Summary form only given. Optical nonlinearities limit the maximum power that can be transmitted through an optical fiber transmission system. The intensity dependent refractive index n/sub 2/ leads to a variety of nonlinear optical phenomena in fibers and is thus a critical parameter in the design of long-haul systems. A number of approaches for measuring n/sub 2/ in both bulk materials and fibers have been reported. One of the simplest and widely used of these approaches to measure the nonlinear refraction is the single beam Z-scan measurement. This technique is widely used in semiconductors films, but due to the confined mode of an optical fiber, the necessary spatial variation of the intensity dependent refractive index is precluded and thus the Z-scan can not be applied to single mode fibers. In a technique developed by Stolen et al. (1978), a transform limited optical pulse is coupled into a fiber and the output spectrum is measured as a function of the input power. As the input power is increased maxima and minima of the central peak occurs in the frequency spectrum at integer orders of n/2. From the known phase shift a value for n/sub 2/ can be deduced. In these experiments, the typical input pulse widths are 100-ps and the fiber lengths range from 100-250 meters of fiber in order to achieve several orders of /spl pi//2 phase shift. Utilizing photorefractive beam coupling, we can easily detect a phase modulated optical pulse in the time domain, using 53-ps pulses and <24-m length of fiber.