3-1 Optical Signal Processing using Fiber Nonlinearity

When intense light is propagated through optical fibers, the refractive index of the fiber is changed almost instantaneously. This effect, know as Kerr nonlinearity has been used extensively in many applications in the area of optical communications. The refractive index in the fiber changes according to the relation, n = no + n2 .I(t, x, y), where n2 is the nonlinearindex coefficient and I is the optical intensity (Power/Area). This results in a change in the phase of light that is given by φNL =γP Leff, where Leff is the effective length of the fiber that includes the effect of absorption loss, P is the optical power, andγ is the nonlinear coefficient as defined by,γ= 2πn2/(λAeff). In order to acquire large nonlinear phase change from a shorter length, it is thus necessary to utilize fibers that have large nonlinear parameter. This is generally achieved by utilizing fibers with large nonlinear coefficient n2 and using fibers specially designed to have a small core size. Table 1 shows summarizes various optical fibers made of silica as well as non-silica glass designed to have nonlinear coefficients more than two order of magnitude larger than the standard silica fiber. Fibers tailored with such a large nonlinearity and with suitable dispersion can lead to a many applications in the areas of ultrafast communications. In this paper, we show some application of nonlinearity of optical fiber in the generation of wavelength-tunable picosecond/femtosecond pulses from fiber lasers, the conversion of wavelength over a broad range, and the retiming of such pulses at high repetition rates using the nonlinear effects in optical fiber. In 3 Physical Layer Technologies / Optical Signal Processing