Chapter 9 - Highly Nonlinear Fibers

Abstract

This chapter deals with highly nonlinear fibers that are useful for a variety of applications. The parameter γ that governs most of the nonlinear phenomena in optical fibers is enhanced considerably in such fibers by confining the optical mode to a narrow central region. In practice, this is realized by introducing micrometer-size air holes within the cladding of a narrow-core fiber. Because of such structural modifications, these fibers are collectively referred to as microstructured fibers. This chapter focuses on several important applications of microstructured fibers. Their nonlinear and dispersive properties are discussed. Microstructured fibers can be used for tuning the wavelength of femtosecond pulses over a wide range by exploiting the nonlinear effects such as stimulated Raman scattering and four-wave mixing. It also describes several designs of microstructured fibers. In each case, the narrow silica core is surrounded by one or more rings of air holes that effectively lower the refractive index of the cladding region and thus help to confine the incident light to the core of the fiber. The size, shape, and the number of air holes vary for each design. It is not easy to analyze the modes of a microstructured fiber because the refractive index of the cladding is far from being homogeneous and changes suddenly at the air–silica interface surrounding each air hole.