12.1 INTRODUCTION

Specialized fibers are increasingly being used to manipulate light and to couple light of different wavelengths into and out of fibers in telecommunications and sensing applications. The development of new communication modalities, such as cellular, satellite, and cable communications, has only spurred the growth of optical fiber networks. Wavelength selectivity is achieved by imposing a periodic modulation of the refractive index along the fiber. This is accomplished most often by exposing photosensitive fiber to modulated ultraviolet (UV) light [1]. In gratings with periods greatly exceeding the wavelength, the refractive index may also be modulated by microbending [2], such as may be produced by squeezing the fiber between corrugated plates or by local heating with a CO ₂ laser [3] or with an electric arc [4].

Chiral fibers employ an alternative means of implementing periodicity into a glass fiber, which allows for polarization and wavelength selectivity. This extends the functionality of optical fibers and is advantageous in a variety of filter, polarizer, sensor, and laser applications. An example of a double-helix chiral fiber is shown in Fig. 12.1. Glass fibers with cores that are either concentric and birefringent, or nonconcentric, are twisted at a high rate as they are passed through a miniature oven in a drawing tower such as the one shown in Fig. 12.2. The fiber preform, typically from 100 to 200