Photolytic Index Changes in Optical Fibers

The ability to change the core refractive index of fibers by optical pro­ cessing allows the fabrication of phase structures in the fibers-the simplest of which is a phase grating obtained by a periodic modulation of index along the core. Such in-line fiber phase gratings fall into two classes: short-pitch Bragg gratings, with periods of order 100 nm with applications including reflectors (1, 2), transmission filters (3), and strain sensors (4, 5), and long-pitch gratings, with periods of order 1 mm with applications including mode converters (6, 7) and polarization rocking filters (8, 9). These devices and applications are reviewed by K. Hill et al in this volume. This review is concerned with understanding the mechanism by which photolysis changes the index, with attention to the role of defects and thermoelastic stress. A photolytic refractive index change is the permanent change in the index induced by irradiation with visible/UV light. This terminology is used to distinguish the effect from photorefractivity, in which the index response to the optical fields is dynamic and reversible, rather than cumulative and irreversible, on the time scale of interest. The first observations of photolytic index changes in germanosilica fibers were reported by Hill and co-workers (I, 1 1 ) in 1978; they described permanent holographic gratings written into the cores of fibers by intense 488 nm laser light launched into the fiber. These gratings had a weak index modulation bn of order 105-106, which gave narrow-band reflection filters at the writing wavelength. Interest in the process was revived by Morey and co-workers (12) in 1 989 through their demonstration of side-