Early Index Growth in Germanosilicate Fiber Upon Exposure to Continuous-Wave Ultraviolet Light

In this paper, high-accuracy measurements of ultraviolet (UV)-induced refractive-index changes (plusmn3times10^-7) in germanosilicate optical fiber as a function of intensity and exposure time are presented. To examine the early growth characteristics of the fiber, samples are irradiated with 244-nm light for 100 s at relatively low intensities (0.007-2.7 W/cm²). The combined growth data is then interpolated to generate a 3-D "index growth surface" of photo-induced index. An empirically derived mathematical expression relates the index growth to the exposure time and intensity. Evidence is presented that, after exposing the fiber at one intensity, additional growth at a different intensity is dictated by the final index change of the first exposure and the intensity of the second exposure. This "compound growth rule" permits the complete calculation of induced-grating structures produced by such a complex exposure history. Using the index-growth surface and the compound-growth rule, the growth and UV erasure of a fiber Bragg grating is successfully predicted using a modified F-matrix algorithm