Abstract
In this paper, high-accuracy measurements of ultraviolet (UV)-induced refractive-index changes
(±3×10<sup>-7</sup>) 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/cm<sup>2</sup>). 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.
© 2007 IEEE
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