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A FBG is completely characterized by
its refractive index distribution along the fiber n(z) [2-16]:
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(2-11) |
where z is the position, n0 the
refractive index prior to grating inscription, Dnac the
refractive index modulation amplitude, L a design grating period, q the period
chirp (slowly varying with z), and Dndc the average change in
refractive index (Fig. 2-5). The refractive index modulation amplitude
remains sinusoidal until the exposed region reaches the maximal refractive
index change. Then, if the UV exposure continues, the modulation change from
sinusoidal to rectangular.
Fig. 2-5 Fiber Bragg grating refractive index
For gratings approaching the
photosensitivity saturation, the modulation shape became rectangular.
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