2.3        FBG properties

2.3.1        Refractive index profile

A FBG is completely characterized by its refractive index distribution along the fiber n(z) [2-16]:

where z is the position, n₀ the refractive index prior to grating inscription, Dn_ac the refractive index modulation amplitude, L a design grating period, q the period chirp (slowly varying with z), and Dn_dc 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.