CW UV fringeless pre- or post-exposures of Bragg gratings written in an H_2 loaded telecommunication fiber

摘要

In 1993, Lemaire et al. reported enhanced photosensitivity in H_2 loaded fibers. It allows the inscription of strong Bragg gratings into weakly germanium doped telecommunication fibers. On the other hand, the UV post-processing method which consists of exposing a part of a fiber grating to uniform UV light, is commonly implemented to fabricate complex grating based components such as apodized or (and) chirped gratings A long as Δn_(mod) follows a saturated exponential function of exposure time (i.e.Δn_(mod) ≈Δn_o(1 ― e~(-t/τ)), it can be concluded that UV post-processing partly bleaches the refractive index modulation while increasing the mean refractive index at the fiber core. The first objective of the communication is to show that using cw UV light to realize a post-exposure of a weak grating written in an H_2 loaded telecommunication fiber can give rise to an amplification (and not an erasure) of the grating strength. This phenomenon can be explained by the shape of the curve connecting the growth of Δn_(mod) to the exposure time. Similarly, we have also observed that pre-exposing a H_2 loaded fiber to cw UV fringeless light, before grating inscription, increases the grating growth kinetics when compared to those in pristine fibers. The second aim of the communication is to show that a phenomenological model qualitatively accounts for these observations.