Amplification of fibre Bragg grating reflectivity by post-writing exposure with a 193 nm ArF laser

<title>Postfabrication wavelength trimming of fiber Bragg gratings written in H2-loaded fibers</title>

“…After 15,960 pulses, the Bragg wavelength increased by about 1 nm and the reflectivity grew from 6 dB to 19 dB, which correspond to an increase of the mean index by 0.93× 10 -3 and an increase of the index modulation by as much as 215% (from 0.65× 10 -4 to 1.4× 10 -4 ), respectively. This growth behavior is similar to the amplifications of the FBG in unloaded fiber by post-writing fringeless exposure with 193 nm laser [4] and the longperiod grating in hydrogen-loaded fiber exposed to uniform UV beam at 248 nm [5] . This is a photosensitization process and can be explained by the two-step sensitization mechanism proposed by Canning [6] .…”

Fibre gratings and devices for sensors and lasers

“…Contributions from defect polarisability changes and structural change or densification. Latent sensitisation behaviour, crucial for grating growth, observed [21]. Stresses usually increase with irradiation -depends on fibre history.…”

“…This relatively simple picture remains somewhat misleading because the reaction pathways involved are not unidirectional -a great portion are reversible and the net index change observed is the outcome of an overall quasi-equilibrium process between the various contributions. Further, there is an intrinsic memory effect in processed glass which allows latent imaging to further develop the grating [21] -this is the basis of hypersensitisation, a process that is particularly important for type IHs gratings (see below). Historically, the interpretation of measured index change was, often exclusively, based largely on the changes in the spectral band shapes of defects in the measurable UV-VIS spectrum [45].…”

“…The details involved with saturation and rollover have been modeled extensively particularly on the basis of fringe contrast, from which a phenomenological description of type I and type In (type IIA) gratings can be obtained [46]. However, a complete model must take into account latent imaging [21]. It turns out this latent imaging, long forgotten by most current researchers, could be an important piece of early evidence indicating that periodic irradiation of glass is locked in even after a Bragg grating is annealed out completely - Figure 3 (online color at: www.lpr-journal.org) A high power air-clad fibre laser with a 75% R Bragg grating -top shows schematic of the experiment whilst the centre output spectrum demonstrates the advantage of narrow line width Bragg gratings.…”

Peculiarities of the photosensitivity of low-loss phosphosilica fibres

“…Recently the effect of an increase in the photosensitivity of ébre and planar waveguides was found, which was observed upon a two-stage exposure [3]. The essence of the effect is that before the main exposure (for example, upon writing refractive-index gratings in the ébre), the ébre is soaked with molecular hydrogen and is irradiated by a homogeneous beam (the exposure dose being relatively low).…”