Abnormal spectral evolution of fiber Bragg gratings in hydrogenated fibers

Abstract

We report the observation of abnormal spectral evolution in regenerated fiber Bragg gratings in hydrogenated B -Ge-codoped and standard telecom fiber with UV overexposure. The behavior of this new type of regenerated grating, which we have designated type IA, contrasts with that of the previously reported type IIA grating in nonhydrogenated fiber by exhibiting a large redshift in Bragg wavelength with increasing exposure of as much as 18 nm from a strong (16-dB) In all observations reported to date, the type IIA gratings were formed in nonhydrogenated fibers. However, the introduction of hydrogen into the fiber to increase photosensitivity is a commonly adopted procedure. In seeking to clarify FBG growth with UV overexposure in hydrogenated standard telecom and B-Ge-codoped f ibers we have observed, for the f irst time to our knowledge, regenerated grating growth with an abnormal, i.e., contrary to the behavior expected on the basis of previously reported observation, spectral evolution that is quite distinct from that of type IIA. We have designated this new grating type IA and found it to exhibit less temperature sensitivity than the type I grating.We inscribed the FBGs reported here by two-beam holographic exposure, using a cw 244-nm laser with 8-mm beam width and 60-mW power. For reference, we produced a type I FBG in nonhydrogenated B-Ge-codoped fiber: a maximum 2.2-dB ref lectivity was obtained in ϳ2 min of UV exposure, as shown in Fig. 1(a); the ref lectivity decreased with subsequent exposure and fell to zero after 7 min. A maximum redshift of ϳ2.0 nm in the Bragg wavelength was recorded. As the UV exposure was maintained, the grating ref lectivity increased from zero, accompanied by a blueshift in the Bragg wavelength that is characteristic of the type IIA grating and a negative induced refractive-index change. Limited by the UV wavelength and power, the type IIA grating attained 8.7-dB ref lectivity and 0.66-nm blueshift after 37-min exposure. Subsequent annealing of the grating for 12 h at 70 ± C induced a further blueshift of 0.55 nm owing to relaxation of the photoinduced positive index change; the ref lectivity remained unchanged, indicating good thermal stability, as shown in Fig. 1(b).This fabrication procedure was repeated with a hydrogenated sample of the same fiber. Type I grating formation and complete erasure were again observed; however, with continued UV exposure a quite different behavior emerged that included formation of an

Fiber Bragg gratings of type I in SMF-28 and B/Ge fibre and type IIA B/Ge fibre under gamma radiation up to 0.54 MGy

“…For the simplest case of Type I FBGs formed in H 2 loaded SMF-28 and non H 2 loaded B co-doped fibre, the Bragg wavelength is red shifted (towards longer wavelength) with increasing UV irradiation and saturates at the maximum reflectivity. Continuing UV irradiation beyond this point leads to two different FBG sub types the so-called regenerated FBGs of type IA [7,8] and type IIA [9] . Type IA FBGs show a strong continued red shift of up to 16nm whereas regenerated FBGs in non-hydrogenated B co-doped fibres show a completely different behaviour on over exposure.…”

Fibre gratings and devices for sensors and lasers

“…Eventually (time measured in terms of longer cumulative fluencies) even hydrogen loaded gratings can show stress relief -the swelling by hydrogen through hydrogen bonding (which is almost certainly crucial for any large index change to be obtained) prevents type 1n (type IIa) formation. In contrast, eventual stress relief with the surrounding glass not exposed to the high intensity regions of the periodic fringe leads to relaxation beyond the region of exposure so that there is a very large dc shift to longer wavelengths [55]. These gratings are labeled type IHp(or type 1A) and have very similar properties, despite the opposite wavelength shift, to type IIa gratings.…”

Comparison of the temperature dependence of different types of Bragg gratings

“…Type I, which is the most commonly observed, and two other regimes: type II (damage-written gratings) and type IIA, which is most often observed in nonhydrogenated high-Germanium (Ge)-doped optical fibers [1]. Recently, a new regime termed type IA has been reported [2], which is obtained in hydrogenated germanosilicate fibers and has received considerable attention since then, due to its low-temperature coefficient, which offers an attractive solution to the problem of strain-temperature discrimination in FBG-based sensors [3].…”