Thermal regeneration of fiber Bragg gratings in photosensitive fibers

Abstract

We report about a thermal regeneration of fiber Bragg gratings written in photosensitive fibers with nanosecond laser pulses. We observe a regenerative process in a highly photosensitive fiber without hydrogen loading which indicates a secondary grating growth in an optical fiber by thermal activation. This process is more temperature stable than the commonly known gratings produced by color center modifications. The writing conditions of such new type of gratings are investigated and the temperature behavior of these regenerated fiber Bragg gratings is analyzed. The application possibilities are in the field of high temperature sensor systems by making use of the combination of good spectral shape of a Type I grating with a Type II like temperature stability.

Regenerated Fibre Bragg Gratings

“…Since our work, others [Linder et al 2009] have shown that regeneration can be obtained at lower temperatures in gratings written without hydrogen -the low thermal stability of these gratings is consistent with type 1n gratings. If crystallisation is occurring it is likely to be via the mechanism we described above to α−quartz, which is stable at ~100Mpa close to the observed 600°C.…”

“…The use of hydrogen was important, but not necessarily essential, to obtain index modulation of useful magnitude for very high temperature operation, since it permitted enhanced localisation of the pressure differences between processed and unprocessed regions. The model proposed in Canning et al 2008b is independent of this and recent work demonstrates that regeneration can be achieved without hydrogen, although for much lower temperature operation [Linder et al 2009]. In this chapter, we briefly review the hypothesis and demonstrate thermal processing of UV-induced templates that retain the nano-scale precision of the template whilst stabilising the glass change to unprecedented levels.…”

“…If the possibility of continuous polymorph transformation above the regeneration threshold is accepted, then it is most unlikely the annealing of laser induced changes is going to lead to complete recovery of the glass system -that is, we expect some memory of the UV written "seed" grating to be retained (contrary to a single Debye relaxation below threshold), in particular at the core/cladding boundary. In order to maximise the possibility of achieving high temperature regeneration, hydrogen loading is used for three reasons: (a) optimise the induced stress differences between processed and unprocessed regions in the Bragg structure since UV-induced OH causes periodic relaxation of the stresses; and (b) minimise the refractive index contribution of the UV written grating to the polarisability changes associated with OH formation; and (c) reduce the chance of eventually relaxing to crystallised α quartz which has the potential of changing to other structures each time the grating is heated to higher temperatures (this can explain why the results of Linder et al 2009, and indeed most type In (type IIa), are unstable above 600°C). Therefore, the experiment is to write a standard grating in photosensitive fibre loaded with hydrogen and post-anneal this gradually to 900°C and beyond.…”

Optical Nanofibers

“…A spatially-dependent refractive index can be imprinted in the fiber by modulating the intensity pattern of the writing laser. Lindner et al (2009) use a Talbot interferometer to create a periodic modulation of the refractive index in a Bragg mirror configuration. Two Bragg reflectors produce an 3 counts per second (cps), and the measurement background noise including the dark count is about 10 kcps.…”

Low loss Type II regenerative Bragg gratings made with ultrafast radiation

“…For high temperature applications, two competing FBG technologies exist: 1) regenerated gratings resulting from high temperature annealing of strong type I laser written gratings in hydrogen loaded fiber [3,4] and 2) type II gratings written with femtosecond pulse duration radiation, either using the point-by-point method [5] or using the phase mask approach [6]. Regenerated gratings typically possess low reflectivity and are cumbersome to produce, requiring high temperature processing in an oxygen free environment and, usually high pressure hydrogen loading of the optical fiber at some point in the process [7].…”