Extreme Silica Optical Fibre Gratings

Abstract

A regenerated optical fibre Bragg grating that survives temperature cycling up to 1,295°C is demonstrated. A model based on seeded crystallisation or amorphisation is proposed.

Regenerated Fibre Bragg Gratings

“…To maximise the chance of the process working, the original experiments focused on strong fibre Bragg gratings written with hydrogen. The results reported in [Canning et al 2008b] verify that a new grating structure which was stable up to 1295°C before the fibre broke could be obtained. Despite the deterioration and breaking of the fibre in an open environment at such temperatures, the remaining regenerated grating remained intact when broken pieces were analysed, consistent with a stronger glass than the original glass.…”

“…In practice, we have confirmed that the stronger the seed grating, the stronger the final regenerated grating. For this case, a conventional Bragg grating inscribed into hydrogen loaded (24hrs, 200atm, 70°C) relatively highly germanium doped step index fibre with no boron (r core ~ 2μm, [GeO 2 ~ 10.5mol%], Δn co/cl = 0.012), using the 244nm output from a frequency doubled Ar + laser (P ~ 50W/cm 2 , f cumulative ~ (6-12)kJ/cm 2 , the same as that reported in Bandyopadhyay et al 2008 andCanning et al 2008b). Figure 6 shows the transmission and reflection of a very strong type I Bragg grating, readily exceeding the noise floor in transmission of our tunable laser and power meter setup (res: 1pm).…”

“…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.…”

“…We soon recognized a more general implication of this result -rather than rely on a diffusive interpretation and subsequent polarisability change as the basis for writing gratings, an alternative approach to engineering the index change based on glass structural transformation arising from relaxation of high internal pressures and high temperature processing was proposed. Whilst much work remains to verify details of this approach, it implicitly did not rely on the fibre glass dopants at all (other than maximizing the seed grating strength) and directly led to the development of regenerated gratings in standard photosensitive fibres with transmission rejection >10%/cm and which can tolerate temperatures as high as 1295°C [Bandyopadhyay et al 2008;Canning et al 2008b]. 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 options available, within certain limitations, are numerous. They include type 1n (type IIa) and type Ip (type Ia) gratings that can operate between 500-800 °C depending on preparation [Canning 2008a and refs therein]; type II damage gratings ] produced by irradiation above the damage threshold of the glass using UV lasers or multiphoton excitation with longer wavelengths and more recent regenerated gratings that are based on glass structural change achieved using type I seed gratings as a template [Bandyopadhyay et al 2008;Canning et al 2008b]. The problem with these various methods is that the gratings produced often compromise the ideal properties offered by type I gratings.…”