Overview of high temperature fibre Bragg gratings and potential improvement using highly doped aluminosilicate glass optical fibres

Abstract

In this paper, various types of high temperature fibre Bragg gratings (FBGs) are reviewed, including recent results and advancements in the field. The main motivation of this review is to highlight the potential of fabricating thermally stable refractive index contrasts using femtosecond (fs) nearinfrared radiation in fibres fabricated with non-conventional techniques, such as the molten core method. As a demonstration of this, an yttrium aluminosilicate (YAS) core and pure silica cladding glass optical fibre is fabricated and investigated after being irradiated by an fs laser within the Type II regime. The familiar formation of nanogratings inside both core and cladding regions are identified and studied using birefringence measurements and scanning electron microscopy. The thermal stability of the Type II modifications is then investigated through isochronal annealing experiments (up to T=1100°C; time steps, Δt=30 min). For the YAS core composition, the measured birefringence does not decrease when tested up to 1000°C, while for the SiO 2 cladding under the same conditions, its value decreased by ∼30%. These results suggest that inscription of such 'Type II fs-IR' modifications in YAS fibres could be employed to make FBGs with high thermal stability. This opens the door toward the fabrication of a new range of 'FBG host fibres' suitable for ultra-high temperature operation. ORCID iDsMaxime Cavillon https:/ /orcid.org/0000-0003-1341-6294 Peter Dragic https:/ /orcid.org/0000-0002-4413-9130 Figure 5. Normalized retardance (averaged over the 0.1 to 1.0 μJ pulse energy range) as a function of temperature for YAG-derived fibre at the core center (YAS) and in the cladding (SiO 2 ), for both // and ⊥ configurations. The measurements were performed at room temperature.

Near-IR- and UV-femtosecond laser waveguide inscription in silica glasses

“…by using 800 nm femtosecond laser pulses [28]. The explanation of this efficiency could be linked to the different energies of the absorbed photons: the multiphoton ionization, that is the main process for the plasma generation, requires ∼3 photons at 343 nm and ∼8 at 1030 nm, making the non-linear absorption much efficient in the UV domain, as already observed in [31].…”

Femtosecond laser direct writing in SiO2‐Al2O3 binary glasses and thermal stability of Type II permanent modifications

“…These results suggest that inscription of such "Type II fs-IR" modifications in SiO 2 -Al 2 O 3 could be employed to make fiber Bragg gratings with high thermal stability. This opens the door toward the fabrication of a new range of "fiber Bragg gratings host fibres" (Al 2 O 3 but also ZrO 2 or YAG-derivated 22 ) suitable for ultra-high temperature operation.…”

“…Recent results indicate the need of >30mol% Al 2 O 3 20 to improve FBG thermal stability above standard fibers like SMF28 from Corning. To go beyond the thermal stability limit 16 imposed by standard SiO 2 -based optical fibers, FBGs can be inscribed in SiO 2 -Al 2 O 3 optical fibers using femtosecond laser direct writing 20,21,22 . The doping that determines the optical fiber core soften the glass far too much and so are not directly involved in the high temperature processing that allows gratings to operate at higher temperatures.…”

Fiber Bragg Grating Wavelength Drift in Long-Term High Temperature Annealing

“…Hence, a more specific aim of this study is to look at the most probable mechanisms of the Bragg wavelength drift in silica fibers and suggest ways to mitigate their effect. This becomes especially important because the initial proof-of-concept successes with 1000 °C-resistant sapphire FBGs [ 33 , 34 , 35 ] and sapphire-derived fibers [ 36 ] were not followed by real-life applications. The use of silica-based FOSs comprising high temperature resistant FBGs thus remains the only practical option for distributed high temperature sensing.…”