K. Cook1 , J. Canning1* , S. Bandyopadhyay2 , M. Lancry3 , C. Martelli4 , T. Jin5 and A. Csipkes6
1 interdisciplinary Photonics Laboratories (iPL), School of Electrical and Data Engineering, University of Technology Sydney, Broadway, NSW 2007 & School of Chemistry, The University of Sydney, NSW 2006, Australia 2Fibre Optics and Photonics Division, Central Glass and Ceramic Research Institute, Council of Scientific and Industrial Research (CSIR), Kolkata 700032, India 3Institut de Chimie Moléculaire et des Matériaux d’Orsay,UMR CNRS-UPS 8182, Université Paris Sud, Orsay France 4Graduate School of Electrical Engineering and Applied Computer Science, Federal University of Technology Paraná, Curitiba PR 80230-901, Brazil 5No 23 Anningzhuang E Rd, Haidian, L1, Bldg 2, YinYan Mansion, Beijing, China 6Technica Optical Components, LLC, 3657 Peachtree Rd NE, Suite 10A, Atlanta, GA, 30319, USA
Abstract—Various types of high temperature fibre Bragg gratings (FBGs) for sensing applications, are briefly reviewed, discussing their various figures of merit and performance. References are provided to currently available commercial grade high temperature FBG sensors.
Keywords—Fibre Bragg Gratings; High Temperature Sensing.
Specialised FBGs are proving themselves in a range of industrial applications such high temperature lathe profiling , monitoring of fuel combustion machinery , regulation of diesel locomotive engine temperature  and to assist the structural health assessment of a building post-fire . The oil, gas and geothermal industries are also set to benefit from such sensors as man’s quest for fossil fuels and the lure of “free” geothermal power forces bore holes to be drilled ever deeper – a new world record only recently set with a drill depth of almost 5 km . At these depths, temperatures in excess of 500°C and extreme pressures are encountered. In this paper, an overview of ultrahigh temperature FBG sensors will be presented, updating past reviews [6,7]. In general, the higher the desired temperature of operation of any grating, the higher the local annealing temperature required, a general consequence arising from the relaxation continuum of an amorphous system. In these systems, the initial FBG creates a template where the glass thermal history has been periodically altered.
In the following paper, four distinct groups of FBGs will be reviewed and their various performance merits discussed. The grating types reviewed are: Type 1 and 1n, regenerated, femtosecond and sapphire gratings.
II. TYPE 1 & 1N FBGs
The simplest high temperature gratings are stabilised Type 1 FBGs. Type I FBGs are stabilised to meet telecommunication performances (-20 ≤ T ≤ +80 °C for t > 25yrs) but can be made to operate at much higher temperatures for shorter but still useful durations. Thermal annealing a Type 1 grating at 700 °C will reduce the FBG strength but will then operate for finite periods up to 600 °C . Other approaches to stabilise Type I FBGs involve photosensitisation  to remove the unstable component generated during grating writing; in addition to similar thermal stability, this offers fine-tailoring of hydrogen and other species. Going to higher T requires regeneration.
Unlike thermal stabilization of Type 1 and Type 1n, the much higher temperatures lead to complete relaxation of fibre stresses, in core and cladding. Although now having superior stability, they will also be brittle since compressive stress between cladding and core is removed. It will happen for all fibre devices operated at similar temperatures – regeneration completes this annealing during grating fabrication rather than during application. Regenerated gratings have been used in a range of applications: high T glass lathe temperature profiling , dual P-T sensing for gas turbines , high T air flow meters for internal combustion engines  and train engine T regulation . They have been used to make the first accurate measurements of fibre viscosity . Complex RFBG structures such as phase-shifted gratings  and chirped gratings  have also be realized, demonstrating the high level of spatial resolution with which the regenerated structure can follow the seed. It is also possible, through the application of controlled tension loads, to even tune the final Bragg wavelength . Regenerated gratings can also be readily multiplexed, with grating arrays used to monitor the distributed temperature profile of an optical fibre preform lathe  and a high temperature tubular furnace . Figure 2 shows a commercial grade High-Temperature FBG array useable in the field for applications up to 1,000 °C.
Figure 3 shows typical properties of regenerated gratings inscribed in SMF-28 fibre: (a) and (b) show the clear reflection and transmission spectral profiles respectively. (c) Shows wavelength stability over several hours of operation at 1000°C.
of fibres that may overcome this involve hybrid mixes of silica with aluminate cores. Nevertheless, the higher melting point of these glasses means sapphire FBGs boast the highest temperature performance to date, up to 1900 °C.
Table 1 compares dv/dT between grating types.
A range of optical fibre Bragg gratings capable of high temperature operation have been reviewed. Gratings inscribed in conventional SMF-28 fibre can have their temperature operational limit extended depending on the fabrication technique, with regeneration yielding tolerances beyond 1295 °C. Sapphire gratings, on the other hand, extend this temperature limit much further, up to 1900 °C, addressing the limitations of conventional temperature measurement approaches such as thermocouples and pyrometers at these high temperatures. In contrast to silica waveguides they presently have major design limitations and some research is working on merging the two technologies .
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Type 1n FBGs, (type IIA, negative index ), are gratings regenerated in various H2-free silicate fibres [11-13]. Thermal regeneration from heating is a consequence of extended irradiation, often using UV, quasi CW or CW; similar results are feasible by using annealing alone . The mechanism behind type 1n formation involves partial fibre relaxation of internal core/cladding stresses, in particular the radial and axial stresses induced by the UV inscription. Early Type In FBGs performed to T ~ 500 °C before decaying [11,12]; using higher intensity exposures, generating higher local T, extends this to (700 – 800) °C in step-index  and photonic crystal fibres . At lower T, regeneration involves the core glass. One of the advantages of type In gratings is that they can be fabricated in a one-step process on existing FBG fabrication rigs, without the requirement of H2-loading or post thermal processing.
III. ULTRA-HIGH T REGENERATED FBGs
In order to extend the functionality of gratings to operate above 800 °C, silica regeneration is key . Regenerated FBGs (RFBGs) of all types are fabricated by annealing a seed structure, typically but not limited to a Type 1 grating. To access the silica, and to make the process more effective, H2 is used to further increase strain in processed regions versus those unprocessed. The hydrogen can be used during seed formation or added later during regeneration – the latter enables even draw tower gratings to be regenerated . A seed Type 1 grating is regenerated (typ. T ~ 800 – 900 °C) in H2-loaded germanosilicate fibre, often using a UV laser  but other sources are possible including fs lasers . Since the changes involve silica, these gratings can survive > 1295 °C , 1450 °C for 20-30 mins . A recent study demonstrated the continuous operation of an RFBG at 890 °C for an impressive 9000 hours . Fig 1 is a reference for commercial HT-FBGs.
IV. FEMTOSECOND FBGs
Femtosecond FBGs (fs-FBGs) are gratings inscribed using ultrafast lasers either by phase mask  or by point-by-point . Multiphoton excitation of the silica band edge in the visible or near IR leads to finer gratings than those reported with two-photon 193 nm light [31,32]. The index change mechanism need not depend on core dopants or H2. Femtosecond gratings fall into two categories: Type I and Type II. Type I gratings are formed by laser pulses with energy below the glass damage threshold; the index change is caused by rapid and highly localized heating and cooling of the glass, leading to localized densification and a positive index change. Type II, on the other hand, occur above the glass damage threshold where the glass is ionized leading to structural changes and a changed refractive index. Type II fs-FBGs demonstrate remarkable thermal stability up to ~ 1000 °C . Furthermore, because of the very high intensity fields possible, highly localized plasma ionization and deoxygenation can lead to strong interference effects with the optical field, generating complex condensed structures such as nano gratings as well as depositing silicon (and germanium) rich regions, offering a novel route to semiconductor fabrication in glass . If not scanned over a larger volume of glass, these gratings can suffer from large spectrally wide scattering losses. In terms of applications, fs-FBGs have recently been trialed as temperature sensors for monitoring fluidized bed combustors  and also use as radiation resistant temperatures sensors . The index change is highly localized and involves significant stressinduced changes around the irradiated regions. For this reason, the higher temperature regime is limited by the thermal response of both the surrounding regions and the fibre itself which has not been relaxed prior to application. Regeneration has been shown improves fs-FBG performance .
V. SAPPHIRE FBGs
To go above the limits imposed by a silica system, FBGs can be inscribed in aluminium oxide or sapphire optical fibre using various approaches but particularly by femtosecond laser fabrication . However, unlike conventional optical fibres that have a core and a cladding, sapphire optical fibre consists simply as a single sapphire fibre with the surrounding air as a cladding. This means there is a huge step index difference (~ 0745) and the fibre is inherently highly multimode. The large number of modes makes analysis of the reflection spectra complex and also impacts on sensitivity. The next generation
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