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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