Fiber Bragg grating sensor for high temperature application

Abstract

Regenerated gratings seeded by type I gratings withstand temperatures beyond 1000 0 C. A new approach to increasing temperature resistance of ultra high T stable gratings is presented. IntroductionDevelopment of fibre Bragg gratings (FBGs) which are suitable for high temperature applications is an increasingly important researcher driver in sensing. Previous studies have already established that operable temperature of FBGs can be increased by several means: e.g., through tailoring the glass composition [1],[2], or the process of pre-irradiation [3], the formation of type-II [4] and type-IIA [5,6] grating or by inscribing grating using femtosecond IR lasers [7]. There is another different variant with superior high temperature stability referred to as chemical composition grating (CCG) [8]. It was shown that a periodic index modulation can be regenerated after erasure of the UV induced type-I grating written in H-loaded germanosilicate fibre that contains fluorine if it is annealed at temperature as high as ~1000 0 C. The prediction was a local reduction of fluorine in the UV-exposed zones at that high temperature through diffusion of hydrogen fluoride. A subsequent study on annealing of type-I gratings at high temperature, however, has shown that the presence of fluorine is not necessary for this regeneration of index modulation [9]. So-called chemical composition gratings (CCG) are found in Er-doped fiber with other dopants as Ge, Al and Sn. Very recently this phenomenon of regeneration has been found in simple H-loaded germanosilicate fibre only [10].We present results while annealing FBGs written in hydrogen loaded Ge-doped fiber at high temperature that have noticeable differences with those reported in [10]. They show that more in depth investigations are required to properly understand and optimise the regeneration processconstituent parameters such as doping concentration, H-loading condition, the optimal strength of the initial "seed" type-I grating, and finally, the annealing conditions to optimize the strength and spectrum of the regenerated grating. This new structure has shown to be operable to the glass softening temperature of the fibre core and is easy to fabricate compared to other methods.

Extreme Silica Optical Fibre Gratings

“…The fibre is placed under tension with a standard load ∼100 g. The transmission and reflection spectra of the ordinary type I gratings (those where the refractive index is largely dictated by polarisability changes from exciation into glass defects [3]) used in this work is shown in Figure 1. As noted in [1, 2] no regenerated gratings were observed without hydrogen loading which suggests that hydrogen plays a critical role, most likely through the formation of OH.…”

“…Recently, we have reported for the first time regenerated fibre Bragg gratings in standard photosensitive boron codoped germanosilicate optical fibre that approach 10% transmission [1, 2]. Regenerated Bragg gratings are gratings that have grown through thermal processing at high temperatures (∼900°C) after the seed grating written by UV light is erased.…”

Thermal monitoring of the diamond deposition process using regenerated FBG

“…The RFBGs used in this work were produced based on regeneration methods developed by Canning et al [11]. Saturated gratings with 3 mm-length were inscribed onto hydrogen loaded standard ITU G.652 fiber, using a KrF excimer pulsed laser emitting at 248 nm.…”

Production of optical notch filters with fine parameter control using regenerated fiber Bragg gratings

“…The considerable shift in the Bragg wavelength can be assigned to the changes in the refractive index profile [17]. However, it shall be mentioned that the obtained gratings with very high saturation level showed always a Bragg wavelength blue shift of the order of 2.8 nm, regardless of grating length, slightly different from the value presented in [18]. One explanation for this fact is the use of hydrogen-loaded fiber.…”

“…One explanation for this fact is the use of hydrogen-loaded fiber. Furthermore, the UV laser wavelength, exposure conditions, and composition of the fiber differ from those used by the authors of [18].…”