Ultrahigh-Temperature Regeneration of Long Period Gratings (LPGs) in Boron-Codoped Germanosilicate Optical Fibre

Abstract

The regeneration of UV-written long period gratings (LPG) in boron-codoped germanosilicate “W” fibre is demonstrated and studied. They survive temperatures over 1000 °C. Compared with regenerated FBGs fabricated in the same type of fibre, the evolution curves of LPGs during regeneration and post-annealing reveal even more detail of glass relaxation. Piece-wise temperature dependence is observed, indicating the onset of a phase transition of glass in the core and inner cladding at ~500 °C and ~250 °C, and the melting of inner cladding between 860 °C and 900 °C. An asymmetric spectral response with increasing and decreasing annealing temperature points to the complex process dependent material system response. Resonant wavelength tuning by adjusting the dwell temperature at which regeneration is undertaken is demonstrated, showing a shorter resonant wavelength and shorter time for stabilisation with higher dwell temperatures. All the regenerated LPGs are nearly strain-insensitive and cannot be tuned by applying loads during annealing as done for regenerated FBGs.

Fiber Optic Sensors Based on Fiber Bragg Gratings for Methanol Steam Reforming Temperature Monitoring

“…The wavelength-temperature relationship is different for each grating and changes above certain temperature due to the non-linearity of glass relaxation. This has reported in the literature previously and is to be expected for elevated temperatures (11,24). For these two sensors, two characterization curves are developed and used independently based on their operating temperature.…”

Fiber Bragg grating regeneration at 450°C for improved high temperature sensing

“…These regeneration temperatures are the lowest used so far in a relatively short time since 450 h has been reported as a required time to decrease the regeneration temperature to 700°C for FBGs in standard SMF28 [16]. As explained previously, the fiber composition, mainly boron doping, which decreases the transition temperature of the core [24], and the 193 nm UV-induced defects type [13] contribute to lower the regeneration temperature. Figure 4 shows the annealing cycles at 600°C and 450°C for two FBGs with initial reflectivities 65% and 98%.…”

“…The negative average change in the refractive index is caused by the first mechanism starting at a low temperature, while the second one operates a positive change in the refractive index of the fiber core. This model did not account for the regeneration mechanism since the maximum used experimental annealing temperature was 475°C for 6 h. In a recent work on the regeneration of long period gratings inscribed in a boroncodoped germanosilicate single-mode fiber [24], a nonlinear temperature response of the grating with three threshold points has been observed. Owing to low transition temperature of germanium and boron, the observed effect has been correlated to the phase transition of glass (T g ) in the core and inner cladding at ∼500°C and ∼250°C, respectively, as well as the melting of inner cladding between 860°C and 900°C.…”

“…The total refractive index change involves different independent effects with different relative contributions [12,25]. These contributions decay at different absolute temperatures and at different times because of their own individual activation energy distribution [21,24,27]. Moreover, the same contribution at a given temperature T 1 will take the required time to reach the activation energy to be involved in the process at another temperature T 2 < T 1 [4,25].…”

“…They erase when the temperature exceeds 1000°C. We assume that this temperature corresponds to the glass softening one, since it is lower in a doped fiber (900°C-1000°C) than in pure silica glass [24]. For seed FBGs, the wavelength shift versus temperature is not linear.…”