Long-term stability decay of standard and regenerated Bragg gratings tailored for high temperature operation

Abstract

Abstract-Thermal stability of both standard and regenerated Bragg gratings written in normal and photosensitive optical fibers was accessed. An apparent spectral wavelength stabilization of common gratings with no thermal hysteresis was reached after thermal treatments. However, after a time interval of 5 months, gratings exhibited a shift in the resonance Bragg wavelength at room temperature, as well as important changes in the thermal sensitivity above 200 ºC. Regenerated gratings proved to be stable only at temperatures below the critical regeneration temperature, with significant loss of reflectivity above that critical value. However, regeneration is performed by heating the FBG at high temperatures making the fiber brittle.A packaged RFBG sensor was recently proposed to provide mechanical stability and the sensor performance and response time were analyzed [15]. Despite the packaging does not interfere with the RFBG short term sensitivity and repeatability, the measured recovering time of 22 seconds is a direct consequence of the packaging.In this work, efficiency of thermal treatments on the spectral stabilization of both standard and Bragg and regenerated gratings written in normal and photosensitive optical fibers was accessed.FBGs were submitted to thermal cycles at temperatures up to 425 °C while RFBGs were heated up to 900 ºC. Gratings long-term performance regarding sensitivity, hysteresis and spectral stability were analyzed. Performance of a packaged RFBG was also evaluated. II. METHODSFBGs were produced in single mode (GF-1 from Nufern) photosensitive and standard (G-652 from Fibers were hydrogenated at room temperature along 10 days in a 110 kgf/cm 2 pressure chamber. Thermal treatments (TT) were carried out in a furnace with a 1200 °C maximum temperature operation. FBGs 1, 2, 3 e 4 were submitted to 3 warm-up/cooling-down cycles. For warm-up temperature ranged from (25.0 ± 0.5) ºC to (425.0 ± 0.5) ºC in 45 minutes-long steps of 50 ºC each.FBG1 and FBG3 were slowly cooled-down (annealing) by keeping them inside the furnace after power was switched-off. FBG2 and FBG4 were removed from the furnace immediately after the last

Fibre Bragg Gratings, towards a Better Thermal Stability at High Temperatures

“…Speciality fibres were drawn using different dopants in the glass composition [Butkov et al (2006), Groothoff and Canning, (2004)] or other materials [Grobnic et al (2007)], quenching and annealing processes were applied either as pre-or post grating inscription [Chisholm et al (1998), Aslund and Canning (2000), Coradin et al (2013)], femtosecond laser pulses were used to write the grating by a point by point method [Martinez et al (2004)], among others. Resulting performance increased from a few hours at 800°C [Groothoff and Canning (2004)] -using a Boron co-doped fibre optic -to about 300 hours at 600°C in standard G.652 telecommunications grade fibre optic [Oliveira et al (2011)], as depicted in the graph on Fig.…”

Optical fibre sensing in metals by embedment in 3D printed metallic structures

“…This characteristic, coupled with the advent of high temperature FBGs, such as regenerated fibre Bragg gratings [10,11,12] and high temperature capable Fabry-Perot type temperature sensors indicated that there is an increased chance that fibres containing such sensors are compatible with embedding into high temperature metallic components made in stainless steel using laser based AM technology. The long term stability of FBG based sensors at temperatures in the order of 1000 °C remains to be demonstrated [13] , however we have developed high temperature capable Fabry-Perot type temperature sensors which exhibit very low drift.…”

Development of regenerated fiber Bragg grating sensors with long-term stability

“…In the last decade, different types of FBGs with superior thermal stability (Type-II, Type-In, Chemical Composition Gratings (CCG) and Regenerated Fiber Bragg Gratings (RFBG)) were developed in attempts to increase the thermal resistance of standard type-I gratings that can function only up to 425°C [4,12,13]. While CCG and RFBG have both shown superior performances, withstanding temperatures above 1000°C with exploitable spectral quality for operation in harsh environments over a few hours [4,13,14] to date there is still a strong controversy about the underlying mechanism of the so-called 'thermal regeneration' [12]. The challenge in understanding the regeneration phenomenon relates to the complex processing steps involving numerous parameters which affect the regeneration efficiency, defined as the percentage of the seed-grating reflectivity that remains upon regeneration [2,4,12,15].…”

“…While CCG and RFBG have both shown superior performances, withstanding temperatures above 1000°C with exploitable spectral quality for operation in harsh environments over a few hours [4,13,14] to date there is still a strong controversy about the underlying mechanism of the so-called 'thermal regeneration' [12]. The challenge in understanding the regeneration phenomenon relates to the complex processing steps involving numerous parameters which affect the regeneration efficiency, defined as the percentage of the seed-grating reflectivity that remains upon regeneration [2,4,12,15]. Gas loading (H 2 or Helium), pre-annealing, UV-inscription power and laser wavelength, the strength of initial (seed) grating and annealing cycle, as well as material properties such as fiber composition were all found to influence thermal recovery [2,4,12,15].…”

“…The challenge in understanding the regeneration phenomenon relates to the complex processing steps involving numerous parameters which affect the regeneration efficiency, defined as the percentage of the seed-grating reflectivity that remains upon regeneration [2,4,12,15]. Gas loading (H 2 or Helium), pre-annealing, UV-inscription power and laser wavelength, the strength of initial (seed) grating and annealing cycle, as well as material properties such as fiber composition were all found to influence thermal recovery [2,4,12,15]. In addition, strain is another important factor which affects the central wavelength and the thermal regeneration efficiency [1,16].…”