Temperature and strain characterization of regenerated gratings

Abstract

Received Month X, XXXX; revised Month X, XXXX; accepted Month X, XXXX; posted Month X, XXXX (Doc. ID XXXXX); published Month X, XXXX Both temperature and strain characterization of seed and regenerated gratings with and without post-annealing is reported. The high temperature regeneration has significant impact to thermal characterization and mechanical strength of gratings whilst the post annealing has little effect. The observed difference is evidence of viscoelastic changes in glass structure.OCIS Codes: 060.3735, 060.2310Codes: 060.3735, 060. , 060.2370 , and used to measure exhaust temperatures from diesel train turbines [7]. In this letter, the temperature and strain properties of regenerated gratings are characterized. Regeneration involves a greater physical contribution through structural relaxation at areas where stresses are altered in the presence of hydrogen, which simply by being present acts to reduce tensile stresses across the core and cladding and between processed regions -a recent review reveals new insight in this regard [8]. The local relaxation differs between areas of high and low laser exposure during fabrication of the seed grating. Hydride and hydroxyl formation helps to accentuate this difference, which leads to stronger grating writing as well as altering stresses [9], and therefore local pressures, periodically.Bragg gratings were inscribed in B-codoped germanosilicate fiber ([GeO2] ~ 33 mol%; [B2O3] ~ 12 mol %) by direct writing through a 10mm optical phase mask using an ArF laser (λ = 193 nm; pulse fluence: fpulse = 95 mJ/cm 2 ; cumulative fluence fcum = 113 J/cm 2 ; RR = 30 Hz; pulse duration τw = 15 ns).Before grating writing, the fiber was hydrogen (H2) loaded (T = 80 °C , P = 180 bar, t = 4 days), avoiding type In formation [10]. For regeneration, two groups of seed gratings were fabricated -Group (1): regenerated gratings, and Group (2): regenerated gratings subjected to post-annealing at 1100 °C. The typical initial spectra with both reflection, R, and transmission, Tr, of the seed gratings are shown in Fig. 1. A high-temperature heater was used for annealing the gratings and temperature monitored with a type K thermocouple. The thermal processing recipe is shown in Fig. 2 along with the grating reflection strengths (normalized to the initial seed grating). The temperature rose to T = 850 °C over t = 60 min before dwelling for t = 40 min. Over this period, the grating decays completely before regenerating and saturating at a peak reflection. At the beginning of the thermal process, a growth in the reflectivity is observed indicating an annealing out of a negative contribution, possibly arising from stress, early on has occurred, reminiscent of behavior observed with type In (type IIA) formation and annealing. For Group (2) gratings, additional annealing (T = 1100 °C, t = 20 min) was undertaken to stabilise the regeneration process for even higher temperature operation, shown in Fig. 2. The strength of the grating decays further before stabilizing. Fig.1 shows the final spec...

Regenerated distributed Bragg reflector fiber lasers for high-temperature operation

“…Above 300°C in the high temperature range, the coefficient increases to ∂λ∕∂T ∼ 0.0142 nm∕°C. This changing temperature dependence in low and high temperature regimes is very similar to that of regenerated gratings [9].…”

“…Thus, thermally regenerated gratings can be fabricated in a wide variety of fibers at low cost for extreme environments. Wang et al has reported linear temperature and strain responses in thermally regenerated gratings from room temperature to 1100°C [9], which render regenerated gratings as a significant enhancement to conventional fiber grating applications, especially for extreme environments. Recently, we have demonstrated a regenerated-grating-based high-temperature pressure sensor, with which hydrostatic pressure 15-2400 psi and high temperature (24°C-800°C) can be accurately measured simultaneously [10].…”

Strain Measurement in Aluminium Alloy during the Solidification Process Using Embedded Fibre Bragg Gratings

“…For the casting experiments, Type I FBGs in GF1B fibre without regeneration were used because they showed a better mechanical stability [29]. For this reason, no other type of FBG was used for these tests.…”

Thermal stress modification in regenerated fiber Bragg grating via manipulation of glass transition temperature based on CO_2-laser annealing

“…The T a is deliberately set at a point between the glass transition temperature ∼500°C and regeneration temperature ∼850°C of B/Ge-co-doped fiber [13][14][15], to ensure the induced temperature is sufficiently high for structural relaxation in the core glass without degrading the grating strength of the RFBG. After 2 min of isothermal annealing, two different cooling approaches, namely slow cooling and rapid cooling, are investigated.…”