Abstract
Fiber Bragg gratings (FBG) are extensively used to perform high-temperature measurements in harsh environments, however the drift of the characteristic Bragg wavelength affects their long-term stability resulting in an erroneous temperature measurement. Herein we report the most precise and accurate measurements of wavelength drifts available up to date on high-temperature FBGs. The measurements were performed with a set of packaged π-phase-shifted FBGs for high wavelength resolution, in caesium and sodium pressure-controlled heat pipes for stable temperature environment and with a tunable laser for stable wavelength measurements with a 0.1 pm resolution. Using this dataset we outline the experimental caveats that can lead to inconsistent results and confusion in measuring wavelength drifts, namely: influence of packaging; interchangeability of FBGs produced under identical conditions; birefringence of π-phase-shifted FBGs; initial transient behaviour of FBGs at constant temperature and dependence on the previous thermal history of FBGs. In addition, we observe that the wavelength stability of π-phase-shifted gratings at lower temperature is significantly improved upon by annealing at higher temperature. The lowest value of the wavelength drift we obtain is +0.014 pm·h−1 at 600 °C (corresponding to +0.001 °C·h−1) after annealing for 400 h at 1000 °C, the longest annealing time we have tried. The annealing time required to achieve the small drift rate is FBG-specific.
Highlights
The majority of high-temperature measurements for applications in the 600 ◦C–1200 ◦C temperature range were done with various thermocouples that measure the electromotive force between two dissimilar metal wires wherever a temperature gradient is present along the entire length of the thermocouple [1]
We proceed to describe the experimental caveats that could lead to inconsistent results and confusion in measuring wavelength drifts using our high-accuracy and highprecision dataset, namely: influence of packaging (Section 3.1); interchangeability of Fiber Bragg gratings (FBG) produced under identical conditions (Section 3.2); initial transient behaviour of FBGs at constant temperature (Section 3.3); birefringence of π-phase-shifted FBGs (Section 3.4) and dependence on the previous thermal history of FBGs (Section 3.5)
When studying the wavelength drifts in a batch of FBGs, one typically spends a great deal of time ensuring that the FBGs are produced under identical conditions to enable a fair comparison
Summary
The majority of high-temperature measurements for applications in the 600 ◦C–1200 ◦C temperature range were done with various thermocouples that measure the electromotive force between two dissimilar metal wires wherever a temperature gradient is present along the entire length of the thermocouple [1] (pp. 239–281). The majority of the reported studies involved Bragg gratings inscribed in doped silica fiber with high-intensity infrared femtosecond lasers or ultraviolet lasers by means of a direct writing or writing through a phase mask, which creates periodic variations of the refractive index in the fiber core [11,12]. These periodic variations with a period Λ produce a condition for interference of light propagating in the fiber: 2 · ne f f · Λ = λB, where λB is known as the Bragg wavelength and ne f f is the effective refractive index of the fiber core at λB. As both ne f f and Λ are functions of temperature, λB is correlated with the temperature
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