Fiber Bragg Grating Wavelength Drift in Long-Term High Temperature Annealing

Abstract

High-temperature-resistant fiber Bragg gratings (FBGs) are the main competitors to thermocouples as sensors in applications for high temperature environments defined as being in the 600–1200°C temperature range. Due to their small size, capacity to be multiplexed into high density distributed sensor arrays and survivability in extreme ambient temperatures, they could provide the essential sensing support that is needed in high temperature processes. While capable of providing reliable sensing information in the short term, their long-term functionality is affected by the drift of the characteristic Bragg wavelength or resonance that is used to derive the temperature. A number of physical processes have been proposed as the cause of the high temperature wavelength drift but there is yet no credible description of this process. In this paper we review the literature related to the long-term wavelength drift of FBGs at high temperature and provide our recent results of more than 4000 h of high temperature testing in the 900 –1000°C range. We identify the major components of the high temperature wavelength drift and we propose mechanisms that could be causing them.

Accurate Measurements of a Wavelength Drift in High-Temperature Silica-Fiber Bragg Gratings

“…It is yet unclear how this re-deposition led to the subsequent FBG failure and further experimentation with the packaging is necessary. On the other hand, our recent results [13] indicate that, once the details of the packaging are rectified, the same trends are observed with packaged and unpackaged FBGs, which shows that a properly designed package minimally affects the long-term behaviour of the FBG temperature sensors inside it.…”

“…Interestingly, after 300 h at 1000 • C for probe 28 we registered a negative drift rate of −0.18 pm•h −1 (blue shift). This pattern in the wavelength dynamics-the initial faster red shift followed by the slower blue shift-is common among various types of high-temperature FBGs [13] and strongly suggests that the underlying cause of the wavelength drifts is related to the fiber itself: residual stresses in the fiber, dopant diffusion, structural changes or a combination of these causes. Experiments are under way to see whether this negative trend continues and is reproducible.…”

“…Continuing with the issues that are typically overlooked in the experiments with hightemperature FBGs, the inevitable variability of the FBG sensors comes up, even though the gratings are produced with a great care under as much identical conditions as possible. In Figure 5, we show our own results from [13] obtained with unpackaged π-phase-shifted Type II FBGs at 1000 • C produced under identical laser-writing conditions. The same conditions were also used to fabricate the packaged fibers that are described earlier.…”

“…However, the first experiments involving FBGs in high-temperature environments had three major drawbacks: they lacked a systematic approach; they were primarily concerned with survivability of the FBG sensors on a short time scale; and they were typically performed in a tube furnace or in a specific application setting (e.g., simulated nuclear reactor environment) with a relatively low wavelength and temperature resolution and a relatively poor temperature stability and homogeneity. Closer examination [13] revealed ubiquitous wavelength drifts present across different grating types at high-temperatures which directly impacts applications involving high-accuracy and high-precision temperature measurements (e.g., temperature metrology) and applications that require long-term temperature monitoring, sometimes at the level of years (e.g., in nuclear reactors). The underlying mechanism for the wavelength drifts so far remains disputed.…”

“…The underlying mechanism for the wavelength drifts so far remains disputed. Three effects that are commonly invoked in the literature to explain the wavelength drifts [13] are dopant diffusion from the core, devitrification (crystallization) of the glass, and release of stresses that are built into the fiber during the fiber manufacturing process. It is likely that not a single cause, but a combination of several factors, are responsible for the observed wavelength drifts.…”