Fiber-optic sensors for high-temperature applications

Fiber-optic sensors enable measurements of a variety of parameters in conditions where other sensor technologies fail or simply cannot operate. This type of sensing device has intrinsic advantages, including resistance to electromagnetic interference, non-electrical conductivity, passive measurements, small size and low weight, and the option of multipoint measurements. Development of fiber-optic sensors for operation in harsh environments (such as for temperatures of up to 1000^°C) is becoming an increasingly important field.

Fiber-Bragg gratings (FBGs) are widely used for structural-health monitoring and ambient sensing. Their main advantages compared with other optical-sensing techniques are their measurement of reflected light, wavelength-encoded sensing, and multiplexing capability.¹ FBGs are constructed by exposing optical fibers to an intense UV interference pattern that creates a periodic refractive-index modulation. However, this modulation is not permanent. Depending on fiber type, for high-temperature sensing applications, the modulation decays until it is completely depleted around 600–700^°C.^2,3

Several techniques have recently been developed to increase FBG temperature stability. One of the most promising methods involves the use of chemical-composition gratings (CCGs).^4–6 CCGs are fabricated from hydrogen-loaded optical fibers that are subjected to a subsequent annealing treatment at high temperature. This process replaces the FBG's refractive-index modulation by a more temperature-stable chemical structure. During annealing, the original FBG is completely wiped out, and a new refractive-index modulation is generated in the zones that were previously exposed to UV radiation. These gratings are, therefore, also known as regenerated fiber-Bragg gratings.

We successfully made CCGs using two different types of fibers, including standard telecommunications germanium (Ge)-doped and photosensitive Ge/boron (B) co-doped fibers. We analyzed the regeneration process, sensing properties, maximum operating temperature, and decay of CCGs in both types of fibers and found significant differences.

Our experimental tests have shown that the annealing process is highly repetitive and predictable for both types of fibers. We found that there is a minimum-temperature requirement to regenerate a FBG as a CCG. Lower temperatures cause the FBG to decay and eventually disappear. The minimum required temperature is around 950^°C for standard fibers (see Figure 1). For photosensitive Ge/B co-doped fibers, the regeneration process is similar but the minimum temperature is approximately 550^°C (see Figure 2). In both cases, the regenerated refractive-index modulation has a lower amplitude but is directly proportional to the original pattern. The sensing properties also display significant differences. The presence of boron in photosensitive fibers changes the thermal behavior of the CCGs. The wavelength shift caused by temperature changes exhibits a linear relationship for photosensitive fibers, while for Ge-doped fibers it has a non-negligible quadratic term.

Figure 1. Chemical-composition-grating (CCG) regeneration process for two fiber-Bragg gratings (FBGs) with different initial amplitudes using germanium (Ge)-doped fibers. The blue and green curves correspond to the CCG-amplitude axis, while the red curve represents the temperature variation.

Figure 2. CCG regeneration for Ge/boron co-doped fibers. The minimum regeneration temperature is 550^°C.

To determine the operational limits of CCGs, we performed isochronal high-temperature tests. We increased the temperature from 700 to 850^°C in steps of 50^°C, using time intervals of 5h. The CCG amplitude decays proportionally to a combination of time and temperature at an increasing rate for higher temperatures. The decay is similar to that reported for standard FBGs, although the operational temperature range is extended by approximately 200^°C. We expect a hotter operational limit in Ge-doped fibers because of their higher regeneration temperature. We performed their associated high-temperature tests between 900 and 1100^°C in 50^°C steps (in 5h time intervals), but did not observe a significant decay (see Figure 3).

Figure 3. High-temperature isochronal tests for CCGs using Ge-doped fibers. The blue and green curves correspond to the grating-amplitude axis, while the red curve represents the temperature variation.

In summary, we have analyzed the regeneration process, sensing properties, and operational limits of CCGs using two different types of optical fibers. The results show clear differences between both fiber types. Ge/B co-doped fibers exhibit a lower regeneration temperature and linear wavelength shift with temperature, but their high decay rate above 800^°C limits the maximum operational temperature. For Ge-doped fibers, the regeneration temperature is 950^°C and the wavelength shift with temperature has a non-negligible quadratic term. However, they exhibit minimal decay below 1100^°C. This technology opens up the possibility to extend realistic fiber-optic sensors to high-temperature environments. Detection of other physical parameters (such as strain or pressure) is also possible at high temperatures with this technology, which is our future goal.