Fiber-optic sensors for high-temperature applications

Abstract

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 1000C) is becoming an increasingly important field. Fiber-Bragg gratings (FBGs) are widely used for structuralhealth 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-indexmodulation. However, this modulation is not permanent. Depending on fiber type, for high-temperature sensing applications, the modulation decays until it is completely depleted around 600–700iC.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, 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.