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Previously, a fused-quartz acoustic waveguide named the suspended core waveguide with tight-field confinement in a small diameter was developed and its sensing potential in harsh environment was demonstrated with the fabricated periodic acoustic fiber Bragg grating. In this paper, the acoustic sensor was exposed to around 70 Gy/h gamma radiation at room temperature for 3550 h at Oak Ridge National Laboratory. For comparisons, a thermocouple and two optical fiber Bragg gratings were tested under the same conditions. The central frequency reading of the acoustic sensor was found to encounter a relatively fast decrease of 0.5 kHz in the first 800 hours. The reading then became stable, centering at 478.5 kHz with a fluctuation of ±0.2 kHz, and responded to small environmental temperature variations less than 1.2 °C. The major effect to the fluctuation was concluded to be radiation-induced material compaction and expansion. For optical fiber Bragg gratings, the same one-directional fast change of readings in the first 500 hours were observed as well. Although the optical gratings were able to track temperature changes, they also showed continuous drifts. The survivability and consistency of the acoustic sensor under long-term gamma radiation could lead to new sensing methods in nuclear applications. [ABSTRACT FROM AUTHOR]

A fused quartz acoustic waveguide structure was reported as the suspended core waveguide. The waveguide featured good acoustic wave confinement within the core and a relatively small diameter. Consistent high-temperature sensing was also demonstrated with the waveguide after a temperature sensor was fabricated inside. The waveguide was fabricated by suspending its core rod in the middle of the hollow cladding tube by discrete thin supporting disks. It was found that the characteristics of acoustic propagation in the waveguide was similar to that in a bare core rod and that the waveguide was largely insensitive to lossy surface perturbations. The diameter of the waveguide was relatively small, little more than the bulk longitudinal acoustic wavelength. For sensing, the periodic structure, the acoustic fiber Bragg grating, was fabricated on the core of the suspended core waveguide. A temperature test up to 1000 °C was performed. The central frequency of the reflected acoustic wave was found to change unidirectionally and consistently with changes in temperature. With its good field confinement and consistent sensing capabilities, the suspended core waveguide could find vast potential in many applications. [ABSTRACT FROM AUTHOR]

A novel concept of acoustic fiber Bragg grating (AFBG) was conceived and experimentally validated as an effective technique for high-temperature sensing. Similar to the well-known optical fiber Bragg grating, the AFBG employs interactions between waves and periodic structures on an elongated waveguide. Acoustic waves reflected from periodic structures interfere with each other and result in the emergence of resonance frequency when acoustic wavelength matches with pitch length of the periodic structures. When the single-mode operation condition is satisfied, the resonance frequency location can be accurately resolved and thus be used as an effective indicator for temperature sensing. To demonstrate this concept, an AFBG sample was fabricated on a telecommunication optical fiber (125- $\mu \text{m}$ diameter) with femtosecond laser micromachining setup, and it demonstrated sensing capability up to 700 °C, which was limited by the fiber material (fused silica) and could be over 1400 °C if other material, such as sapphire, was used as the waveguide. With the features of various applicable material, low system cost, and potential multiplexing capability, this AFBG technology is a promising candidate for high-temperature sensing, even in a distributed manner. [ABSTRACT FROM AUTHOR]