Development of photothermal vibrating sensor using optical fiber with a built-in Bragg grating

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A quartz-core cantilever fabricated from an optical fiber tip was discussed as a practical sensor for measurement of various physical quantities. Its operation is based on the principle that the resonance characteristics of the photothermally vibrated cantilever vary with the surrounding physical quantities. In the measurement of resonance frequency, the vibration was detected from light returned from the quartz-core cantilever through an optical fiber. However, the limit existed in the measurement region of resonance frequency on this method. In this study, a quartz-core cantilever was fabricated from an Bragg grating fiber for the purpose of enhancing the detection sensitivity of vibration.First, a quartz-core cantilever (diameter D-32 μm, length L=2.4 mm) was fabricated from multi-mode (GI type) grating fiber. Alhough the resonance point was detected, the characteristics were different from usual one. The amplitude of vibration were varied even at frequency with the exception of a resonance point. Also, the amplitude decreased at the resonance point. Next, a quartz-core cantilever (diameter D=9μm, length L=1.1 mm) was fabricated from single-mode grating fiber. Alhough measuerment of the half power width of the resonance was impossible, the resonance point was detected, As the results, if a quartz-core cantilever with unsymmetrical cross section would be realized using a grating fiber, it must be effective to enhance the detection sensitivity of vibration.