A novel method for the temperature-insensitive measurement of the bending of a cantilever plate is proposed. It is based on the radio-frequency (RF) power measurement of the beat signal that has been generated from a dual fiber Bragg grating (FBG)-based sensing head. Another noticeable feature is that a sine-modulated amplified spontaneous emission from a directly modulated reflective semiconductor optical amplifier (RSOA) is employed as a broadband light source.
A photonic crystal fiber with two large air holes outside the holey cladding region is fabricated to induce an effective long periodic grating (LPG) in the core by an electric arc discharge. We believe that the two large air holes lead to the asymmetric perturbation in the core under the electric arc discharge, thereby introducing the coupling to the first higher-order mode. The transmission characteristics of the PCF with the LPG for the external perturbation such as strain, curvature, and temperature are also investigated. It was found that the shift of resonance peak in the transmission spectrum depends on the bending direction. The curvature of 8.55 m -1 results in the center wavelength shifts of 1.8, 4.3, and 11 nm for a vertical, diagonal, and horizontal direction of the curvature to the large air-hole alignment, respectively.
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