Abstract
An enhanced distributed optical fiber sensor based on broadband ultra-weak fiber Bragg grating array (UWFBG) for dynamic strain measurement with large temperature tolerance range was proposed and experimentally demonstrated. The reflectivity of UWFBG is about -50 dB, and the intensity of the reflected signal is 17 dB higher than the Rayleigh back-scattering. Using an unbalanced Michelson interferometer (MI) and 3 × 3 coupler phase demodulation technique, multiple vibration events can be detected simultaneously. We experimentally demonstrated the effectiveness of the technique by monitoring dynamic strain with a large temperature range of 20-200 °C. The minimum phase detection of the system is 1.02 × 10 - 3 rad/√Hz, and the signal-to-noise ratio (SNR) is above 59.2 dB @100 Hz at different temperatures. Meanwhile, the system has a wide frequency response from 10 Hz to 4.5 kHz.
Highlights
Fiber optic distributed acoustic sensor (DAS) is a promising technology for detection acoustic signals along optical fiber [1], [2]
An enhanced distributed optical fiber sensor based on broadband ultra-weak fiber Bragg grating array (UWFBG) for dynamic strain measurement with large temperature tolerance range was proposed and experimentally demonstrated
Continuous light from the narrowband laser is modulated by an electronic-optic modulator (EOM) to generate a probe pulse, and the pulse width W meets the formula W ≤ 2neL/c, where c is the speed of light in vacuum, ne is the effective refractive index of the fiber, L is the distance between two adjacent UWFBGs
Summary
Fiber optic distributed acoustic sensor (DAS) is a promising technology for detection acoustic signals along optical fiber [1], [2]. For example in downhole vertical seismic detection, the downhole temperature maybe exceed 100 °C and increases with the depth of the well, even over 200 °C, the spectra of UWFBG array will drift as temperature rise, and the intensity of the reflection signal will gradually decrease due to the mismatch of the light source wavelength. This will result in detection dead zone when the temperature is too high. The proposed DAS with good stability which can demodulate vibration signals over a wide temperature tolerance range of 20–200 °C
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