We propose a Brillouin optical correlation domain reflectometry (BOCDR) technique with a lock-in detection scheme in this paper. By designing a new system using the lock-in detection scheme and amplifying a small spontaneous Brillouin signal with a lock-in amplifier, a Brillouin scattering spectrum with a stable shape is obtained. By further introducing a periodical on/off phase modulation for chopping for lock-in detection, the undesired optical background spectrum is effectively reduced, and a 20 cm section with 7,000 µε strain is clearly measured.
Compressed sensing (CS) is proposed in Brillouin optical correlation-domain reflectometry (BOCDR) to enhance the effective repetition rate at any single position along an optical fiber. Through the random selection of modulation frequencies applied to the laser source in each epoch that the entire fiber is traversed and the reconstruction of the under-sampled data, a picture of the distributed and time-varying Brillouin frequency shift values can be achieved. Dynamic strains at frequencies of 20 Hz and 40 Hz are measured with CS at different under-sampling rates. The effective repetition rate is enhanced by 10 times with an improved temporal resolution.
A BOTDA with the capacity of break interrogation is proposed and demonstrated experimentally. In our configuration, coherent detection and double sideband probe method are employed to enhance the signal-to-noise ratio (SNR) and to effectively reduce nonlocal effects, respectively. Without amplification, a 72 km sensing range with 5-meter resolution and an estimated temperature uncertainty of 1.8 °C are obtained. Benefiting from the flexible optical configuration, this sensor system has the capacity of break interrogation as a coherent optical time domain reflectometry (COTDR) if there is a break in the fiber under test (FUT). The sensor achieves a dynamic range of 36 dB with a 100 m spatial resolution, which offers an excellent solution for the requisite of two-end-access in BOTDA, and significantly enhances the robustness of the sensing system.
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