A novel concept of distributed temperature-strain sensors is proposed using inter-mode Kerr four-wave mixing of polarization-maintaining fiber (PMF), in which the dominated physical mechanism is basically different from reported Brillouin dynamic grating (BDG). The independent measurement for temperature and strain sensitivities (-0.431 MHz/℃ and 0.067 MHz/10 µɛ) was completed by detecting the change in birefringence under pump-Stokes frequency separation at 370 MHz. As a preliminary proof-of-concept, sensing with spatial resolution of ~ 5 m was demonstrated along ~210 m PMF. Compared with BDG, the proposed sensor features potential of multiple parameters measurement including temperature-strain and liquid acoustic impedance.

The impacts of nonlinear effects on sensing performance of forward stimulated Brillouin scattering (FSBS) were investigated, using opto-mechanical time-domain analysis (OMTDA) sensor as an example. The excitation of FSBS often requires high pulse power (Watt level) because of the lower gain coefficient. Due to the copropagation of reading pulse and scattered light, high-power activation pulses will induce various nonlinear effects in FSBS sensing system. Using the reported method based on activation-reading time-domain separation, the influence of nonlinear effects due to activation pulses can be effectively avoided. However, the nonlinear effects of reading pulses have direct impact on sensing performance. Based on this consideration, we studied the influence of nonlinear effects on FSBS sensing and its physical mechanism under different peak power of reading pulses; the variation process of the 1st- and 2nd-order FSBS spectrums along~4.7 km standard single-mode fiber (SMF) was shown in detail. Finally, the optimization region was found, inside which a perfect FSBS local spectrum was obtained, and the sensing distance extension can be achieved.

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Brillouin optical time-domain analysis (BOTDA) using distributed Brillouin amplification (DBA) only requires a milliwatt-level pump to achieve a sensing range beyond 100 km, which provides a powerful tool for temperature/strain sensing. However, similar to the majority of other long-range BOTDAs, the state-of-the-art reports require > 1000 times average, severely restricting the sensing speed. The blind area over tens of kilometers caused by the nonuniform Brillouin response and parasitic amplitude modulation (AM) are crucial factors affecting the signal-to-noise ratio (SNR). Here, a comprehensive performance optimization and substantial enhancement for BOTDA sensors was presented by the direct demodulation of an injection-locked dual-bandwidth probe wave. Injection locking (IL) can completely eliminate the impact of AM noise; dual-bandwidth probe enables self-adaptive pulse loss compensation, thereby intensifying the SNR flatness along the ultralong fiber, and direct probe demodulation can overcome nonlocal effects and allows ∼19.7 dB enhancement of probe input power. Therefore, using only 100 times average, ∼148.3 km sensing, and ∼5 m spatial resolution were achieved with < ∼0.8 MHz standard deviation of Brillouin frequency shift (BFS) over a broad range (∼131.7 km). The reduction in averages was more than 10 times that of the reported majority of long-range BOTDAs. Such performances were achieved without using time-consuming or post-processing techniques, such as optical pulse coding and image denoising. Because this approach is compatible with optical chirp chain technique without frequency sweeping, fast acquisition (0.3 s) was also realized, which has the potential for fast sensing at 3.3 Hz along a ∼150 km fiber.

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