A distributed acoustic sensor (a phase optical time-domain reflectometer) configuration with a low noise level in the hertz and sub-hertz frequency ranges is proposed. The sensor scheme uses a Mach–Zehnder interferometer to generate a dual-pulse probe signal and implements the frequency stabilization of a laser source using the same interferometer as a frequency etalon. The scheme simultaneously provides a low noise level owing to the compensation of the optical path difference of interfering backscattered fields and low drift of the output signal. It has been shown experimentally that the stabilization of the laser frequency provides up to 35 dB signal/noise gain in the sub-hertz frequencies, which are of interest for seismology. The applicability of the proposed scheme is demonstrated experimentally by teleseismic earthquakes recorded by a fiber-optic cable deployed on the seabed of the Black Sea.

In the paper, we present a qualitative analysis of the dual-pulse phase optical time domain reflectometry (phase-OTDR) response to uniform and nonuniform propagating fiber strain. It is found that on average over all realizations of scattering centers the response of the dual-pulse phase-OTDR is linear with respect to an external perturbation. Meanwhile, individual responses contain random phase jumps, which are an intrinsic property of phase-OTDR. These jumps are the result of nonlinear responses of the scattering fiber segments and arise due to interference of random backscattered fields varying in time. Two types of phase jumps are considered: π jumps and 2 π jumps; the first type is caused by the fading in phase-OTDR spatial channel, while the second type occurs when a nonuniform perturbation propagates along the fiber. The origin of the phase jumps is explained by considering the simulated response on the complex plane. It is shown that the distribution of 2 π jumps can be well described by the Gaussian probability mass function (PMF), provided the number of 2 π jumps is large. The conducted experiments on the registration of uniform and nonuniform fiber strain confirm the presence of the jumps in the phase-OTDR response.

A numerical model describing the effect of an external longitudinal disturbance acting on a sensitive fiber of a dual-pulse phase-OTDR is considered. In the proposed theoretical description it is assumed that the external disturbance forms a region of the elastic deformation in the optical fiber and the corresponding strain amplitude changes along this region in accordance with the shape of the Ricker wavelet. The region with the externally induced strain propagates along the fiber axis and causes the changes of the phases of the optical fields backscattered by this fiber. The resulting measured response in each spatial channel of the phase-OTDR consists of linear and nonlinear random contributions with respect to the external disturbance. The linear contribution is formed due to the change of the optical path within the gauge length-the distance between the beginnings of two scattering segments of a spatial channel. The nonlinear random contributions are produced due to the responses of the scattering segments themselves. These contributions degrade the phase-OTDR response fidelity. In particular, they lead to an error in the registration of the first arrival time of the propagating external disturbance.To verify the results of the numerical modeling, an experiment on registration of the external disturbance propagating along the fiber using a dual-pulse phase-OTDR has been carried out. In the experimental setup, the strained fiber regions with the same spatial shape of the strain propagating along the fiber were repeatedly excited. As a result, the random nature of the response measured at the phase-OTDR output has been confirmed, the statistical parameters of this response are in good agreement with the simulation results.

A novel arrangement for fiber optic distributed stress and temperature sensing based on the Rayleigh scattering spectra correlation method is proposed. The principal feature of the arrangement is usage of low-coherence light in probe pulses, which ensures a wide dynamic range for measurements at moderate sensitivity. Such a characteristic corresponds to performance specifications for infrastructure monitoring systems. A theory of optical time domain reflectometry for arbitrary coherence light is developed describing the contrast in reflectograms and Rayleigh scattering spectra properties. The experimental setup uses a wideband source of light pulses and an electronically controlled micro-electro-mechanical system optical filter for wavelength tuning. Temperature change experiments show root mean square (RMS) noise levels of 0.13 °C, 0.24 °C and 0.3 °C for fiber lengths of 2 km, 8 km and 25 km, respectively, at a spatial resolution of about 1 m (for 10 min data collection). As much as 2000 µstrain dynamic range is demonstrated in the stress measurement experiment while the noise level (RMS error) is estimated to be 2 µstrain. Our experimental results are compared with the theory and a satisfactory match is demonstrated.

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