A novel configuration of a phase-sensitive optical time-domain reflectometer (OTDR) utilizing dual-pulse phase modulations of the probe signal is presented and experimentally demonstrated. The proposed modulation method enables one to perform the demodulation and reconstruction of an external perturbation signal which impacts the fiber using the phase diversity technique. The proposed phase-sensitive OTDR has some advantages in comparison with conventional solutions, which are discussed. The feasibility of a double pulse OTDR with phase modulation is demonstrated and theoretically proved.

In the present communication we propose a novel approach to the realization of a phase sensitive optical time-domain reflectometer (OTDR) which is capable of a precise reconstruction of the phase signal which impacts the arbitrary point of a fiber-optic line. The method uses a dual-pulse probe signal with diverse carrier optical frequency within each half of the double pulse. The quasi-periodic intensity pattern which emerges as a result of double frequency backscattered signal interference contains the information of the external action over the fiber. The phase signal is extracted with the aid of an I/Q quadrature demodulation scheme, realized at the receiving side of the OTDR. The feasibility and limitations of the proposed scheme are theoretically proved and experimentally demonstrated.

In the present paper we propose a novel method for optical time-domain reflectometer (OTDR)-reflectogram contrast enhancement via compensation of nonlinear distortions of propagating probe pulse, which arise due to the self-phase modulation (SPM) effect in optical fiber. The compensation is performed via preliminary frequency modulation (chirp) of the initial probe pulse according to the specific law. As a result the OTDR contrast at some distant predefined fiber point is fully restored to the value of non-distorted probe pulse at the beginning of the fiber line. As a result, the performance of the phase OTDR increases. The point of full SPM compensation could be shifted to any other point of the fiber line via preliminary frequency modulation index change. The feasibility of the proposed method is theoretically proved and experimentally demonstrated.

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.