A Fiber Phase-Sensitive Optical Time-Domain Reflectometer for Engineering Geology Application

A new architecture of a fiber phase-sensitive optical time-domain reflectometer (φ-OTDR, i.e., a distributed acoustic sensor) suitable for engineering geology application is proposed. The sensor is based on a double-pulse scheme in which a pair of pulses is formed using an unbalanced Michelson interferometer. A symmetrical 3 × 3 coupler built into the Michelson interferometer is used to obtain the phase delay needed for the demodulation of the backscattered light. Using the unbalanced Michelson interferometer in the circuit for dual-pulse probe signal generation, it is possible to reduce the requirements for the degree of coherence of the light source, since the delay line introduced between the dual-pulse parts is compensated in the φ‑OTDR fiber under test. As a result, it is possible to use a laser with a wide spectral line (~1 GHz) and generate short (7-ns-wide) laser pulses by directly modulating the laser-diode injection current. In order to reduce the signal fading in the φ-OTDR and to improve the linearity of its response, responses are averaged over 16 optical frequencies. The efficiency of the proposed distributed acoustic sensor has been demonstrated by detecting a strong impact on a cable that was horizontally buried in the ground as well as by detecting seismic waves using a cable inserted in a well at the sea bottom.