A Frequency-Scanned Φ-OTDR Implemented by a Frequency Shifting Loop Used for Distributed Strain Sensing

In this paper, a frequency-scanned Φ-OTDR based on a frequency shifting loop (FSL) is proposed. The FSL is a fiber optic loop containing an acousto-optic frequency shifter (AOFS) and a fiber optic amplifier that converts a single-frequency pulse into a train of pulses with a fixed time and frequency spacing. By introducing bias light and precise switching control the AOFS, the FSL outputs low ASE noise pulses with a frequency spacing of 20 MHz and a scanning range of 460 MHz, which are used as the probe light for the frequency-scanned Φ-OTDR. In addition, a real-time update of the reference Rayleigh scattering pattern (RSP) is proposed to extend the strain measurement range. Experiments on a 3.96-km fiber shows that the strain ASD noise is better than 0.4 nϵ/Hz
^1/2
, and approximate white noise at frequencies higher than 2 Hz with a spatial resolution better than 2.4 m. This frequency-scanned Φ-OTDR is suitable for low-frequency and quasi-static distributed strain measurements, and is expected to be applied to microseismic, teleseismic, and structural health monitoring.