Stable dynamic phase demodulation in a DAS based on double-pulse φ-OTDR using homodyne demodulation and direct detection

Distributed Acoustic Sensing (DAS) is a technology with interesting features for real-time safety and security monitoring applications, and constitutes a steadily growing share of the optical fiber sensing market. Recently, the quantitative measurement of disturbances using DAS schemes based on Phase-Sensitive Time Domain Reflectometry (phi-OTDR) has become a focus of investigation. In this contribution, we propose and experimentally demonstrate a stable homodyne phase demodulation scheme in a fiber optic phi-OTDR sensor using a double pulse probe and a direct detection receiver. We show that a carrier for the distributed dynamic phase change induced by an external perturbation can be generated by selective phase modulation of one of the probing pulses. The local phase is then retrieved from the backscattering signal using a demodulation technique robust against light intensity disturbances, which have been limiting factors in existing phase demodulation schemes. In addition, the method is independent of the phase modulation depth and does not require computationally costly multi-dimensional phase unwrapping algorithms necessary when using I-Q demodulation in DAS, and is a suitable candidate for analogue signal processing. We demonstrate the capacity of the sensor to measure the distributed dynamic phase change induced by a nonlinear actuator generating a 2 kHz perturbation at a distance of 1.5 km with an SNR of similar to 24 dB. The demodulated multi-frequency response is also shown to be consistent with one obtained using a point senor based on an FBG and a commercial reading unit.