Frequency-Shift Phase Generation Carrier Demodulation for Noise Reduction in ϕ-OTDR

We propose a frequency-shift phase generation carrier (FS-PGC) demodulation algorithm for reduction of noise and sampling rate in phase-sensitive optical time-domain reflectometry (
$\Phi $
-OTDR). By applying phase modulator and 90° hybrid at the transmitter side, a quadrature signal pair carried with multi-frequency phase information is generated for frequency diversity. Moreover, high frequency component is eliminated, thus significantly decreasing the sampling rate in following digital demodulation process. Frequency diversity is further utilized for noise reduction based on our proposed FS-PGC algorithm. Furthermore, the influence of modulation depth and carrier phase delay are mitigated. Simulation and experimental results demonstrate that interference fading is effectively suppressed and the external vibration signal is well reconstructed. Compared with traditional
$\Phi $
-OTDR system, sampling rate is decreased by 10 times and signal-to-noise ratio (SNR) is increased by 12.3 dB and strain resolution is about
$1.96~{p} \varepsilon \surd $
Hz. With the advantages of strong robustness, low sampling rate, high SNR and strain resolution, the proposed FS-PGC algorithm could meet the detection requirements for weak signals in specific scenarios.