Ultra-Long Range Distributed Acoustic Sensor Using Poly-Phase Coding Approach

Synchronous intensification of spatial resolution and sensing range has attracted widespread concern for distributed acoustic sensor. The pulse compression approach as a representative technology is adopted for addressing this issue, while the performance is suffered from the Rayleigh side-lobe noise due to the insufficient side-lobe suppression ratio (SLSR). In this work, the poly-phase coding phase-sensitive optical time domain reflectometer (Φ-OTDR), possessing an ultra-high SLSR, is proposed to immensely alleviate the Rayleigh side-lobe noise influence. The spatial resolution is relevant to the single code width, as well as the sensing range and SLSR are relevant to the coding length. An instructive model is deduced to evaluate the influence of SLSR to Φ-OTDR system's SNR. According to the model, a theoretical SLSR value is derived, making the Rayleigh side-lobe noise could be neglected. In the proof-of-concept experiments, the modulated poly-phase coding with ultra-high 127.08 dB SLSR guarantees a 204.3 km sensing range and 2 m spatial resolution. Two sinusoidal perturbances with 20 Hz and 60 Hz are exerted at 173.3 km and 203.8 km on the fiber. As a result, the strain resolutions are achieved at 287.3 pϵ/√Hz and 142.1 pϵ/√Hz, respectively. This work demonstrated that ultra-high SLSR is the foundation for achieving excellent performance for compression pulse distributed acoustic sensors.