Long-reach, multi-channel passive audio transmission system enabled by reflectivity-compensated EFPI arrays

Addressing the urgent demand for large-capacity, long-distance passive acoustic field monitoring in complex industrial environments, this paper proposes and demonstrates a 20-km, eight-channel passive audio sensing system based on extrinsic Fabry-Perot interferometer (EFPI) arrays over a single-mode fiber. To break through the physical bottleneck of “bandwidth-cavity length-loss” imposed by high-density coarse wavelength division multiplexing (CWDM, with single-channel bandwidth compressed to ∼5 nm), we fundamentally reconfigured the probe at the physical level. The physical cavity length was extended to approximately 240.25 μm to compress the free spectral range (FSR), and an approximately 30-nm-thick gold film (∼90% reflectivity) was innovatively coated on the inner surface of the silicon nitride MEMS sensing diaphragm. This design, based on a near-optimal opto-mechanical balance, effectively compensates for the significant Gaussian beam divergence attenuation induced by the long cavity, and achieves favorable optical intensity matching with the fiber end-face. Furthermore, a concurrent adaptive three-wavelength demodulation algorithm based on an FPGA was introduced at the receiving end to eliminate signal fading caused by operating point drift. Experimental results demonstrate that the system achieves high-fidelity parallel transmission of eight-channel audio with an excellent frequency response between 20 Hz and 4 kHz. The maximum signal-to-noise ratio (SNR) reaches 67.55 dB, and the peak acoustic pressure sensitivity is 722.1 mV/Pa. The system also features dynamic channel routing (approximately 4.7 ms switching latency) and maintains stable performance over 20–60 °C. By doubling the multiplexing capacity while maintaining exceptional sound pickup performance, this study provides an ideal solution for deploying large-scale, high-density passive acoustic sensing networks.