Phase Demodulation Optimization Method for Multi-Frequency COTDR System

Objective Optical fiber distributed acoustic sensing (DAS) system is a type of acoustic sensor built upon optical fibers and optoelectronic technology. It primarily utilizes optical coherence detection technology to convert acoustic vibration signals into optical signals, which are then transmitted through optical fibers to a signal processing system, thereby obtaining valuable acoustic wave data. They possess advantages such as high sensitivity, wide dynamic range, resistance to electromagnetic interference, structural flexibility, scalability into large-scale arrays, and suitability for extremely harsh conditions. Consequently, since their advent, driven by significant military and civilian applications, they have rapidly developed into an important direction in modern optical sensing and acoustic sensing technology. In recent years, DAS has been widely applied in fields such as oil and gas pipeline monitoring, national defense security surveillance, power cable monitoring, and structural health monitoring of large-scale infrastructure. With the expanding applications of DAS, there is a growing need for acoustic measurement solutions with higher sensitivity and reduced noise floors. To meet these demands, researchers have employed techniques to reduce system phase noise and suppress the fading phenomenon in the context of coherent optical time-domain reflectometry (COTDR) systems. The traditional COTDR systems struggle to achieve ideal signal demodulation due to the presence of signal fading phenomena. We propose a phase demodulation technique based on the multi-frequency optimized diversity (MFOD) algorithm. Methods This experiment employed a multi-frequency coherent optical time-domain reflectometry (MF-COTDR) system based on the frequency-shifted loop structure. Firstly, a detailed description of the phase demodulation method in the traditional COTDR systems was provided. This was followed by the utilization of multiple matching bandpass filters to separate detection pulses of different frequencies, achieving phase demodulation in the MF-COTDR system. Subsequently, we designed an MFOD algorithm based on the MF-COTDR system and established a coherent detection system for MF-COTDR, incorporating a frequency-shifted loop structure and a polarization diversity receiver. This reduced the influence caused by signal fading due to polarization fading and phase fading. Then, we verified the influence of the MFOD algorithm on the signal demodulation performance of the MF-COTDR system and tested the frequency response range of the MF-COTDR system. Results and Discussions Since the MFOD algorithm filters out phase signals in frequency bands unfavorable for gain aggregation, the phase variation information exhibits significantly improved SNR, rendering the phase fluctuations of the fiber under test (FUT) extremely weak in the absence of external disturbances (Fig. 4). Subsequently, a comparative analysis is conducted on the phase demodulation signals of single detection frequency COTDR, MF-COTDR, and MF-COTDR based on MFOD. The results indicate that the adoption of the MFOD algorithm not only suppresses signal fading but also reduces the noise floor of the COTDR system (Fig. 5). Further testing of the frequency response range of the MF-COTDR system demodulated by the MFOD algorithm shows that the system can achieve excellent broadband frequency response (Fig. 6). Conclusions We propose an optimized phase demodulation algorithm for the MF-COTDR system based on the frequency-shifted loop structure. This algorithm can selectively and automatically identify favorable phase demodulation signals from multiple detection frequencies with statistically independent coherent rayleigh noise patterns. The adoption of the MFOD algorithm enables the creation of a distributed sensing system with a uniform noise floor across all sensing channels. Through experiments, the impact of the MFOD algorithm on the overall noise floor of phase demodulation signals of the COTDR system is evaluated. The results show that when combined with the MFOD algorithm, the overall noise floor of the MF-COTDR system is significantly improved; the average signal-to-noise (SNR) is increased by about 9 dB, and the minimum strain resolution reaches 33. 3 p epsilon/Hz(1/2). In addition, we study the response of the MF-COTDR system based on the MFOD algorithm to vibration signals in different frequency ranges. The results show that the MF-COTDR system has larger response bandwidth and better linear response. The research has a good reference value for improving the phase signal demodulation performance of the COTDR system and promoting its application in practical engineering.