Significance Phase-sensitive optical time-domain reflectometry (
Ф-OTDR) represents a significant branch of distributed fiber sensing technology. This technique facilitates the quantitative analysis of vibration and acoustic signals along the sensing fiber by real-time demodulation of the phase evolution of backscattered Rayleigh light. It is distinguished by its rapid response and high detection sensitivity, making it particularly suitable for applications such as perimeter security, oil and gas pipeline leak detection, and marine monitoring. However, owing to the utilization of a narrow line-width laser light source in the
Ф-OTDR system, the Rayleigh scattered light signal generated within the sensing fiber exhibits high coherence. This results in random fluctuations of the Rayleigh scattered light signal along the sensing fiber due to interference between scattered light, thereby creating sensing dead zones at positions with low signal-to-noise ratios. Additionally, polarization mismatch between the Rayleigh backscattered light signal and the local reference light further contributes to these sensing dead zones, leading to phase demodulation distortion. In recent years, suppressing the fading effect in
Ф-OTDR systems has emerged as a research hotspot, with significant academic and practical importance.
Progress The
Ф-OTDR system includes two main types of structures: self-coherent detection and intrinsic heterodyne detection. Common demodulation methods include IQ phase demodulation, Hilbert transform demodulation, PGC phase demodulation, and 3 × 3 coupling demodulation. The existing polarization fading suppression schemes for the
Ф-OTDR system primarily encompass three approaches:
Ф-OTDR system mainly consist of three approaches: Full polarization maintenance, probe pulse polarization control, and polarization diversity. The full polarization maintenance approach entails significant structural costs and demonstrates low cost-effectiveness in long-distance engineering monitoring applications. The probe pulse polarization control method requires complex modulation of optical signals within the
Ф-OTDR system. In contrast, the polarization diversity scheme only necessitates implementing diversity at the receiving-end optical path, effectively mitigating partial fading while offering a simpler structure and lower costs. Therefore, it is currently the most prevalent solution for Polarization fading suppression.
The fundamental principle of interference fading suppression involves generating and synthesizing multiple independent signals with low correlation in order to accurately demodulate high-fidelity phase information. The primary approaches encompass wavelength, frequency, or phase diversity, specialized sensing units, and algorithmic enhancements. The frequency, phase, or wavelength diversity scheme realizes interference fading suppression based on the different Rayleigh scattering distribution characteristics of probe pulses with different frequencies, phases or wavelengths, it is a pioneering and most widely adopted interference fading suppression schemes. The special sensor unit scheme uses multi-mode fiber, few-mode fiber, multi-core fiber and scattering enhanced fiber as the sensor units of the
Ф-OTDR system to obtain multiple independent signals with low correlation or a stable Rayleigh scattering distribution, which is one of the effective means to realize interference fading suppression without necessitating complex optical path. The algorithmic enhancement scheme does not require complex probe pulse modulation and special sensing units, and only performs signal processing in the digital domain to achieve high-fidelity phase information demodulation. The classical algorithms include spectrum extraction and remixing, phase shift transformation, rotation vector sum method, etc.
Conclusions and Prospects Researchers have proposed and validated numerous schemes to suppress polarization and interference fading effects, which significantly mitigate phase demodulation errors caused by fading in the
Ф-OTDR system. This has facilitated the application of
Ф-OTDR technology in industrial monitoring. With the continuous expansion of the application field of
Ф-OTDR technology, end-users are imposing increasingly stringent requirements on this technology. In the future, real-time demodulation of vibration or acoustic information along the sensing fiber will be of critical importance. This is because the machine-OTDR system generates a substantial volume of data during high-frequency signal monitoring or long-distance monitoring. Furthermore, the distributed optical fiber sensing system, which supports multi-parameter hybrid measurement, is anticipated to offer significant advantages in the realm of industrial monitoring in the future.