Zhaoyong Wang

Ultra-broadband phase-sensitive optical time-domain reflectometry with a temporally sequenced multi-frequency source.

A phase-sensitive optical time-domain reflectometry (Φ-OTDR) with a temporally sequenced multi-frequency (TSMF) source is proposed. This technique can improve the system detection bandwidth without the sensing range decreasing. Up to 0.5 MHz detection bandwidth over 9.6 km is experimentally demonstrated as an example. To the best of our knowledge, this is the first time that such a high detection bandwidth over such a long sensing range is reported in Φ-OTDR-based distributed vibration sensing. The technical issues of TSMF Φ-OTDR are discussed in this Letter. This technique will help Φ-OTDR find new important foreground in long-haul distributed broadband-detection applications, such as structural-health monitoring and partial-discharge online monitoring of high voltage power cables.

High sampling rate multi-pulse phase-sensitive OTDR employing frequency division multiplexing

A novel high sampling rate multi-pulse phase-sensitive OTDR (Φ-OTDR) employing frequency division multiplexing (FDM) is proposed to increase the sampling rate of the long distance sensor system. Compared with the conventional Φ- OTDR, the new system owns much higher detection bandwidth as more probe pulses are allowed simultaneously traveling in the sensing fiber. The feasibility of the technique is experimentally verified. By multiplexing four different frequencies, we realize a experimental system with 20kHz vibration detection bandwidth over 10km sensing range.

Novel Slow-Light Reflector Composed of a Fiber Ring Resonator and Low-Reflectivity Fiber Bragg Grating

Slow-light components are attractive for many practical applications, such as narrow linewidth lasers and optical delay line. Characteristics of a novel slow-light reflector, which is composed of a fiber resonator and an extremely low reflectivity fiber Bragg grating (LRFBG), are proposed and analyzed theoretically and experimentally. Due to the light recirculating propagation in the fiber resonator by using a low split ratio coupler, the LRFBG inserted ring provides not only high reflectivity, but also an enlarged group delay, which shows a good slow-light property. In our research, the effective reflectance up to unity can be obtained when the FBG reflectivity and coupler split ratio reach to the critical values for lossless cases. Double peak resonant effects are also observed when the reflectivity of FBG is larger than the critical value. A supernarrow-linewidth fiber laser using the proposed slow-light reflector as its output cavity mirror is built experimentally to verify the analyzed results. It is believed that the proposed slow-light component will have some good applications due to the merits of compactness and low cost.

Brillouin Frequency Shift of Fiber Distributed Sensors Extracted from Noisy Signals by Quadratic Fitting

It is a basic task in Brillouin distributed fiber sensors to extract the peak frequency of the scattering spectrum, since the peak frequency shift gives information on the fiber temperature and strain changes. Because of high-level noise, quadratic fitting is often used in the data processing. Formulas of the dependence of the minimum detectable Brillouin frequency shift (BFS) on the signal-to-noise ratio (SNR) and frequency step have been presented in publications, but in different expressions. A detailed deduction of new formulas of BFS variance and its average is given in this paper, showing especially their dependences on the data range used in fitting, including its length and its center respective to the real spectral peak. The theoretical analyses are experimentally verified. It is shown that the center of the data range has a direct impact on the accuracy of the extracted BFS. We propose and demonstrate an iterative fitting method to mitigate such effects and improve the accuracy of BFS measurement. The different expressions of BFS variances presented in previous papers are explained and discussed.

Pulse compression phase sensitive optical time domain reflectometer with sub-meter resolution

We propose and experimentally demonstrate a pulse compression phase sensitive optical time domain reflectometer (φ-OTDR) with sub-meter resolution. Principle and theoretical analysis on the spatial resolution, the feasibility to obtain the phase information are provided. This technique can break the tradeoff between spatial resolution and measurement range in the traditional φ-OTDR. As example, our verifying experiment achieves 30cm spatial resolution and 20km measurement range, and the signal to noise ratio (SNR) reaches 10dB. To our knowledge, this is the first time that such a high spatial resolution over such a long sensing range is reported in φ-OTDR-based distributed vibration sensing.

Novel railway-subgrade vibration monitoring technology using phase-sensitive OTDR

High-speed railway is being developed rapidly; its safety, including infrastructure and train operation, is vital. This paper presents a railway-subgrade vibration monitoring scheme based on phase-sensitive OTDR for railway safety. The subgrade vibration is detected and rebuilt. Multi-dimension comprehensive analysis (MDCA) is proposed to identify the running train signals and illegal constructions along railway. To our best knowledge, it is the first time that a railway-subgrade vibration monitoring scheme is proposed. This scheme is proved effective by field tests for real-time train tracking and activities monitoring along railway. It provides a new passive distributed way for all-weather railway-subgrade vibration monitoring.

Characteristics of double fiber ring incorporated with a fiber Bragg grating

A novel slow-light reflector composed of a double ring resonator with a fiber Bragg grating (FBG) incorporated is proposed in this paper, which can be used as a mirror of narrow linewidth fiber laser. Due to the Vernier effect, it is shown that the amplitude of resonance peaks is modulated by the simulations and the mode spacing is expanded greatly. Compared with the case of single ring, the mode hopping is mitigated effectively, while the line narrowing function is maintained. The light recirculating propagation in the fiber resonator provides not only high reflectivity, but also an enlarged group delay, which shows a good slow-light property.

Coherent detection based Φ-OTDR and its application

Ultra-narrow linewidth laser, the influence of interference noise and fast pattern recognition algorithm are obstacles in developing coherent detection based Φ-OTDR. This paper presents our recent works on solving these problems and gives some practical engineering applications.

Vehicle tracking by Φ-OTDR used in safety monitored areas

This paper proposes two algorithms, dynamic frequency-space image and threshold sieving to deal with signal of vehicles moving in Φ-OTDR. Satisfactory results of field experiments for tracking of single car and multiple cars are presented, indicating Φ-OTDR is suitable for vehicle tracking.