Optical Time Domain Reflectometer Research Articles

Pattern recognition based on pulse scanning imaging and convolutional neural network for vibrational events in Φ-OTDR

Feature extraction method of a phase-sensitive optical time-domain reflectometer distributed optical fiber vibration detection system requires a priori knowledge. A lack of feature evaluation methods leads to a low pattern recognition accuracy. Traditional pattern recognition methods cannot be widely applied. This paper presents the implementation of a deep learning-based method to identify vibration signal categories. First, the vibration signal was reconstructed in the time and space domain, which is regarded as a pulse scanning image. Secondly, moving average was used to reduce noise, and seeking the signal envelope surface as an image sample. Finally, the image sample was inputted into the trained convolutional neural network (CNN) to obtain recognition results. Experiments showed that the phase-sensitive optical time-domain reflectometer pulse scanning imaging pattern recognition method based on deep learning proposed in this paper improved recognition accuracy while ensuring recognition efficiency. The algorithm is easy to implement and apply and satisfies the requirements of real-time online monitoring.

Integrated optical time-domain reflectometer with low overhead

This paper introduces an integrated optical time-domain reflectometry (iOTDR) design with low overhead based on several innovations, such as analog-to-probability conversion and probability density modulation, as well as well-known techniques. Due to the small form factor, low overhead, and CMOS compatibility of the proposed system, it holds the promise to be integrated with any optical communication transceiver. The prototyping system clearly demonstrates the feasibility of the proposed instrumentation by comparing its performance to a conventional OTDR setup that is based on an analog-to-digital converter. In addition, the system is integrated onto a custom printed circuit board to demonstrate the promise of future integration into optical transceivers. The experimental results demonstrate a spatial resolution of 23 cm, a sensitivity of − 60 d B , and a dynamic range of 24.15 dB.

A distributed optical fiber sensor system for intrusion detection and location based on the phase-sensitive OTDR with remote pump EDFA

In this paper, a distributed optical fiber senor system for multi-intrusion monitoring has been proposed along a 25 km distance based on phase-sensitive optical time domain reflectometer (Δt=100 ns) and a remote optically pump amplifier (ROPA). The ROPA is based on Raman pump relay free amplification and remote connected in erbium-doped fiber (EDF). It makes the detection distance increase 108 %, from 12km to 25km. The best amplification effect is achieved when the EDF is connected in the middle area. In the experiments, several vibrations of the fiber at different locations given by piezoelectric transducers (PZTs) simulate the vibrations caused by intrusions. Three remote intrusions have been detected and located simultaneously along a 25km optical fiber with a spatial resolution of 10m. In order to improve the signal-to-noise ratio (SNR) and detection distance, the semiconductor optical amplifier (SOA) with high extinction ratio and narrow pulse width has been used to modulate light into pulses. The SNR of 9.5dB has realized for intrusion detection at remote tail end of 25km distance. The experimental results can be the references for improving the remote intrusion detection and location technology of Φ−OTDR sensor system.

Sensing within the OTDR dead-zone using a two-mode fiber.

An optical time-domain reflectometer (OTDR) is incapable of providing sensing or diagnostic information within dead-zones. We use a two-mode fiber (TMF) and a photonic lantern to completely overcome the main OTDR's dead-zone originating from the front facet of optical fiber. This is achieved by injecting the optical pulses of the OTDR in the form of the fundamental ${{\rm LP}_{{01}}}$ mode and meanwhile collecting the Rayleigh signals associated with the higher-order modes. Using the reported TMF-based OTDR, we accurately sense the position and frequency of a vibration event located within the dead-zone as a proof-of-concept demonstration.

Drying-induced soil shrinkage and desiccation cracking monitoring with distributed optical fiber sensing technique

Monitoring of drying-induced volume shrinkage and desiccation cracking in clayey soils is of great importance in geological and geotechnical engineering. Compared with conventional strain monitoring methods providing discrete measurements, the Brilliouin optical time domain reflectometer (BOTDR) technique enables continuous measurement of the distributed strain generated along optical fibers. In this study, a physical model test is conducted to investigate the feasibility of monitoring drying-induced soil shrinkage and desiccation cracking using BOTDR. Three optical fibers with different surface protections (thermoplastic polyester elastomer (TPEE) jacket, nylon jacket, and acrylate coating) are tested and compared. Experimental results validate that BOTDR is applicable for the direct strain monitoring of desiccation cracking soils. Monitored strain values are strongly influenced by water content, soil cracking, and fiber types. The strain measured by the optical fiber reaches only several micro strains when the soil is over-saturated, gradually increases with the decreasing water content and the increasing soil-fiber interfacial shear stresses, and drops rapidly after the occurrence of decoupling between fiber and soil resulting from the mature development of desiccation cracks. The optical fiber with acrylate coating is not suitable because of its fragility and the poor interfacial coupling with the soil. Optical fibers covered with TPEE jacket or nylon jacket are both applicable for soil strain monitoring, with the former one more sensitive to water content variations. The study is the first attempt to apply the BOTDR technique for the direct and continuous monitoring of drying-induced soil shrinkage and desiccation cracking process. It is expected bring new insights into the fundamental understanding of volumetric shrinkage and desiccation cracking in clayey soils.

Extending the operation range of a phase-sensitive optical time-domain reflectometer by using fibre with chirped Bragg gratings

>A record long external-disturbance detection distance of 140 km without employing optical amplifiers in the fibre line is demonstrated by a phase-sensitive optical time-domain reflectometer (Φ-OTDR) using a fibre section with inscribed chirped fibre Bragg gratings (CFBGs). This solution provides simultaneous operation of Φ-OTDR with a standard single-mode fibre at a distance of up to 100 km and with a CFBG fibre section located from the fibre line input point by at least 140 km.

Fast Acquisition Tunable High-Resolution Photon-Counting OTDR

A 15 dB dynamic range and 4.6 cm spatial resolution tunable photon-counting optical time-domain reflectometer (PC-OTDR) is presented along with a Field Programmable Gate Array (FPGA)-based detection management system that allows several regions of the fiber to be interrogated by the same optical pulse, increasing the data acquisition rate when compared to previous solutions. The optical pulse generation is implemented by a tunable figure-8 passive mode-locked laser providing pulses with the desired bandwidth and center wavelength for WDM applications in the C-band. The acquisition rate is limited by the afterpulse effect and dead time of the employed gated avalanche single-photon detectors. The devised acquisition system not only allows for centimeter-resolution monitoring of fiber links as long as 12 km in under 20 minutes but is also readily adapted to any other photon-counting strategy for increased acquisition rate. The system provides a 20-fold decrease in acquisition times when compared with state-of-the-art solutions, allowing affordable times in centimeter-resolution long-distance fiber measurements.

Performance enhancement for a self-heterodyne detection Brillouin optical time-domain reflectometer

Aiming at improving signal-to-noise ratio (SNR) and system performance for a Brillouin optical time-domain reflectometer (BOTDR) based on self-heterodyne detection of Rayleigh and Brillouin scattering lights, an approach assisted by modulating probe light and shift-averaging laser frequency, is proposed and demonstrated experimentally. The principles of modulating multifrequency probe light and shift-averaging laser frequency are analyzed theoretically, which are able to increase the signal intensity and reduce the coherent Rayleigh noise, respectively. The experimental results show that the amplitude fluctuations of Brillouin signals and spectrum parameters are decreased effectively, and SNR is enhanced significantly using three-frequency probe light with 22-time shift-averaging compared to that using single-frequency probe light without shift-averaging, and the highest accuracy of temperature measurement obtained on the 50-m-heated fiber near the far end is 0.36°C, which indicates the proposed approach is effective to enhance performance for a self-heterodyne detection BOTDR.

Spatial resolution enhancement of DFT-BOTDR with high-order self-convolution window

Abstract Brillouin optical time-domain reflectometer based on discrete Fourier transform has attracted interest for the simple structure, low cost, and fast data processing. In this paper, a 4th-order Hanning self-convolution window method is demonstrated to enhance the spatial resolution of DFT-BOTDR system. The Brillouin backscattering is profile-weighted by convolving with a 4th-order Hanning self-convolution window. The center frequency of the Brillouin spectrum is determined by the weighted function, where the spectrum power is concentrated in the main lobe, leading to an optimized spatial resolution. Experiments of strain sensing on a 5 km long sensing fiber with pulse width of 300 ns and sampling rate of 625 MS/s are carried out. The theoretical spatial resolution is 41 m with traditional rectangular window. The experimental results show that the spatial resolution is improved by ∼31 m with the proposed 4th-order Hanning self-convolution window. The sensing system is promising in accurate location of distributed sensing.

Investigation on the improvement of Brillouin optical time-domain reflectometer with Golay pulse codes in radiation environment

The application of Golay pulse coding technique on Brillouin optical time-domain reflectometer (BOTDR) in a radiation environment is experimentally analyzed. By applying Golay pulse codes on the BOTDR system in the radiation experiment, the enhancement of detection range, accuracy of Brillouin frequency shift (BFS), and reduction of stress measurement error are characterized. By using 64-bit Golay coding, signal-to-noise ratio can be improved significantly and the variation of BFS is more accurate. These results are beneficial for the application of the Golay pulse code technique on BOTDR in the space station.

The OTDR With High Dynamic Range Based on LFM Signal and FDM

In this paper, we proposed a kind of novel optical time-domain reflectometer (OTDR) which employed the linear frequency modulation (LFM), the fractional Fourier transform (FrFT), and electrical domain frequency division multiplexing (FDM) technologies. In our proposed OTDR, the intensity of the probe pulse was modulated by the LFM signal. At the receiver of the OTDR, the LFM signal was recovered through FrFT after the direct detection. The LFM and FrFT were helpful to overcome the deterioration induced by the multipath interference of Rayleigh backscattering and improve the signal-to-noise ratio (SNR) of the backscattered probe signal to realize a higher dynamic range (DR). With the FDM technology, the DR could be expanded furthermore. Experiments showed that about 11dB DR improvement was realized by our method with the same spatial resolution (SR) compared with a single pulse OTDR.

Performance enhancement method for phase-sensitive optical time-domain reflectometer system based on suppression of fading induced false alarms

The phase-sensitive optical time-domain reflectometry (Φ-OTDR) has been developed rapidly as a fully fiber-optic distributed vibration sensing technology. However, the demodulation technique based on the phase term would induce a serious false alarm problem due to the signal fading effect. An effective method to suppress fading-induced false alarms in the Φ-OTDR system is proposed, which is based on the suppression mask and numerical relationship between phase and amplitudes of Rayleigh backscattering. The performance of the proposed method has been experimentally demonstrated in both laboratory environment and in-field situation test. Without any hardware addition in a traditional Φ-OTDR equipment, false alarms rate can be reduced from 4.81% to 0.15%, whereas low missing alarms rate can be achieved at the same time. In-field results show that this work provides a low-cost solution to enhance the performance for real-life engineering application of the phase-discrimination Φ-OTDR system.

Monitoring Vehicles on Highway by Dual-Channel φ-OTDR

As a fully distributed sensor, the phase-sensitive optical time domain reflectometer (φ-OTDR) has attracted remarkable attention in real-time vibration detection. We present a dual-channel φ-OTDR (DC-φ-OTDR), formed by two single-channel φ-OTDRs (SC-φ-OTDR), to monitor running vehicles on a highway. In the double-channel system, an improved algorithm (will be referred to as the CDM&V) is proposed to alleviate the strong dependence of vibration detection on the differential step as in the widely used conventional differential method (CDM). The DC-φ-OTDR is first tested over campus road before applying it to locate moving vehicles on the highway. For comparison purposes, both the DC-φ-OTDR and SC-φ-OTDR are used to monitor the vehicles with respective signal processing methods of the CDM and CDM&V. The experimental results at campus show that the dual-path scheme can undoubtedly reduce vibration misjudgment relative to the single one due to the very small possibility of false measurements occurred simultaneously at the same location in both channels. In signal demodulation, the CDM&V greatly relaxes the constraints on the differencing interval for identifying the vehicle-caused vibration. With a step size of 5 or lower, the CDM fails to locate the running vehicle at z=~8.5 km, but the CDM&V successfully demonstrates the feasible capability of locating the vibration. With an increase in the differential interval, both the CDM and CDM&V are able to detect the vibration signal, but with the latter showing a much better noise suppression performance and hence a larger SNR. Importantly, in comparison with the SC-φ-OTDR system, the DC-φ-OTDR exhibits a considerable enhanced SNR for the detection signal regardless of which processing algorithm (i.e., CDM, CDM&V) is used. The vehicle locations positioned by the DC-φ-OTDR are confirmed by the monitoring cameras.

Co-Processing Parallel Computation for Distributed Optical Fiber Vibration Sensing

Rapid data processing is crucial for distributed optical fiber vibration sensing systems based on a phase-sensitive optical time domain reflectometer (Φ-OTDR) due to the huge amount of continuously refreshed sensing data. The vibration sensing principle is analyzed to study the data flow of Rayleigh backscattered light among the different processing units. A field-programmable gate array (FPGA) is first chosen to synchronously implement pulse modulation, data acquisition and transmission in parallel. Due to the parallelism characteristics of numerous independent algorithm kernels, graphics processing units (GPU) can be used to execute the same computation instruction by the allocation of multiple threads. As a conventional data processing method for the sensing system, a differential accumulation algorithm using co-processing parallel computation is verified with a time of 1.6 μs spent of the GPU, which is 21,250 times faster than a central processing unit (CPU) for a 2020 m length of optical fiber. Moreover, the cooperation processes of the CPU and GPU are realized for the spectrum analysis, which could shorten substantially the time of fast Fourier transform analysis processing. The combination of FPGA, CPU and GPU can largely enhance the capacity of data acquisition and processing, and improve the real-time performance of distributed optical fiber vibration sensing systems.

Investigation of radiation recovery effect on sensing fibre in BOTDR

The radiation recovery effect on sensing fibre in Brillouin optical time-domain reflectometer (BOTDR) has been experimentally investigated. In the experiment, three kinds of commercial single-mode G.652.D fibres: Yangtze Optical Fibre and Cable, Corning and Fujikura have been used as sensing fibre in BOTDR system. After 200 Gy dose gamma-ray radiation absorbed by sensing fibre, the performance of BOTDR at different time intervals has been tested. Moreover, by analysing Brillouin gain spectra along with fibre sample, the recovery in radiation-induced attenuation, the variation of Brillouin frequency shift and the variation of stress coefficient are characterised. The results will be helpful for the error correction in the simulation experiment and are a reliable basis for the introduction of BOTDR into the space station.

Distributed single fiber optic vibration sensing with high frequency response and multi-points accurate location

ABSTRACT A distributed fiber optic vibration sensing system with high frequency response and multi-points accurate location is proposed and demonstrated by combining a feedback loop-based interferometer (FLI) and a phase sensitive optical time domain reflectometer (φ-OTDR). The FLI and the φ-OTDR are merged together based on a wavelength division multiplexing (WDM) scheme. Assisted by the feedback loop, the interferometer structure can achieve a high frequency detection using a single fiber only. Meanwhile, the vibration position is obtained by the φ-OTDR, which shares the same fiber with the FLI. Experimental results demonstrate that the frequency response of the merged system reaches up to 300 kHz, and the spatial resolution is 15 m at the sensing length of 4 km. Compared with previous reported systems, the structure of our proposed system is simpler, only one fiber is used to detect the frequency and the position of the vibration. In addition, the single end sensing has also been achieved in the proposed system.

Spectral Properties of the Signal in Phase-Sensitive Optical Time-Domain Reflectometry With Direct Detection

The spectral properties of the Rayleigh backscattered traces measured by a phase-sensitive optical time-domain reflectometer ( φ OTDR) with direct detection are theoretically and experimentally analyzed. The spectrum of the measured φ OTDR signal is found to be strictly dependent on the spectral shape of the probing optical pulse. Furthermore, the visibility, spatial resolution, fading rate, and correlation spectrum of the traces are analyzed using different detection bandwidths. Results point out that the quality of φ OTDR traces and target spatial resolution are secured only if the electrical bandwidth of the photodetector is broad enough to cover at least 80% of the total power contained in the electrical spectral density function of the measured trace. This means that in the case of using direct detection of the Rayleigh backscattered light induced by rectangular-shaped optical pulses, the minimum bandwidth required for a proper detection of the traces is equal to the reciprocal of the pulse temporal width (which is larger than the pulse spectral width). Although the theoretical analysis and numerical simulations are here experimentally validated for rectangular and sinc-shaped optical pulses, the results and methodology presented in this article can be applied to optimize the direct-detection bandwidth of φ OTDR sensors using any optical pulse shape.

Modeling and Optimization of a Cost-Efficient Distributed Temperature Sensor Based on Temperature Sensitive Optical Fibers

A cost-efficient distributed temperature sensor based on Rayleigh scattering in temperature sensitive optical fibers was proposed. The temperature dependence of the irradiated commercial off-the-shelf single mode fiber was analyzed. By combining a temperature dependent model of the sensitive fibers and loss resolution model of optical time domain reflectometer, a general sensor model has been developed. In order to optimize the temperature resolution, the length of each sensing segment and the overall sensing distance, three models are derived from the general model. Based on the models and parameters optimization, a novel design method is proposed. Using the method, a prototype was built and tested. The test results indicate that the temperature resolution is better than 1 °C in the temperature range from −40 °C to 150 °C, which is consistent with the optimal results.

Event Identification by F-ELM Model for $\varphi$ -OTDR Fiber-Optic Distributed Disturbance Sensor

A novel event identification method, which combines the extreme learning machine (ELM) and fisher score feature selection method, is proposed to reduce the nuisance alarm rate (NAR) in fiber-optic distributed disturbance sensors based on phase-sensitive optical time-domain reflectometer (φ-OTDR). Through constructing 25.05 km long φ-OTDR experimental system and analyzing the selected features with the ELM, four kinds of real disturbance events, including watering, climbing, knocking and pressing, and a false disturbance event can be effectively identified. Experimental results show that the average identification rate of five disturbance events exceeds 95%, the identification time is below 0.1 s and the NAR is 4.67% through selecting 25 features. Compared with the ELM model without feature selection, the ELM model with feature selection by fisher method has several distinguished advantages of higher identification rate, shorter identification time, and lower NAR.

Investigation of a quasi-distributed displacement sensor using the macro-bending loss of an optical fiber

Abstract In this paper a quasi-distributed fiber-optic displacement sensor (QDFODS) for landslide monitoring using an optical time domain reflectometer (OTDR) was demonstrated. The new sensor was connected in series by several fiber-optic displacement sensors (FODSs). Their configurations introduce power loss through the decrease of their fiber bowknot radius when displacement was applied. The decrease of the light intensity with displacement variation was reported. The proposed FODS had a maximum measurement distance and an effective initial measurement displacement of 36 mm and 0.98 mm. The authors performed the laboratory experiments, and the sliding damage of the rock mass with two-stage sliding was effectively predicted by the QDFODS. Further, a field monitoring application of the QDFODS was conducted to confirm the feasibility for determining the progressive deformation characteristics of the slope, such as the magnitude of movement and positions of the potential sliding surfaces. This capability demonstrates the promising applications of the QDFODS in the monitoring of civil engineering, especially slopes, foundation pits and tunnels.