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Abstract
In this study, a long-distance phase-sensitive optical time domain reflectometry (Φ-OTDR) with a flexible frequency response based on time division multiplexing is proposed and experimentally demonstrated. Distributed flexible frequency vibration sensing over long distance can be realized by reconfiguring the system layout in a time-division-multiplexed manner by re-routing the Rayleigh backscattered signals for segmented processing with extra erbium-doped fiber amplifiers added only instead of any other complex signal amplification or pulse modulation mechanisms. Through time-division-multiplexed reconfiguration, the tradeoff between sensing distance and vibration frequency response in Φ-OTDR system is largely relieved. Compared with the traditional system layout, the proposed system allows a flexible frequency response in each sensing fiber segment without any crosstalk among them. In experiments, distributed vibration sensing with a frequency response up to 4.5 kHz is achieved over a sensing distance of 60km by the proposed system, which is not possible in a conventional Φ-OTDR system. Furthermore, the frequency response flexibility of the proposed system is further verified by successfully identifying a vibration event with a frequency of up to 20 kHz at the end of a 52-km-long fiber.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Phase-sensitive optical time domain reflectometry (Φ-OTDR) is a powerful fully distributed optical fiber sensor (DOFS) based on detecting the interference effects of the Rayleigh backscattered signals in a sensing fiber [1–3]. Benefitted from its long sensing length and high-density distributed sensor arrays, Φ-OTDR can be potentially applied in perimeter intrusion detection [4], structural health monitoring of pipelines [5–7], and communication or power cables surveillance [8]. In most applications of Φ-OTDR, sensing distance and vibration frequency response bandwidth are two crucial technical parameters that determine the practical performance of the sensing system, as in many long-haul systems such as pipelines or power cables, the length is on the order of tens of kilometers and high frequency vibration events frequently occur [9,10]. Thus, the ability to realize high frequency response over a long sensing range is highly desired for Φ-OTDR-based distributed sensing network.
Similar to most of the time-of-flight based DOFSs using optical pulses as interrogators, the sensing distance of traditional Φ-OTDR is also limited by the peak power of the probe pulses as low peak power does not guarantee a long transmission length, while high peak power easily induces undesired nonlinear effects in optical fibers, such as modulation instability [11]. Many valuable works have been carried out in the past few years to improve the maximum measurable distance of DOFSs. Raman amplification has been proved to be an effective way to extend the sensing distance thanks to its capability of distributed amplification that keeps the power of the probe pulses almost unchanged along the sensing fiber. A quantity of different systems using first-order or second-order Raman amplification have been proposed to prolong the sensing distance of DOFS [5,12–18]. Besides, Brillouin amplification has also been proposed to increase the sensing range of DOFS [19]. In addition, the two can be combined to achieve an ultra-long sensing distance [20]. However, these amplification methods require the pump light to be injected into both ends of sensing fiber, which reduces the flexibility of system design and increases the cost of the system. Furthermore, both Raman amplification and Brillouin amplification will introduce noise to the sensing system, such as relative intensity noise transferred from the pumps when the probe pulses are being amplified [21,22], which sometimes completely masks the signal if no careful signal processing is conducted.
Another research focus is to improve the bandwidth of the vibration frequency response in long-distance sensing. Conventional Φ-OTDR system suffers from an inherent trade-off between the maximum measurable distance and the highest vibration frequency response. In the time-of-flight-based distributed sensing system like Φ-OTDR system, the theoretical maximum detectable vibration frequency equals to half of the repetition rate of the injected probe pulses [23] according to the Nyquist sampling theorem. However, considering the influence of the environmental noises in practical applications, the maximum achievable frequency response bandwidth will be lower than that of the theoretical prediction. Furthermore, as the time interval between two successive injected probe pulses must be greater than the round trip time of the light wave travelling through the entire sensing fiber to avoid generating mixed Rayleigh scattering signals, and given the fact that the time interval of the pulses is inversely proportional to the injection rate, the repetition rate of the probe pulses is strictly limited by the sensing fiber length, thus making it impossible to detect high-frequency vibration events at far distances. Hybrid sensing systems combining Φ-OTDR and Mach-Zehnder interferometer (MZI) have been proposed [24–26] to enhance the frequency response for long-distance sensing. But a reference fiber with a similar length to the sensing fiber is needed to construct the MZI, which will introduce additional noises and uncertainties to the systems since noisy vibrations from the external environment can also affect the reference fiber. Temporally sequenced multi-frequency light source and linear-frequency-modulated probe pulses have also been used in Φ-OTDR system to extend the vibration response bandwidth in long-distance sensing [27,28]. However, drawbacks such as low spatial resolution of 90m or complex system structure with high cost for probe pulse modulations make the system not practically applicable.
In this paper, we introduce a long-distance Φ-OTDR with flexible frequency response based on time division multiplexing. With proper time-division-multiplexed reconfiguration of the sensing system by re-routing the Rayleigh backscattered signals for segmented processing, high frequency response over long distances is achieved without any other complex pulse modulation or amplification add-ons. In addition, the frequency response can be flexibly adjusted for a single segment of optical fiber in different application environments. Compared with the traditional system layout, the proposed system allows flexible frequency response in each segment without any crosstalk among them. In experiments, the system is able to detect a vibration event with a maximum frequency up to 4.5 kHz over a sensing distance of 60km. Furthermore, vibration with frequency up to 20 kHz can be reliably detected at the end of a 52-km-long fiber, further verifying the frequency response flexibility of the proposed sensing system.
2. Principle
In this section, we explain how time division multiplexing can be applied in Φ-OTDR system to extend the sensing range for vibration signals with flexible frequency response. The schematic diagram of the sensing principle is shown in Fig. 1. In order to break through the tradeoff between the maximum sensing distance and the detectable vibration frequency in Φ-OTDR system and to realize high-frequency vibration detection for long sensing range, the entire sensing fiber is divided into several segments with lengths marked as x1, x2, x3, …, xn. The number of the fiber segments and the length of each segment can be flexibly adjusted according to the practical needs. By inserting light steering elements such as optical circulators between the neighboring fiber segments, the Rayleigh backscattered signals generated in each fiber segment as the probe pulse transmits through the entire sensing fiber are no longer all guided back to the very beginning of the sensing fiber for data collection and sampling as in the conventional Φ-OTDR, but collected in a chronological manner that the local Rayleigh backscattered signals within each fiber segment are directed to the corresponding input port of the local fiber segment. Therefore, time delays as marked with t1, t2, t3, …, tn in Fig. 1 exist between the collected Rayleigh signals from neighboring fiber segments, with delay values determined by the length of the former fiber segment in every pair of adjacent fiber segment. Provided that the lengths of all fiber segments are known, Rayleigh backscattered signals in each segment can be detected in a time-division-multiplexed way. As a result, each fiber segment acts as an independent distributed sensing sub-system and the Rayleigh backscattered signals are handled in a segment processing manner, which means that probe pulses with repetition rates up to the maximum allowable value determined by the local fiber segment length can be launched according to the practical needs. Crosstalk-free high frequency response with uncompromised spatial resolution and other performance parameters can thus be achieved at far distance without the limitations imposed by other sub-systems. In addition, the time-division-multiplexing-based segment processing has a more valuable advantage, that is, each sub-system can be implemented as either an independent distributed vibration sensing system (DVS) or distributed acoustic sensor system (DAS), and all state-of-art pulse modulation methods can be applied in each sub-system to improve the overall system performance.
3. Experimental setup
The experimental setup employed in this work is shown in Fig. 2. A narrow linewidth (<3 kHz) semiconductor laser at 1550nm was used. The laser output passed through an optical isolator and was then modulated to probe pulses with a repetition rate of 10 kHz and pulse width of 100ns, which corresponds to a theoretically detectable frequency response up to 5 kHz according to the Nyquist sampling theorem and a spatial resolution of 10m. An acoustic-optic modulator (AOM) was used to introduce a frequency shift of 150MHz to the probe pulse frequency. Then the probe pulse power was boosted by an erbium doped fiber amplifier (EDFA) before launched into the fiber under test (FUT) via the first circulator. In the proof-of-concept experiment, the entire FUT was divided into six segments concatenated with optical circulators, each with a length of 10km. In addition, another EDFA was added after the 3rd fiber segment for pulse re-amplification to compensate the fiber loss. Note that additional EDFAs can be placed after any fiber segment whenever it is necessary for signal enhancement. Rayleigh backscattered signals from each fiber segment were assembled into the same data collection port by connecting the output ports of all the circulators with proper fiber delay lengths. The purpose of adding delay fiber between the output ports of neighboring circulators is obvious, that is to guide all Rayleigh signals generated within far fiber segments to the user side for data collection and sampling in practical applications. Optical switches (OS), each with a maximum insertion loss of ∼0.3dB, are added at the output port of each optical circulator for switching on/off the local Rayleigh backscattered signal in each fiber segment in order to avoid signal mixing during the signal sampling process. Furthermore, amplifying modules consisting of EDFA and 1×2 OS were inserted into the delay fiber to amplify the Rayleigh backscattered signal, making ensure that the signal has sufficient power to reach the detector. The amplifying modules were controlled by the 1×2 OS to ensure only one EDFA was working to amplify the signal in the assembling line and avoid extra introduced noise. The switching time and switching channel of the optical switches were controlled by the upper computer software according to the time delays determined by the time-division-multiplexed configuration of the sensing system to interrogate each fiber segment. The Rayleigh backscattered signal was finally detected by a balanced photo-detector (BPD) and sampled with a data acquisition card (DAQ) in a speed of 125MSamples/s. To verify that vibration can be accurately detected at each fiber segment, three piezoelectric transducers (PZT) winded with 5-m fiber were placed at the end of the 1st, the 3rd, and the 6th fiber segment, corresponding to overall sensing distances of 9.95km, 29.95km and 59.95km respectively.
Fig. 2. Schematic of the experimental setup, Laser: Narrow linewidth semiconductor laser, ISO: Isolator, AOM: Acousto-optic modulator, EDFA: Erbium-doped fiber amplifier, Cir. Circulator, FUT: Fiber under test (10 km), PZT: Piezo-transducer, OS: Optical switch, BPD: Balanced photo-detector, DAQ: Data acquisition card, AFG: Analog function generator.
4. Results and discussion
In conventional Φ-OTDR, a single EDFA is used to amplify the probe pulse before it enters the sensing fiber. Due to the increased loss induced by optical fiber as the pulse travels farther, the pulse power becomes weaker, leading to the fading of the Rayleigh backscattered signal, and the vibration induced fluctuation in the Rayleigh signal will be submerged in the noise and difficult to be identified. Boosting the peak power of the probe pulse in conventional Φ-OTDR system to a high level is also not suggested for long-distance sensing for the sake of avoiding undesired nonlinear effects. However, these limitations can be overcome in the proposed sensing system. In our experiment, with the time-division-multiplexed configuration and segmented processing, probe pulses with proper peak power are sent to each sensing fiber segment and the Rayleigh backscattered signal is boosted by the amplifying module. The amplification parameters could be adjusted in accordance with the sensing requirements in different segments. Therefore, the Rayleigh backscattered signal in each segment maintains with almost the same order of magnitude, rendering high signal-to-noise ratio (SNR) signals in each fiber segment. Two collected Rayleigh backscattered traces along a 30km-long sensing fiber are compared as shown in Fig. 3. The bottom trace (blue) was detected in the conventional Φ-OTDR system and the upper trace (orange) in the proposed system by tailoring signals from the first 30km sensing fiber. Both signals are collected using probe pulses with a repetition rate of 3 kHz and pulse width of 100ns. It can be seen that the intensity of the Rayleigh backscattered signal decreases rapidly and fades close to zero at 20km in the blue trace. On the contrary, the Rayleigh backscattered signal detected by the time division multiplexing technique maintains high signal-to-noise ratios on all fiber segments after signal normalization.
Fig. 3. Measured Rayleigh backscattered traces using conventional system (blue) and the proposed system (orange).
In order to verify the effectiveness of the proposed technique in improving the frequency response for long distance sensing using Φ-OTDR system, the repetition rate of the injected probe pulse was set to 10 kHz in our experiment by considering that the length of each fiber segment is 10km long. Thus, the theoretical maximum measurable frequency of vibrations is 5 kHz for each fiber segment according to the Nyquist sampling theorem. Taking various environmental and systematic noises into account, the maximum vibration frequency that can be actually measured is lower than 5 kHz. In the tests using the proposed time-division-multiplexed system, vibrations with three different frequencies including 4.3 kHz, 4.0 kHz, and 4.5 kHz were generated by the three PZTs, respectively and applied onto the winded fiber for multiple vibration events detection. 1000 consecutive Rayleigh backscattered traces were collected and then processed using moving averaging and difference method to extract the vibration location information. Wave threshold denoising method was also adopted to improve the SNR of the Rayleigh signals. The extracted vibration location information is shown in Fig. 4, which clearly points out the positions of the three vibration events with high visibility. It is worth noting that the SNR of the three vibration events decreases slightly with the sensing distance, which is due to the increased loss of the probe pulse as it travels through the fiber. The SNR of the third vibration event did not decrease significantly compared with the second vibration event because the probe pulse was amplified by the EDFA inserted between the 3rd and the 4th fiber segments. According to the above results, as long as EDFAs are properly added into the system, the SNR of the detected vibration signal will not drop to the noise level and can be guaranteed for event identification. Theoretically, the sensing distance of the experimental system can be extended indefinitely. However, in practical application, the amplified spontaneous emission introduced by multiple EDFAs and nonlinear effects induced by high probe pulse power will limit the sensing performance.
Fig. 4. Extracted position information of the three vibration events located at 9.95 km, 29.95 km and 59.95 km using the proposed sensing system.
By transforming the acquired interference signal at the vibrating points in time domain into the frequency domain via fast Fourier transform (FFT), the recovered frequency information of all the three vibration events is shown in Fig. 5. The results show that the detected frequencies are clearly located at 4.3kHz, 4.0kHz, and 4.5kHz with SNRs well above 20 dB, agreeing well with the vibration frequencies of the three PZTs. This demonstrates that frequency response up to 4.5kHz can be achieved over a sensing distance of 60 km using the proposed sensing system, which however is impossible to be realized using a conventional Φ-OTDR system as the pulse repetition rate is strictly limited below 2kHz in this case. In other words, the bandwidth of the vibration frequency response of the Φ-OTDR system is increased by about 6 times with the time-division-multiplexed configuration.
Fig. 5. Retrieved frequency spectra for the detected three vibration events with frequencies of (a) 4.3 kHz, (b) 4.0 kHz, and (c) 4.5 kHz.
The previous experiments have demonstrated the successful detection of three vibration events with correct retrieved vibration locations and frequencies, indicating that the proposed sensing system is able to break through the tradeoff between the sensing length and the detectable vibration frequency with a vibration frequency response bandwidth up to 4.5 kHz over a 60-km-long sensing fiber. As mentioned in the earlier part, each sensing segment is independent and acts as a distributed sensing subsystem so that the vibration frequency response for each fiber segment is flexible, which is only subject to the segment length. Another proof-of-concept experiment is carried out to show the frequency response flexibility of the proposed system. In the test, the 6th segment was replaced with a 2km-long sensing fiber with PZT 3 placed at the end of this fiber segment, corresponding to an overall sensing distance of 51.8km. The vibration frequency of PZT 3 is increased to 20kHz. In order to monitor and locate this vibration event, the repetition rate of injected probe pulse was increased to 50kHz. Due to the high repetition rate of the probe pulse, the time interval between the two adjacent probe pulses is less than the maximum round trip time for the light wave travelling through the first five fiber segments. Their Rayleigh backscattered signals are mixed together and cannot be distinguished. But by controlling the optical switches, only the Rayleigh backscattered signal from the last 2km-long FUT can be received for detection. One EDFA was inserted into the delay fiber to amplify the Rayleigh backscattered signal to ensure that the signal has sufficient power to reach the detector. 1000 consecutive Φ-OTDR traces were collected with a total recording time of 0.02s. After applying moving averaging and difference method and wave threshold denoising method to process the traces, the vibration position and frequency information are obtained as shown in Fig. 6 and Fig. 7. The recovered position result shows a peak located at 51.8km with high visibility in Fig. 6. After performing FFT on the time-domain signals, one dominant peak is observed at 20 kHz in the frequency spectrum in Fig. 7, which is consistent with the applied frequency on PZT 3. The results prove that the proposed system has the greatest advantage of flexible frequency response, which is only limited by the local fiber segment length but free from the impacts from other fiber segments, providing abundant choices for constructing agile distributed sensing systems for practical applications.
5. Conclusion
In conclusion, we have proposed and experimentally demonstrated a long-distance phase-sensitive optical time domain reflectometry with flexible frequency response based on the time division multiplexing technique. Neither additional complex amplification mechanisms such as Raman or Brillouin amplification nor probe pulse modulations are added to the system to achieve high frequency vibration sensing over long distance, greatly reducing the complexity and the cost of the system. Taking advantage of time-division-multiplexed segmented sensing algorithm, the tradeoff between sensing distance and vibration frequency response is significantly alleviated. Furthermore, independent interrogation of vibration sensing in each fiber segment has been achieved. The proposed Φ-OTDR system is able to locate the vibration point in long-distance sensing fiber with high frequency response that surpasses the limit imposed on the conventional system. Specifically, distributed vibration sensing with frequency up to 4.5 kHz over a 60km-long sensing range with 10 m spatial resolution has been demonstrated in the proof-of-concept experiment. In addition, the frequency response flexibility of the proposed system is verified by a test detecting a vibration event with a frequency of 20 kHz located at 51.8 km. It is of great value to practical applications that the entire sensing fiber in Φ-OTDR system can be segmented by time division multiplexing technique, so that each fiber segment forms an independent sensing subsystem. Any existing technology can be incorporated into any segment to build up an Φ-OTDR subsystem with better sensing performance. Therefore, it is believed that the system will have great potential for long-distance distributed vibration sensing applications with flexible frequency responses.
Funding
Natural Science Foundation of Shandong Province (ZR2020MF110, ZR2020MF118); Shandong Provincial Key Research and Development Program (Major Scientific and Technological Innovation Project) (2020CXGC010204); China Scholarship Council (202006220139); Qilu Young Scholar Program of Shandong University.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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