Open Access
Add to BibSonomy
Abstract
A novel fiber-optic distributed acoustic sensor (DAS) utilizing a LiNbO3 straight through waveguide phase modulator as phase generation carrier (PGC) modulation module for the detection of acoustic signal is presented. The sensitive principle and the phase demodulation method of the system based on phase-sensitive optical time domain reflectometer (Φ-OTDR) are described. This scheme solves the problems of low modulation frequency and unstable performance of piezoelectric transducer (PZT) in the traditional homodyne detection system and depends only on the pulse repetition frequency. The efficacy of the new approach is demonstrated experimentally, showing that the weak acoustic signal can be demodulated accurately. The noise level of the system is < 4.2×10−3 rad/√Hz, the signal to noise ratio (SNR) is > 16 dB, and the spatial resolution is 10 m, as well as a detection frequency can theoretically achieve 25 kHz at 2 km sensing fiber. It provides a new research idea for DAS and is expected to replace PZT to achieve a high-frequency response, which has good potential in the applications of low cost, long distance and high frequency detection.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Fiber-optic distributed acoustic sensor (DAS) can detect acoustic signal along the whole sensing fiber, which has been widely applied in pipeline detection, seismic monitoring, border security and so on [1–4]. There are many types of DAS, including various optical fiber interferometers, fiber grating sensor, optical frequency domain reflectometry (OFDR), and optical time domain reflectometry (OTDR) [5,6]. Among them, phase-sensitive optical time domain reflectometry (Φ-OTDR) based on Rayleigh scattering has attracted considerable interest since it was firstly reported in 1993 [7]. Φ-OTDR has been studied for over two decades due to its unique advantages, which meets the requirements of real-time multi-point measurement, high sensitivity and strong environmental adaptability [8–11].
Generally, the photoelectric detection methods of the Φ-OTDR include direct detection and heterodyne detection. Up to now, the progress of studying them never stop, and a variety of methods of acoustic wave phase demodulation have been derived simultaneously. For heterodyne detection system, Φ-OTDR is realized through coherent heterodyne demodulation technique, which is limited by the polarization states controlling between the backscattered light and the reference light [12,13]. Besides, the introduction of polarization controller (PC), acousto-optic modulator (AOM), balanced photo-detector (BPD), and polarization-maintaining fiber (PMF), etc. has significantly increased the hardware cost and system complexity. For direct detection system, Φ-OTDR needs to combine with a Michelson interferometer (MI) at the receiving part of the system to implement the quantitative measurement of acoustic signal. 3×3 demodulation algorithm and PGC demodulation algorithm are used to restore the amplitude and frequency information of the acoustic signal. 3×3 demodulation scheme requires three high sensitivity photo-detectors with consistent performance and an acquisition device containing at least three channels. Besides, it must satisfy the polarization matching when the backscattered light interfering in the optical coupler [14,15]. Φ-OTDR can also extract the useful acoustic signal through the PGC demodulation algorithm by introducing a phase modulator to generate the PGC signal on one arm of MI [16,17]. The commonly used phase modulator includes PZT and LiNbO3 phase modulator. PZT is widely used in research because of its simple structure and principle. However, PZT element size is larger and has hysteresis characteristics during use. The manufacturing process of the wound fiber is complicated and the stability of the MI composed of PZT is not high, which is unfavorable for the modularization and integration of the system. Furthermore, as far as we know, the working frequency of PZT can reach several hundred kHz, but its performance is not stable. Generally, the frequency of the PZT often used is about dozens of kHz, which limits the maximum detectable frequency of the acoustic wave. In the published literature whose system uses the PZT, the maximum measurable frequency is only 1.5 kHz over 3 km sensing fiber [16,17]. Hence, using PZT to achieve high frequency response over long distance is not an ideal choice and how to improve the carrier frequency and detectable frequency is important for the traditional PZT-based homodyne PGC demodulation system in some specific application backgrounds.
In this paper, we demonstrate a novel PGC modulation structure based on LiNbO3 straight through waveguide phase modulator whose modulation bandwidth can generally reach several hundred MHz to several GHz. The system consists of Φ-OTDR and unbalanced MI. Compared with Refs. [13] and [18], the system has simpler structure and lower cost because it does not need two AOMs or acoustic frequency shifter (AFS) or hybrid etc. and needs only a common photo-detector (PD) with high sensitivity and a low sampling rate data acquisition (DAQ) card. Compared with Refs. [16] and [17], the difference is that a PGC signal is introduced by LiNbO3 phase modulator whose modulation frequency is much higher than the pulse repetition frequency to apply to one arm of MI. It makes the detectable frequency of our system is unrestricted by modulation frequency, but depends on the pulse repetition frequency. Therefore, the system can achieve a higher frequency response than that in Refs. [16] and [17]. In addition, because the system is aimed at the traditional homodyne demodulation system without other technologies, specifically the PZT-based PGC modulation scheme, the system does not introduce the frequency division multiplexing (FDM) technology or merge with Mach-Zehnder interferometer (MZI) etc. compared with Refs. [19–21]. Moreover, the unbalanced MI structure in our scheme is all polarization-maintaining so that the system can eliminate the phase fading and have a more stable phase sensitivity.
The remainder of the paper is organized as follows: Section 2 describes the principles of the Φ-OTDR utilizing LiNbO3 straight through waveguide phase modulator and PGC demodulation algorithm. Section 3 presents the experimental results and discussion. Ultimately, conclusions and prospects are given in Section 4.
2. Principle
The working principle of the Φ-OTDR system with an unbalanced polarization-maintaining MI is shown in Fig. 1. Generally, a narrow-linewidth and highly coherent continuous-wave (CW) laser source is modulated to light pulses [22,23]. While input light pulses propagate along the sensing fiber, Rayleigh backscattering light will self-interfere and propagate backwards to the unbalanced MI with the arm length difference d introduced at the receiving end. The spatial resolution is determined by the pulse width Tp and d simultaneously. Coherent Rayleigh backscattering light signal carrying all sensing information is detected by a PD, and is sampled by a DAQ card, finally is processed by a computer. Rayleigh backscattering light is caused by the scattering centers from the sensing fiber. Assuming that every scattering unit has i scattering centers, the electric field of the scattering centers at the position Li = iΔL (ΔL is the distance from each of the scattering centers) can be derived as [16,17]:
Fig. 1. Working principle of the Φ-OTDR with an unbalanced MI. PM, phase modulator; FRM, faraday rotator mirror.
When EL(t) enters into the unbalanced MI, there is a delay between two paths because of the optical path difference d. The electric field of the interference light from the position L can be expressed as:
where A and B are the coefficients after simplified respectively, Δφ(t) = φL-d (t) - φL (t). The output interference light intensity from MI is:The phase of interference light will be added a phase modulation term mΔdcos(w0t) when a PGC signal whose frequency is w0 modulates one arm of MI. Where m is the modulation coefficient and Δd is the length difference between the two arms of MI. Supposing that the modulation depth C = mΔd, the Eq. (4) can be rewritten as:
In the system, the phase modulator (PM) we used is the LiNbO3 straight waveguide phase modulator whose structure is shown in Fig. 2, different from the traditional PGC system based on PZT. LiNbO3 straight waveguide, as the key component in the novel modulation module, is made by depositing Aluminium as electrodes on the Ti-diffused LiNbO3 planar optical waveguide. It has the advantages of small response delay, low driving voltage, low back reflection, high modulation bandwidth and easy compatibility with polarization-maintaining fiber. Furthermore, its modulation bandwidth can generally reach hundreds of MHz to several GHz. As shown in Fig. 2, the incident light travels along the z-axis and the electric field is applied along the y-axis. When the crystal is subjected to the modulation voltage, the electro-optical effect will occur to change the refractive index of the waveguide. The modulation voltage will directly affect the phase of the light passing through the electrode region, so as to realize the phase modulation. We know that straight waveguide can transmit light of both transverse electric (TE) and transverse magnetic (TM) modes. According to the transverse electro-optical effect, the phase difference generated by the two orthogonal rays along the x’ and y’ directions can be written as [24]:
According to Eq. (6), the half-wave voltage of LiNbO3 straight waveguide under transverse modulation can be obtained as follows:
Based on Eqs. (6) and (7), the phase delay of LiNbO3 straight waveguide is:
PGC demodulation algorithm has been proposed to recover the acoustic information in the interference signal of the homodyne demodulation scheme [16,17,25]. In our system, PGC-Arctangent demodulation algorithm is applied, which is shown in Fig. 3. The w0 in Eq. (5) is equal to the modulation frequency of the straight waveguide and the modulation depth C depends on the amplitude of modulation voltage. The interference signal I(t) passes through direct current (DC) filter, the multiplier, and is filtered by a low pass filter (LPF). Then, the two signals obtained can be respectively expressed as:
where J1(C) and J2(C) are the 1st order and the 2nd order Bessel function of the 1st kind, respectively. To meet J1(C) = J2(C), the value of C should be adjusted to 2.63 rad, which is the optimal phase modulation depth [16]. After an arctangent operation and a high pass filter (HPF), the phase change caused by the acoustic signal can be demodulated as follows: where D and ws are the amplitude and frequency of the acoustic signal, respectively.3. Experiments and discussion
Based on the principle analysis in Section 2, we firstly establish the experiment setup to confirm the feasibility of LiNbO3 straight waveguide phase modulator, which is described in Fig. 4. The CW light from narrow linewidth laser (NLL) passes through the MI, and interference optical signal returns to PD. In the experiment, the power of NLL is about 10 mW and a sinusoidal modulation signal with frequency of 200 kHz is applied to the straight waveguide. The interference signal is acquired and processed in personal computer (PC), which is shown in Fig. 5 (a). In addition, Fig. 5 (b) is the spectrum after fast Fourier transform (FFT).
The results of Fig. 5 present that the FFT spectrum of the interference light contains the baseband and multi-octave items of the modulation frequency. Experimental results show that the PGC signal can be added to the interference signal and demodulated through the demodulation algorithm. It demonstrates the feasibility of generating the PGC signal by LiNbO3 straight waveguide phase modulator and the applicability of the PGC demodulation algorithm to restore the phase information of the interference light.
Then, we build an experimental system including Φ-OTDR and the unbalanced polarization-maintaining MI with LiNbO3 straight waveguide, as shown in Fig. 6. In the experiment, a narrow line-width (3kHz) laser light source with 10 mW output emitting at 1550.92 nm is used. The reflected light can be isolated by an optical ISO. The high extinction ratio (ER) SOA (>50dB) not only increases the SNR, but also reduces the intra-band coherent noise [9,26]. The pulse width is set to 100 ns corresponding to the highest spatial resolution of 10 m. The pulse repetition frequency is 50 kHz. Er-doped fiber amplifier (EDFA) is employed to amplify the light pulses. The fiber under test (FUT) is the standard single mode fiber (SMF) of 2 km. The unbalanced MI is a fully polarization-maintaining structure to realize the modulation of the interference signal. It consists of a 2×2 optical coupler, a LiNbO3 straight waveguide, and two FRMs of consistent performance. The path difference of MI is 10 m. The half-wave voltage of the straight waveguide in the experiment is 3.75 V. To realize C = 2.63 rad, the electrode voltage V obtained based on Eq. (8) and experimental test is about 4 V. Hence, the modulation voltage and frequency applied to the straight waveguide are 4 V and 200 kHz, respectively. After photoelectric conversion by a PD, the interference signal is transmitted to a DAQ card with a sampling rate of 10 MHz for data processing.
Fig. 6. Experimental system based on Φ-OTDR and LiNbO3 straight waveguide. SOA, semiconductor optical amplifier; FBG, fiber Bragg grating.
In our experiment, a PZT as a vibration source is placed at the 1 km position over 2 km sensing fiber. A sinusoidal signal with magnitude of 2.5 V and frequency at 2 kHz is applied to the PZT to simulate the generation of acoustic event. The demodulated results of the acoustic signal is shown in Fig. 7. Figure 7 (a) is the fitted curve and its amplitude is related to the optical phase modulation coefficient of PZT. In Fig. 7 (b), to compare the results under different frequencies, the spectrum is normalized and then calculated through the logarithm operation. We can see clearly that the system can accurately demodulate the phase information and obtain the amplitude and frequency of the acoustic signal.
Fig. 7. The demodulation result on the original PZT source of 2 kHz with 2.5 V. (a) time domain result; (b) frequency domain result.
In order to further verify the system’s ability to detect the dynamic acoustic events, we set the frequency of the sinusoidal signal applied to the PZT to 5 kHz and 10 kHz, respectively, and set the same magnitude to 2.5 V. From Fig. 8, we can see that the similar peaks also exist at 5 kHz and 10 kHz, respectively. Furthermore, the SNRs of frequency domain results are 20.79 dB, 17.71 dB and 16.23 dB and the noise levels are 3.9×10−3 rad/√Hz, 4.0×10−3 rad/√Hz and 4.2×10−3 rad/√Hz corresponding to the signals of 2 kHz, 5 kHz and 10 kHz, respectively. Here the SNR = 20 lg(Asignal/Anoise), where Asignal and Anoise respectively represent the amplitude (rad) of the signal and the background noise. The noise level NL is the environment noise level at a certain frequency point f. NL = Anoise /√f, which can be calculated according to the spectrum. Due to the non-uniform frequency response of the PZT, the SNR of demodulated signals at different frequencies has slight deviation.
In summary, Fig. 7 and Fig. 8 have proved the capability of the system of detecting the acoustic signal by demodulating the phase information of the measured signals. The amplitude and the frequency of the applied signal can be demodulated accurately and the experimental results are consistent with the applied signals.
Through the above experimental results, the feasibility of the system has been verified. The phase, amplitude and frequency of the acoustic signal can be well demodulated simultaneously. Previously, Refs. [16] and [17] have demonstrated fiber-optic DAS based on Φ-OTDR using the unbalanced MI to detect of Rayleigh backscattering light. It is similar to the basic principle of our system. The difference is that we use a novel PGC modulation module in the receiving part of the system. The modulation frequency of the LiNbO3 straight waveguide phase modulator is much larger than that of PZT. In Refs. [16] and [17], the maximum measurable frequency of the homodyne detection system based on the PZT is 1.5 kHz. However, according to the Nyquist sampling theorem, the maximum detectable frequency of our system is equal to half of the pulse repetition frequency theoretically. Therefore, we have reasons to believe that the novel PGC modulation structure with straight waveguide can be used in DAS system and help to achieve the high-frequency response. Moreover, compared with heterodyne detection scheme [12,13,18,22], the system not only has simpler structure and lower cost, but also significantly reduces the data volume and computational complexity. At present, the SNRs of the proposed system at different frequencies are not high enough due to the inherent loss of the LiNbO3 straight waveguide. In the future, an optical amplifier can be used to achieve optical amplification. Next, we will also try to add experimental conditions and data, and make further research on each indicator to improve the performance of system.
4. Conclusion
In this paper, a novel PGC modulation structure based on LiNbO3 straight waveguide phase modulator for fiber-optic DAS is proposed. Experimental results prove that the scheme is feasible and is expected to replace PZT to help the DAS system to achieve a high-frequency response. The system overcomes the bottleneck of the traditional homodyne demodulation system based on PZT and the maximum measurable frequency is limited only by the pulse repetition frequency. Theoretically, the detection frequency can achieve 25 kHz at 2 km sensing fiber. In the experiment, the background noise is < 4.2×10−3 rad/√Hz, the SNR is > 16 dB and the spatial resolution is 10 m. In addition, due to the fully polarization-maintaining structure and two FRMs used in the receiving part of the system, it can suppress the effect of polarization fading well. In future work, we will focus on improving the SNR and the dynamic range of the frequency response, and try to adopt LiNbO3 phase modulator to achieve MHz frequency response of Φ-OTDR. Further experiments will be done to study and improve the performance indicators in practical application, such as oil and gas exploration, health monitoring of structures, pipeline detection, and seismic monitoring.
Funding
National Key Research and Development Program of China (2016YFF0102400).
Acknowledgments
The authors would like to thank the Institute of Opto-electronic Technique in Beihang 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.
References
1. S. Liehr, S. Münzenberger, and K. Krebber, “Wavelength-scanning coherent OTDR for dynamic high strain resolution sensing,” Opt. Express 26(8), 10573–10588 (2018). [CrossRef]
2. F. Peng, H. Wu, X. Jia, Y. Rao, Z. Wang, and Z. Peng, “Ultra-long high-sensitivity Φ-OTDR for high spatial resolution intrusion detection of pipelines,” Opt. Express 22(11), 13804–13810 (2014). [CrossRef]
3. G. Marra, C. Clivati, R. Luckett, A. Tampellini, J. Kronjäger, L. Wright, A. Mura, F. Levi, S. Robinson, A. Xuereb, B. Baptie, and D. Calonico, “Ultrastable laser interferometry for earthquake detection with terrestrial and submarine cables,” Science 361(6401), eaat4458 (2018). [CrossRef]
4. T. Chang, Z. Wang, Y. Yang, Y. Zhang, Z. Zheng, L. Cheng, and H. Cui, “Fiber optic interferometric seismometer with phase feedback control,” Opt. Express 28(5), 6102–6122 (2020). [CrossRef]
5. T. Liu, F. Wang, X. Zhang, Q. Yuan, J. Niu, L. Zhang, and T. Wei, “Interrogation of ultra-weak FBG array using double-pulse and heterodyne detection,” IEEE Photonics Technol. Lett. 30(8), 677–680 (2018). [CrossRef]
6. X. Bao, D. Zhou, C. Baker, and L. Chen, “Recent development in the distributed fiber optic acoustic and ultrasonic detection,” J. Lightwave Technol. 35(16), 3256–3267 (2017). [CrossRef]
7. H. F. Taylor and C. E. Lee, “Apparatus and method for fiber optic intrusion sensing,” U.S. Patent, 5194847, (1993).
8. B. Redding, M. J. Murray, A. Davis, and C. Kirkendall, “Quantitative amplitude measuring Φ-OTDR using multiple uncorrelated Rayleigh backscattering realizations,” Opt. Express 27(24), 34952–34960 (2019). [CrossRef]
9. F. Ma, N. Song, X. Wang, H. Ma, X. Wei, and J. Yu, “Study on the generation and influence factors of the new wavelength components in backscattering light of Φ-OTDR,” Opt. Commun. 474, 126106 (2020). [CrossRef]
10. Y. Wang, P. Lu, S. Mihailov, L. Chen, and X. Bao, “Distributed time delay sensing in random fiber grating array based on chirped pulse Φ-OTDR,” Opt. Lett. 45(13), 3423–3426 (2020). [CrossRef]
11. F. Ma, X. Wang, X. Liu, J. Yu, T. Wang, and W. Zhang, “Application of Segmentation Threshold Method and Wavelet Threshold Denoising Based on EMD in Φ-OTDR System,” Proc. SPIE 10964, 141 (2018). [CrossRef]
12. Z. Qin, T. Zhu, L. Chen, and X. Bao, “High sensitivity distributed vibration sensor based on polarization-maintaining configurations of phase-OTDR,” IEEE Photonics Technol. Lett. 23(15), 1091–1093 (2011). [CrossRef]
13. X. He, S. Xie, F. Liu, S. Cao, L. Gu, X. Zheng, and M. Zhang, “Multi-event waveform-retrieved distributed optical fiber acoustic sensor using dual-pulse heterodyne phase-sensitive OTDR,” Opt. Lett. 42(3), 442–445 (2017). [CrossRef]
14. A. Masoudi, M. Belal, and T. P. Newson, “A distributed optical fibre dynamic strain sensor based on phase-OTDR,” Meas. Sci. Technol. 24(8), 085204 (2013). [CrossRef]
15. A. Masoudi and T. P. Newson, “High spatial resolution distributed optical fibre dynamic strain sensor with enhanced frequency and strain resolution,” Opt. Lett. 42(2), 290–293 (2017). [CrossRef]
16. G. Fang, T. Xu, S. Feng, and F. Li, “Phase-sensitive Optical Time Domain Reflectometer Based on Phase Generated Carrier Algorithm,” J. Lightwave Technol. 33(13), 2811–2816 (2015). [CrossRef]
17. Z. Yu, Q. Zhang, M. Zhang, H. Dai, J. Zhang, L. Lin, L. Zhang, X. Jin, G. Wang, and G. Qi, “Distributed optical fiber vibration sensing using phase-generated carrier demodulation algorithm,” Appl. Phys. B 124(5), 84 (2018). [CrossRef]
18. Z. Wang, L. Zhang, S. Wang, N. Xue, F. Peng, M. Fan, W. Sun, X. Qian, J. Rao, and Y. Rao, “Coherent Φ-OTDR based on I/Q demodulation and homodyne detection,” Opt. Express 24(2), 853–858 (2016). [CrossRef]
19. D. Iida, K. Toge, and T. Manabe, “High-frequency distributed acoustic sensing faster than repetition limit with frequency-multiplexed phase-OTDR,” in Optical Fiber Communication Conference, OSA Technical Digest Series (Optical Society of America, 2016), paper M2D.6.
20. G. Yang, X. Fan, Q. Liu, and Z. He, “Frequency response enhancement of direct-detection phase-sensitive OTDR by using frequency division multiplexing,” J. Lightwave Technol. 36(4), 1197–1203 (2018). [CrossRef]
21. Q. He, T. Zhu, X. Xiao, and B. Zhang, “All fiber distributed vibration sensing using modulated time-difference pulses,” IEEE Photonics Technol. Lett. 25(20), 1955–1957 (2013). [CrossRef]
22. X. He, X. Xu, M. Zhang, S. Xie, F. Liu, L. Gu, Y. Zhang, Y. Yang, and H. Lu, “On the phase fading effect in the dual-pulse heterodyne demodulated distributed acoustic sensing system,” Opt. Express 28(22), 33433–33447 (2020). [CrossRef]
23. X. Lu, M. A. Soto, L. Zhang, and L. Thévenaz, “Spectral Properties of the Signal in Phase-Sensitive Optical time domain Reflectometry With Direct Detection,” J. Lightwave Technol. 38(6), 1513–1521 (2020). [CrossRef]
24. J. Zhao, Advanced optics, (National Defence Indestry Press, 2002), Chap. 10.
25. A. Dandridge, A. B. Tveten, and T. G. Giallorenzi, “Homodyne demodulation scheme for fiber optic sensors using phase generated carrier,” IEEE J. Quantum Electron. 18(10), 1647–1653 (1982). [CrossRef]
26. H. F. Martins, S. Martín-López, P. Corredera, M. L. Filograno, O. Frazão, and M. Gonzalez-Herráez, “Phase-sensitive optical time domain reflectometer assisted by first-order Raman amplification for distributed vibration sensing over >100 km,” J. Lightwave Technol. 32(8), 1510–1518 (2014). [CrossRef]
