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We demonstrate a novel distributed acoustic sensing (DAS) system based on phase-sensitive optical time-domain reflectometry (Φ-OTDR). Both the phase and the amplitude of the Rayleigh scattering (RS) light can be demodulated in real-time. The technique is based on I/Q demodulation and homodyne detection using a 90° optical hybrid. The theoretical analysis is given, and as a proof of the concept, the dynamic strain sensing is experimentally demonstrated, with a sensing range of 12.566 km and a spatial resolution of 10 m.
© 2016 Optical Society of America
1. Introduction
The distributed optical fiber sensing (DOFS) systems have been widely used in industries involved with security surveillance and structural health monitoring, owing to its large-scale monitoring range, low cost per monitored point, simple installation and geometric versatility [1]. It has been extensively studied in terms of the monitoring range, sensing parameter, spatial resolution and so on [2–7 ].
As one of the typical DOFS systems, phase sensitive optical time-domain reflectometer (Φ-OTDR) has been demonstrated as a promising technique, because of its ability to monitoring dynamic process and its high sensitivity [8]. Recently, distributed acoustic sensing (DAS) technology based on Φ-OTDR has attracted much attention due to its ability to measure the phase variation of the Rayleigh scattering (RS) signal and its promising industrial applications [9–12 ]. The reported demodulation schemes for DAS include 3 × 3 coupler demodulation [9,10 ], phase generated carrier algorithm [11], and digital coherent detection [12]. In [9–11 ], an interferometer structure is needed to retrieve the phase variation along the fiber. The interferometer structure is sensitive to the environment hence the performances of the systems may be influenced. In [12], the researchers use digital coherent detection. There is no interferometer structure in this scheme, and the phase of the signal is demodulated using software calculation, however, it requires high-speed analog to digital converter (ADC) and the large amount of data makes the data analysis quite challenging.
In the paper, an efficient phase demodulation scheme is demonstrated for the Φ-OTDR system using I/Q demodulation and homodyne detection. With the 90° hybrid, both the in-phase (I) and the quadrature (Q) components of the backscattering light field can be detected, then the phase information of the light signal can be demodulated without an interferometer structure. Moreover, the electrical signal is in baseband since the frequencies of the signal and the local-oscillator (LO) are the same, therefore only low-speed ADC is sufficient. Dynamic strain sensing is experimentally demonstrated with a sensing range of ~12.6 km and a spatial resolution of 10 m, as a proof of concept.
2. Principle
Figure 1 shows the general block diagram of I/Q demodulation used in our scheme. In the DAS system based on Φ-OTDR, the detected signal is the coherent Rayleigh scattering (RS) of the probe pulse. Assuming that the RS light can be described as
where , ,is the scalar amplitude, the angular frequency and initial phase of the signal, respectively. is the phase change induced by the refraction index variation along the fiber. The signal is split into two beams by the hybrid, and the accurate 90° phase shift is induced in one beam of the signal [13] and the corresponding light field changes into
Then the two signal beams are respectively superposed with the LO. The LO can be described as
where , , is the scalar amplitude, the angular frequency and initial phase of LO, respectively. Each of the two outputs of the hybrid is then detected by a bandwidth-limited photodetector, and the photo-currents should be [14]:In the equation, the coefficient 1/2 comes from the splitting of the signal and R is the responsivity of the two detectors (providing the responsivities of the two detectors are the same), I/Q mean the in-phase and the quadrature components of the electrical signal, respectively. The AC components of the photo-currents are:In our experiment, the signal and the LO come from the same laser, and no frequency different is present, so . is the difference between the initial phases of the LO and the signal, and we assume that it is a constant. Then we have:Then the amplitude and the phase of the RS light can be obtained:Here, the value of the arctan function is from to. By inducing the phase unwrapping algorithm [15], the range of the demodulated phase becomes from negative infinite to positive infinite. Then the phase and amplitude of the RS along the fiber can be demodulated. By subtracting phases of two certain positions, the phase change of the corresponding part of the fiber can be demodulated aswhich is linearly associated with the strain induced by the acoustic field [16].3. Experimental setup
The experimental scheme is depicted in Fig. 2 . An ultra-narrow-linewidth (100 Hz) laser operating at 1550.1 nm is used as the light source with polarization-maintaining (PM) output. The laser output is split into two branches by a 1:99 PM coupler. The 99% branch is modulated by an acoustic-optic modulator (AOM) with 80 MHz frequency shift to generate the pulsed probe wave (with 100 ns pulse width, allowing for 10 m spatial resolution). The repetition rate of the probe pulse is 5 kHz. The probe pulse is injected into the sensing fiber through a circulator and the RS signal is injected into the 90° hybrid. The 1% branch of the laser output is used as the LO. An acoustic frequency shifter (AFS) is used in order to introduce 80 MHz frequency shift into LO, to make sure the signal and the LO are with the same frequency. Before the LO is injected into the hybrid, a polarization controller (PC) is inserted in order to match the LO with the selected polarization branch of the hybrid. The two outputs of the hybrid are converted into electrical signals by a two ports AC coupled detector and then sampled by a 25 MS/s ADC. The process of phase unwrapping algorithm and signal reconstruction are completed in real-time with a personal computer. The sensing fiber is 12.56 km long. As represented in Fig. 2, at the position of ~12.4 km, 10 m bare fiber is coiled over a cylindrical PZT, used as the test point. A two-channel arbitrary waveform generator (AWG) is used in our experiment. One channel of the AWG is used to provide RF signal to the AOM driver and the trigger of the ADC, and the other one is used to drive the PZT.
It should be noted that a dual polarizations 90° hybrid is used in the system. In optical communication systems, the coherent receiver with a dual polarization 90° hybrid has been proved to be an effective method to measure the amplitude and phase of optical signals with arbitrary polarization [17]. Therefore, essentially this scheme is able to cope with the polarization fading phenomenon in OTDR systems, if all the outputs of the hybrid are used. The polarization dependent attributes of the approach are the topic of a further study. In this work, only one polarization channel is used since we only focus on the demonstration of the phase recovery.
4. Results and discussions
From Eq. (8) one can see that the interval between Z1 and Z2 can actually limit the spatial resolution. Typically, the interval is set to a length no longer than the spatial resolution limited by the pulse width of the probe light. In our experiment, the pulse width is 100 ns and it allows the highest spatial resolution of 10 m. Besides, the sampling rate of data acquisition card is 25 MS/s, which means the spatial interval between every two adjacent data points is 4 m. To match the 10 m spatial resolution we deliberately choose an interval of 8 m, using the #3100 data point and the #3102 data point to demodulate the phase difference.
Figure 3 shows the trace of the RS light along the fiber without PZT perturbation. The trace shows the speckle pattern as in conventional Φ-OTDR. To test the ability of strain change detection of the system, sinusoidal signals are applied on the PZT with different voltage magnitudes or different frequencies. By applying the phase demodulation approach proposed in Section 2, the phase difference at the relevant part of the fiber can be demodulated. The results are filtered by a low pass filter with a bandwidth of 2.5 kHz.
First, the sinusoidal signal that we apply on the PZT has a magnitude of 2 V and a frequency of 500 Hz. The results are showed in Fig. 4 . The dotted blue line in Fig. 4(a) represents the experimental data, while the red solid line represents the sinusoidal fitting curve. The demodulation phase differences present a clear sinusoidal variation over time. The amplitude of the fitting curve is about 2.51 rad. The pink dash line in Fig. 4(b) shows the FFT of the demodulated signals. Clear peak can be seen at 500 Hz and the calculated SNR is about 34.1 dB (SNR is calculated as 20*log(As/A n),where As is the amplitude of the signal, An is the root mean square of the background noise [11]).
Then we change the magnitude of the sinusoidal signal applied on the PZT, in order to test the relationship between the magnitude of the applied voltage on the PZT and the demodulated phase change (at 500 Hz). The magnitude of the applied voltage is set from 1 V to 2.4 V with a step of 0.2 V. The experimental results are shown in Fig. 5 . The blue dots are the amplitudes of the sinusoidal fitting curve of the phase difference traces and the red solid line is the linear fitting line of the amplitudes. The R-square of this fitting is 0.974, which shows a reasonable linear response of the system.
To further demonstrate the ability of the system to sense the dynamic events, we change the frequency of the sinusoidal signal applied on the PZT as 400 Hz and 600 Hz, respectively, and set the magnitude to 2 V. As it can be seen clearly in Fig. 6 , similar peaks stand at 400Hz and 600 Hz, respectively. The blue solid line represents experimental result with frequency of 400 Hz, and green dotted line represents experimental result with frequency of 600 Hz. Accurate frequency responses of the system is verified. Moreover, the SNRs in frequency domain are 42.0 dB, 34.1 dB and 34.8 dB corresponding to signals of 400 Hz, 500 Hz and 600 Hz, respectively. The differences between the SNRs are considered to be mainly induced by the non-uniform frequency response of the PZT.
Figure 4-6 illustrate the capability of the system of detecting the acoustic event by measuring the phase and amplitude of the RS signals in real-time. The amplitude and the frequency of the applied signal can be measured correctly and the results fit the applied signals well.
There are several advancements in this scheme. First, the data processing load is reduced. From the principle shown in Section 2 one can see that, the I/Q demodulation is mainly realized in optical domain rather than in electrical domain. Consequently, calculating amount in the scheme can be relatively small compared with the digital coherent detection in [12]. In addition, compared with the heterodyne coherent detection [11, 12 ], the signals in the system with our scheme are located in baseband, which allows the use of an ADC with relatively low sampling rate, hence the data amount is significantly reduced. Second, the mature telecom technique can assure the hybrid to induce accurate 90° phase shift and it is much less sensitive to environmental vibration and temperature drift compared with the interferometer structure. Moreover, the demodulation proposed in this paper also benefits from the amplification from the LO. The LO can improve the sensitivity of the system, as a result, no pre-amplifier is needed in the experimental system and a sensing range of more than 10 km is achieved.
6. Conclusion
In this paper, we propose a DAS system by introducing an efficient phase demodulation scheme in Φ-OTDR. Both the amplitude and the phase of the RS light are recovered by I/Q demodulation and homodyne detection utilizing a 90° hybrid. The dynamic phase difference induced by the PZT is detected. A DAS system with a sensing range of 12.56 km and a spatial resolution of 10 m is demonstrated.
Acknowledgments
This work is supported by Natural Science Foundation of China (61205048, 61290312, 41527805), Research Fund for the Doctoral Program of Higher Education of China (20120185120003), the PCSIRT project (IRT1218), and the 111 project (B14039).
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