High-fidelity distributed fiber-optic acoustic sensor ... - OSA Publishing

Vol. 26, No. 13 | 25 Jun 2018 | OPTICS EXPRESS 16138

High-fidelity distributed fiber-optic acoustic sensor with fading noise suppressed and sub-meter spatial resolution D IAN C HEN , Q INGWEN L IU , *

AND

Z UYUAN H E

State Key Laboratory of Advanced Optical Communication Systems and Networks, Department of Electronic Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China * [email protected]

Abstract: In order to solve fading problem and realize sub-meter spatial resolution in DAS, this paper proposes a novel configuration of time-gated digital optical frequency domain reflectometry (TGD-OFDR) based on optical intensity modulator (IM). IM has a large modulation bandwidth and the positive and negative harmonics can be fully used to suppress fading while the spatial resolution remains unchanged. In experiments, with fading suppressed, the spatial resolution of √ DAS is 0.8 m and the strain resolution is about 245.6 pε/ Hz along the total 9.8-km sensing fiber. The response bandwidth of vibration is 5 kHz, only limited by the fiber length. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement OCIS codes: (060.2370) Fiber optics sensors; (120.7280) Vibration analysis; (290.5870) Scattering, Rayleigh Fibers.

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#327631 Journal © 2018

https://doi.org/10.1364/OE.26.016138 Received 4 Apr 2018; revised 26 May 2018; accepted 30 May 2018; published 11 Jun 2018


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1.

Introduction

The phases of highly coherent light and its Rayleigh backscattering (RBS) traveling in optical fiber are sensitive to external perturbations occurring along the fiber. Distributed fiber-optic acoustic sensor (DAS) is developed from this feature, combined with reflectometry techniques. DAS is a very useful tool having been widely applied in many fields in recent years, such as intrusion detection [1], railway monitoring [2], pipeline surveillance [3], oil exploration [4], and structural health monitoring [5]. This is mainly owing to its long sensing length, the capability of high-density multiplex of acoustic sensors, the strong robustness against harsh environments [6], and so on. Most of DAS are based on phase-sensitive optical time domain reflectometry (ϕ-OTDR) and lots of important researches about ϕ-OTDR have been reported in the past few years [7–12]. In some applications, such as oil exploration and structural health monitoring [13], the spatial resolution of DAS is expected to be better than 1 m, and meanwhile the sensing length of a few kilometers is required, which is a challenge for ϕ-OTDR. According to the principle of ϕ-OTDR, the width of the optical pulse has to be shorter than 10 ns to achieve sub-meter spatial resolution. As a result, the sensing length and the strain resolution deteriorate severely. The response bandwidth of vibration decreases too, since temporal average method has to be adopted to improve the signal-to-noise ratio (SNR). Additionally, fading noise is a fatal problem in ϕ-OTDR. Fading includes interference fading [14] and polarization fading [15]. They both result in the drastic fluctuation of the intensity of RBS along the fiber. At the points where the RBS intensity is extremely weak, the phase of RBS is dominated by fading noise and can’t be used for sensing any more, since its phase is calculated from the intensity by I/Q demodulation [16]. In 2017, B. Lu et al. reported a DAS system with 30-cm spatial resolution and 19.8-km sensing length [17], but fading noise wasn’t discussed; In 2016, H. F. Martins et al. reported a DAS with 2.5-cm spatial resolution by using the backreflection of live PSK data, and polarization fading was solved by polarization diversity receiver (PDR) [18], but the sensing length is only 500 m in the experiment. In 2015, we proposed a novel DAS based on time-gated digital optical frequency domain reflectometry (TGD-OFDR) [19]. Both its sensing length and strain resolution are independent with the spatial resolution, since the probe is optical linear-frequency-modulated (LFM) pulse,


of which the frequency sweeping range decides the spatial resolution. In our previous reports, the spatial resolution is meter-scale, because the maximum effective frequency sweeping range is only 60 MHz, limited by the bandwidth of acousto-optic modulator (AOM) [20]. We have also proposed methods to solve the fading noise problem [21], which are very effective, but they degrade the spatial resolution, making it harder to realize sub-meter spatial resolution. The electro-optic modulators (EOM) have already been widely applied in optical fiber communication and sensing to modulate the lightwave with a large frequency bandwidth up to tens of GHz. They can also function as optical frequency shifters, providing a larger frequency bandwidth than AOM. However, unlike the AOM, the monochromatic lightwave becomes polychromatic after the EOM, and many unwanted harmonics arise together with the required one. In order to suppress the unwanted harmonics, optical bandpass filter, injection locking technique [17] and single-side band modulation are used. These techniques make systems more complicated and the potential of harmonics is wasted. In this paper, we introduce a novel DAS system based on TGD-OFDR configuration and LiNbO3 intensity modulator (IM). High spatial resolution is realized owing to the large modulation bandwidth of the IM, and harmonics induced by the IM are fully used to suppress fading noise by matched filter and rotated-vector-sum method, while the spatial resolution remains unchanged. In experiments, with fading noise suppressed well, the spatial resolution of DAS is 0.8 m, and the sensing length is about 9.8 km. The response bandwidth of vibration is 5 kHz, only limited by the length of the sensing fiber. Two vibration events are detected with high SNR by the phase √ traces of RBS. The strain resolution is about 245.6 pε/ Hz along the whole sensing fiber. Due to competitive performances of high strain resolution, high spatial resolution, linear response, broad bandwidth, and fading suppression, the proposed DAS system is capable of high-quality reproduction of the acoustic signal under detection, and thus it is named as high-fidelity DAS. 2.

System and principle

The system is shown in Fig. 1. The power and frequency of the laser are constant. The highly coherent light from the laser is split into two parts by a polarization-maintaining (PM) coupler. One part is sent to a 90◦ hybrid & PDR and acts as the local oscillator (LO). The other part is sent into an optical IM as the probe. In the 90◦ hybrid & PDR, the LO is divided into two

LFM Pulse Sequence AWG τp

Trigger & Reference Clock

Amp.

Tp Laser

Circulator

SG

C1

IM

Sensing Fiber

PM Coupler

C2 PZT1 PZT2

EDFA DC 90° Hybrid & PDR

BPD1 X-I BPD2 X-Q Y-I Osc. BPD3 Y-Q BPD4

Signal Process

Fig. 1. The experiment configuration. Red solid lines are polarization-maintaining (PM) fiber and black solid lines are normal single mode fiber. AWG: arbitrary waveform generator; Amp.: ratio-frequency amplifier; BPD: balanced photodetector; C: connector; DC: direct-current source; EDFA: erbium-doped fiber amplifier; IM: intensity modulator; LFM: linear frequency modulated; Osc.: oscilloscope; PZT: cylinder piezoelectric transducer; PDR: polarization diversity receiver; SG: signal generator.


beams with two orthogonal polarization states, and then each beam is divided again into two quadrature beams [22]. An arbitrary waveform generator (AWG) generates a LFM pulse sequence to drive the IM. The width of each LFM pulse is τp , and the period of the sequence is Tp . A ratio-frequency amplifier is used to change the modulation depth of the IM and a direct-current (DC) source is used to adjust the bias point of the IM. An erbium-doped fiber amplifier (EDFA) is implemented to boost the optical power of probe pulses. The sensing fiber is commercial single mode fiber without any special treatment. The RBS reflected by the sensing fiber enters the 90◦ hybrid & PDR and is divided into four beams too. These beams from the RBS beat with their corresponding beams from the LO and four beating signals are converted into four photocurrent signals by balanced photo-detectors (BPDs), marked as i X I (t), i XQ (t), iY I (t) and iYQ (t) respectively. A 4-channel oscilloscope is used to acquire the photocurrent signals and the data is processed by a personal computer off-line. The system parameters are listed in detail in Section 3. The IM is based on a Mach-Zehnder interferometer configuration. The optical phase change induced to either arm of the interferometer is expressed as π β πα + · cos(2π f0 t + πκt 2 ), t ∈ [0, τp ], (1) 2 2 where α is the modulation depth of the IM; β is the normalized bias point of the IM; f0 is the initial frequency, and κ is the sweeping rate. For shorthand, ϕ(t) is used to represent (2π f0 t + πκt 2 ) hereinafter. The electric fields of optical pulses entering the fiber [23] is s(t) =

E P (t) = exp j [ωc t + s(t)] + exp j [ωc t − s(t)] ,

t ∈ [0, τp ],

(2)

and the LO is E L (t) = exp j [ωc t] ,

(3)

where ωc is the center angular frequency of the laser. Unimportant scale factors are normalized. We assume there are vast independent scattering points randomly distributed along the sensing fiber [6]. The total RBS in X polarization state reflected from all scattering points is expressed as Õ EX (t) = ai cos(θ i )E p (t − τi ), (4) i

where i represents the i-th scattering point in the fiber model; τi is the round trip time of optical pulse traveling from the fiber’s incident end to the position of the i-th point; ai is the amplitude coefficient, determined by the reflectivity of the i-th point and the attenuation along the light path; θ i is the polarization angle. The RBS enters the 90◦ hybrid & PDR and the outputs from BPDs are i X I (t), i XQ (t), iY I (t), and iY Q (t), as described above. We combine two quadrature signals with the same polarization state into a complex signal as follows, ®i X (t) = i X I (t) + j · i XQ (t) = < EX (t) · E ∗ (t) + j · = EX (t) · E ∗ (t) L L Õ (5) = ai cos(θ i ) {exp j [−ωc τi + s(t − τi )] + exp j [−ωc τi − s(t − τi )]}, i

where ∗ is conjugate symbol;