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Frequency-comb-based Brillouin optical time-domain analysis (BOTDA) sensors were developed to achieve acquisition-time reduction and high-spatial-resolution/long-distance sensing simultaneously. We found that, for the standard frequency-comb-based BOTDA, the use of a double-sideband (DSB) pulse generates a series of pulse pairs that simultaneously propagate along the sensing fiber, leading to a nonlinear interaction between the two sidebands of each frequency comb pulse, and a significant splitting of the Brillouin gain spectrum (BGS). This problem prevents its application in high-spatial-resolution sensing due to the higher pulse power requirement. Thus, one of the sidebands of DSB pulse was proposed for greatly suppressing the BGS distortion. In combination with the phonon pre-excitation technique based on phase-shifted pulse, a sensor with a spatial-resolution approximately 60 cm along a fiber approximately 592 m in length was demonstrated. Furthermore, we explored the detailed performance of long-distance sensing by frequency- comb-based BOTDA. The use of a frequency comb for the probe wave can suppress the pulse distortion and non-local effect, which is helpful for extending the sensing distance. A spatial resolution of approximately 6 m along a sensing fiber approximately 74.2 km in length was successfully demonstrated.
© 2017 Optical Society of America
1. Introduction
Brillouin optical time-domain analysis (BOTDA) has been found to be suitable for many applications such as fire alarms, oil pipeline leaking, and power establishment security monitoring due to its high response sensitivity in measuring the temperature/strain over very long fibers. One of the challenges faced by conventional BOTDA is the time-consuming data acquisition resulting from the step-by-step frequency sweeping. To solve this problem, two types of frequency-comb-based BOTDA have been proposed.
The first involves launching many pulse probe frequency comb pairs into the fiber under test (FUT) and performing simultaneous tone interrogations [1–4]. As the space between adjacent frequency comb pairs for the pulse and probe lights are different, the equivalent pulse probe frequency difference varies for different frequency components, thus enabling the reconstruction of the Brillouin gain spectrum (BGS). In order to decrease the pressure of the Erbium-doped fiber amplifier (EDFA) due to multiple pulse amplification, a time shift for various frequency comb pulses has been proposed [2]. An additional advantage of using time-shifted pulses is that the nonlinear interaction, including cross-phase modulation (XPM), four-wave mixing (FWM), and stimulated Raman scattering (SRS) among different frequency comb pairs are thoroughly avoided, due to the sequential propagation of pulses along the FUT. In addition, distributed sensing with a spatial resolution of a few meters using pulse probe frequency comb pairs has been demonstrated [2].
In another setup, the frequency comb is introduced only for the probe light. Each comb can be demodulated based on the coherent beating between the optical source and received probe wave. The proof-of-concept experiment for <1 km sensing has been demonstrated [5]. However, the feasibility and detailed performance analysis of this scheme for long-distance sensing has not yet been reported.
In this paper, we attempt developing a frequency-comb-based BOTDA for high-spatial-resolution/long-distance sensing. It is well known that for conventional BOTDA, the acoustic lifetime (~10 ns) determines the limit of achievable spatial resolution (~1 m) due to the convolution between the pulse spectrum and intrinsic BGS. To obtain high-spatial-resolution sensing, various phonon pre-excitation methods, such as differential pulse pair (DPP) [6, 7], phase-shifted pulse (PSP) [8, 9], dark pulse [10], bright pulse [11], etc., have been proposed. In a recent and preliminary study, to simultaneously obtain high-spatial-resolution sensing and acquisition time reduction, the authors proposed a scheme that combines the phonon pre-excitation with the frequency-comb-based BOTDA [12]. In section 2, we provide a more detailed analysis and design for this technique. In particular, the residual nonlinear interaction for the double-sideband (DSB) pulse components and its impact for sensing are explored for the first time. One of the sidebands of DSB pulse is proposed to suppress the BGS distortion due to such nonlinearity.
In section 3, the performance of the frequency-comb-based BOTDA for long-distance sensing is presented. It is found that the non-local effect [13–19] that exists in conventional BOTDA can be significantly reduced using a frequency comb probe and single-frequency pulse. Considering that the amplified pulse due to the distributed Brillouin amplification (DBA) [20–23] (the Brillouin loss spectrum is used) is almost undistorted, the sensing distance of approximately 74.2 km is demonstrated.
2. Frequency-comb-based BOTDA for high-spatial-resolution sensing
2.1 Principle and experimental setup
The schematic diagram of the BOTDA using pulse probe frequency comb pairs is shown in Fig. 1(a). Multiple frequency comb components for both the pulse and probe waves are injected into the FUT simultaneously. The adjacent frequency comb intervals for the pulse and probe waves are denoted by Δνpulse and Δνprobe, respectively. In experiment, Δνpulse = 220MHz, Δνprobe = 200MHz. Although the DSB pulse was generated by an electro-optic modulator (EOM) (see Fig. 2), only one of the sidebands (solid lines of pulse combs in Fig. 1(a)) were used for the interaction of stimulated Brillouin scattering (SBS). This is different to the reported structure of the frequency-comb-based BOTDA [1–4]. The coherent detection based on heterodyne method should be used for simultaneously demodulating each frequency comb for the received probe wave by the beating between the reserved carrier and probe light. Due to the available acquisition bandwidth (~1.5 GHz) in our experiment, the maximal number of probe frequency comb components is limited. For the selected Δνprobe and number of combs (N = 6), since the first frequency is downshifted to 150 MHz after the mixer2 (see Fig. 2), the total frequency range for all comb components after detection is 150 + 5×200 = 1150 MHz, which is close to the available acquisition bandwidth. In this case, only 6 frequency points of BGS are reconstructed for the one-time sweep (more specifically, the frequency shift of each comb in Fig. 1(a) is νb + 50, νb + 30, νb + 10, νb -10, νb - 30, and νb -50 MHz). To increase the number of BGS reconstruction points, the frequency shift νb was swept four times around the Brillouin frequency shift (BFS). Because Δνpulse - Δνprobe = 20MHz was chosen for the one-time sweep, the final reconstruction step of BGS is 20/4 = 5 MHz.
Fig. 1 (a) Principle of BOTDA using pulse probe frequency comb pairs. (b) Time-domain waveform used to generate the pulse comb. One of the sidebands of DSB pulse is used for SBS interaction and BGS reconstruction (solid lines of pulse combs).
Fig. 2 Experimental setup of frequency-comb-based BOTDA. DFB-LD: Distributed feedback laser diode; EOM: Electro-optic modulator; AOM: Acousto-optic modulator; EDFA: Erbium-doped fiber amplifier; PS: Polarization scrambler; VOA: Variable optical attenuator; ISO: optical isolator; CIR: circulator; FUT: Fiber under test; TBSF: Tunable band-stop filter; BPF: band-pass filter; PD: photodetector; DAQ: Data acquisition card; RF: Microwave source; DC: direct current port; RF-AMP: Microwave amplifier; AWG: Arbitrary waveform generator; LPF: low-pass filter.
In order to realize a high-spatial-resolution, several phonon pre-excitation approaches can be combined [6–11]. Figure 1(b) shows the time-domain waveform used to produce the electronic output pulse for driving the EOM. The EOM for pulse light was biased at null point so that two DSB optical pulses can be generated by each electronic comb. Considering that the response intensity for PSP is twice that of DPP [8], PSP modulation was used in our experiment. Every pulse comb is shifted by one pulse width to reduce the pressure of the EDFA and prevent nonlinear interactions among different frequency comb pairs [2]. To decrease the impact of environmental change on the BGS reconstruction during acquisition, the pulse frequency combs with and without phase shifts of π were injected in sequence [7]. The final Brillouin response was obtained by performing the differential operation with and without the phase shifts. The relative time delay of the different Brillouin responses can be simply removed by the reversed time shift when reconstructing the BGS [2].
The experimental setup of the frequency-comb-based BOTDA is shown in Fig. 2. The output of the distributed feedback laser diode (DFB-LD) is divided into two parts. The upper part is modulated by EOM1, which is driven by the mixed signal between the frequency comb and microwave source 1 (RF1). The carrier is retained by appropriately adjusting the direct current port input. A tunable band-stop filter (TBSF) with a <0.1 nm bandwidth (realized by a fiber Bragg grating) is used to suppress the low-frequency sideband. The high-frequency counterpart is used as the probe wave. The frequency of RF1 is determined by νb - (N + 1)Δνpulse /2, where νb is swept four times around the BFS and N = 6 is the number of pulse frequency-combs used, as shown in Fig. 1(a). The single-sideband (SSB) probe light together with the carrier is then launched into the FUT. Note that the utilization of the SSB probe avoids BFS distortion during coherent detection, which is induced by the frequency-dependent response due to group velocity dispersion [24].
The lower 50% part acts as a pulse frequency comb by passing through EOM2 that is driven by the waveform output given in Fig. 1(b). Both the pulse and probe frequency combs are amplified by the EDFA before being launched into the FUT. Two polarization scramblers (PSs) are used to average the polarization-dependent gain noise.
The received probe light and carrier are detected by an approximate 12 GHz photodetector (PD) after a band-pass filter (BPF) was used to exclude the amplified spontaneous emission (ASE) noise. The coherent detection system consists of microwave amplifier 2 (RF-AMP2), microwave source 2 (RF2), frequency mixer 2, a low-pass filter (LPF), and a data acquisition card with a 4 GSa/s sampling rate. The first beating frequency is downshifted to 150 MHz by the mixer through the proper frequency selection of RF2, as shown in Fig. 1(a). The sum frequencies are removed by the LPF. The BGS reconstruction includes the following steps: (I) each of the heterodyne frequency components is separated in the frequency domain by a fast Fourier transform (FFT); (II) the time-domain trace of each frequency comb is recovered by an inverse fast Fourier transform (IFFT), then the intensity of each received probe comb can be acquired by square of modulus; (III) a differential operation of the Brillouin trace for the pulse with and without phase shift is performed; and (IV) the inverse time delay over one pulse width, averaging, and Lorentz fitting are carried out for the BGS reconstruction. The rising time of the demodulated signal is given by 0.5/(Δνprobe/2). Therefore, the limit of the spatial resolution due to the restriction from the demodulation bandwidth is 50 cm.
2.2 Impact of nonlinearity for BGS reconstruction using DSB pulse frequency comb
The previously reported frequency-comb-based BOTDA structures reconstruct the BGS based on the DSB pulse frequency comb [1–4], the schematic diagram of which is shown in Fig. 3. The principle is similar to the case shown in Fig. 1, except that the central frequencies of the pulse (νpulse) and carrier (νcarrier) are identical. Considering that, for the BOTDA using phonon pre-excitation, a higher pulse peak power (larger than a few hundreds of milliwatts) is often required to achieve a higher signal-to-noise ratio (SNR) due to the narrower pulse width difference, it is expected that the impact of residual nonlinearity between the two sidebands for each comb is not negligible.
Initially, we tested the BGS along a standard single-mode fiber, approximately 592 m in length, using DSB pulse frequency combs. No phase shift was introduced in this case. The pulse width was set to approximately 25 ns. The pulse peak and total probe input powers were approximately 27 and 0 dBm, respectively. The BFS of the FUT was approximately 10850 MHz. The frequency of RF1 (νb) was swept four times from 10850 to 10865 MHz and there were 4096 averaging times. Figure 4(a) depicts the reconstructed BGS. A significant BGS splitting is observed due to the nonlinear effect because the DSB pulses for each frequency comb overlap in the time domain and propagate along the FUT with a similar velocity. The observed BGS exhibits a noticeably visible energy transfer between the two DSB components.
Fig. 4 (a) Reconstructed BGS of frequency-comb-based BOTDA. (b) Brillouin responses for various frequency shifts. The DSB pulse is used.
Figure 4(b) displays the reconstructed Brillouin response for various frequency shifts. It is observed that the Brillouin responses at 10840–10855 MHz clearly decrease along the fiber. Conversely, a significant energy enhancement is observed at 10860–10875 MHz. Note that the frequency of the pulse decreases from νcarrier + 550 MHz to νcarrier - 550 MHz with the increased frequency shift, as shown in Fig. 3. Thus, SRS and FWM might play a crucial role except for XPM and self-phase modulation (SPM). Another feature is the rising or falling speed in Fig. 4(b), which is similar for various Brillouin responses. Hence, it is expected that the BGS splitting due to the residual nonlinear effect could be suppressed using one of the sidebands of DSB pulse frequency comb.
Figure 5 presents the separated BFS along the FUT after Lorentz fitting using the DSB pulse and one of the sidebands. The first subsection of the BFS that displays a sudden peak is caused by the additional strain during the winding process. We have verified the BFS using the standard BOTDA without any frequency comb technique. The result agrees well with that of the frequency-comb-based BOTDA using one of the sidebands of DSB pulse. However, a large measurement error was obtained for the DSB pulse.
In order to further suppress the impact of nonlinear interactions, a possible improvement configuration is the use of an SSB EOM for the pulse frequency comb. Although it is not available in our laboratory, a better sensing performance can be expected. In the following, we demonstrate the results of high-spatial-resolution sensing based on the scheme described in section 2.1.
2.3 High-spatial-resolution sensing using one of the sidebands of DSB pulse
To test the capability of high-spatial-resolution sensing, a subsection of fiber, approximately 50 cm in length, was heated in a water bath from approximately 23 to 45 °C. The phase shift point was set to 20 ns for the 25-ns pulse width, corresponding to the nominal spatial resolution of 50 cm. Figures 6(a) and 6(b) respectively present the 2D BGS distribution along the FUT before and after the differential operation. Figures 6(d) and 6(e) display the magnified views near the hotspot. No BGS splitting due to the residual nonlinear effect is observed. The hotspot could not be identified completely for the 25-ns pulse, however, after being combined with the PSP technique, it was clearly visible. Figure 6(c) presents the separated temperature distribution along the FUT. The temperature distribution around the hotspot is shown in Fig. 6(f). The maximum standard deviation of the temperature is approximately ± 0.5 °C. The achieved spatial resolution is approximately 60 cm from the full-width at half maximum (FWHM). The approximate 10 cm discrepancy from the nominal value may be due to the limited bandwidth when performing the demodulation, as mentioned in section 2.1.
Fig. 6 (a) 2D BGS distribution using 25-ns pulse without phase shift. (b) 2D BGS distribution after the differential operation. (c) Separated temperature distribution. The magnified views of (a), (b), and (c) around the hotspot are respectively shown in (d), (e) and (f).
3. Frequency-comb-based BOTDA for long-distance sensing
3.1 Principle and experimental setup
Figure 7 shows the schematic diagram of the frequency-comb-based BOTDA for long-distance sensing. The frequency comb is introduced only for the probe light. An acousto-optic modulator (AOM) is used to generate high extinction ratio (ER) pulses (~52 dB), as shown in Fig. 2. The central frequency difference between the carrier and probe should be νb + ΔνAOM, where ΔνAOM (200 MHz) is the frequency shift of the AOM. The Brillouin loss spectrum is used to realize the DBA, which enables the SNR enhancement along the FUT. Contrary to Ref [5], the carrier and probe lights follow identical optical paths, which ensures the best coherence and thus decreases the linewidth requirement (~1.5 MHz was used in the experiment). For example, for the used fiber length of approximately 74.2 km, if the structure of Ref [5]. is used, the required linewidth should theoretically be smaller than approximately 0.9 kHz according to , where vg is the group velocity, and Lc and L represent the lengths of the coherence and FUT, respectively. Figure 8(a) shows the received electronic spectrum, in which the SNR is larger than approximately 36 dB at a 300-kHz resolution. The space of the adjacent comb is 20 MHz. Note that the carrier power should be less than the threshold to avoid noise due to random-lasing in the long-distance case [25], which results from the common role of Rayleigh random distributed feedback and DBA [26]. In the experiment, both powers of the carrier and probe lights were set to approximately 1.5 dBm.
Fig. 7 Schematic diagram of frequency-comb-based BOTDA for long-distance sensing. ΔνAOM is the frequency shift of the AOM. DBA is introduced to enhance the SNR.
Fig. 8 (a) Received electronic spectrum from the beating of carrier and probe light. (b) Pulse output waveforms for various frequency shifts.
3.2 Suppressed non-local effect for long-distance sensing
The structure is different with our previously reported BOTDA using DBA [22, 23]. In addition to the coherent detection, the Brillouin pump in the DBA acts as a probe light, which is helpful for suppressing the non-local effect induced by pump depletion (due to another probe input, see Refs [22, 23].). Figure 8(b) shows the pulse waveforms passing through the FUT four times while frequency sweeping, where the input peak power and pulse width are approximately 10 dBm and 50 ns, respectively. The BFS of the FUT is approximately 10843 MHz at 26 °C. Because of the expanded gain spectrum for the pulse, its shape and peak power are almost unchanged, implying that the non-local effect is reduced significantly. Note that the power of each comb (~) is far larger than the onset threshold (~-14 dBm) of the non-local effect in the conventional BOTDA [15]. The observed relative time delay of approximately 5 ns may be caused by the slow-light effect. The value of should be smaller than the BGS width to achieve the flattened pulse gain response for suppressing the non-local effect. However, the smaller would restrict the achievable demodulation bandwidth and spatial-resolution. So there exists an optimized range.
3.3 Long-distance sensing experiment
For the long-distance sensing experiment, 15-bit Simplex return-to-zero (RZ) coding [27] was utilized to further enhance the SNR (~3 dB). The selection of lower pulse peak power and coding length is based on the reduction of BGS expansion due to nonlinear accumulation (mainly including SPM [28] and modulation instability (MI) [29]) and transient gain saturation due to DBA [22]. The space of the coding pulse was 200 ns to avoid the influence of phonon lifetime. An approximate 5-m section was heated from approximately 26 to 43 °C. The received signal was pre-averaged 300 times, resulting in 4500 total averaging times. The sampling rate of 500 MSa/s was chosen. Figures 9(a) and 9(b) show the obtained distributions of the BGS and BFS along the FUT. A clear hotspot can be observed. The FWHM of the BGS along the FUT is less than approximately 45 MHz, indicating that the nonlinear effect impact is well restricted. The extracted temperature distribution is shown in Fig. 9(d). The measured maximum standard uncertainty is approximately ± 0.7 °C for the 40–60 km range. The ~6-m real spatial resolution is observed from Fig. 9(d). Again, an approximate 1-m discrepancy with the nominal spatial resolution is due to the limited demodulation bandwidth.
Fig. 9 Distributions of (a) BGS, (b) BFS, and (c) extracted temperature. The magnified view of (c) around hotspot is shown in (d).
The data acquisition-time of DAQ is decreased to 1/6 of the conventional BOTDA since the 6 frequency components are acquired simultaneously. Considering that the switching time of RF1 is in the millisecond scale and is negligible, the acquisition-time is determined by TperiodNaNcNs, where Tperiod = 800 is the period of pulse train, Na = 300 is the pre-averaging times, Nc = 15 is the coding length, Ns = 4 is the sweeping times. Hence, the acquisition-time of DAQ is ~15 s. In the experiment, as a proof-of-concept, we performed the data processing (including average, FFT, IFFT and decoding) by Matlab software, leading to the required total time of ~20 minutes for a complete BGS reconstruction. The processing time could be largely decreased if high-speed and real-time digital signal processing (DSP) is used.
4. Conclusions
A frequency-comb-based BOTDA has been developed for realizing high-spatial-resolution/ long-distance sensing. For high-spatial-resolution sensing, the higher peak power causes a non-negligible impact of nonlinearity from a DSB pulse. In addition, BGS splitting was observed. One of the sidebands of DSB pulse was thus proposed to reconstruct the undistorted BGS. As a proof-of-concept, sensing with a spatial resolution of approximately 60 cm along a fiber approximately 592 m in length was realized. The detailed characteristic of frequency-comb-based BOTDA for long-distance sensing was also investigated. The shape and power of the pulse can be maintained during the frequency sweep due to the expanded gain spectrum for the pulse light, causing the non-local effect to be suppressed significantly. Long-distance sensing along a fiber approximately 74.2 km in length with a spatial resolution of approximately 6 m was demonstrated.
Funding
National Natural Science Foundation of China (NSFC) under Grants 61205079 and 11474233; Construction Plan for Scientific Research Innovation Teams of Universities in Sichuan Province under Grant 12TD008; 251 Talents Program of Sichuan Normal University; Open Experimental Program of Sichuan Normal University under Grant KFSY2016-52; Scientific Research Foundation of Sichuan Normal University under Grant 16ZP08.
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