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Abstract
Firstly, Erbium-doped fiber amplifier (EDFA) served as pre-amplifier may distort Brillouin gain spectrum (BGS) in coded Brillouin optical time domain analysis sensor. Here, we found that the EDFA has negligible impact on the shape of Brillouin phase spectrum (BPS). Experimental results show that a ∼5.4 MHz Brillouin frequency shift error caused by EDFA has been avoided by using BPS instead of BGS. Secondly, after eliminating phase fluctuation caused by optical fiber, the combination of Golay coding and coherent detection has been realized.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In the past three decades, Brillouin optical time domain analysis (BOTDA) sensor [1,2] has been employed in multiple applications, such as structural health monitoring and geotechnical applications. Since the temperature and strain distribution along a sensing fiber has been proved to be linearly related to the local Brillouin frequency shift (BFS), when the frequency difference between Brillouin pump and probe light is tuned to the BFS of a sensing fiber, continuous-wave (CW) probe light is amplified by pulse pump light through stimulated Brillouin scattering (SBS). The overall sensing performance is highly related to the signal-to-noise ratio (SNR) of Brillouin gain [3]. Therefore, a lot of methods have been proposed for SNR enhancement, including distributed Raman amplification [4], digital signal processing [5], optical pulse coding (OPC) [6–13] and so on [14,15]. Among them, the OPC method has received extensive attention mainly due to its capability to enhance the SNR by a factor of L (pulse pump coding length). But final effective SNR improvement of OPC is ∼7.5dB [13]. In addition, a BOTDA with coherent detection [16–18] has a unique advantage in reducing in-band noise. However, the combination of OPC and coherent detection has not been realized yet in BOTDA. In a BOTDA setup, EDFAs are usually hired to amplify the CW probe before photodetectors (PDs) to increase the SNR of converted electrical signals [3]. However, an EDFA served as pre-amplifier may distort BGS due to EDFA slow transient effects (EDFA-STE).
In this paper, firstly, we propose a method for overcoming EDFA-STE by using BPS instead of BGS in a combined Golay-coding and coherent detection BOTDA fiber sensor. Previous studies [19,20] have indicated that an EDFA served as pre-amplifier may distort BGS in coded BOTDA. Here, through theoretical analysis and experimentally verified, we found that the EDFA has negligible impact on the shape of BPS. By detecting a hotspot at the far end of fiber over a temperature range from 25°C to 70°C, a ∼5.4 MHz BFS error caused by EDFA is avoided by using BPS instead of BGS, fully demonstrating the effectiveness of BPS in overcoming EDFA-STE. Secondly, the combination of OPC and coherent detection has been realized after eliminating phase fluctuation caused by optical fiber. There is a measurement uncertainty problem when we directly hire Golay-coding technique in coherent detection BOTDA. Experimental results show that traditional coherent detection scheme would cause very large measurement uncertainty (∼5.71 MHz at the far fiber end). Thus, we employ a phase fluctuation cancellation (PFC) method [17] to enhance system measurement certainty. After the PFC [17] by using an optical wavelength-multiplexing scheme, the measurement uncertainty reduces to 0.52 MHz at the same fiber location. Finally, only 256-time average is required, the SNR of the sensing system reaches 15.3 dB.
2. Operation principle
According to previous studies [19,20], EDFA presents a slow transient behavior (as shown in Fig. 1) because of amplified CW light depletes the population of erbium ions that inversion takes a long time. The EDFA’s different instantaneous gain distorts BOTDA trace. In detail, the leading part of trace depletes ions in EDFA, thus the following part of trace is less amplification than it should be. Assuming that the amplitude of local gain trace (without the effects of EDFA) is $g(t )$ (blue line in Fig. 1) and the function of EDFA-STE is d$(t )$, actual gain trace with the effects is $g(t )d(t )$ (red line in Fig. 1). Then optical filed sent into PD is
where ${\varphi _{SBS}}(t )$ is phase shifts caused by SBS and ${f_s}$ is the frequency difference between the local light and probe light. After a I/Q-demodulator, the output I trace and Q trace areThen we could calculate out the amplitude of gain (Amp) and phase ($\varphi $). Equation (3) clearly shows that the EDFA-STE distorts $g(t )$ by $d(t )$. However, this problem not occurs in $\varphi $ due to that d$(t )$ from I and Q cancels each other out during the calculation. As described in Eq. (3), the transient behavior has few effects on BPS, which will also be verified by subsequent experimental results.
Fig. 2. (a) BGS with the EDFA-STE. (b) BGS’s distortion induces BFS estimation error (ve) when there is frequency shift (δν).
3. Experimental setup
The experimental setup for using the coherent detection in Golay-coded BOTDA with PFC (path ‘a’) and without PFC (path ‘b’) shows in Fig. 3. A narrow linewidth (∼100 kHz) light source’s central wavelength and output power are ∼1550 nm and ∼13 dBm, respectively. The output of the light source is divided into two branches by a 3-dB optical coupler. The upper branch in the figure is working as CW Brillouin probe signal and the lower branch is working as pulse Brillouin pump. On the upper branch, a polarization controller (PC) is placed at the input of intensity modulator (IM) to assist the modulator to achieve maximum modulation efficiency. An IM biased at Vπ is used to obtain a carrier-suppressed dual-sideband (CS-DSB) probe wave. The IM is driven by a mixed frequency radiofrequency (RF) signal (i.e., fL + fS and fL). The next few steps generate the mixed frequencies RF signal: 1. fL mixes fS, so we obtain fL + fS and fL - fS frequencies. 2. Filtering out the fL + fS frequency component by band-pass filter (BPF). 3. Combining fL + fS with fL. Finally, we obtain the mixed frequencies RF signal consisted of fL and fL + fS. Moreover, the frequency fL is sweeping from 8.130 GHz to 8.330 GHz with a 4 MHz step and the frequency fS is a fixed frequency at 2.45 GHz. The modulated light of IM pass through an isolator into to a sensing fiber. The light interacts with pulse pump light, is amplified by pre-amplifier (EDFA2) and then the upper and lower sidebands of CW probe light are separated by DWDM and sent into PD1 and PD2, respectively. After photoelectric conversion, the RF signal fS is selected by band-pass filters (BPF) and amplified by low noise amplifiers (LNA). On the lower branch, the light wave is firstly amplified by a high-power EDFA1 with adjustable gain. Then that, the light is further coded by an acousto-optic modulator (AOM) with 50-dB extinction ratio and then sent into a sensing fiber as pulse pump light through an optical circulator (OC). By the way, the AOM also induces 200 MHz optical frequency shift. The optical pulse width of each bit and bit duration are 20 ns and 200 ns, respectively, it is return-to-zero (RZ) modulation format [6]. The path ‘b’ is the local oscillator (LO) port of IQ-demodulator connecting to fS signal generator directly, i.e., the case without PFC. On the other path (i.e., path ‘a’) shows the case with PFC, when the LO port connects to LNA1. The outputs of IQ-demodulator (i.e., I-trace and Q-trace) are sampled at 100 MSa/s and averaged 256-time by an oscilloscope (OSC). Then sampled data are processed to obtain Brillouin spectra and further fitted BFS.
Fig. 3. The experimental setup for using the coherent detection in Golay-coded BOTDA with PFC (path ‘a’) and without PFC (path ‘b’). EDFA: erbium-doped fiber amplifier; AOM: acousto-optic modulator; PC: polarization controller; PS: polarization scrambler; FUT: fiber under test; IM: intensity modulator; OC: optical circulator; DWDM: dense wavelength division multiplexer; BPF: band-pass filters; LNA: low noise amplifier; OSC: oscilloscope.
4. Experimental results and discussion
4.1 The combination results of Golay-coding and coherent detection
In our experiment, ∼39.1-km standard single mode fiber with ∼10.68-GHz BFS is employed as a sensing fiber. We directly hire Golay-coding technique in coherent detection BOTDA in order to show the measurement uncertainty problem. Thus, we compare the fitting BFSs of the case with PFC (i.e., path ‘a’) and the case without PFC (i.e., path ‘b’). Experimental results in Fig. 4(a) show the case without PFC (i.e., path ‘b’) which will cause very large measurement uncertainty (∼ 5.71 MHz at the far end of the fiber) due to phase fluctuation caused [17]. This measurement uncertainty reduces to ∼0.52 MHz as shown in Fig. 4(b) with the assistance of PFC. After eliminating phase fluctuation caused by optical fiber, coherent detection is successfully used in Golay-coded BOTDA sensor for BPS measurement.
Fig. 4. Fitting BFSs in consecutive 10 times measurements (a) without PFC (path ‘b’) and (b) with FPC (path ‘a’), respectively; (a) and (b) are hiring the 128-bit Golay-coding.
By tuning fL over a span from 8.130 GHz to 8.330 GHz with a 4-MHz step, fL + fS is sweeping from 10.580 GHz to 10.780 GHz to acquire the Brillouin spectrum distribution along the whole sensing fiber. The measured BGS and BPS are shown in Figs. 5 and 6, respectively, in which also show the BGS and BPS at the near and far fiber end in the insets. Those figures show clean BGS and BPS experimental results.
Fig. 5. Measured BGS along a ∼39.1-km fiber, at 0.68-km and 39.105-km locations; where is hiring the Golay-coding and PFC.
Fig. 6. Measured BPS along a ∼39.1-km fiber, at 0.68-km and 39.105-km locations; where is hiring the Golay-coding and PFC.
By calculating standard deviation values at each fiber location in consecutive 10 measurements, measurement error shows in Fig. 7 by using BPS. It is clearly observed that Golay-coding reduces BFS measurement error from ∼3.3 MHz to ∼0.59MHz, matching well with the 5.6 times SNR enhancement that benefitting from the 128-bit Golay coding.
Fig. 7. Frequency measurement error of consecutive 10 estimation BFS profiles along a ∼39.1 km fiber by using 128-bit Golay-Coding (red line) and single pulse (blue line), respectively; where employs the PFC; the black lines are corresponding exponential fitting.
4.2 The compensated BGS
In order to verify that the BFS error of BGS is derived from the EDFA-STE and not for other reasons [22,23], a compensated BGS (BGSC) is proposed as described in Eq. (5).
where $\mbox{BGS}({v,z = 39.346km} )$ is the baseband spectra out of the sensing fiber and close to the hotspot, which is displayed in Fig. 8(a). Note that, this method only works for the BGS at the far end of fiber for now. If the BFS error of BGS comes from the EDFA-STE, the BGSC could avoid the error and get the similar result with BPS. By the way, the results from BGSC is not good as BPS’s due to the bad SNR of $\mbox{\; BGS}({v,z = 39.346km} )$.Fig. 8. (a) Fitting BGSs at 39.091 km (middle point of ∼50 °C hotspot) and 39.346 km. (b) Fitting BPS at 39.091 km (middle point of ∼50 °C hotspot) and 39.346 km; where employs the Golay-coding and PFC.
4.3 Comparison results from BGS, BGSC and BPS
Comparing the impact of BGS and BPS caused by the EDFA-STR, BGS at middle of hotspot (∼50 °C) and the BGS out of the sensing fiber are displayed in Fig. 8(a). Note that the total length fiber is 39.105 km, so the BGS at 39.346 km actually is the baseband spectra without gain. The depletion BGS at 39.346 km is a Lorentz shape. However, this problem not appears in corresponding BPS as shown in Fig. 8(b). Comparing the depletions from Figs. 8(a) and 8(b), it also explains that using BPS instead of BGS could overcome EDFA-STE. By the way, the gained trace of center frequency has 15.3-dB SNR at the far end of fiber. The BFSs are got from the Lorentzian fitting technique, which refers to Eqs. (6) and (7).
A 10-m-testing fiber at far end of ∼39.1-km fiber is heated up from 25 °C to 70 °C with a step of ∼5°C and the remaining fiber is kept at room temperature. Figure 9(a) shows the hotspot (50 °C) BFSs, about 2-m spatial resolution (from 10% to 90%) is observed, agreeing well with the 20-ns optical pulse width. Since the results from BGS has a large BFS estimation error, the BFS from BGS and BPS have 5.4 MHz difference at the hotspot. As previous analysis expected, the BFS from BGSC has similar result with the BFS from BPS. The averaged BFS (5 points from 39.089 km to 39.093 km) versus temperature change displays in Fig. 9(b). The line of BGSC is also similar with the BPS’s. However, the dull-red line (i.e., BFS from BGSC) is not straight than the red line (i.e., BFS from BPS). The maximum BFS error by using BGS is ∼5.4 MHz when temperature is 50 °C. However, the ∼5.4 MHz error caused by slow transient response of EDFA could be avoided by using BPS instead of BGS.
Fig. 9. (a) The hotspot (50 °C) BFSs, (b) The hotspot averaged BFS at different temperature, blue line is from BGS; red line is from BPS; dull-red line is from BGSC; where employs the Golay-coding and PFC.
5. Conclusions
In this work, we aim for overcoming EDFA-STE in a combined Golay-coded and coherent detection BOTDA sensor. After eliminating phase fluctuation caused by optical fiber, the combination of OPC and coherent detection has been realized. Experimental results show that a ∼5.4 MHz BFS error caused by EDFA-STE is avoided by using BPS instead of BGS.
Funding
National Natural Science Foundation of China (61735015); 111 Plan (B18045).
Disclosures
The authors declare no conflicts of interest.
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