PHOTONIC SENSORS / Vol. 6, No. 1, 016: 58 6 Enhancement for Φ-OTDR Performance by Usng Narrow Lnewdth Lght Source and Sgnal Processng Meng ZHANG 1, Song WANG *, Yuanwe ZHENG 1, Yao YANG 1, Xngje SA 1, and L ZHANG 1 Guzhou Power Grd Informaton and Communcaton Company, Guzhou, 550003, Chna Key Laboratory of Optcal Fber Sensng & Communcatons (Mnstry of Educaton), Unversty of Electronc Scence and Technology of Chna, Chengdu, 611731, Chna * Correspondng author: Song WANG E-mal: 394858749@qq.com Abstract: In order to enhance the sgnal-to-nose-rato of a dstrbuted acoustc sensng system based on phase-senstve optcal tme-doman reflectometry (Φ-OTDR), we have proposed a combnaton of segmented unwrappng algorthm, averagng estmaton of phase dfference, and nfnte mpulse response (IIR) flterng method. The enhancement of sgnal qualty s numercally demonstrated. Moreover, we have studed the nfluence resulted from the lght source nose on the Φ-OTDR performance. The result has shown that when the lnewdth of lght source used n the Φ-OTDR system s narrower, the performance of the system s better. In a word, such a Φ-OTDR system could obtan hgher qualty demodulated sgnals when the narrower lnewdth lght source s chosen and the method of averagng estmaton phase dfference s used. Keywords: Phase unwrap; frequency flter; phase nose Ctaton: Meng ZHANG, Song WANG, Yuanwe ZHENG, Yao YANG, Xngje SA, and L ZHANG, Enhancement for Φ-OTDR Performance by Usng Narrow Lnewdth Lght Source and Sgnal Processng, Photonc Sensors, 016, 6(1): 58 6. 1. Introducton Phase-senstve optcal tme-doman reflectometry (Φ-OTDR) s a powerful technque that allows fully dstrbuted vbraton sensng wth fast response and hgh senstvty. It has been appled n many felds such as cvl structure health montorng and securty guardng [1, ]. Accurately, acqurng vbraton locaton has been acheved n the reported Φ-OTDR system by montorng the ampltude change of backscattered lght [3 5]. However, for some specal applcatons such as sesmc wave detecton, t could not satsfy requrements wthout the ablty of provdng phase nformaton. Recently, an I/Q phase demodulaton technology usng 90 optcal hybrd was proposed to acheve phase demodulaton [6]. In order to mprove the performance of I/Q phase demodulaton of the Φ-OTDR, we have proposed the combnaton of segmented unwrappng algorthm, averagng estmaton of phase dfference, and nfnte mpulse response (IIR) flterng method. Further, we have studed the nfluence on the Φ-OTDR by ntroducng lght source nose wth zero mean and dfferent varance.. Schematc setup Compared wth other Φ-OTDR demodulaton regmes, the most sgnfcant dfference n our work s that a hybrd component s ntroduced to the system structure, as shown n Fg. 1 [6]. One of the Receved: 6 September 015 / Revsed: 16 November 015 The Author(s) 015. Ths artcle s publshed wth open access at Sprngerlnk.com DOI: 10.1007/s1330-015-083-7 Artcle type: Regular
Meng ZHANG et al.: Enhancement for Φ-OTDR Performance by Usng Narrow Lnewdth Lght Source and Sgnal Processng 59 vacuum, and the value of the rect functon s one when 0<t kt<w, otherwse t wll be zero. The delay τ corresponds to the dstance z from the nput end to the th backscatter through the relaton τ =n f z /c. The phase θ(kt+τ ) stands for the lght source nose at the tme t=kt+τ, whch obeys Fg. 1 Schematc setup. hybrd outputs s the result of the nterference normal dstrbuton wth zero mean and varance σ=πτδν [8], where τ and Δν are the perod of the between the local oscllator lght and the seed lght pulse and the lnewdth of the lght source, backscattered lght, whch s called I channel. The other hybrd output called Q channel s the result of the nterference between the π/ phase-shfted local oscllator and the backscattered lght. In order to demodulate the nformaton mposed on the optcal fber by smulaton, t s necessary to obtan the arthmetc express of the wave of backscattered lght [7]. When a coherent lght pulse wth a pulse wdth W and an optcal frequency f s launched nto the fber at the tme t=kt, where k and T stand for the numbers and the perod of the lght pulse, respectvely, we obtan the wave of the backscattered lght at the respectvely. Wth the help of the hybrd and the local oscllator, t s easy to get the lght sgnals of I channel and Q channel, whch are converted to analog electrc sgnals by PD1 and PD, respectvely. Here, the analog electrc sgnals are sampled wth 50 MHz samplng rate by the data acquston (DAQ) card. As for the segmented unwrappng algorthm, t means successvely dvdng the dgtal sgnals of I channel and Q channel of the traces nto groups,.e., every ten dgtal sgnals of I channel and Q channel as a group before unwrappng, whch makes the nput end that s gven by phase φ ( n) between Q channel and I channel contnuous and beyond [ π, π]. In comparson wth N cτ e( t) = = 1aexp( α )exp{j[ π f ( t kt τ) unwrappng all dgtal sgnals of I channel and Q n f (1) channel, the segmented unwrappng algorthm lmts t kt τ + θ( kt + τ )]} rect( ) the nfluence of the unwrappng error wthn the W specfc group and reduces the nfluence of total where ɑ and τ are the ampltude and delay of the th backscattered wave, respectvely, N s the total scatterng tmes of one lght pulse, α s the fber attenuaton constant, c s the velocty of lght n a unwrappng errors. Another mportant algorthm s averagng estmaton of phase dfference, whch operated after the segmented unwrappng algorthm. It s expressed as φ( y x) = y x+1 fx( ) () y x+1 = fx( ) y x+1 =1 { φ[ y fx( ) + ] φ( x+ 1)} where y and x (y>x) are not larger than N, and n the same group, the dstance between y and x s not larger than the group length. In addton, the fx(x) functon s to get the nteger part of x. The thrd algorthm s to desgn a low-pass IIR flter wth 8 khz stop-frequency to get the smooth φ( y x) sgnals from pulse to pulse. 3. Smulaton results and dscussons In smulaton, as shown n Fg. 1, we set the total fber length as 100 m, and only the fber secton between 60 m and 70 m bears external sne acoustc
60 Photonc Sensors wave, whch produces snusodal stran between 00 nε and +00 nε at 1500 Hz, as shown n Fg.. The repetton rate and wdth of lght pulses are 50 khz and 100 ns, respectvely. The samplng rate of the DAQ card s confgured as 50 MHz, n other words, the DAQ card could get the data of I channel and Q channel every two-meter length of the optcal fber, respectvely. Fg. Snusodal stran sgnal. For the sgnal processng, specfc mplement of the segmented unwrappng algorthm s to obtan the carng data n the same group from the sampled data and then unwrap t. Here, the carng data, fve dgtal data, are the data of the fber between 60 m and 70 m, but ten dgtal data of the fber between 60 m and 80 m are n the same group, so the carng data n the same group are the data of the fber between 60 m and 80 m. In order to get phase dfference of the unwrapped data of the fber between 70 m and 60 m, the drect phase dfference of two ponts s the dfference between the unwrapped data at 70 m and that at 60 m. At the moment, the drect phase dfference of two ponts s rough, and t becomes smooth when t passes the 8 khz low-pass IIR flter, as shown n Fg. 3. The drect smooth phase dfference of two ponts n Fg. 3 s transformed by fast Fourer transform algorthm (FFT) to those n Fg. 3(b) when the spectrum of drect phase dfference s needed to calculate the sgnal-tonose-rato (SNR) (the calculaton method of SNR s reported n [9]). However, the averagng estmaton smooth phase dfference of two ponts s dfferent from the drect smooth phase dfference of two ponts, and the dfference s the process of gettng the phase dfference. The process whch s descrbed as () has been done, and we could qualtatvely fnd that the curve s smoother than that of drect phase dfference n Fg. 3. Moreover, the peak value of the spectrum of averagng estmaton phase dfference s about tmes larger than that of the spectrum of drect phase dfference. In order to quanttatvely compare the qualty of two methods [9], the method of averagng estmaton phase dfference and the method of drect phase dfference, we calculate the SNRs of sgnals demodulated by them, and the result s that the SNR usng the method of averagng estmaton phase dfference s 35.8 db, and the SNR usng the method of the drect phase dfference s 3.7 db. Obvously, the method of averagng estmaton phase dfference s better than the other demodulaton method (n ths stuaton, the lnewdth of the lght source s typcal value 100 Hz). Tme (ms) Freqency (Hz) Fg. 3 Sgnals of the Φ-OTDR system wth the lnewdth of 100 Hz lght source: demodulated sgnals and (b) spectra of demodulated sgnals. Next, we dscuss the effect that the dfferent lnewdths of lght sources play on the demodulated
Meng ZHANG et al.: Enhancement for Φ-OTDR Performance by Usng Narrow Lnewdth Lght Source and Sgnal Processng 61 sgnals usng both the methods of averagng estmaton phase dfference and the drect phase dfference. We set the lnewdth as 100 Hz and 500 Hz, and then we compare the phase dfferences n the each method, respectvely. The results are presented n Fg. 4. It s easly found that the dot lnes wth 100 Hz lnewdth are smoother and more perfectly ft the sne curve than the sold lnes wth 500 Hz lnewdth, no matter whch method s used. In order to accurately further study how the lnewdth of the lght source affects the performance of both methods, the SNRs of sgnals demodulated by both methods wth dfferent lnewdths of the lght source are calculated, as shown n Fg. 5. From Fg. 5, two characterstcs are found, one s that the SNR descends when the lght source lnewdth ncreases, and the other characterstc s that the SNR of the demodulaton sgnal usng the method of averagng estmaton phase dfference s always larger than that of the demodulaton sgnal usng the method of the drect phase dfference on the condton of the same lnewdth of the lght source. Besdes, the SNR of the sgnal demodulated Tme (ms) by the method of averagng estmaton phase dfference s up to 31.6 db when the lnewdth of the lght source s 3500 Hz. In a word, the SNR descends when the lght source nose ncreases, however, the method of averagng estmaton phase dfference n performance s better than the other method. Fg. 5 SNRs of sgnals demodulated by two methods wth dfferent lnewdths. 4. Conclusons In order to enhance the performance of the dstrbuted acoustc sensng system based on Φ-OTDR, we have proposed the method of averagng estmaton phase dfference, whch s proved better than the method of the drect phase dfference by addng the lght source wth dfferent lnewdths (dfferent level lght source noses) to the Φ-OTDR system. Moreover, by comparng the SNRs of the demodulated sgnals of dfferent lnewdths of the lght source, we fnd that demodulated sgnal s better when the lnewdth of the chosen lght source s narrower. Hence, a lght source of a narrower lnewdth s preferred to enhance the performance of the system. In summary, the Φ-OTDR system could obtan hgher qualty demodulated sgnals when we choose narrower lnewdth lght source and use the method of averagng estmaton phase dfference. Tme (ms) (b) Fg. 4 Demodulated sgnals of usng two methods: the method of averagng estmaton phase dfference and (b) the method of drect phase dfference. Open Access Ths artcle s dstrbuted under the terms of the Creatve Commons Attrbuton 4.0 Internatonal Lcense (http:// creatvecommons.org/lcenses/by/4.0/), whch permts unrestrcted use, dstrbuton, and reproducton n any medum, provded you gve approprate credt to the orgnal author(s) and the source,
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