Phase based high spatial resolution and distributed

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Vol 25 No 5 6 Mar 2017 OPTICS EXPRESS 5377, 14 Y Lu T Zhu L Chen and X Bao Distributed vibration sensor based on coherent detection of phase OTDR. J Lightwave Technol 28 22 3243 3249 2010, 15 D Arbel and A Eyal Dynamic optical frequency domain reflectometry Opt Express 22 8 8823 8830. 16 A Masoudi M Belal and T P Newson A distributed optical fiber dynamic strain sensor based on phase. OTDR Meas Sci Technol 24 8 085204 2013, 17 J Urricelqui A Zornoza M Sagues and A Loayssa Dynamic BOTDA measurements based on Brillouin. phase shift and RF demodulation Opt Express 20 24 26942 26949 2012. 18 A Lopez Gil X Angulo Vinuesa A Dominguez Lopez S Martin Lopez and M Gonzalez Herraez. Exploiting nonreciprocity in BOTDA systems Opt Lett 40 10 2193 2196 2015. 19 A Minardo A Coscetta R Bernini and L Zeni Heterodyne slope assisted Brillouin optical time domain. analysis for dynamic strain measurements J Opt 18 2 025606 2016. 20 W Li X Bao Y Li and L Chen Differential pulse width pair BOTDA for high spatial resolution sensing. Opt Express 16 26 21616 21625 2008, 21 A Motil Y Peled L Yaron and M Tur Fast and distributed high resolution Brillouin based fiber optic. sensor Opt Fiber Commun Conf pp OM3G 2 2013, 22 J Urricelqui M Sagues and A Loayssa Phasorial differential pulse width pair technique for long range.
Brillouin optical time domain analysis sensors Opt Express 22 14 17403 17408 2014. 23 K Y Song W Zou Z He and K Hotate All optical dynamic grating generation based on Brillouin scattering. in polarization maintaining fiber Opt Lett 33 9 926 928 2008. 24 S Chin N Primerov and L Thevenaz Sub centimeter spatial resolution in distributed fiber sensing based on. dynamic Brillouin grating in optical fibers IEEE Sens J 12 1 189 194 2012. 25 A Bergman L Yaron T Langer and M Tur Dynamic and distributed slope assisted fiber strain sensing. based on optical time domain analysis of Brillouin dynamic gratings J Lightwave Technol 33 12 2611 2616. 26 A Zornoza M Sagues and A Loayssa Self heterodyne detection for SNR Improvement and Distributed. phase shift measurements in BOTDA J Lightwave Technol 30 8 1066 1072 2012. 27 L Yaron E Shahmoon A Bergman T Langer and M Tur Spontaneous anti Stokes backscattering in. Brillouin dynamic gratings Proc SPIE 9634 96342X 2015. 28 A M Scott and K D Ridley A review of Brillouin enhanced four wave mixing IEEE J Quantum Electron. 25 3 438 459 1989, 29 Y Dong H Zhang D Zhou X Bao and L Chen Chapter 5 Characterization of Brillouin grating in optical. fibers and their applications in Fiber Optic Sensors Intech Publisher 2012 pp 115 136. 30 L Th venaz ed Advanced Fiber Optics Concepts and Technology EPFL press 2011 Chap 9. 31 H Kogelnik Theory of Optical Waveguides in Guided Wave Optoelectronics T Tamir ed Springer 1988. 32 A Bergman T Langer and M Tur Coding enhanced ultrafast and distributed Brillouin dynamic gratings. sensing using coherent detection J Lightwave Technol 34 24 5593 5600 2016. 33 W Zou Z He and K Hotate Complete discrimination of strain and temperature using Brillouin frequency. shift and birefringence in a polarization maintaining fiber Opt Express 17 3 1248 1255 2009. 34 A Othonos and K Kalli Fiber Bragg Gratings Fundamentals and Applications in Telecommunications and. Sensing Artech House 1999, 35 P Dragic T Hawkins P Foy S Morris and J Ballato Sapphire derived all glass optical fibres Nat. Photonics 6 9 629 635 2012, 36 J Sancho N Primerov S Chin Y Antman A Zadok S Sales and L Th venaz Tunable and reconfigurable. multi tap microwave photonic filter based on dynamic Brillouin gratings in fibers Opt Express 20 6 6157. 1 Introduction, Brillouin dynamic sensing is of importance in many applications 1 Recent implementations. of the Brillouin Optical Time Domain Analysis BOTDA 2 and Brillouin Optical. Correlation Domain Analysis BOCDA 3 techniques have demonstrated sampling rates of. the order of kilohertz s with a centimetric spatial resolution 10cm over a range of 145m for. the fully distributed case of 2 and 3cm over 6m for the random access approach of 3. Both techniques however require some form of time consuming scanning of the probe. frequency against that of the pump which limits their acquisition speed In contrast slope. assisted SA techniques using a single or at most a few pair s of pump and probe. frequencies can be much faster As such they have played a key role in taking the Brillouin. distributed fiber optic sensing to the fast dynamic regime 1 4 including demonstrations of. its practical utilization for monitoring the propagation of mechanical waves 5 6 for the use. of slope assisted interrogation of a fiber Bragg grating see 7. Vol 25 No 5 6 Mar 2017 OPTICS EXPRESS 5378, Most commonly the SA techniques employ a tunable laser source TLS adjusted to the.
linear region of the slope of either the reflection spectrum of a fiber Bragg grating FBG 8. or the intrinsic Brillouin gain spectrum BGS 9 such that changes induced by measurand. variations e g strain are translated to changes in the measured quantity usually optical. power However SA techniques are inherently sensitive to source optical power fluctuations. and frequency drifts fiber bend losses and spectral shape longitudinal inhomogeneity. introducing errors to the strain measurement Much ingenuity has been spent on finding. sophisticated solutions for these problems such as using the ratio between readings taken on. both slopes of the BGS 10 locking the laser frequency via a feedback loop 11 and. tailoring the probe frequency to the BGS profile of the fiber 12 However problems still. remain and new ones are frequently discovered as evidenced by 13 where it was shown. that the BGS linewidth broadens with increasing pump power with obvious ramifications on. its shape and slopes which affects the performance of the slope assisted Brillouin optical. time domain analysis SA BOTDA techniques indicating an additional drawback of. techniques based on the direct detection of optical power. An alternative method which might avoid such problems is to exploit the measurand. information encoded in the optical phase which is widely recognized as the workhorse of. distributed acoustic sensors DAS based on Rayleigh backscattering in optical fibers These. methods employ a coherent interference between the backscattered components of the. interrogating pulse resulting in a speckle like trace whose amplitude and phase can be. detected by means of coherent detection 14 15 To obtain quantitative information of the. measurand rather than merely detect dynamic perturbations the phase difference between. two reflections can be measured using an imbalanced Mach Zehnder interferometer with. predetermined path difference 16, Recently interesting SA BOTDA techniques harnessing Brillouin phase shift have. emerged 17 19 It should be noted that the spatial resolution of both gain and phase based. slope assisted BOTDA techniques is practically limited by the phonon lifetime to 1m. Recently proposed combinations of the differential pulse width pair DPP 20 with either. the gain 21 or the phasorial 22 BOTDA techniques showed an improved spatial resolution. of 1m at the expense of a decreased signal to noise ratio leading to an increased number of. averages and slower dynamic capabilities, A quite different distributed approach to enhance the spatial resolution without sacrificing. the sampling speed is to take advantage of Brillouin dynamic gratings BDGs in polarization. maintaining PM fibers 23 These moving Bragg gratings are generated by two strong. counter propagating pumps whose polarizations are aligned with the slow axis of the fiber. While both the magnitude and phase of the gratings are affected by the measurand all recent. demonstrations of this high spatial resolution sensing technique e g 24 static and 25. dynamic slope assisted have only used the gratings magnitude as measured by the. reflectivity of an orthogonally polarized narrow probe pulse While offering the advantage of. probe power independent measurements the correct estimation of the local Brillouin phase. shift BPS in BDG setups is quite challenging mainly due to non local contributions to. phase of the reflected probe from which the measurand induced BPS is to be deduced. Indeed the phase of the gratings at the location of interest is critically affected not only by the. measurand but also by the phase of the interference pattern generated by the counter. propagating pumps This latter phase is governed by the environmentally dependent optical. lengths of the down lead fibers feeding the two writing pumps As for the probe itself on its. journey to the point of interest and back it also collects non local phase contributions. Furthermore it will be shown below that the probe phase is also affected by inherent. longitudinal non uniformity of the birefringence in PM fibers 4 Proper measurement of the. phase is also an issue While in BOTDA setups operating in transmission measurement of. the BPS can be accomplished with minimum phase drifts by interference with a co. propagating reference 26 BDG setups operate in reflection By the same reasoning and due. Vol 25 No 5 6 Mar 2017 OPTICS EXPRESS 5379, to the fact that in BDG setups the reflected probe is also shifted in frequency the technique. that employs the nonreciprocal phase shift between the two paths of Sagnac interferometer. allowing for the measurement of BPS in BOTDA setup 18 cannot be efficiently harnessed. in BDG setups, In this paper we present a novel technique which practically combines the benefits of. phasorial measurements and high spatial resolution BDG reflectometry Using coherent. addition of the Stokes and anti Stokes reflections from two simultaneously counter. propagating BDGs in the fiber the technique advantageously offers distributed Brillouin. induced Phase Shift BiPS measurement with high spatial resolution The technique is. largely immune to variations in laser optical power and frequency drifts fiber bend losses. and similarly to phasorial SA BOTDA techniques offers an extended dynamic range. Detrimental non local phases and birefringence non uniformity induced contributions are. shown to be significantly reduced if not completely cancel out Finally a measurement of. static and dynamic strain fields is demonstrated,2 Theoretical analysis.
2 1 Principle of operation, BDGs are optically generated longitudinal density acoustic waves in optical fibers 23. whose magnitude and phase depend on the amplitudes phases and frequency difference of. the optical pump waves that generate them as well as on the electrostrictive properties of the. interaction medium Most commonly BDG based sensors employ PM fibers where two. counter propagating optical pump waves PumpH and PumpL PumpH PumpL are polarized. along the slow axis of the fiber and the Probe pulse is orthogonally polarized and propagates. along the fast axis of the fiber For a Stokes BDG scenario the Probe pulse is launched into. the fiber from the same side as PumpH It is then reflected from a co propagating refractive. index grating the BDG which was generated by PumpH and PumpL The reflected signal is. also Doppler downshifted by the BDG frequency PumpH PumpL The grating amplitude. and phase depend on the frequency difference between the writing pumps as well as on the. local strain temperature of the fiber Therefore in classical BDG sensing to obtain the. measurand information the frequency difference between the writing pumps is scanned. looking for the frequency difference that maximizes the intensity of the reflected probe Much. like the case of the SA BOTDA technique a major speed advantage can be achieved if the. frequencies of the signals involved in the interaction are tuned to the slope of the BDG. spectrum 25 so that rapid strain variations are translated to changes in the intensity of the. probe reflection However the intensity based slope assisted BDG SA BDG and SA. BOTDA techniques share the same disadvantage of measurand dependence on the local. optical power which impairs their performance Furthermore in SA BDG setups the PM. fibers birefringence longitudinal variations introduce errors to the measurement through the. modification of the conversion factor between the intensity and strain temperature While. cannot be mitigated using the pre compensation technique of 12 these manufacturing. related and measurand induced birefringence variations introduce additional impediments to. dynamic strain measurements To address these disadvantages we hereby propose a. phasorial SA BDG technique which overcomes most if not all these disadvantages. In our proposal two counter propagating BDGs are generated by the same PumpH and. PumpL both of which are now launched from both sides of the PM fiber polarized along its. slow axis Fig 1 To attain maximum gratings strength the frequency difference between the. pumps is tuned to the Brillouin frequency shift BFS of the slow axis of the fiber B. 11GHz An orthogonally polarized dual tone Probe pulse can be launched from either side. of the fiber and propagates along the fast axis of the fiber The Probe pulse carrier frequency. comprises two tones a higher frequency tone Probe HF which is reflected from the Stokes. BDG a reflection from a receding grating attaining maximum reflection for. Vol 25 No 5 6 Mar 2017 OPTICS EXPRESS 5380, Probe Stokes PumpH BDG BDG primarily depends on the fiber birefringence 23 n nslow. Phase based high spatial resolution and distributed static and dynamic strain sensing using Brillouin dynamic gratings in optical fibers ARIK BERGMAN TOMI LANGER AND MOSHE TUR School of Electrical Engineering Tel Aviv University Ramat Aviv Tel Aviv 6997801 Israel

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