Sweep-free distributed Brillouin time-domain analyzer ( SF-BOTDA )

A frequency-sweep-free method for distributed Brillouin sensing is proposed, having the potential for fast dynamic strain measurements. In this reported implementation of the method, multiple probe waves with carefully chosen optical frequencies simultaneously propagate in the fiber against an equal number of sequentially-launched, short pump pulses of matching frequencies, where each of pump-probe pair replaces one sweeping step in the classical BOTDA technique. Experimentally, distributed sensing is demonstrated with a spatial resolution of a few meters. ©2011 Optical Society of America OCIS codes: (060.2330) Fiber optics communications; (060.2370) Fiber optics sensors. References and links 1. R. W. Boyd, Nonlinear Optics (Academic Press, 2008), Chap. 9. 2. M. Barnoski and S. Personick, “Measurements in Fiber Optics,” Proc. IEEE 66(4), 429–441 (1978). 3. S. Diaz, S. Foaleng Mafang, M. Lopez-Amo, and L. Thevenaz, “A high performance Optical Time-Domain Brillouin Distributed Fiber Sensor,” IEEE Sens. J. 8(7), 1268–1272 (2008). 4. R. Bernini, A. Minardo, and L. Zeni, “Dynamic strain measurement in optical fibers by stimulated Brillouin scattering,” Opt. Lett. 34(17), 2613–2615 (2009). 5. Y. Peled, A. Motil, and M. Tur, “Distributed and dynamical Brillouin sensing in optical fibers”, Optical Fiber Sensor conference, OFS21, Ottawa, Canada, (2011). 6. P. Chaube, B. G. Colpitts, D. Jagannathan, and A. W. Brown, “Distributed fiber-optic sensor for dynamic strain measurement,” IEEE Sens. J. 8(7), 1067–1072 (2008). 7. A. Voskoboinik, J. Wang, B. Shamee, S. Nuccio, L. Zhang, M. Chitgarha, A. Willner, and M. Tur, “SBS-based fiber optical sensing using frequency-domain simultaneous tone interrogation,” J. Lightwave Technol. 29(11), 1729–1735 (2011). 8. A. Voskoboinik, J. Wang, A. E. Willner, and M. Tur, “Frequency-domain simultaneous tone interrogation for faster, sweep-free Brillouin distributed sensing”, 21st Inter. Conference of Optical Fiber Sensors, Proc. Of SPIE, 7753 (2011) 9. A. Voskoboinik, O.F. Yilmaz, A.E. Willner and M. Tur, “Sweep-free distributed Brillouin sensing using multiple pump and probe tones”, ECOC, Geneva, Switzerland, September 2011. 10. S. Damzen, V. Vlad, V. Babin and A. Mocofanescu, Stimulated Brillouin Scattering, Fundamentals and Applications (Institute of Physics Publishing, 2003), Chap. 1. 11. K. -D. Chung, G. -C. Yang, and W. C. Kwong, “Determination of FWM products in unequal-spaced-channel WDM lightwave systems,” J. Lightwave Technol. 18(12), 2113–2122 (2000).


Introduction
Stimulated Brillouin based sensors are of great interest in various environmental and security applications.Sensors based on this technology use the Brillouin nonlinear process [ 1] in which acoustic phonons in the fiber mediate, via a stimulated interaction, power transfer between counter propagating waves ('pump' and 'probe').For a given pump, the spectral characteristics of the Brillouin-amplified light are uniquely tied to the strain and temperature dependent acoustic velocity in the fiber, thereby allowing the extraction of the temperature and strain distribution along optical fibers [2][3].
One of the most prevalent techniques is the Brillouin Optical Time Domain Analysis (BOTDA) [ 3], which requires frequency sweeping of one of the counter-propagating waves in order to find the frequency at which the energy mediation is maximal.This frequency is called the Brillouin frequency shift (BFS).The need to make consecutive multiple frequency steps in order to map the Brillouin gain spectra (BGS) and locate its maximum may potentially limit the ability to resolve fast, dynamic changes in the BFS distribution along the optical fiber.
Several dynamic Brillouin sensing concepts have been already proposed and demonstrated [4][5][6].Recently, [ 7,8], we described a novel concept, called SF-BOTDA (for Sweep-Free BOTDA), which retains all the advantages of the classical BOTDA technique together with the potential to be much faster.In [ 7] we demonstrated the concept in which multiple CW pump and probe tones Brillouin-interact pair-wise in the fiber to simultaneously probe different parts of its BGS.Once the tones are detected, the Brillouin amplification experienced by each of them can be determined, resulting in an accurate reconstruction of the BGS, and, consequently, the determination of the BFS.Then in [ 8] we discussed an extension, where the multiple pump tones are concurrently pulsed to achieve distributed sensing, faster than classical BOTDA by a factor, which could be as high as the number of simultaneous tones used.It turns out, though, that such a scheme has some practical disadvantages, having to do with fiber nonlinearities and overloading of optical amplifiers.Instead, sequential pumping is shown in this paper to achieve true distributed sensing without significantly compromising the sensing speed.While sequential pumping, described in Sec. 2, has been used in our first demonstration of distributed sensing using SF-BOTDA [ 9], here we report an important improvement of the experimental setup together with new results, Sec. 3.

Description of method
The concept of the SF-BOTDA sensor, first described in [ 7], is illustrated in Fig. 1.Multiple pump tones are produced, each generating the ~30 MHz-wide BGS with the corresponding BFS, as determined by the fiber type and strain/temperature environment [ 10].In addition, simultaneous multiple probe tones are launched in the opposite direction with slightly larger spacing than the pump tones.The idea is to arrange the probe tones in such a way that each of them is located in a different region of the BGS of the corresponding pump tone.The probe tones, which are located closer to the BGS center, will see higher Brillouin gain.In this manner the BGS can be reconstructed without the need for frequency sweeping, where the i th tone of the probe is amplified by the i th tone of the pump through SBS.The pump frequency spacing has to exceed the width of the BGS, and was set to 100MHz in Fig. 1.Consequentially, the corresponding generated BGSs are equally 100MHz spaced and of equal width [ 10].In order to appropriately position the probe tones in and around the corresponding BGSs, their optical frequencies are downshifted from the pump tones by the approximate BFS.The progression of the frequency spacings between corresponding pump and probe tones determines the resolution of the BGS reconstruction.In our example, the probe frequency spacing is chosen to be 103 MHz, such that the frequency difference between the pump and the probe tones is decreased by 3MHz from the i th tone to the (i+1) th tone.As a result, the different probe tones experience different amounts of Brillouin amplification and thus, simultaneously provide information about the shape of the BGS in a single measurement, without the need for probe frequency sweeping.As in classical BOTDA,  The use of many simultaneous pump tones, as in [ 7], may strain the Erbium-doped fiber amplifiers (EDFAs).Furthermore, pump inter-tone modulation effects may also become detrimental.To address these issues, we introduce here the use of sequentially launched pump tones, where the single pump pulse of width T, comprising the N frequency tones, is replaced by a compact sequence of N equally-wide sub-pulses, each with a different frequency, as described in Fig. 2 and Sec. 4 below.

Experimental setup and results
The experimental setups used in [ 9] and in this paper are shown in Figs.3(a) and 3(b), respectively.The current setup will be described in detail and its modifications with respect to the old one will be emphasized.A narrow linewidth (80 KHz) tunable laser is split into probe and pump arms.For the probe wave, a radio-frequency (RF) frequency comb is generated by a wideband arbitrary waveform generator (AWG).This RF signal, comprising a superposition of multiple (N) sinusoidal waves (tones) at the desired frequencies, feeds the intermediate frequency (IF) input of the RF mixer, while the latter's local oscillator (LO) input is driven by a synthesizer running at the average BFS of the fiber (e.g., 10870MHz), resulting in 2N RF tones, symmetrically arranged around 10870MHz.This compound signal now drives the MZM1 Mach-Zehnder modulator, operating at its quadrature point.MZM1 translates this microwave signal to the optical domain, generating two optical multiple-tone sidebands (2Ntone each) around the laser frequency, with the lower sideband to serve as the probe.Note that the laser frequency is also present in the probe wave: It will have no pump counterpart for Brillouin amplification but it will later serve for heterodyne detection of the amplified probe tones.In the older setup MZM1 was biased near zero transmission, allowing only for a much weaker laser frequency component to propagate through the fiber.
The pump pulsed wave is also derived from the same laser frequency.Another channel of the AWG generates the signal of Fig. 2(a-b), comprising a compact sequence of equally-wide sub-pulses, each with a different frequency.Modulator (MZM2), operating at its zerotransmission point, is used to translate the RF pump pulse train to the optical domain.Each sub-pulse generates two optical pulsed tones, as seen in Fig. 2(c).For example, the first RF sub-pulse, riding on an RF carrier of 105MHz will generate a pair of time-overlapping pulses, one riding on an optical carrier having a frequency 105MHz higher than that of the laser frequency, while the other is 105MHz below the laser frequency.Using this technique, the overall required instantaneous optical power used for amplification is just 3dB higher than in conventional BOTDA, since at each time slot only two optical tones (generated from a single RF tone) are generated by MZM2.Furthermore, since pulsing of the pump tones is performed in the RF domain, and since MZM2 is a high extinction ratio modulator, biased at its zero transmission point, very high extinction ratio can be achieved for the pump pulses.Noting that normally, the fiber-under-test (FUT) length far exceeds the pump pulse length, no measurement speed is lost if we use sequential pumping, thereby increasing the few nanosecond wide sub-pulse only by a factor of N. The sequential launching of the pump tones results in time-shifted Brillouin-amplified probe tones, to be realigned at the post processing stage by a simple temporal correction (Fig. 4(a)).Continuing now with the pump channel, an Erbium-doped fiber amplifier (PRE-EDFA2) preceding MZM2, serves to reach the maximum allowed input power into MZM2.From MZM2 the pump signal continues to EDFA2 for further amplification, and finally to the FUT.A polarization-scrambling device (SC) is located prior to EDFA2, to average out the influence of the polarization dependence of the Brillouin gain.Unlike the older setup, now the narrow linewidth fiber Bragg grating (FBG) is used to precisely block the upper optical sideband, rather than transmitting the lower sideband (Fig. 3(b)), and consequently significantly attenuating the co-propagating laser frequency.This modification along with the previous one significantly improved the obtainable signal to noise ratio.After the Brillouin spectrally selective amplification and additional flat amplification by EDFA2, the probe signal hits the wideband detector, where it mixes with the laser signal that co-propagated with it along the fiber.The resulting ~11GHz electrical signal is amplified by an RF amplifier and sent to a fast 20GHz bandwidth real-time acquisition system for further post-processing.In the experiments below, two pump sub-pulses were used: 500ns and 50ns.For the former the chosen RF pump tone frequencies for the compound pump pulse were: 80, 155, 275, 375, 450, 535, 625, 725, 810, 900, 985, 1080, 1160, 1250 and 1350 MHz.This unequal spacing was used to reduce the detrimental effect of intermodulation products [ 11].The corresponding RF probe tone frequencies were 77, 149, 266, 363, 435, 517, 604, 701,783, 870, 952, 1044, 1121, 1208 and 1305MHz, having a 3-MHz spacing decrement relative to the pump tone spacing.Since the shorter 50ns pump sub-pulses have wider spectral bandwidth, a more spacious selection of (N = 10) pump (probe) tones was used: 105 (102), 240 (234), 390 (381), 525 (513), 670 (655), 810 (792), 955 (934), 1090 (1066), 1230 (1203) and 1370 (1340) MHz.After up-conversion, the resulting optical probe comb included 30 (for the 500ns case) or 20 (for the 50ns case) different frequencies located around 10877MHz, corresponding, respectively, to a sweep-free range of 90 or 60MHz, with a 3-MHz resolution, respectively.showing the time evolution of each of the 20 optical tones used.The width of each pump subpulse was 50ns and the center of the tones (i.e., the frequency of the microwave source in Fig. 3) was chosen to coincide with the fiber BFS.(b) Reconstruction of the BGS using 20 frequency tones with 5-m resolution with no stretching applied to the fiber.The distance axis is obtained from the temporal axis of using: Position = Group-velocity x Time/2 For each launched compound pump pulse, the Brillouin return from the fiber had to be collected from the beginning of the first pump sub-pulse until the contribution of the last pump sub-pulse returned from the fiber end.In practice, we have collected data beyond the return from the last sub-pulse, thereby recording the unamplified probe signal, to be used later for the estimation of the Brillouin gain of the individual probe tones.The recorded signal was then averaged over 22 launched compound pump pulses.For a sub-pulse of duration T, the recorded signal was divided into T-long sections, each representing a fiber segment of length V g T/2, where V g is the group velocity of the optical signals in the fiber.Each T-long section was then Fourier transformed [ 8] to obtain the spectrogram of Fig. 4(a), showing the time evolution of each of the 20 optical tones, for an essentially Brillouin-homogeneous, 20m-long fiber.Here T = 50ns and 20 (N = 10) tones were used, centered around the fiber BFS.As expected, the first two sub-pulses arrived first at the detector.As per our choice, they carry frequencies closest to the fiber BFS, thereby measuring the gain near the peak of the BGS.The last to arrive sub-pulses have frequencies farthest from the BFS and provide information on the gain in the wings of the BGS.Following proper time-shifting and Lorentzian fitting (with peak normalization after fitting) we obtain the commonly recognized distancefrequency distribution of a uniform fiber, Fig. 4(b).The first distributed sensing to be reported dealt with a 2-km FUT, comprising two spliced together, 1-km SMF-28 fiber spools with different BFSs of 10877 and 10892MHz.To minimize temperature effects on the BFS, the FUT was kept in a 35°C inside the temperature chamber.Figure 5(a) shows the resultant BGS variation near their joint.Longer, 500ns sub-pulses were used here, along with 30 tones.Lorentzian fit was performed, giving an average BGS FWHM of 29.1±2MHz.Sweep-free measurements compared favorably with classical BOTDA results of a FWHM of 27.6 ±1.5 MHz, measured using the same frequency sweep step of 3MHz and pulse width, see Fig. 5(b).The slight change in BFS near the end of the first fiber is not due to the effective spatial resolution of 50m, but rather to strain variations resulting from manual spooling.
In a higher spatial resolution experiment, we stretched the middle 4m part of the 20m fiber of Fig. 4(b).The pump sub-pulse width was set to 50ns, which reflects an effective resolution of 5m. Figure 4(b) described the unstretched state of the fiber.Stretching, Fig. 5 (c), gave rise to a 14MHz shift in the BFS, indicating a local average strain of ~280µε.Only 22 integrations performed resulting in a total sensing time of ~30µsec, assuming pump pulses at a repetition rate of 1MHz.

Discussion and conclusions
The choice of the number of tones, N , their spacing, Pump , should not exceed the BFS (~11GHz) in order to avoid disturbing effects of the lowest frequency pump tone on the highest frequency probe tone.While larger frequency spans cause the BFS to slightly change as a result of its dependence on the optical frequency [7], this dependence can be corrected for in the signal processing stage; (ii) The inter-tone spacing, Pump ν ∆ , must be larger than the width of the BGS at its bottom (~60MHz).Otherwise, a probe tone experiences gain from more than one BGS; (iii) In the current implementation of the proposed technique, the sweep-free dynamic range for strain or temperature measurements is given by must be at least of the order of the natural width of the BGS, which is ~30MHz or larger, as determined by the inverse of the width of the pump sub-pulse.
In summary, we successfully demonstrated a novel technique for distributed Brillouin sensing, which does not require the classical step-by-step mapping of the fiber BGS.This proposed method is potentially faster than classical BOTDA by a factor equal to the number of pump-probe pairs used for the BGS reconstruction, since each pair replaces one sweeping step in the classical technique.In the current implementation and for a given number of tones, N, there is a trade-off between the achieved resolution for strain/temperature measurements, and the available sweep-free frequency dynamic range.Current research aims at the removal of this trade-off.

Fig. 1 .
Fig. 1.BGS reconstruction using the newly proposed sweep-free concept accomplished in a single measurement using multiple (N) pump and probe tones.Here the pump spacing is 100MHz, while the additional incremental spacing of the probe tones is 3MHz Probe δν

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155558 -$15.00USD Received 28 Sep 2011; revised 17 Nov 2011; accepted 21 Nov 2011; published 7 Dec 2011 (C) 2011 OSA distributed Brillouin sensing measurements could, in principle, be achieved by gating the multiple pump tones with a pulse, whose width would determine the spatial resolution.

Fig. 2 .
Fig. 2. Sequential pump launching.For N multiple tones, the pump waveform comprises a sequence of T-wide, N sub-pulses, each riding on a different frequency tone.As described in Sec. 3, the figure describes the RF waveform, which is then upconverted to an optical compound pulse.(a) Sub-pulse amplitude vs. time; (b) Sub-pulse frequency vs. time.(c)A spectrogram of the compound optical pulse used for sequential pumping.The laser frequency is located at the center of the horizontal optical frequency axis

Fig. 4 .
Fig. 4. (a) Spectrogram of the Brillouin return from an essentially uniform 20m-long fiber,showing the time evolution of each of the 20 optical tones used.The width of each pump subpulse was 50ns and the center of the tones (i.e., the frequency of the microwave source in Fig.3) was chosen to coincide with the fiber BFS.(b) Reconstruction of the BGS using 20 frequency tones with 5-m resolution with no stretching applied to the fiber.The distance axis is obtained from the temporal axis of using: Position = Group-velocity x Time/2

Fig. 5 .
Fig. 5. (a) Reconstruction of the BGS of a 2-km FUT, comprising two 1km fiber segments with different BFS, spliced together at the position 1000m.30 frequency tones, spanning 90MHz, were used with a sub-pulse width of T = 500ns, resulting in frequency and spatial resolutions of 3MHz and 50m, respectively.The zero frequency is 10877MHz (b) Results of classical BOTDA also with 3MHz sweeping step.(c) Reconstruction of the BGS using 20 frequency tones with 5-m resolution with the central 4-m of the fiber being stretched

. 1 )
depends on several considerations[ 7]: (i) The total frequency range spanned by the tones, of GHz's.With N being limited by the two previous considerations, wider dynamic range could be achieved by a larger Probe δν , but only at the expense of the obtained frequency granularity, potentially resulting in limited strain/temperature resolution.On the other hand, for proper Lorentzian fitting, Probe δν cannot be made too small since the sweep-free range Probe N δν ⋅