High Spatial Resolution BOTDR Based on Differential ... - IEEE Xplore

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 28, NO. 14, JULY 15, 2016

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High Spatial Resolution BOTDR Based on Differential Brillouin Spectrum Technique Qingyu Li, Jiulin Gan, Yuqing Wu, Zhishen Zhang, Jiong Li, and Zhongmin Yang Abstract— A novel differential-technique Brillouin optical time domain reflectometry with sub-meter spatial resolution is proposed and demonstrated. By analyzing the time-space related property of the pulse excited backward spontaneous Brillouin scattering light, a weighting factor distribution of the Brillouin spectrum along the fiber is obtained. Based on this distribution, a two-step-subtraction technique is proposed. A pulse pair with slightly width difference is employed as the probe pulse. At each corresponding location, for each pulse of the pulse pair a Brillouin-spectrum pair is extracted with two different time sequence length. By performing a two-step subtraction on these two Brillouin-spectrum pairs, the differential Brillouin spectrum is theoretically and experimentally proved to be spatially related with the width difference of the pulse pair. A spatial resolution of 0.4 m utilizing 60/56 ns pulse pair is experimentally achieved over 7.8-km sensing length with 4.1-MHz Brillouin frequency accuracy. Index Terms— Brillouin fiber sensor, Brillouin spectrum analysis, differential technique.

I. I NTRODUCTION RILLOUIN fiber sensor has the capability of distributed sensing of temperature/strain of fiber. Tens of kilometers distance distributed fiber strain and temperature sensing system based on time-domain spontaneous Brillouin scattering (BOTDR) [1]–[3] or stimulated Brillouin scattering (BOTDA) [4]–[6] have been developed. Conventionally, the spatial resolution of BOTDR/BOTDA equals half of the probe pulse width. When the pulse is shorter than the phonon lifetime (10ns), the decrease in Brillouin gain makes it hard to measure the Brillouin frequency shift (BFS) accurately, further affecting the sensing accuracy, which limits the spatial resolution to 1 m. To improve the spatial resolution, prepump-BOTDA [7], dark-pulse BOTDA [8] and differentialtechnique BOTDA [9], [10] are proposed and demonstrated. Centimeter-scale spatial resolution has been achieved and demonstrated over kilometer long sensing fiber based on

B

Manuscript received February 20, 2016; revised April 11, 2016; accepted April 13, 2016. Date of publication April 15, 2016; date of current version May 12, 2016. This work was supported in part by the High-Level Personnel Special Support Program of Guangdong Province under Grant 2014TX01C087, in part by the Fundamental Research Funds for Central Universities under Grant 2015ZP019, in part by the Opened Fund of the State Key Laboratory on Integrated Optoelectronics under Grant IOSKL2015KF04, in part by the National Science Fund for Distinguished Young Scholars of China under Grant 61325024, and in part by the National Natural Science Foundation of China under Grant 61575064. The authors are with the State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China, and also with the Special Glass Fiber and Device Engineering Technology Research and Development Center of Guangdong Province, Guangzhou 510640, China (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2016.2555078

differential-technique BOTDA [10], [11]. For field application, BOTDR technique owns the attractive feature of one-end access. Some signal processing methods based on BOTDR have been applied for improving the spatial resolution, such as Brillouin spectrum analyzing [12] or iterative subdivision method [13]. Since differential-technique in BOTDA system can achieve high spatial resolution with long sensing length, it also has great potential in BOTDR system. However, as far as the author knows, the study of differential-technique on BOTDR has not been reported. In this letter, we present a high spatial resolution BOTDR system with a two-step-subtraction differential Brillouin spectrum technique. Tens of nanoseconds pulse pair with slightly width difference is employed and successively launched into the sensing fiber. Through analyzing the energy distribution of the backward spontaneous Brillouin scattering (SpBS) light, direct subtraction of the pulse pair related Brillouin spectrums fails to improve the spatial resolution. But with the two-stepsubtraction technique illustrated in this letter, sub-meter spatial resolution can be achieved. For each pulse of the pair, a Brillouin-spectrum pair has been obtained by Brillouin spectral analysis with two different time sequence lengths. By properly setting related time sequence length and performing the twostep-subtraction technique, the differential Brillouin spectrum is theoretically and experimentally proved to be spatially related with the sensing information excited exactly by the difference of pulse pair, which indicates that the spatial resolution can be greatly improved. Differential Brillouin spectrums with 40cm spatial resolution are extracted continuously along the 7.8km sensing fiber, while the signal-to-noise ratio and BFS accuracy are 33.5 dB and 4.1 MHz, respectively. II. P RINCIPLES In the BOTDR systems, the backward SpBS light excited by an injected pulse is recorded as a function of time, and from a sequence of detected time-domain Brillouin signal through spectral analysis we can extract the sensing related Brillouin spectrum, which is given as gb g(v, v b ) = (1) 1 + 4(v − v b )2 /v b2 where v is the frequency, gb is the peak gain, v b is sensing related center frequency, v b is spectrum linewidth. When the temperature or strain varies along the sensing fiber, we assume that the measured Brillouin spectrum is given by  z0 +D G(v) = a(x)g(v, v b (x))d x (2) z0

where a(x) is the weighting factor, D is the length of the corresponding region of the sensing fiber. The distribution of weighting factor a(x) is determined by optical pulse width τ and time sequence length T . As illustrated in Fig. 1(a), at each time instant t0 , the detected

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Fig. 1. Illustration of the superposition property of SpBS light. (a) The detected SpBS light at instant of time t0 is the superposition of the sub SpBS lights generated within a fiber section L; (b) the detected SpBS light within time range from t0 to t0 + T is the superposition of the sub SpBS lights generated within a fiber section D.

SpBS light can be considered as the superposition of sub SpBS lights generated from different position by different portion of the pulse, which indicates the spatial resolution L = cτ/2n in fiber (where c is the light velocity in vacuum, n is the refractive index of fiber). In the real signal processing illustrated in Fig. 1(b), Brillouin spec- tral analysis (commonly FFT used) requires a definite length T of sequence t0 ∼ t0 +T , the related SpBS light covers location from z 0 to z 0 + D (D = c(τ + T )/2n), where the spatial resolution is apparently degraded by time sequence length T . It is assumed that the power of SpBS lights are linearly proportional to the energy of the optical pulse [14]. According to the energy distribution of the corresponding sub SpBS lights, the weighting factor a(x) within location from z 0 to z 0 + x is given as: a(x) ⎧ 2nkx/c, 0 ≤ x < cTmin /2n ⎪ ⎪ ⎨ cTmin /2n ≤ x ≤ cTmax /2n kTmin , = ⎪ k(T + T − 2nx/c), cTmax /2n < x max ⎪ ⎩ min ≤ c (Tmin + Tmax ) /2n (3) where Tmin = mi n{T, τ }, Tmax = max{T, τ }, k is a constant coefficient. Intuitively, a pulse pair (τ1 and τ2 ) with slightly width difference (τ = τ1 − τ2 ) is employed as the probe pulses, FFT with time sequence length T1 and T2 (where T1 = τ1 and T2 = τ2 ) has been utilized to obtain the Brillouin spectrum G 1 (v) and G 2 (v), respectively, and the differential Brillouin spectrum G d (v) is obtained by the one-step direct frequency domain waveform subtraction as shown in Eq. (4). G d (v) = G 1 (v) − G 2 (v)  z0 +D = [a1 (x) − a2 (x)]g(v, v b (x))d x

(4)

z0

The difference of the pulse pair with width τ is defined as equivalent residual pulse. The spatial resolution can be proved to be greatly improved, only if G d (v) is spatially related with the sensing information excited exactly by equivalent pulse. However, the differential weighting factor ad (x) = a1 (x) − a2 (x) shown in Fig. 2(a) still cover a fiber section with the length larger than L 1 = cτ1 /2n. The main reason is that, in the sequence of detected signal for Brillouin spectral analysis, the energy of sub SpBS light generated from different position varies along the corresponding section, inducing the non-uniform weighting factor distribution of the measured

Fig. 2. The weighting factor distribution of differential Brillouin-spectrum. (a) One-step subtraction illustration, where a1 (x) and a2 (x) is the weighting factor of the Brillouin-spectrum pair in case of T = τ = τ1 and T = τ = τ2 , respectively; (b) two-step subtraction illustration, where a1 (x) is the case of τ = τ1 and T = T1 ; a2 (x): τ = τ1 , T = T2 ; a3 (x): τ = τ2 , T = T1 ; a4 (x): τ = τ2 , T = T2 .

Fig. 3. The scheme of differential Brillouin spectrum technique based BOTDR.

Brillouin spectrum. Therefore, the time-space related differential Brillouin spectrum could not be extracted by direct one-step subtraction. In order to substantially improve the spatial resolution, a two-step-subtraction technique has been proposed. As illustrated in Fig. 3, at the case of longer pulse with width τ1 , FFT with different time sequence lengths T1 and T2 = T1 − τ (where T1 > τ1 and T2 > τ2 ) has been utilized, Brillouin spectrums G 1 (v) and G 2 (v) have been obtained; the same procedure has been implemented for the case of the narrower pulse with width τ2 , which has a delay of τ relative to the longer pulse, and the Brillouin spectrums G 3 (v) and G 4 (v) have been obtained. The resultant differential Brillouin spectrum has been obtain by two-step spectrum subtraction as following: G dd (v) = [G 1 (v) − G 2 (v)] − [G 3 (v) − G 4 (v)]

(5)

As shown in Fig. 2(b), utilizing this two-step spectrum subtraction technique, the resultant weighting factor add (x) highlights the red area with the full width at half maximum (FWHM) of L R = cτ/2n, indicating that the BFS of the fiber area with spatial resolution L R l can be estimated by obtaining the differential Brillouin spectrum G dd (v). The spatial resolution L R is decided by the width difference τ of pulse pair. However, as can be seen in Fig. 2(b), the proportion of the weighting factor add (x) is very small compared with a1 (x), which is reduced by (τ/τ )2 . Therefore, the SNR has also been greatly reduced by this two-step-subtraction differential Brillouin spectrum technique, which will deteriorate the sensing range and measuring accuracy. To keep the SNR of the received Brillouin scattering signal at an acceptable level and avoid broadening the Brillouin spectrum, the pulse width τ should be set to 30∼100 ns and width difference τ < 10 ns for achieving sub-meter spatial resolution. In order to keep

LI et al.: HIGH SPATIAL RESOLUTION BOTDR BASED ON DIFFERENTIAL BRILLOUIN SPECTRUM TECHNIQUE

Fig. 4. Experimental setup: SFNL Laser, single frequency narrow linewidth laser; OC, optical coupler; EDFA, erbium-doped fiber amplifier; AWG, arbitrary waveform generator; EOM, electro-optic modulator; BPF, bandpass filter; Cir, circulator; DB-PD, double balanced photo detector; DAQ, data acquisition card.

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Fig. 5. The experiment effect of differential Brillouin spectrum technique. (a) A heated section with length of 0.8m in the fiber under test; (b) the measured Brillouin frequency shift distribution.

spectrum analysis at nearly the same condition, the FFT time sequence length T is set to be just a little larger than τ (∼5 ns). III. E XPERIMENT AND R ESULT The experimental setup is shown in Fig. 4. The laser source is a self-developed 1550 nm single- frequency fiber laser with 2 kHz linewidth and low intensity noise [15]. The source laser with 40 mW output power is split into the probe and reference paths by a 50/50 optical coupler (OC). After amplified to 100 mW by an erbium doped fiber amplifier (EDFA), the contin- uous probe light is modulated by electro-optic modulator (EOM) to form a pulse light with tens of nanoseconds width and 10 KHz repetition rate. The probe pulse is optically ampli-fied by another EDFA and filtered by an optical bandpass filter (1 nm bandwidth) in order to reduce amplified spontaneous emission (ASE) noise. The peak power of the pulse launched into the sensing fiber is above 30 dBm. The continuous pump light is launched into a fiber ring cavity to stimulate Brillouin fiber lasing [16]. Compared with the pump light, the Brillouin laser has an 11 GHz frequency downshift. Due to the very small doping concentration difference between the Brillouin gain fiber in the cavity and the sensing fiber, the BFSs have a slight difference about hundreds of MHz between the Brillouin fiber laser and the spontaneous Brillouin scattering light. The Brillouin fiber laser is used as the reference light and mixed with the spontaneous Brillouin scattering light by a 50:50 OC. The beat component with a frequency less than 800 MHz is measured at the double balanced photo detector (DB-PD) associated with the high speed data acquisition card (DAQ), in which the data acquisition is synchronous with the signal generator of the EOM. Acquisition and analysis of the sensing signal are real-time processed by a self-designed program. As illustrated in the theory part, when the injected optical pulses are ideal rectangular shape, the spatial resolution of the differential Brillouin spectrum technique based BOTDR is determined by width difference of the pulse pair. The pulse pair with width τ = 68/60 ns and rise/fall-time (10%-90%) of 125 ps is successively launched into the fiber under test. The time sequence length pair for FFT is set to be T = 78/70 ns. The fiber length under test is over 7800 m, including a heated section of 0.8m at the location 7514 m, as schematically shown in Fig. 5(a). The temperature applied on this section is 70 °C, which induces temperature related BFS. Figure 5(b) shows the measured BFS distribution. Figure 6 shows the localized Brillouin spectrums of the heated section fiber.

Fig. 6. The experimental result of differential Brillouin spectrum technique based BOTDR. (a) At the 7514 m position, the related Brillouin spectrums are obtained, curve G 1 and G 2 represent τ1 = 68 ns pulse case, and T1 = 78 ns, T2 = 70 ns, respectively; curve G 3 and G 4 represent τ2 = 60 ns pulse case, and T1 = 78 ns, T2 = 70 ns, respectively; (b) G dd = (G 1 −G 2 )−(G 3 −G 4 ), and curve G f is the Lorentzian fitting of curve G dd .

Fig. 7. Temperature distribution measurement. (a) Two heated sections along the fiber under test; (b) the demodulated differential Brillouin spectrums related with location; (c) the measured temperature distribution.

As shown in Fig. 6(b), the common term of 192 MHz peak is eliminated and the 142 MHz peak is highlighted, which relates directly towards the special location of the 0.8 m heated section. Curve G dd clearly shows the Brillouin spectrum of 0.8 m heated section with 40 MHz linewidth. Meanwhile, if single 8 ns probe pulse is injected, the insufficient Brillouin scattering effect will greatly enlarge the linewidth of the Brillouin spectrum and reduce the accuracy of the measurement. Through the spectrum differential method, the spatial resolution is enhanced greatly and the signal-to-noise ratio is kept at the acceptable level. The temperature sensing experiment is conducted to confirm the spatial resolution improvement. The fiber under test is about 7.8 km long, including two heated sections of 0.4 m and 0.8 m, separated by 0.8 m unheated section, as schematically shown in Fig. 7(a). The temperature applied to the two heated sections is 65 °C. The injected pulse pair is τ = 60/56 ns, and the FFT time sequence length pair is set to be T = 64/60 ns. Based on the procedure introduced above, the differential Brillouin spectrums related with the location are demodulated as shown in Fig. 7(b). The temperature-frequency shift coefficient of this testing fiber is about 1.01 MHz/°C and the room

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Fig. 8. Strain distribution measurement. (a) Different strain applied on the fiber under test; (b) the demodulated differential Brillouin spectrums related with location; (c) the measured strain distribution. TABLE I B RILLOUIN F REQUENCY R ESOLUTION W ITH 214 AVERAGES

IV. C ONCLUSION A two-step differential technique BOTDR sensing system is proposed and demonstrated with spatial resolution of 0.4 m over 7.8 km long fiber. Through analyzing the weighting factor distribution of the Brillouin spectrum along the fiber, direct subtraction of the pulse pair related Brillouin spectrums fails to improve the spatial resolution. The two-step-subtraction technique is proposed for achieving sub-meter spatial resolution. Tens of nanoseconds pulse pair with slightly width difference are employed and successively launched into the sensing fiber. For each pulse of the pair, a Brillouin-spectrum pair has been obtained by spectral analysis with two different time sequence lengths. By properly setting related time sequence length and performing the two-step-subtraction technique, the differential Brillouin spectrum is theoretically and experimentally proved to be spatially related with higher spatial resolution location. R EFERENCES

temperature is about 22 °C, based on these environmental and material parameters, the temperature distribution along the fiber is demodulated as shown in Fig. 7(c). The two heated sections and the isolated section can be clearly distinguished. The rise/fall length of the temperature distribution curve is 0.4 m, which implies that 0.4m spatial resolution has been achieved. The strains distribution experiment is also conducted. The fiber under test is after 7.5 km position, including two stressed sections of 0.4 m and 0.8 m, separated by 0.8 m loose fiber, as schematically shown in Fig. 8(a). The strain applied to the two sections is about 1000 με and 2000 με, respectively. The injected pulse pair is τ = 60/56 ns and the FFT time sequence length pair is set to be T = 64/60 ns. Based on the procedure introduced above, the differential Brillouin spectrums related with the location are demodulated, and the strain-frequency shift coefficient of this testing fiber is about 0.05 MHz/με. The differential Brillouin spectrums related with the location are demodulated as shown in Fig. 8(b). The strain distribution along the fiber is shown in Fig. 8(c), where the two stressed sections and the loose section can be clearly distinguished. During the strain distribution measurement experiments, different pulse pairs have been applied to the BOTDR system. The SNR is defined and measured to be the electrical signal to noise power ratio of the Brillouin spectrum. The Brillouin spectrum adopting single pulse of 60 ns, 56 ns, 54 ns and 52 ns have similar SNR, while the SNR of the differential Brillouin spectrums in the cases of 60/52 ns, 60/54 ns and 60/56 ns pulse pairs are quite different, as shown in Table I. In the experiments, 214 averages have been taken to obtain the differential Brillouin spectrum and the data shown in table I. The last case τ = 60/56 ns has the highest spatial resolution due to the smallest width difference of pulse pair. The Brillouin frequency accuracy (that is measure error) is calcu√ lated by  f B = ν B /( 2S N R 1/4 ), where v B = 40 MHz is the differential Brillouin spectrum linewidth measured in the experiment. The spatial resolution L R is obtained by calculating the rise/fall length of the strain distribution curve. It is shown in table I that as the decrease of the width difference of the pulse pair, the Brillouin frequency accuracy reduces obviously, this is mainly caused by the decrease of SNR. However, if 218 averages have been taken, the Brillouin frequency accuracy will be improved to 4.1MHz for the case of τ = 60/56ns.

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