- Email: [email protected]
Effects of modulated pulse format on spontaneous Brillouin scattering spectrum and BOTDR sensing system
Optics & Laser Technology 46 (2013) 37–41
Contents lists available at SciVerse ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Effects of modulated pulse format on spontaneous Brillouin scattering spectrum and BOTDR sensing system Yunqi Hao a,b, Qing Ye a,n, Zhengqing Pan a, Haiwen Cai a,n, Ronghui Qu a, Zhongmin Yang c a
Shanghai Key Laboratory of All Solid-state Laser and Applied Techniques, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China Department of Technology Physics, Zhengzhou University of Light Industry, Zhengzhou, Henan 450002, China c Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510641, China b
a r t i c l e i n f o
abstract
Article history: Received 9 March 2012 Received in revised form 5 April 2012 Accepted 18 April 2012 Available online 17 May 2012
The signal noise ratio (SNR) enhancement effects of spontaneous Brillouin scattering spectrum on Brilloluin optical time domain reflectometry (BOTDR) sensing system have been analyzed theoretically and demonstrated experimentally through changing the modulated pulse format. With the same pulse width or same spatial resolution, the SNR is larger for triangular pulse. Take the width of 200 ns as an illustration, the SNRs of the coherent detection power spectrum for trapezoidal pulse and triangular pulse increase 3 dB and 4.8 dB relative to that of rectangular pulse respectively. The corresponding spectral linewidthes are narrowed and the sensing distances are also increased by about two times from the rectangular pulse to the triangular pulse. This phenomenon will be helpful to improve the spatial resolution or achieve longer sensing distance in the BOTDR sensing system at the same systemic conditions. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Brillouin optical time domain reflectometry (BOTDR) sensing system Modulated pulse format Signal noise ratio (SNR) enhancement effect
1. Introduction Recently, a number of studies [1–6] have focused on the longdistance distributed fiber sensors with Brillouin scattering in optical fiber for their potential applications of monitoring temperature and strain, which mainly include Brillouin optical time domain analysis (BOTDA) [1–3] and Brillouin optical time domain reflectometry (BOTDR) [4–6]. The power spectrum of the Brillouin backscattering induced by a launched pulse usually includes the strain and temperature information along the whole sensing fiber link, which may be used to detect the variations of strain and temperature simultaneously in different positions. In BOTDR sensing system, the frequency-shift and amplitude variation of the power spectrum are proportional to the changes of strain and temperature. In Ref. [7], the Brillouin scattering was fitted by a Lorenzian function to improve the accuracy of peak-power frequency measurements. High SNR power spectrum for spontaneous Brillouin scattering (SBS) will decrease the peak-power frequency measurement error [8] and it may increase the sensing distance while preserving the spatial resolution. Therefore enhancing the SNR for the same launched pulse width is one of the effective solutions to improve the peak-power frequency measurement accuracy without worsening the spatial resolution [9]. Pulse code technology [10] has been used to improve the Brillouin scattering SNR, but the coded and decoded process is very complicated for coherent detection. Furthermore, amplification techniques
n
Corresponding authors. E-mail addresses: [email protected] (Q. Ye), [email protected] (H. Cai).
0030-3992/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2012.04.025
[11,12] were used to increase the Brillouin backscattering intensity, but the SNR of signal itself has not been improved. In this paper, we propose and investigate experimentally the effects of modulated pulse formats on the SBS power spectrum and BOTDR sensing systems. The pulse width is defined by the full width half maximum (FWHM) of the light intensity, and the rise/ fall time is defined by the optical intensity of launched pulse rising from 0 to 1 and falling from 1 to 0. With the same pulse width of 200 ns for instance, the SNRs of the SBS/local oscillator (LO) beat frequency signal for trapezoidal pulses (80 ns rise/fall time) and triangular pulses (200 ns rise/fall time) increase 3 dB and 4.8 dB, respectively, relative to that of rectangular pulses (about 5 ns rise/fall time). Moreover, the spectrum linewidthes, obtained by the Lorentz fitting, are narrower when the pulse format is closer to a triangular shape. A BOTDR sensing system is built to test the influence of the modulated pulse format, and results show that the sensing distance of triangular pulse is about triple than that of the rectangular pulse. We believe that our study will be helpful to improve spatial resolution or sensing distance in BOTDR/BOTDA sensing systems.
2. Theoretical analysis and simulations For the backscattering Brillouin light, the frequency-dependent factor H(n) is [13] HðnÞ ¼
Z
1
P p ðf Þ 1
hðo=2Þ2
nðf SB Þ2 þ ðo=2Þ2
df
ð1Þ
38
Y. Hao et al. / Optics & Laser Technology 46 (2013) 37–41
where SB is Brillouin frequency shift, PP(f) is the power spectrum of launched pulse, o is the typical FWHM, and n is the abscissa of backward Brillouin power spectrum. For different pulse width and different modulation formats, the corresponding peak values of the Brollouin power spectra are shown in Fig. 1. It can be seen that the peak values become larger with wider pulse width. But with the same pulse width, i.e., the same spatial resolution, the triangular pulse has bigger value than that of the rectangular pulse. In this paper the pulse width is 200 ns for instance. When the modulated pulse formats are rectangular, trapezoidal, and triangular, their rise/fall time are 0 ns, 80 ns, and 200 ns
58
Triangular pulse Rectangular pulse
57
Peak values (dBm)
56 55 54 53 52 51 50 50
100
150 Pulse width (ns)
200
250
Intensity (a.u.)
Fig. 1. Peak values for different pulse width and different modulated formats.
Intensity (a.u.)
Rectangular Pulse Trapezoidal Pulse Triangular Pulse
1.0 0.8 0.6 0.4 0.2 0.0 -300
-200
-100
0 Pulse width (ns)
100
200
0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -0.02
300
Rectangular Pulse Trapezoidal Pulse Triangular Pulse
1.0 0.8 0.6 0.4 0.2 0.0 -0.02
Intensity (dB)
respectively as shown in Fig. 2(a). Fig. 2(b) shows the corresponding pulse power spectra by the fast Fourier transform (FFT). Launching these pulses into the long distance sensing fiber, the power spectra of SBS are simulated numerically according to Eq. (1) and the corresponding results are shown in Fig. 2(c). Comparing these power spectra, it is found that the triangular pulse shape has the highest peak intensity and narrowest spectrum bandwidth for the single SBS light. Difference of the Brillouin power spectra could be explained as follows: from Eq. (1), it can be seen that parameters h, o and SB are inherent properties of the optical fiber, related to the doping concentration, etc. PP(f) and f are the power spectrum and frequency of the launched pulse, which can be changed through launching different lights, and it is the only factor we can used to improve the Brillouin power spectrum. To the same optical source, the difference of Brillouin power spectra in Fig. 2(c) is resulted from power spectra of launched pulse formats, as shown in Fig. 2(b). The power spectra are normalized by the peak of triangular pulse power spectrum. The triangular pulse power spectrum is biggest and narrowest, and the rectangular is of smallest and widest. The Brillouin power spectrum is the convolution of the pulse and the SBS in the time domain, which means the product of the pulse power spectrum and the standard Brillouin power spectrum in the frequency domain. As shown in Fig. 2(b), the pulse power spectrum of triangular format has much larger peak value, narrower linewidth, and smaller sidelobes. In the backscattered SBS signal, product of these sidelobes and the ideal Lorentzian function is of no use and decreases the intensity of main peak because of energy conversation. For the equal input pulse energy, if more light energy concentrates in the main peak, where the Brillouin gain is better, the Brillouin power spectrum would also be bigger. So the Brillouin power spectrum from the sensing fiber is best for triangular, and vice versa.
-0.01
0.00 Frequency (GHz)
0.01
0.02
Rectangular Pulse Trapezoidal Pulse Triangular Pulse
-0.01
0.00 0.01 Brillouin power spectra (GHz)
0.02
Fig. 2. (a) Different modulated pulse formats with the same pulse width of 200 ns; (b) pulse power spectra; (c) corresponding Brillouin scattering spectra for three kinds of pulse modulated formats.
Y. Hao et al. / Optics & Laser Technology 46 (2013) 37–41
In order to test the effects of modulated pulse formats on the SBS spectrum and on the BOTDR fiber sensing system, an experimental setup is constructed (Fig. 3). A short-cavity single-frequency stable linear-polarization fiber laser (SFFL) [14] at 1548.976 nm is used as the primary light source in the sensing system. Part of the laser beam is amplitude modulated by an acousto-optic modulator (AOM, Model MT160-IIR10-FIO, AA Inc.) to generate pulse sequence with repetition rate of 4 kHz and pulse width of 200 ns. The repetition rate is relative to the sensing range, where 4 kHz corresponds to 25 km for accurate sensing location. For 1550 nm wavelength band, the Brillouin frequency shift is about 11 GHz. In order to detect the Brillouin scattering with the conventional detector (bandwidth o800 MHz), the other part of the source is applied to pump a compact singlefrequency Brillouin fiber laser (BFL) with about 11 GHz Brillouin frequency shift, which is used as LO for coherent detection [15,16]. The different pulse formats (e.g. rectangular, trapezoidal and triangular) are achieved through the programmable modulating drive signal imposed on the AOM drive module, changing the rise/fall time from a minimum of 5 ns (limited by the signal generator) to a maximum of 200 ns, as shown in Fig. 4. The modulated pulse is appropriately amplified (avoiding stimulated Brillouin scattering) by an Erbium doped fiber amplifier (EDFA) and then launched into the long sensing fiber to generate the backscattered spontaneous Brillouin signal. The SBS signal, carrying local information of temperature or stain along the whole sensing fiber, can be detected by the coherent heterodyne detection. The beat signal between SBS and LO are injected into a double balanced photodetector (DB-PD, New Focus 1617-AC, Newport) through a 1:1 coupler, and then amplified by a microwave amplifier (MWA, ALPHALAS BBA-100-VG, bandwidth 0.01– 2 GHz). All the data is collected by high-speed data-acquisition card (DAQ, Gage, Model CS82G, 3 GS/s sample rate), where the data acquisition is synchronized with the signal generator for the AOM. Acquisition and analysis of sensing signal are real-time processed by a customized LabVIEW program. In our experiment, the modulated pulses with 200 ns pulse-width are launched into the sensing fiber and the backscattered timedomain pulse signal is obtained with 30 sampling points every meter. As the length of the sensing fiber is 20 km, the corresponding spontaneous Brillouin pulse is about 200 ms. While acquiring the backward pulse signal, the beat frequency of Rayleigh scattering with LO and other DC components will be filtered out because the DB-PD response bandwidth is in the range 10 kHz–800 MHz. Fig. 5 shows the typical fast Fourier transform (FFT) spectra of the SBS/LO beat signal in the whole sensing fiber, averaged over 100 SBS pulses, with different pulse modulated formats. The center frequency is about 420 MHz, which is decided by the Brillouin frequency-shift difference between LO and the sensing fiber. Through Lorentz fittings of these FFT spectra, ratios of the peak and the bottom give SNRs. By changing the modulated pulse format from rectangular to triangular
for the same pulse-width of 200 ns, the SNRs of the beat frequency signal have obviously improved. In the BOTDR sensing system, optical intensity and frequency of LO are stable, so the SNR increment of the beat frequency signal is in essence the SNR improvement of SBS light. Through analyzing the Lorentz fitting curves in Fig. 5, the peak values for the trapezoidal pulse and triangular pulse are about two and three times respectively relative to that of rectangular pulse. Correspondingly the SNR have about 3 dB and 4.8 dB increasements. Moreover, it is also found that the 3 dB linewidths of the beat signals are 22 MHz, 19 MHz and 18 MHz for rectangular pulse, trapezoidal pulse and triangular pulse, respectively. In other words, when the pulse modulation format is a triangular shape, the Brillouin power spectrum has the maximum peak-power and the narrowest FWHM, so the fitting accuracy will be the best, and the fitting error is the least. In contrast, when the pulse format is close to ideal rectangular, the Lorentzian fitting accuracy and the peak-power frequency estimation would have maximum error. Moreover, it should also be illuminated about the relation of the spatial resolution and the complex pulse shape. In the distributed fiber sensing system (e.g. BOTDR), the spatial resolution Dz is related to the pulse width t, the PD bandwidth Dfdetector and the sampling rate Dfsample of signal, i.e. ( ) c c c , Dz ¼ max t, , 2n nDf detector nDf sample where c¼3 108 m/s and nE1.5. In the common experiment, the PD bandwidth and the data acquisition are fixed. So the only factor for spatial resolution is the pulse width t. For the rectangular pulse, no effect exists in the close pulses (e.g. in Fig. 4) if the pulse space is larger than the pulse width. However, the triangular pulse or trapezoidal pulse could be influenced by the gradient sides of the close pulse. Then the corresponding SNR will be reduced due to the pulse crosstalk. However, in the condition of the same pulse width
1.2 1.0 Intensity (a.u.)
3. Experiment results and discussion
39
Rectangular pulse Trapezoidal pulse Trianglar pulse
0.8 0.6 0.4 0.2 0.0 -250 -200 -150 -100 -50 0 50 100 150 200 250 Pulse width (ns) Fig. 4. Different pulse modulated formats from AOM.
Fig. 3. Schematical diagram of BOTDR sensing system.
40
Y. Hao et al. / Optics & Laser Technology 46 (2013) 37–41
0.000003
Rectangular pulse and Lorentz fitting
0.000002
Intensity (a.u.)
0.000001 0.000000 350 0.000003
375
400
425
450
475
500
Trapeziodal pule and Lorentz fitting
0.000002 0.000001 0.000000 350 0.000003
375
400
425
450
475
500
Triangular pulse and Lorentz fitting
0.000002 0.000001 0.000000 350
375
450 400 425 Beat frequency(MHz)
475
500
Fig. 5. FFTs of backward SBS/LO beat signals with pulse width 200 ns for different modulated pulse format. The black curve is FFTs of beat signals, and the fitting curves are the respective Lorentz fittings.
Frequency (MHz)
465 450 435 420 405 465 450 435 420 405 465 450 435 420 405
For rectangular pulse
0
5000 For trapezoidal pulse
10000
15000
20000
0
5000 For triangular pulse
10000
15000
20000
0
5000
10000 Distance (m)
15000
20000
Fig. 6. Effects of modulated pulse format on the sensing distance in the BOTDR system.
and pulse space for the rectangular pulse and triangular pulse, the effect of the crosstalk will be smaller than the SNR enhancement for the pulse shape modulation by comparing the theoretical analysis and the experimental results. Therefore, the SNR increased by different modulated pulse format will be an effective solution for improve the sensing system performance. We adjusted the LO optical power from 1.8 mW to 3.746 mW in order to illustrate the effects of modulated pulse format on the BOTDR sensing system. Fig. 6 shows the effects of modulated pulse format on the sensing distance. The vertical axis signifies the peak-power frequency of Lorentz-fitting. When the SNR is too small to take Lorentz fitting, the peak-power frequency would be the set-frequency (450 MHz). Frequency-mutation in different positions is because of the SNR variation resulted from polarization effect. The sensing distance is indicated by the fiber length before the first frequency-mutation. According to the SNR analysis, when we change the modulated pulse format from the rectangular to the triangular, the SNR of the beat frequency signal
becomes larger, so the sensing range also increases accordingly. With the same pulse width of 200 ns, the effective sensing distances in BOTDR system are 3.7 km, 5.8 km and 9.5 km for the rectangular, trapezoidal, and triangular pulses, respectively. Therefore, for the same pulse width (i.e. the same spatial resolution), the triangular pulse format increases the system sensing range obviously. Based on the same principle, for the same sensing distance, certain modulated pulse format may also improve the system spatial resolution.
4. Conclusion We have proposed and investigated experimentally the effects of different modulated pulse format on spontaneous Brillouin scattering spectrum and BOTDR sensing system. With the same pulse width, the SNRs of the SBS/LO beat frequency signal for trapezoidal pulse and triangular pulse have an obvious increment
Y. Hao et al. / Optics & Laser Technology 46 (2013) 37–41
relative to that of the rectangular pulse. Correspondingly the spectrum linewidthes by the Lorentz fitting are narrower when the pulse format is closer to triangular shape. Changing the modulated pulse format, performance of BOTDR sensing system may be improved obviously in the sensing distance or spatial resolution.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant no. U0934001), the key cooperation project of CAS and Guangdong province (Grant no. 2009B091300127) and the project of STCSM (Grant no. 11DZ1140202). References [1] Culverhouse D, Farahi F, Pannel CN, Jackson DA. Potential of stimulated Brillouin scattering as sensing mechanism for distributed temperature sensors. Electronics Letters 1989;25:913–5. [2] Bao XY, Chen L. Recent progress in optical fiber sensors based on Brillouin scattering at university of Ottawa. Photonic Sensors 2011;1:102–17. [3] Dong YK, Chen L, Bao XY. Time-division multiplexing-based BOTDA over 100 km sensing length. Optics Letters 2011;36:277–9. [4] Kurashima T, Horiguchi T, Izumita H, Furukawa S, Koyamada Y. Brillouin optical-fiber time domain reflectometry. International Quantum Electronics Conference 1992;E76-B:42–4. [5] Geng JH, Staines S, Blake M, Jiang SB. Distributed fiber temperature and strain sensor using coherent radio-frequency detection of spontaneous Brillouin scattering. Applied Optics 2007;46:5928–32.
41
[6] Koyamada Y, Sakairi Y, Takeuchi N, Adachi S. Novel technique to improve spatial resolution in Brillouin optical time-domain reflectometry. IEEE Photonics Technology Letters 2007;19:1910–2. [7] Pannell CN, Dhliwayo J, Webb DJ. The accuracy of parameter estimation from noisy data, with application to resonance peak estimation in distributed Brillouin sensing. Measurement Science and Technology 1998;9:50–7. [8] Horiguchi T, Shimizu K, Kurashima T, Tateda M, Koyamada Y. Development of a distributed sensing technique using Brillouin scattering. Journal of Lightwave Technology 1995;13:1296–302. [9] Stella Foaleng Mafang, Fe´lix Rodriguez Barrios, Sonia Martin Lopez, Herra´ez Miguel Gonza´lez, Luc The´venaz. Impact of self phase modulation on the performance of Brillouin distributed fibre sensors. Proceedings of SPIE1996756X 2010;7653:76532U. [10] Soto MA, Bolognini G, Pasquale FD. Analysis of optical pulse coding in spontaneous Brillouin-based distributed temperature sensors. Optics Express 2008;16:19097–111. [11] Alahbabi MN, Cho YT, Newson TP. 150 km range distributed temperature sensor based on coherent detection of spontaneous Brillouin backscatter and in-line Raman amplification. Journal of the Optical Society of America B 2005;22:1321–4. [12] Cho YT, Alahbabi MN, Brambilla G, Newson GT. Distribution Raman amplification combined with a remotely pumped EDFA utilized to enhance the performance of spontaneous Brillouin-based distributed temperature sensors. IEEE Photonics Technology Letters 2005;17:1256–8. [13] Naruse H, Tateda M. Launched pulse-shape dependence of the power spectrum of the spontaneous Brillouin backscattered light in an optical fiber. Applied Optics 2000;39:6376–84. [14] Yang F, Ye Q, Pan ZQ, Chen DJ, Cai HW, Qu RH, et al. 100 mW linear polarization single-frequency all-fiber seed laser for coherent Doppler lidar application. Optics Communications 2012;285:149–52. [15] Harun SW, Shahi S, Ahmad H. Compact Brillouin-erbium fiber laser. Optics Letters 2009;34:46–8. [16] Hao YQ, Ye Q, Pan ZQ, Yang F, Cai HW, Qu RH, et al. Design of wide-band frequency shift technology by using compact Brillouin fiber laser for BOTDR sensing system. Optics Communications 2012 [In press].
Contents lists available at SciVerse ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Effects of modulated pulse format on spontaneous Brillouin scattering spectrum and BOTDR sensing system Yunqi Hao a,b, Qing Ye a,n, Zhengqing Pan a, Haiwen Cai a,n, Ronghui Qu a, Zhongmin Yang c a
Shanghai Key Laboratory of All Solid-state Laser and Applied Techniques, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China Department of Technology Physics, Zhengzhou University of Light Industry, Zhengzhou, Henan 450002, China c Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510641, China b
a r t i c l e i n f o
abstract
Article history: Received 9 March 2012 Received in revised form 5 April 2012 Accepted 18 April 2012 Available online 17 May 2012
The signal noise ratio (SNR) enhancement effects of spontaneous Brillouin scattering spectrum on Brilloluin optical time domain reflectometry (BOTDR) sensing system have been analyzed theoretically and demonstrated experimentally through changing the modulated pulse format. With the same pulse width or same spatial resolution, the SNR is larger for triangular pulse. Take the width of 200 ns as an illustration, the SNRs of the coherent detection power spectrum for trapezoidal pulse and triangular pulse increase 3 dB and 4.8 dB relative to that of rectangular pulse respectively. The corresponding spectral linewidthes are narrowed and the sensing distances are also increased by about two times from the rectangular pulse to the triangular pulse. This phenomenon will be helpful to improve the spatial resolution or achieve longer sensing distance in the BOTDR sensing system at the same systemic conditions. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Brillouin optical time domain reflectometry (BOTDR) sensing system Modulated pulse format Signal noise ratio (SNR) enhancement effect
1. Introduction Recently, a number of studies [1–6] have focused on the longdistance distributed fiber sensors with Brillouin scattering in optical fiber for their potential applications of monitoring temperature and strain, which mainly include Brillouin optical time domain analysis (BOTDA) [1–3] and Brillouin optical time domain reflectometry (BOTDR) [4–6]. The power spectrum of the Brillouin backscattering induced by a launched pulse usually includes the strain and temperature information along the whole sensing fiber link, which may be used to detect the variations of strain and temperature simultaneously in different positions. In BOTDR sensing system, the frequency-shift and amplitude variation of the power spectrum are proportional to the changes of strain and temperature. In Ref. [7], the Brillouin scattering was fitted by a Lorenzian function to improve the accuracy of peak-power frequency measurements. High SNR power spectrum for spontaneous Brillouin scattering (SBS) will decrease the peak-power frequency measurement error [8] and it may increase the sensing distance while preserving the spatial resolution. Therefore enhancing the SNR for the same launched pulse width is one of the effective solutions to improve the peak-power frequency measurement accuracy without worsening the spatial resolution [9]. Pulse code technology [10] has been used to improve the Brillouin scattering SNR, but the coded and decoded process is very complicated for coherent detection. Furthermore, amplification techniques
n
Corresponding authors. E-mail addresses: [email protected] (Q. Ye), [email protected] (H. Cai).
0030-3992/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2012.04.025
[11,12] were used to increase the Brillouin backscattering intensity, but the SNR of signal itself has not been improved. In this paper, we propose and investigate experimentally the effects of modulated pulse formats on the SBS power spectrum and BOTDR sensing systems. The pulse width is defined by the full width half maximum (FWHM) of the light intensity, and the rise/ fall time is defined by the optical intensity of launched pulse rising from 0 to 1 and falling from 1 to 0. With the same pulse width of 200 ns for instance, the SNRs of the SBS/local oscillator (LO) beat frequency signal for trapezoidal pulses (80 ns rise/fall time) and triangular pulses (200 ns rise/fall time) increase 3 dB and 4.8 dB, respectively, relative to that of rectangular pulses (about 5 ns rise/fall time). Moreover, the spectrum linewidthes, obtained by the Lorentz fitting, are narrower when the pulse format is closer to a triangular shape. A BOTDR sensing system is built to test the influence of the modulated pulse format, and results show that the sensing distance of triangular pulse is about triple than that of the rectangular pulse. We believe that our study will be helpful to improve spatial resolution or sensing distance in BOTDR/BOTDA sensing systems.
2. Theoretical analysis and simulations For the backscattering Brillouin light, the frequency-dependent factor H(n) is [13] HðnÞ ¼
Z
1
P p ðf Þ 1
hðo=2Þ2
nðf SB Þ2 þ ðo=2Þ2
df
ð1Þ
38
Y. Hao et al. / Optics & Laser Technology 46 (2013) 37–41
where SB is Brillouin frequency shift, PP(f) is the power spectrum of launched pulse, o is the typical FWHM, and n is the abscissa of backward Brillouin power spectrum. For different pulse width and different modulation formats, the corresponding peak values of the Brollouin power spectra are shown in Fig. 1. It can be seen that the peak values become larger with wider pulse width. But with the same pulse width, i.e., the same spatial resolution, the triangular pulse has bigger value than that of the rectangular pulse. In this paper the pulse width is 200 ns for instance. When the modulated pulse formats are rectangular, trapezoidal, and triangular, their rise/fall time are 0 ns, 80 ns, and 200 ns
58
Triangular pulse Rectangular pulse
57
Peak values (dBm)
56 55 54 53 52 51 50 50
100
150 Pulse width (ns)
200
250
Intensity (a.u.)
Fig. 1. Peak values for different pulse width and different modulated formats.
Intensity (a.u.)
Rectangular Pulse Trapezoidal Pulse Triangular Pulse
1.0 0.8 0.6 0.4 0.2 0.0 -300
-200
-100
0 Pulse width (ns)
100
200
0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -0.02
300
Rectangular Pulse Trapezoidal Pulse Triangular Pulse
1.0 0.8 0.6 0.4 0.2 0.0 -0.02
Intensity (dB)
respectively as shown in Fig. 2(a). Fig. 2(b) shows the corresponding pulse power spectra by the fast Fourier transform (FFT). Launching these pulses into the long distance sensing fiber, the power spectra of SBS are simulated numerically according to Eq. (1) and the corresponding results are shown in Fig. 2(c). Comparing these power spectra, it is found that the triangular pulse shape has the highest peak intensity and narrowest spectrum bandwidth for the single SBS light. Difference of the Brillouin power spectra could be explained as follows: from Eq. (1), it can be seen that parameters h, o and SB are inherent properties of the optical fiber, related to the doping concentration, etc. PP(f) and f are the power spectrum and frequency of the launched pulse, which can be changed through launching different lights, and it is the only factor we can used to improve the Brillouin power spectrum. To the same optical source, the difference of Brillouin power spectra in Fig. 2(c) is resulted from power spectra of launched pulse formats, as shown in Fig. 2(b). The power spectra are normalized by the peak of triangular pulse power spectrum. The triangular pulse power spectrum is biggest and narrowest, and the rectangular is of smallest and widest. The Brillouin power spectrum is the convolution of the pulse and the SBS in the time domain, which means the product of the pulse power spectrum and the standard Brillouin power spectrum in the frequency domain. As shown in Fig. 2(b), the pulse power spectrum of triangular format has much larger peak value, narrower linewidth, and smaller sidelobes. In the backscattered SBS signal, product of these sidelobes and the ideal Lorentzian function is of no use and decreases the intensity of main peak because of energy conversation. For the equal input pulse energy, if more light energy concentrates in the main peak, where the Brillouin gain is better, the Brillouin power spectrum would also be bigger. So the Brillouin power spectrum from the sensing fiber is best for triangular, and vice versa.
-0.01
0.00 Frequency (GHz)
0.01
0.02
Rectangular Pulse Trapezoidal Pulse Triangular Pulse
-0.01
0.00 0.01 Brillouin power spectra (GHz)
0.02
Fig. 2. (a) Different modulated pulse formats with the same pulse width of 200 ns; (b) pulse power spectra; (c) corresponding Brillouin scattering spectra for three kinds of pulse modulated formats.
Y. Hao et al. / Optics & Laser Technology 46 (2013) 37–41
In order to test the effects of modulated pulse formats on the SBS spectrum and on the BOTDR fiber sensing system, an experimental setup is constructed (Fig. 3). A short-cavity single-frequency stable linear-polarization fiber laser (SFFL) [14] at 1548.976 nm is used as the primary light source in the sensing system. Part of the laser beam is amplitude modulated by an acousto-optic modulator (AOM, Model MT160-IIR10-FIO, AA Inc.) to generate pulse sequence with repetition rate of 4 kHz and pulse width of 200 ns. The repetition rate is relative to the sensing range, where 4 kHz corresponds to 25 km for accurate sensing location. For 1550 nm wavelength band, the Brillouin frequency shift is about 11 GHz. In order to detect the Brillouin scattering with the conventional detector (bandwidth o800 MHz), the other part of the source is applied to pump a compact singlefrequency Brillouin fiber laser (BFL) with about 11 GHz Brillouin frequency shift, which is used as LO for coherent detection [15,16]. The different pulse formats (e.g. rectangular, trapezoidal and triangular) are achieved through the programmable modulating drive signal imposed on the AOM drive module, changing the rise/fall time from a minimum of 5 ns (limited by the signal generator) to a maximum of 200 ns, as shown in Fig. 4. The modulated pulse is appropriately amplified (avoiding stimulated Brillouin scattering) by an Erbium doped fiber amplifier (EDFA) and then launched into the long sensing fiber to generate the backscattered spontaneous Brillouin signal. The SBS signal, carrying local information of temperature or stain along the whole sensing fiber, can be detected by the coherent heterodyne detection. The beat signal between SBS and LO are injected into a double balanced photodetector (DB-PD, New Focus 1617-AC, Newport) through a 1:1 coupler, and then amplified by a microwave amplifier (MWA, ALPHALAS BBA-100-VG, bandwidth 0.01– 2 GHz). All the data is collected by high-speed data-acquisition card (DAQ, Gage, Model CS82G, 3 GS/s sample rate), where the data acquisition is synchronized with the signal generator for the AOM. Acquisition and analysis of sensing signal are real-time processed by a customized LabVIEW program. In our experiment, the modulated pulses with 200 ns pulse-width are launched into the sensing fiber and the backscattered timedomain pulse signal is obtained with 30 sampling points every meter. As the length of the sensing fiber is 20 km, the corresponding spontaneous Brillouin pulse is about 200 ms. While acquiring the backward pulse signal, the beat frequency of Rayleigh scattering with LO and other DC components will be filtered out because the DB-PD response bandwidth is in the range 10 kHz–800 MHz. Fig. 5 shows the typical fast Fourier transform (FFT) spectra of the SBS/LO beat signal in the whole sensing fiber, averaged over 100 SBS pulses, with different pulse modulated formats. The center frequency is about 420 MHz, which is decided by the Brillouin frequency-shift difference between LO and the sensing fiber. Through Lorentz fittings of these FFT spectra, ratios of the peak and the bottom give SNRs. By changing the modulated pulse format from rectangular to triangular
for the same pulse-width of 200 ns, the SNRs of the beat frequency signal have obviously improved. In the BOTDR sensing system, optical intensity and frequency of LO are stable, so the SNR increment of the beat frequency signal is in essence the SNR improvement of SBS light. Through analyzing the Lorentz fitting curves in Fig. 5, the peak values for the trapezoidal pulse and triangular pulse are about two and three times respectively relative to that of rectangular pulse. Correspondingly the SNR have about 3 dB and 4.8 dB increasements. Moreover, it is also found that the 3 dB linewidths of the beat signals are 22 MHz, 19 MHz and 18 MHz for rectangular pulse, trapezoidal pulse and triangular pulse, respectively. In other words, when the pulse modulation format is a triangular shape, the Brillouin power spectrum has the maximum peak-power and the narrowest FWHM, so the fitting accuracy will be the best, and the fitting error is the least. In contrast, when the pulse format is close to ideal rectangular, the Lorentzian fitting accuracy and the peak-power frequency estimation would have maximum error. Moreover, it should also be illuminated about the relation of the spatial resolution and the complex pulse shape. In the distributed fiber sensing system (e.g. BOTDR), the spatial resolution Dz is related to the pulse width t, the PD bandwidth Dfdetector and the sampling rate Dfsample of signal, i.e. ( ) c c c , Dz ¼ max t, , 2n nDf detector nDf sample where c¼3 108 m/s and nE1.5. In the common experiment, the PD bandwidth and the data acquisition are fixed. So the only factor for spatial resolution is the pulse width t. For the rectangular pulse, no effect exists in the close pulses (e.g. in Fig. 4) if the pulse space is larger than the pulse width. However, the triangular pulse or trapezoidal pulse could be influenced by the gradient sides of the close pulse. Then the corresponding SNR will be reduced due to the pulse crosstalk. However, in the condition of the same pulse width
1.2 1.0 Intensity (a.u.)
3. Experiment results and discussion
39
Rectangular pulse Trapezoidal pulse Trianglar pulse
0.8 0.6 0.4 0.2 0.0 -250 -200 -150 -100 -50 0 50 100 150 200 250 Pulse width (ns) Fig. 4. Different pulse modulated formats from AOM.
Fig. 3. Schematical diagram of BOTDR sensing system.
40
Y. Hao et al. / Optics & Laser Technology 46 (2013) 37–41
0.000003
Rectangular pulse and Lorentz fitting
0.000002
Intensity (a.u.)
0.000001 0.000000 350 0.000003
375
400
425
450
475
500
Trapeziodal pule and Lorentz fitting
0.000002 0.000001 0.000000 350 0.000003
375
400
425
450
475
500
Triangular pulse and Lorentz fitting
0.000002 0.000001 0.000000 350
375
450 400 425 Beat frequency(MHz)
475
500
Fig. 5. FFTs of backward SBS/LO beat signals with pulse width 200 ns for different modulated pulse format. The black curve is FFTs of beat signals, and the fitting curves are the respective Lorentz fittings.
Frequency (MHz)
465 450 435 420 405 465 450 435 420 405 465 450 435 420 405
For rectangular pulse
0
5000 For trapezoidal pulse
10000
15000
20000
0
5000 For triangular pulse
10000
15000
20000
0
5000
10000 Distance (m)
15000
20000
Fig. 6. Effects of modulated pulse format on the sensing distance in the BOTDR system.
and pulse space for the rectangular pulse and triangular pulse, the effect of the crosstalk will be smaller than the SNR enhancement for the pulse shape modulation by comparing the theoretical analysis and the experimental results. Therefore, the SNR increased by different modulated pulse format will be an effective solution for improve the sensing system performance. We adjusted the LO optical power from 1.8 mW to 3.746 mW in order to illustrate the effects of modulated pulse format on the BOTDR sensing system. Fig. 6 shows the effects of modulated pulse format on the sensing distance. The vertical axis signifies the peak-power frequency of Lorentz-fitting. When the SNR is too small to take Lorentz fitting, the peak-power frequency would be the set-frequency (450 MHz). Frequency-mutation in different positions is because of the SNR variation resulted from polarization effect. The sensing distance is indicated by the fiber length before the first frequency-mutation. According to the SNR analysis, when we change the modulated pulse format from the rectangular to the triangular, the SNR of the beat frequency signal
becomes larger, so the sensing range also increases accordingly. With the same pulse width of 200 ns, the effective sensing distances in BOTDR system are 3.7 km, 5.8 km and 9.5 km for the rectangular, trapezoidal, and triangular pulses, respectively. Therefore, for the same pulse width (i.e. the same spatial resolution), the triangular pulse format increases the system sensing range obviously. Based on the same principle, for the same sensing distance, certain modulated pulse format may also improve the system spatial resolution.
4. Conclusion We have proposed and investigated experimentally the effects of different modulated pulse format on spontaneous Brillouin scattering spectrum and BOTDR sensing system. With the same pulse width, the SNRs of the SBS/LO beat frequency signal for trapezoidal pulse and triangular pulse have an obvious increment
Y. Hao et al. / Optics & Laser Technology 46 (2013) 37–41
relative to that of the rectangular pulse. Correspondingly the spectrum linewidthes by the Lorentz fitting are narrower when the pulse format is closer to triangular shape. Changing the modulated pulse format, performance of BOTDR sensing system may be improved obviously in the sensing distance or spatial resolution.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant no. U0934001), the key cooperation project of CAS and Guangdong province (Grant no. 2009B091300127) and the project of STCSM (Grant no. 11DZ1140202). References [1] Culverhouse D, Farahi F, Pannel CN, Jackson DA. Potential of stimulated Brillouin scattering as sensing mechanism for distributed temperature sensors. Electronics Letters 1989;25:913–5. [2] Bao XY, Chen L. Recent progress in optical fiber sensors based on Brillouin scattering at university of Ottawa. Photonic Sensors 2011;1:102–17. [3] Dong YK, Chen L, Bao XY. Time-division multiplexing-based BOTDA over 100 km sensing length. Optics Letters 2011;36:277–9. [4] Kurashima T, Horiguchi T, Izumita H, Furukawa S, Koyamada Y. Brillouin optical-fiber time domain reflectometry. International Quantum Electronics Conference 1992;E76-B:42–4. [5] Geng JH, Staines S, Blake M, Jiang SB. Distributed fiber temperature and strain sensor using coherent radio-frequency detection of spontaneous Brillouin scattering. Applied Optics 2007;46:5928–32.
41
[6] Koyamada Y, Sakairi Y, Takeuchi N, Adachi S. Novel technique to improve spatial resolution in Brillouin optical time-domain reflectometry. IEEE Photonics Technology Letters 2007;19:1910–2. [7] Pannell CN, Dhliwayo J, Webb DJ. The accuracy of parameter estimation from noisy data, with application to resonance peak estimation in distributed Brillouin sensing. Measurement Science and Technology 1998;9:50–7. [8] Horiguchi T, Shimizu K, Kurashima T, Tateda M, Koyamada Y. Development of a distributed sensing technique using Brillouin scattering. Journal of Lightwave Technology 1995;13:1296–302. [9] Stella Foaleng Mafang, Fe´lix Rodriguez Barrios, Sonia Martin Lopez, Herra´ez Miguel Gonza´lez, Luc The´venaz. Impact of self phase modulation on the performance of Brillouin distributed fibre sensors. Proceedings of SPIE1996756X 2010;7653:76532U. [10] Soto MA, Bolognini G, Pasquale FD. Analysis of optical pulse coding in spontaneous Brillouin-based distributed temperature sensors. Optics Express 2008;16:19097–111. [11] Alahbabi MN, Cho YT, Newson TP. 150 km range distributed temperature sensor based on coherent detection of spontaneous Brillouin backscatter and in-line Raman amplification. Journal of the Optical Society of America B 2005;22:1321–4. [12] Cho YT, Alahbabi MN, Brambilla G, Newson GT. Distribution Raman amplification combined with a remotely pumped EDFA utilized to enhance the performance of spontaneous Brillouin-based distributed temperature sensors. IEEE Photonics Technology Letters 2005;17:1256–8. [13] Naruse H, Tateda M. Launched pulse-shape dependence of the power spectrum of the spontaneous Brillouin backscattered light in an optical fiber. Applied Optics 2000;39:6376–84. [14] Yang F, Ye Q, Pan ZQ, Chen DJ, Cai HW, Qu RH, et al. 100 mW linear polarization single-frequency all-fiber seed laser for coherent Doppler lidar application. Optics Communications 2012;285:149–52. [15] Harun SW, Shahi S, Ahmad H. Compact Brillouin-erbium fiber laser. Optics Letters 2009;34:46–8. [16] Hao YQ, Ye Q, Pan ZQ, Yang F, Cai HW, Qu RH, et al. Design of wide-band frequency shift technology by using compact Brillouin fiber laser for BOTDR sensing system. Optics Communications 2012 [In press].