Distributed fiber sensor based on modulated pulse base reflection and Brillouin gain spectrum analysis Qingsong Cui,1,2,* Sibel Pamukcu,2 Wen Xiao,1 Cyril Guintrand,3 Jean Toulouse,3 and Mesut Pervizpour4 1
School of Instrument Science and Opto-Electronics Engineering, Beihang University, Beijing 100083, China
2
Department of Civil and Environmental Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, USA 3 4
Department of Physics, Lehigh University, Bethlehem, Pennsylvania 18015, USA
Department of Civil Engineering, Widener University, Chester, Pennsylvania 19013, USA *Corresponding author:
[email protected] Received 12 May 2009; revised 10 August 2009; accepted 24 September 2009; posted 2 October 2009 (Doc. ID 111275); published 16 October 2009
In recent years, several distributed sensor systems based on stimulated Brillouin scattering in optical fibers have been proposed [ J. Intell. Mater. Syst. Struct. 10, 340 (1999); Proc. SPIE 5855, 555 (2005) ]. We propose a simpler scheme based on fiber-end reflection and Brillouin gain spectrum analysis. In this setup, only one optical source is necessary to provide both the pump and the probe wave; the latter is provided by the modulated pulse base. First, the physical mechanisms for two different Brillouin scattering processes in our sensor system are analyzed and an approximate theory model is proposed. In addition, it is demonstrated that the simple system configuration allows simultaneous acquisition of the time-domain and the frequency-domain information. It is experimentally demonstrated that this configuration is effective for strain measurements and could as well be applied to temperature monitoring. © 2009 Optical Society of America OCIS codes: 060.2370, 060.2630, 290.5900.
1. Introduction
Distributed sensor systems based on stimulated Brillouin scattering (SBS) have attracted much interest in past years because of their increasingly varied applications in the civil engineering area, such as health monitoring of civil infrastructure, vibrations measurement, or environmental effects [1–4]. Our sensor configuration relies on the SBS effect in optical fiber. When an optical pump wave is injected into a fiber, incident light is partially backscattered due to its interaction with acoustic phonons. The backscattered light frequency is shifted toward a lower frequency compare to the incident light. It is also pos0003-6935/09/305823-06$15.00/0 © 2009 Optical Society of America
sible to amplify a counterpropagating signal when its optical frequency falls in the SBS gain spectrum. The frequency shift is directly related to the acoustic properties of the fiber in which the waves interact. The typical frequency shift value is ∼10 GHz in silica fibers. Change in temperature or strain conditions of the fiber induces variations in the acoustic properties of the medium and, by consequence, can be monitored by examination of the SBS frequency shift observed. A number of research groups have engaged in the design of distributed sensor system based on Brillouin scattering. For instance, Horiguchi and his colleagues reported the first distributed sensor, called Brillouin optical time-domain analysis (BOTDA), using a pulsed pump wave and a cw Stokes probe in a counterpropagation scheme [1]. Brillouin optical 20 October 2009 / Vol. 48, No. 30 / APPLIED OPTICS
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time-domain reflectometry (BOTDR) was introduced by Kurashima et al. [5,6]. They analyzed the spontaneous Brillouin backscattered light instead of the Brillouin amplification signal using coherent detection. Niklès and coworkers presented a distributed Brillouin sensor consisting of a single laser and a single fiber based on a pump-and-probe technique [7,8]. One important feature of the system was the generation of pump-and-probe signals in a single LiNbO3 intensity electro-optic modulator separately in a time-sharing method. All those studies have shown their advantages and disadvantages. The disadvantage of the conventional BOTDA is that it requires a laser to be placed at each end of the fiber being tested and a fixed frequency relationship with each other [9]. Using only one end of the sensing fiber is the advantage of the BOTDR system. The drawback is that the spontaneous Brillouin scattering signal is much weaker than that of BOTDA and the spatial resolution is limited to 1 m, which is related to the phonon lifetime. The single-laser BOTDA system [7,8] is simpler than the conventional BOTDA system. However, in the presented design, the RF port is shared for generation of both the pump and the probe waves, which increases the complexity of the system. This timedivision-multiplexing method requires an electric circuit to switch between the microwave and the pulse generator and requires a stable circuit design, especially for high resolution measurements. In this paper, we propose a distributed fiber sensor system based on fiber-end reflection and Brillouin gain spectrum analysis, and we demonstrate the efficiency of this system. Using theoretical simulation and experimental results, we attempt to show that the modulated pulse base reflection BOTDA is a simpler sensor system. In our configuration, the probe is generated by the modulated pump base reflection. Pulse and microwave signals are simultaneously connected to the electro-optic modulator (EOM) electrodes and no time multiplexing is required. Brillouin interactions are analyzed to detect the optimal electrical power values necessary to drive the EOM. In order to demonstrate the effectiveness of our system and ensure the correctness of the interaction mechanism, the Brillouin scattering process is simulated numerically to confirm the experiment results. 2. Principle and Experimental Setup
A schematic diagram of our proposed system is illustrated in Fig. 1. In this technique, the light beam from a distributed feedback (DFB) laser is injected into the EOM. A polarization controller (PC) is positioned before the EOM to insure correct polarization of the input light. A sinusoidal RF signal with frequency f m is used to modulate the optical wave; this creates sidebands on each side of the laser frequency f 0 . Simultaneously, a pulse generator is also connected to the bias port of the EOM to create optical pulses. No logical gate and electric circuit is applied at this stage, which is a unique feature of our system. 5824
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Fig. 1. Diagram of the distributed fiber sensor system tested.
A 30 dBm output power erbium-doped fiber amplifier (EDFA) is used to amplify the input signal to a high peak power. We use a circulator placed at the fiber under test (FUT) input to collect the backscattered light. Reflection of the light at the fiber output end is provided by Fresnel reflection from the glass/air interface (∼4% in our case). A fiber Bragg grating (FBG) is used to filter the Stokes component from the total backscattered light. Finally a PIN detector and an oscilloscope are used to monitor the filtered backscattered light power. The process of interaction in the fiber is divided into two steps: (1) the pulse base (low state) preceding the pulse (high state) interacts with the reflected portion of the base and (2) the pulse interacts with the reflected base. When the modulation frequency f m approaches the Brillouin frequency of the fiber, SBS interaction occurs and the Stokes component in the reflected probe signal is amplified. The time-dependent output power detected by the oscilloscope carries local information about the temperature and strain change along the sensing fiber. Figure 2 shows the EOM principle for our system configuration. Figure 2(a) is the structure of the push–pull intensity modulator. The phase difference between the two arms can be written as ΔϕðtÞ ¼ Δϕbias ðtÞ þ ΔϕRF ðtÞ:
ð1Þ
Δϕbias ðtÞ and ΔϕRF ðtÞ are the phase difference between the optical waves from the two EOM branches induced by the pulse and microwave signal, respectively. This method is superior to the one presented in [7], since it does not require a time-division multiplexing of pulse and sinusoidal waves. The sinusoidal modulation and the pulse modulation are superimposed, as shown in Fig. 2(b). The pulse base approach has already been used by other groups as prepump wave to improve the spatial resolution [10]. In our configuration, the pulse base is not only the prepump wave but, more importantly, it is used as the probe wave. The energy transfer mechanism is schematized in Fig. 3; on top, we have represented the probe wave beam (reflected pulse base) spectrum, and on the bottom, the pump wave (pulse) spectrum. The pulse finite extinction ratio is defined as ER ¼ Ppulse =Pcw , where Ppulse and Pcw are the power of the pulse top and the pulse base, respectively.
Fig. 3. Energy transfer mechanism between pump-and-probe light.
Fig. 2. (Color online) EOM structure and transfer function: (a) balanced push–pull EOM structure and (b) optical modulation transfer function.
When the modulation frequency f m is equal to the Brillouin frequency νB, the counterwaves interact with each other and energy transfer occurs between the waves. From Fig. 3, it is observed that there are mainly four Brillouin scattering energy transfer processes occurring in the fiber, Ep1 → Es0 , Es0 → Ep2 , Es1 → Ep0 , and Ep0 → Es2 . The four processes could be separated to two groups: (1) Ep1 → Es0 and Es0 → Ep2 and (2) Es1 → Ep0 and Ep0 → Es2 . The intensity of group (2) is much higher than group (1); thus, the Brillouin interaction of group (1) is much less than group (2). In group (2), probe Es1 corresponds to loss spectrum and probe Es2 corresponds to gain spectrum. Our sensor system is based on gain spectrum analysis, thus Es2 is the probe component detected in our sensor system. By monitoring the power of Es2, we can obtain direct information on the fiber stress and temperature variations. 3. Mathematical Model and Numerical Simulations
In Section 2, we have shown that the main SBS interaction in the fiber involves the pump wave component Ep0 and the reflected signal component Es2 (Stokes wave). A three coupled-wave equation system constitutes the basic theoretical model used to simulate the Brillouin scattering in the sensing fiber. The equations, using the slowly varying amplitude approximation, can be written as [11]
∂ n∂ 1 − − α Ep0 ¼ ig1 QEs2 ; ∂z c ∂t 2
ð2Þ
∂ n∂ 1 þ þ α Es2 ¼ −ig1 Q Ep0 ; ∂z c ∂t 2
ð3Þ
∂ þ Γ Q ¼ −ig2 Ep0 Es2 : ∂t
ð4Þ
Equations (2) and (3) are descriptions of the optical fields and Eq. (4) is the acoustic wave equation, assuming the acoustic amplitude varies slowly. Where Ep0 , Es2 , and Q are the amplitudes of the pulse beam (pump), the pulse base reflection (Stokes) beam, and the acoustic wave fields, g1 and g2 are the photon– phonon coupling coefficients, α is the attenuation constant of the fiber, and Γ ¼ Γ1 þ iΓ2 , with Γ1 ¼
1 ; 2τph
Γ2 ¼ 2πðν − νB Þ: Γ1 is the damping rate with phonon lifetime τph ∼ 10 ns and Γ2 represents the detuning frequency. The detected optical power could be defined as P ¼ Aeff · jEj2 , where Aeff is the fiber effective core area and E is the optical wave field. The numerical method of transient SBS is applied to solve the equations [12]. In the simulation, we assumed a 50 m fiber length, an unstressed fiber Brillouin shift νB ¼ 10866 MHz, and the effective fiber core area Aeff ¼ 54 μm2 . The peak power, ER, and pulse width are 24 dBm, 20 October 2009 / Vol. 48, No. 30 / APPLIED OPTICS
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27 dB, and 10 ns, respectively. The fiber-end reflection is 4%. The length of the stressed section is 1 m, corresponding to the spatial resolution of the 10 ns pulse and positioned between the 20 and 21 m linear positioning of the fiber, and the Brillouin frequency shift of the stressed fiber is 10; 950 MHz. Figure 4 presents the simulation results of the Brillouin scattering interaction. The figure illustrates the Brillouin spectrum that carries the pulse base pump wave and its reflected probe wave interaction, and the pulse pump and the reflected pulse base probe interaction, including the information on spatial resolution. Figure 4(a) describes the stimulated gain Brillouin spectrum for the Stokes component (Es2 ) at the stressed section by scanning the frequency f m . The lower peak is the pulse base component contribution and the higher peak is the pulse pump contribution to the Brillouin spectrum for the stressed spectrum. In Fig. 4(b), the base of the time-domain signal corresponds to the steady interaction between the pulse base and its reflection. The peak section is the transient Brillouin interaction from the stressed section (f m ¼ 10; 950 MHz), which is much stronger than other frequencies in the same stressed fiber. 4. Experimental Results
We carried out some experiments based on the principle presented in Section 2. We proved its efficiency in terms of precise localization and the quantitative value of the stress applied in the fiber. The experimental results presented here were conducted using the experimental setup from Fig. 1. In this experiment, we use a laser wavelength of 1551 nm and the linewidth is less than 1 MHz. The FUT is ∼270 m of SMF-28 (silica fiber); low cost and easy availability dictate our choice. We measured a Brillouin shift of 10; 866 MHz for the SMF-28 when no stress was applied to the fiber (room temperature). The sinusoidal RF signal was provided by a signal generator with a 0–20 GHz frequency range, and a 2 MHz sweeping step was applied. The pulse generator was set at a 10 ns pulse width with a 1 KHz repetition rate. We used a three-point travelling pulley system to apply axial strain to a specific section of the fiber. Figure 5(a) shows the experimental spectrum at the fiber input in the pulse high state and the pulse low state (around V π ) [Fig. 5(b)]. The interaction of group (1) can be highly suppressed by adjusting the pulse voltage and the microwave signal power. Figure 5(a) shows that the intensity of pump Ep0 is 16 dB higher than the sidebands, Ep1 and Ep2 . Figure 5(b) shows that the intensity of probe Es2 is 18 dB higher than the carrier wave Es0 . Figure 6 illustrates the correlation between the effective axial strain applied (in term of mass force) and the Brillouin frequency shift. Figure 6(a) was obtained by monitoring the power of the backscattered light for two different positions (z ¼ 45 m and z ¼ 74 m) in the same fiber, in which a stress was applied at z ¼ 74 m. The left curve (circles) corresponds 5826
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Fig. 4. (Color online) Energy transfer mechanism simulation between pump-and-probe light: (a) Brillouin gain spectrum and (b) time-domain signal output power.
to the position 1 (at z ¼ 45 m, where no stress was applied) and the right curve (squares) is related to the position 2 (at z ¼ 74 m, where stress is applied). The sinusoidal frequency modulation f m was tuned and the backscattered light powers corresponding to those two positions were monitored. For position 1, the peak power is observed for a frequency modulation of 10; 866 MHz, which corresponds to the natural fiber Brillouin shift. When a stress was applied (540:4 g) to the fiber section (position 2), the peak power shifted to 11; 016 MHz, a result of the fiber properties change. Looking closely at the curve corresponding to position 2 (stressed section), we notice a second small peak (circled portion) located at 10; 866 MHz. The results displayed in Fig. 6(b) were obtained when we applied a strain with a 540:4 g weight (on 1 m of fiber) at a distance of 74 m from the fiber input. The sinusoidal modulation was set at 11; 016 MHz and the time-dependent backscattered Stokes power was monitored on the oscilloscope. Then the time variable was translated in fiber distance. Figure 6(a) exhibits a strong peak around 74 m, which indicates a stress at this point.
Fig. 5. (Color online) Spectrum detection for pump-and-probe signal from optical spectrum analyzer: (a) pulse high level spectrum, (b) pulse low level spectrum.
Figure 7 shows the measured Brillouin frequency shift for different strain values induced by the application of the weight. The measurement error is about 4 MHz with a 2 MHz sweeping step. The solid line is the linear fit of experimental data (blue stars). The relationship between Brillouin frequency shift and the strain exhibits a linear variation given by
Fig. 6. (Color online) Brillouin frequency shift /strain dependence for standard SMF-28 fiber: (a) Brillouin spectrum for two positions (with and without stress) and (b) time-domain signal output power (modulation frequency, 11; 016 MHz; weight, 540:4 g).
Another advantage of our system is that the pulse leakage (base) is used as a prepump, as in [10]. From Fig. 6(a), it is obvious that the spectrum has a nar-
ΔvB ðGHzÞ ¼ 2:615 × 10−4 × WeightðgÞ: These results show that the proposed system is an effective sensor system for strain measurements. Under the conditions of the experiment and the frequency scanning step (2 MHz), the root-meansquare deviation (RMSD) between the linear fit line and the experimental data is 23:6 g, and the strain coefficient of the Brillouin frequency shift is about 0:26 MHz=g. Hence the strain (weight) resolution under our certain experiment conditions could be treated as 30 g.
Fig. 7. (Color online) Weight strain and Brillouin frequency shift relation results. 20 October 2009 / Vol. 48, No. 30 / APPLIED OPTICS
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rower top peak than the normal BOTDA system (the bandwith for 10 ns is about 80 MHz [13]). Those properties lead us to think that our system could offer better accuracy on stress localization of the order of submeters. 5. Conclusion
We have presented a distributed fiber sensor using a single optical source with simultaneous pulse and sinusoidal modulation. This is more convenient than the systems previously used and reported, which include either two optical sources or more complex RF circuits. We have derived and numerically solved the coupled equations for the fields involved in the process. By using this configuration of pulse base reflection as the probe wave, a Brillouin frequency shift induced by strain or temperature has been observed. This system allows accurate localization of stress and quantitative evaluation of the strain magnitude. Temperature monitoring could also be used with the same experiment setup. Q. Cui is supported by the China Scholarship Council, China, and performs this research as a visiting Ph.D. student at Lehigh University. The work is supported financially by grant CMMI-0855603 from the National Science Foundation (NSF). We also want to thank Prof. Xiaoyi Bao, Dr. Yongkang Dong, Dr. Yun Li, and Dr. Feng Wang at the University of Ottawa for providing a visiting opportunity and valuable suggestions. References 1. T. Kurashima, T. Horiguchi, and M. Tateda, “Distributedtemperature sensing using stimulated Brillouin scattering in optical silica fibers,” Opt. Lett. 15, 1038–1040 (1990).
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2. A. W. Brown, J. P. Smith, and X. Bao, “Brillouin scattering based distributed sensors for structural applications,” J. Intell. Mater. Syst. Struct. 10, 340–349 (1999). 3. S. Texier, S. Pumukcu, and J. Toulouse, “Advances in subsurface water-content measurement with a distributed Brillouin scattering fibre-optic sensor,” Proc. SPIE 5855, 555–558 (2005). 4. S. Anastasio, S. Pamukcu, and M. Pervizpour, “Chemical Selective BOTDR Sensing for Corrosion Detection on Structural Systems,” in Proceedings of the Seventh International Workshop on Structural Health Monitoring (International Workshop on Structural Health Monitoring, 2007), pp. 1701–1708. 5. T. Kurashima, T. Horiguchi, and M. Tateda, “Thermal effects of Brillouin gain spectra in single-mode fibers,” IEEE Photonics Technol. Lett. 2, 718–720 (1990). 6. T. Kurashima, T. Horiguchi, H. Izumita, S. Furukawa, and Y. Koyamada, “Brillouin optical-fiber time domain reflectometry,” IEICE Trans. Commun. E76-B, 382–390 (1993). 7. M. Niklès, L. Thevenaz, and P. Robert, “Simple distributed fiber sensor based on Brillouin gain spectrum analysis,” Opt. Lett. 21, 758–760 (1996). 8. A. Fellay, L. Thevenaz, M. Facchini, M. Niklès, and P. Robert, “Distributed sensing using stimulated Brillouin scattering: towards ultimate resolution,” in Optical Fiber Sensors, Vol. 16 of 1997 OSA Technical Digest Series (Optical Society of America, 1997), paper OWD3. 9. T. Horiguchi and M. Tateda, “BOTDA—nondestructive measurement of single-mode optical fiber attenuation characteristics using Brillouin interaction: theory,” J. Lightwave Technol. 7, 1170–1176 (1989). 10. K. Kishida, CH. Li, and K. Nishiguchi, “Pulse pre-pump method for cm-order spatial resolution of BOTDA,” Proc. SPIE 5855, 559–562 (2005). 11. R. W. Boyd, Nonlinear Optics, 2nd ed. (Academic, 2003). 12. R. Chu, M. Kanefsky, and J. Falk, “Numerical study of transient stimulated Brillouin scattering,” J. Appl. Phys. 71, 4653–4658 (1992). 13. X. Bao, A. Brown, M. DeMerchant, and J. Smith, “Characterization of the Brillouin-loss spectrum of single-mode fiber by use of very short (
