Analysis of Stimulated Brillouin Scattering Characteristics in Frequency Domain M.Kasinathan, C.Babu Rao, N.Murali, T.Jayakumar and Baldev Raj Indira Gandhi Centre For Atomic Research (IGCAR), Kalpakkam -603102, India Aleksander Wosniok and Katerina Krebber Federal Institute for Materials Research and Testing(BAM), Berlin,Germany email: mkasi@igcar.gov.in Abstract: A Brillouin Optical Frequency Domain Analysis (BOFDA) system was developed using a single semiconductor Distributed Feed Back (DFB) laser source, two electro optical modulators, Erbium Doped Fiber Amplifier (EDFA) and Optical Spectrum Analyser (OSA). The Brillouin frequency shift, gain and line width were measured as a function of temperature. Brillouin amplification is optimized by varying pump power and stokes powers. This system was tested using 2.5km length of single mode fiber in the temperature range -20 o C to 80 o C. The temperature coefficient of the system was measured. It is found to be df B /dt =1.12 MHz/ºC for SMF- 28 Corning fiber. 1. INTRODUCTION The Brillouin Optical Frequency Domain Analysis (BOFDA) technique is used to measure the distributed temperature and strain in power cables, oil pipe lines, large scale structures such as dams, river dikes, land slippage over kilo meter ranges [1-3]. The theoretical and experimental studies were done by Bao et al.[4]. Our present interest is to extend the research in the analysis of Stimulated Brillouin Scattering (SBS) characteristics in frequency domain [5-7]. In this paper, we study gain optimization by varying the pump and stokes powers and also gain saturation as a function of temperature in Brillouin lines [8-10]. This system consists of a single laser source and two electro optical modulators (EOM), to optimize the cost effective solution. Using brillouin system frequency shift, amplitude and line width were measured as a function of temperature. 2. EXPERIMENTAL DESCRIPTION To perform the Brillouin characteristics analysis, a BOFDA system was developed. Figure 1 shows the schematic of the fiber optics BOFDA system. The fiber pigtailed Distributed Feedback Laser (DFB) is employed and it is operates on Continuous Wave mode. This laser source has wavelength 1551.93nm, line width of 88kHz and optical power of 26mW. The Laser output is divided by equal amount by using 3dB fiber fusion coupler for generation of pump and probe signals from single laser source. Fiber coupler port gives inputs to pump signal and probe signal to Fiber Under Test (FUT). The pump signal is coupled to the Polarization Scrambler (PS) to improve the brillouin gain and to reduce the amplitude fluctuations due to changes in the state of polarization. The output of the PS is connected to EOM1, which provides amplitude modulation by applying suitable modulation frequency to produce pump pulse and generate the carrier signal. This pump pulse is amplified by EDFA to improve the brillouin gain in frequency domain, pump signal needs high power to propagate long distance. The probe signal, coupled to PS, sets the CW light in linear polarization mode before coupling to the EOM2. The probe signal frequency is set at RF generator, which is equivalent to brillouin frequency shift for corresponding fiber. This method is called side band suppression technique by setting the proper DC bias voltage at EOM2. The probe signal is launched into the fiber after passing through optical isolator, which prevents the pump signal from entering the EOM2 and DFB laser diode. In this experimental setup, the single mode fiber acrylate coated SMF28 fiber is used. The operating temperature of the fiber is 100 C, 2.5km length optical fiber is subjected to different temperatures and the distributed temperature response is monitored.
SBS gain bandwidth is recorded as a function of temperature. The relation between SBS gain and temperature is plotted in figure 3. The variation of brillouin frequency as a function of temperature is shown in figure 4. We see that the response of gain and the frequency to the temperature is linear. Figure 1. Schematic of BOFDA system 14mw pump power and 450uw stokes power are used to realize the stimulated brillouin effect in frequency domain. The CW laser becomes pulsed after passing through the EOM1 and the amplitude modulation frequency range is 40kHz to 20MHz, which corresponds to 2.5Gb/sec and produces the 50ns optical pulse. The spatial resolution of the system is 5m. The EOM1 is driven by pulse generator with suitable DC bias voltage. The other part of the brillouin is down shifted in frequency, namely stokes line. For side band suppression, the DC bias voltage is applied to EOM2 and the stokes signal is amplified by changing the modulation frequency of about 10.853 GHz, which corresponds to 20Gb/sec to get the proper amplification at room temperature of 20 C. 3. RESULTS AND DISCUSSION The Stimulated Brillouin Scattering (SBS) gain is measured by varying the modulation frequency. The SBS gain spectrum frequency is within the range of brillouin shift and the shape of the spectrum is fitted with Lorenzian function. The typical gain spectrum of 2.5km fiber at 40 o C is shown in figure 2. Figure 3. SBS gain vs Temperature Figure 4. Frequency shift vs Temperature Figure 5 FWHM vs Temperature Figure 2. SBS gain spectrum at 40 C The SBS line width as a function of temperature is shown in figure 5. The three dimensional temperature profile is shown in figure 6. The brillouin gain is a
function of pump power and stokes power. The Figure 7 and 8 show the variation of SBS gain as a function of stokes power and pump power respectively at room temperature. amplified as a function of temperature and strain. In this study, we have observed the amplitude of stokes, anti stokes and central wavelength at different temperatures. Figure 6, 3D Picture of Temperature response on 2.5km fiber at 40 C The pump power is varied using EDFA to achieve different SBS gain. To adjust the stokes power, attenuators are used. Figure 8. SBS gain vs Pump power Based on the SBS gain, the freqeuency measurements are made at room temperature. The brillouin frequency fb is 10.853GHz and in this frequency band, the minimum frequency fb min is 10.823GHz and the maximum frequency fb max is 10,883GHz. In which the centre frequency amplitude is always high and fb-min and fb-max are always low and almost equal also. Using these frequencies, brillouin shift is derived. Figure 7. SBS gain vs Stokes power The wavelength and amplitude response of the Brillouin stimulated back scattering signals is captured using optical spectrum analyzer (figure 9). The pump wavelength is 1551.671nm. The stokes, which is down shifted frequency signal by frequency side band suppression technique, is at higher wavelength, 1551.758nm. The Pump wavelength and stokes wavelength are shown in figure 10. Usually the stokes signals amplitude is less. However, due to the three signal interaction the stokes signal get amplified. This spectrum contains the anti stokes signal at lower wavelength at 1551.584nm. The intensity of anti stokes signal is less compared to stokes signal due to less amplification in the three wave interaction. The wavelength of the brillouin scattering undergoes wavelength change due to nonlinear scattering at different temperature and strain conditions. But in frequency domain the wavelength is kept constant and corresponding frequency is Figure 9. BOFDA Spectrum Figure 11 shows the different temperature response for Brillouin signals. It is observed from figure 11 that the Centre wavelength and stokes lines amplitudes are nearly equal at temperature range from 10 C to 80 C, SBS gain is low at low temperature range due to poor amplification. In the case of anti stokes signal, amplitude is always smaller compared to stokes.
Figure 10. Pump and stokes wavelength spectrum Figure 13 Anti Stokes amplitude vs temperature The anti stokes signal is not amplified in the brillouin frequency domain. Its frequency is high compared to stokes and the central wavelength line. Figure 13 shows the anti stokes signal amplitude response at different temperatures. This antistokes amplification is negligible at stimulated scattering and it is not sensitive to temperature or strain compared to stokes signal. 4. CONCLUSION Figure 11. BOFDA Amplitude vs temperature The distributed sensor for temperature measurement is demonstrated and its functional characteristics are discussed. The fiber optic BOFDA sensor system was designed with single laser source, which is cost effective solution for distributed temperature monitoring applications. The optical fiber of 2.5km length is tested at different temperatures, the temperature coefficient is 1.12 MHz/ºC for corning SMF-28. REFERENCES Figure 12. Stokes amplitude vs temperature [1] G. P. Agarwal, Nonlinear Fiber Optics, Academic Press,San Diego, 2001, 3 rd Edition.B. [2] [Torsten Gogolla and Katerina Krebber, Fiber Sensors for distributed temperature and strain measurements using brillouin scattering and frequency domain methods, SPIEVol.3105.0277-786X/97. [3] Dieter Garus, Torsten Gogolla, Katerina Krebber and frank Schliep, Brillouin Optical Fiber Frequncy Domain Analysis for Distributed Temperature and Strain Measurements, Journal of Light wave Technology, Vol.15, No.4, April 1997. [4] X. Bao,,Jabulani Dhilwayo, Nicol Heron, David J. Webb and D.A. Jackson, Experimental and Theoritical Studies on a Distributed Temperature
sensor based on Brillouin Scattering. Journal of Light wave Technology, Vol.13, No.7 July 1995. [5] Romeo Bernini, Aldo Minardo, Luigi Zeni, Distributed fiber-optic frequency-domain Brillouin sensing, Sensors and Actuators A 123 124, 2005. [6] S.Diaz, S. Foaleng Mafang, M.Lopez Amo,L.Thevenaz, High performance Brillouin distributed fiber sensor, SPIE Vol.661938-1/2007. [7] Il-Bum Kwon, Chi-Yeop Kim, and Man-Yong Choi, Demonstration of Temperature Measurement Distributed on a Building Using Fiber Optic Sensor, Proceedings, XVII IMEKO World Congress, Dubrovnik, Croatia, June 2003. [8] Zang Zai-xuan, Wang Jian-feng, Liu Hong-Lin, Xu HAi-fen, Dai Bi-zhi, Li Chen-xia, Li Lanxiao, Geng Dan and Insoo S.Kim, The long range distributed fiber raman photon temperature sensor, Optoelectronice letters Vol 3, No.6, Nov 2007. [9] Kellie Brown, Anthony W. Brown, Bruce G. Colpitts, Characterization of optical fibers for optimization of a Brillouin scattering based fiber optic sensor, Optical Fiber Technology 11, 2005. [10] Kazuyuki Shiraki, Masaharu Ohashi, Mitsuhiro Tateda, SBS Threshold of a Fiber with a Brillouin Frequncy shift distribution, Journal of Light wave Technology, Vol.14, No.1, Jan 1996.