Proceedings of Meetings on Acoustics

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1 Proceedings of Meetings on Acoustics Volume 19, ICA 2013 Montreal Montreal, Canada 2-7 June 2013 Underwater Acoustics Session 3aUWa: Acoustic Sensing Via Fiber Optics 3aUWa2. Measurements of Brillouin gain spectra in erbium-doped optical fibers for long-distance distributed strain and temperature sensing Mingjie Ding*, Yosuke Mizuno, Neisei Hayashi and Kentaro Nakamura *Corresponding author's address: Tokyo Institute of Technology, Yokohama, , Kanagawa, Japan, Brillouin scattering, one of the most important acousto-optical phenomena, occurs when lightwave interacts with periodical refractive-index change caused by the acoustic wave. The frequency downshift from the incident light to the backscattered Stokes light is called the Brillouin frequency shift (BFS), which depends on the strain/temperature applied to the optical fiber. Fiber-optic distributed Brillouin sensors have already been widely utilized to monitor strain/temperature conditions of structures including buildings, bridges and cables. The measurement range of the sensing system is partially limited by the optical propagation loss inside the sensing fiber. One solution is to employ special fibers capable of amplifying Brillouin signal. In this work, we employ erbium-doped fibers (EDFs) as sensing fibers, which are commonly used as 1550 nm optical amplifiers. As the first step, we investigated the strain/temperature dependence of Brillouin gain spectra (BGS) in EDFs with three different erbium concentrations. For all the samples, with increasing strain/temperature, the BGS shifted toward higher frequency, which was the same as that of silica fibers. Moreover, as the erbium concentration increased, the strain coefficient of BFS was increased, while its temperature coefficient was reduced. These results will definitely contribute to the future research of EDF-based distributed strain/temperature sensing systems. Published by the Acoustical Society of America through the American Institute of Physics 2013 Acoustical Society of America [DOI: / ] Received 30 Jan 2013; published 2 Jun 2013 Proceedings of Meetings on Acoustics, Vol. 19, (2013) Page 1

2 INTRODUCTION Brillouin scattering is one of the most significant acousto-optical phenomena in optical fibers, 1,2 and, since Brillouin frequency shift (BFS) is sensitive to temperature and strain, 3,4 it has been applied to fiber-optic temperature/strain distributed sensing for smart materials and structures, including buildings, bridges, dams, pipelines, cables, and aircraft wings. Several schemes of Brillouin fiber-optic distributed sensing systems have been developed, and the parameters for evaluating their performances, such as spatial resolution, measurement range, measurement speed, and signal-to-noise ratio (SNR), have also been improved by enhancing the Brillouin signal power. For example, in the time-domain techniques, 5,6 by using Raman amplification and/or inserting optical fiber amplifiers, the optical attenuation effect was mitigated to elongate the measurement range. 7 With enhanced signal, the measurement time, which is closely related to the time of signal integration, can also be shortened. As for the correlation-domain techniques, 8-10 in Brillouin optical correlation-domain reflectometry (BOCDR), 9,10 the spatial resolution was partially limited to ~13 mm by the weak Brillouin signal, 10 and was enhanced to ~6 mm by employing a tellurite glass fiber with a high Brillouin gain coefficient, i.e., with strong Brillouin signal. 11 Thus, the performance of Brillouin fiber-optic distributed sensing is expected to be improved by Brillouin signal amplification. Another methodology to enhance the Brillouin signal is to use pumped optical fibers doped with rareearth ions, such as neodymium, thulium, samarium, holmium, ytterbium, and erbium ions. With their optical amplification capability, Brillouin signal is predicted not only to be enhanced but also to be controlled at high speed by adjusting the pumping light, which will be of great interest in other Brillouin application fields. Although the reports of fundamental Brillouin properties in neodymium- and thulium-doped fibers have been already supplied, 12 erbium-doped fibers (EDFs), which are most widely used as optical amplifiers of the all rare-earth doped fibers, 13,14 have not been fully investigated yet. In this work, the Brillouin gain spectra (BGSs) in ~19-m-long EDFs with three different erbium concentrations (0.72, 1.20 and 2.28 wtppt) are measured at 1.55 µm without pumping light, and the BFSs and their dependences not only on temperature and strain but also on erbium concentration are investigated. The BFS and its temperature and strain coefficients in the EDF with 0.72-wtppt erbium concentration were GHz, 0.87 MHz/K, and 479 MHz/%, which changed with increasing erbium concentration, respectively. PRINCIPLE Brillouin scattering occurs when lightwave interacts with periodical refractive-index change caused by the acoustic wave in an optical fiber. Such acousto-optical interaction generates backscattered light beam called Stokes lights, and its spectrum is called BGS. 1,2 The center frequency of the BGS is lower than that of the incident light by the amount called BFS. Since the BFS and its temperature and strain dependences drastically differ according to the fiber materials and structures, they have been investigated in various kinds of special fibers to improve the sensor performance, which are well summarized in Ref. 12. Table 1 provides the physical properties of the three EDFs that were used in the experiment as fibers under test (FUTs). The erbium concentrations of the EDF samples were 0.72, 1.20 and 2.28 wtppt. A fusion splicer (FSM-50S, Fujikura) was utilized to splice 1-m-long silica single-mode fibers (SMFs) to both ends of the FUTs and the optical loss at the splicing point was suppressed to lower than 0.1 db by adjusting the arc duration. In addition, we submerged the open end of the spliced 1-m-long SMF into the matching oil (n=0.46) to supress the Fresnel reflection. The experimental setup for investigating the BGSs in EDFs was almost the same as that reported in Ref. 15 based on self-heterodyne detection. We used a laser diode (LD) at nm as a light source and the input optical power into the FUT was set to 20 dbm with an EDF amplifier (EDFA). The polarization states of the incident light and the reference light were optimized with polarization controllers (PCs). The Stokes light also includes the Brillouin signal generated in the 1-m SMF between the circulator and the FUT, but it did not influence the measurement due to the fact that the BFS in the EDF was ~500 MHz lower than that in the EDF, which is discussed later. Different strains were applied to the whole length of the EDF fixed on a translation stage using epoxy glue, while the temperature along the whole length of the EDF was adjusted with a heater. Proceedings of Meetings on Acoustics, Vol. 19, (2013) Page 2

3 TABLE 1. Physical properties of EDF samples. EDF sample No Length (m) Erbium concentration (wtppt) 0.72 (low) 1.20 (moderate) 2.28 (high) Peak µm (db/m) EXPERIMENTAL RESULTS The temperature dependence of the BGS in the EDF with low erbium concentration is shown in Fig. 1(a). The temperature was controlled from 20 C up to 100 C with a step of 20 C. The BFS at 20 C, calculated using Lorentzian fitting, was GHz, which is ~500 MHz higher than that of standard silica SMF. With increasing temperature, the BGS shifted toward higher frequency. The same measurements were performed for the EDFs with moderate and high erbium concentrations, as shown in Figs. 1(b) and (c), where similar behavior was observed. Since the optical propagation losses in these EDFs at 1.55 µm (without pumping) are higher than that in the EDF with low erbium concentration, the Stokes power or the signal-to-noise ratio (SNR) was lower. Figure 2 shows the BFS dependence in the EDFs at 20 C on erbium concentration. As the erbium concentration was increased, the BFS was decreased. This trend is not consistent with the fact that the BFS in standard silica SMFs (without erbium doping) is ~10.8 GHz, which is probably caused by alumina co-doped to suppress the clustering effect of erbium ions 16 (Note that alumina doped to some fibers is known to raise the BFS). 17 Figure 3 shows the measured BFS dependences in the three EDFs on temperature, from which each slope, temperature coefficient can be plotted as a function of erbium concentration, as shown in Fig. 4. With increasing erbium concentration, the BFS temperature coefficient was linearly decreased with a slope of -1.0 MHz/K/wtppt. The BFS temperature coefficients of ~0.8 MHz/K were lower than that of ~1.18 MHz in silica SMFs. 4 (a) (b) (c) FIGURE 1. Measured BGS dependence on temperature in EDF with: (a) low erbium concentration (0.72 wtppt), (b) moderate erbium concentration (1.20 wtppt), (c) high erbium concentration (2.28 wtppt). FIGURE 2. BFS in EDFs at 20 C as a function of erbium concentration. Proceedings of Meetings on Acoustics, Vol. 19, (2013) Page 3

4 Ding et al. FIGURE 3. Temperature dependences of BFS in EDFs with different erbium concentrations. FIGURE 4. Temperature coefficient of BFS in EDF as a function of erbium concentration. Next, the strain dependence of the BGS in the EDF with low erbium concentration was measured, as shown in Fig. 5(a). As the applied strain was increased, the BGS shifted toward higher frequency. The same measurements were performed for the other EDFs, as shown in Figs. 5(b) and (c), where similar behavior was observed. Figure 6 shows the measured BFS dependences in the three EDFs on strain, from which the dependence of the BFS strain coefficient on erbium concentration was derived, as shown in Fig. 7. With increasing erbium concentration, the strain coefficient was slightly enhanced with a slope of 5.92 MHz/%/wtppt. The BFS strain coefficients of ~485 MHz/% were lower than that of 580 MHz/% in silica SMFs. (a) (b) (c) FIGURE 5. Measured BGS dependence on strain in EDF with: (a) low erbium concentration (0.72 wtppt). (b) moderate erbium concentration (1.20 wtppt). (c) high erbium concentration (2.28 wtppt). Proceedings of Meetings on Acoustics, Vol. 19, (2013) Page 4

5 FIGURE 6. Measured BFS dependence on strain in EDFs with three erbium concentrations. FIGURE 7. Strain coefficient of BFS in EDF as a function of erbium concentration. CONCLUSION The BGSs in EDFs with three different erbium concentrations were investigated at 1.55 µm without pumping light. Besides, the BFSs and their dependences on strain, temperature, and erbium concentration were also measured. The BFS and its temperature and strain coefficients in the EDF with low erbium concentration were GHz, 0.81 MHz/K, and 479 MHz/%. It was also found that, by changing the erbium concentration, these parameters could be moderately controlled. We believe that these measurement results are fundamental and important information in achieving both pumped-edf-based Brillouin sensing and high-speed control of the Stokes power by pumping light in future. ACKNOWLEDGMENTS This work was partially supported by a Grant-in-Aid for Research Activity Start-up ( ) from the Japan Society for the Promotion of Science (JSPS), and by research grants from the Murata Science Foundation, the Hattori-Hokokai Foundation, the Mazda Foundation, the JFE 21st Century Foundation, the General Sekiyu Foundation, the Iwatani Naoji Foundation, and the Foundation of Ando Laboratory. REFERENCES 1. E. P. Ippen and R. H. Stolen, Stimulated Brillouin scattering in optical fibers, Appl. Phys. Lett. 21, (1972). 2. G. P. Agrawal, Nonlinear Fiber Optics, (Academic Press, New York, 1995). 3. T. Horiguchi, T. Kurashima, and M. Tateda, Tensile strain dependence of Brillouin frequency shift in silica optical fibers, IEEE Photon. Technol. Lett. 1, (1989). 4. T. Kurashima, T. Horiguchi, and M. Tateda, Thermal effects of Brillouin gain spectra in single-mode fibers, IEEE Photon. Technol. Lett. 2, (1990). 5. T. Horiguchi and M. Tateda, BOTDA-nondestructive measurement of single-mode optical fiber attenuation characteristics using Brillouin interaction: theory, J. Lightwave Technol. 7, (1989). 6. T. Kurashima, T. Horiguchi, H. Izumita, S. Furukawa, and Y. Koyamada, Brillouin optical-fiber time domain reflectometry, IEICE Trans. Commun. E76-B, (1993). 7. X. Jia, Y. Rao, L. Chang, C. Zhang, and Z. Ran, Enhanced sensing performance in long distance Brillouin optical time- Proceedings of Meetings on Acoustics, Vol. 19, (2013) Page 5

6 domain analyzer based on Raman amplification: theoretical and experimental investigation, J. Lightwave Technol. 28, (2010). 8. K. Hotate and T. Hasegawa, IEICE Trans. Commun. Measurement of Brillouin gain spectrum distribution along an optical fiber using a correlation-based technique--proposal, experiment and simulation--, E83-C, (2000). 9. Y. Mizuno, W. Zou, Z. He, and K. Hotate, Proposal of Brillouin optical correlation-domain reflectometry (BOCDR), Opt. Express 16, (2008). 10. Y. Mizuno, Z. He, and K. Hotate, One-end-access high-speed distributed strain measurement with 13-mm spatial resolution based on Brillouin optical correlation-domain reflectometry, IEEE Photon. Technol. Lett. 21, (2009). 11. Y. Mizuno, Z. He, and K. Hotate, Distributed strain measurement using a tellurite glass fiber with Brillouin optical correlation-domain reflectometry, Opt. Commun. 283, (2010). 12. Y. Mizuno, N. Hayashi, and K. Nakamura, Dependences of Brillouin frequency shift on strain and temperature in optical fibers doped with rare-earth ions, J. Appl. Phys. 112, (2012). 13. E. Desurvire, J. R. Simpson, and P. C. Becker, High-gain erbium-doped traveling-wave fiber amplifier, Opt. Lett. 12, (1987). 14. R. J. Mears, L. Reekie, I. M. Jauncey, and D. N. Payne, Low-noise erbium-doped fibre amplifier operating at 1.54 µm, Electron. Lett. 23, (1987). 15. Y. Mizuno and K. Nakamura, Experimental study of Brillouin scattering in perfluorinated polymer optical fiber at telecommunication wavelength, Appl. Phys. Lett. 97, (2010). 16. B. J. Ainslie, A review of the fabrication and properties of erbium-doped fibers for optical amplifiers, J. Lightwave Technol. 9, (1991). 17. P. Dragic, T. Hawkins, P. Foy, S. Morris, and J. Ballato, Sapphire-derived all-glass optical fibres, Nat. Photonics 6, (2012). Proceedings of Meetings on Acoustics, Vol. 19, (2013) Page 6