Long-distance remote simultaneous measurement of strain and temperature based on a Raman fiber laser with a single FBG embedded in a quartz plate

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1 Long-distance remote simultaneous measurement of strain and temperature based on a Raman fiber laser with a single FBG embedded in a quartz plate Young-Geun Han, Thi Van Anh Tran, Ju Han Lee, and Sang Bae Lee Photonics Research Center, Korea Institute of Science and Technology 39-1 Hawolgok-dong, Seongbuk-gu, Seoul , Korea yyghan@kist.re.kr ABSTRACT We propose and experimentally demonstrate long-distance, simultaneous measurement of strain and temperature based on a Raman fiber laser with a single FBG embedded on a quartz plate. Most of all, FBG based sensing technology has attracted considerable attention in the field of optical sensors since FBG based sensing probe can provide the most simple and attractive methods to monitor the external perturbation change like temperature, strain, and pressure due to its high sensitivity, electro-magnetic immunity, compactness, and ease of fabrication. In order to enhance the measurement resolution of sensing systems, fiber lasers based sensor schemes with narrow bandwidth and high extinction ratio have been considered as promising technologies. A novel and practical Raman laser based long-distance sensing scheme for simultaneous measurement of strain and temperature using FBGs is investigated. High-quality Raman laser output with a high extinction ratio of more than 50 db is obtained at a long distance of over 50 km. Lasing wavelength shift and separation occur as the temperature and strain increase, respectively. To induce these phenomena, half of the FBG is fixed steadily on the quartz plate to respond to temperature only, while the other half of the FBG is free to respond to both temperature and strain. The temperature and strain sensitivities are measured to be 8.96 pm/ 0 C and 1.47 pm/µε, respectively. This allows simultaneous measurement of strain and temperature for long-distance sensing applications of more than 50 km. Keywords: Raman fiber laser, strain and temperature sensor, long distance sensor 1. INTRODUCTION Fiber gratings have been emerged as a promising technology in the field of fiber optic sensors due to due to their versatile advantages such as their high sensitivity, electro-magnetic immunity, compactness, and ease of fabrication [1-3]. Fiber gratings based sensing probes can provide the most simple and attractive methods to monitor the external perturbation change like temperature, strain, and pressure [1-3]. However, they still have some limitations like the concurrent sensitivities to both stain and temperature. In order to resolve this problem, many techniques have been proposed, including reference fiber Bragg gratings (FBGs) method [2], specialty long-period fiber gratings (LPFGs) [3], and combined FBG and LPFG [4], and so on. These techniques require a broadband light source for a sensing signal. In fact, the distance of transmission signal in the conventional sensing system with broadband source is limited to 25 km due to Rayleigh scattering, which induces an optical noise and degrades the quality of the transmission signal in the fibre [5-6]. In order to increase the distance and enhance the performance of fiber grating sensor system, several methods have been proposed [6]. A passive FBG sensing scheme based on distributed Raman amplification with the transmission fiber has been proposed in order to enhance the sensor performance for long-distance remote sensing applications [5]. However, it has drawbacks since it still requires a broadband light source as an input signal. Passive Components and Fiber-based Devices III, edited by Sang Bae Lee, Yan Sun, Kun Qiu, Simon C. Fleming, Ian H. White, Proc. of SPIE Vol. 6351, , (2006) X/06/$15 doi: / Proc. of SPIE Vol

In order to enhance the measurement resolution of sensing systems, active fiber laser based sensing schemes with narrow bandwidth and high extinction ratio have been proposed [6].

2 In order to enhance the measurement resolution of sensing systems, active fiber laser based sensing schemes with narrow bandwidth and high extinction ratio have been proposed [6]. However, it is not easy to discriminate two sensitivities between strain and temperature even if the previously proposed Raman laser configuration effectively removes the requirement of an additional broadband light source and significantly improves the sensing signal quality. In this paper, we proposed and experimentally demonstrate long-distance, simultaneous measurement of stain and temperature based on a Raman fiber laser with a single fiber Bragg grating embedded on the quartz plate. The high quality Raman laser output with high extinction ratio more than 50 db is obtained at the long distance over 50 km. The lasing wavelength shift and separation occur as the temperature and strain increase, respectively. To induce these phenomena, the half of the FBG is fixed steadily on the quartz plate in order to respond to temperature only, whilst the other half of the FBG are settled freely to respond to both temperature and strain. The temperature and strain sensitivity are measured to be 8.96 pm/ 0 C and 1.47 pm/µε, respectively. This allows simultaneous measurement of strain and temperature for long-distance sensing applications more than 50 km. 2. LONG DISTANCE SIMULTANEOUS MEASUREMENT OF STRAIN AND TEMPERATURE BASED ON A RAMAN FIBER LASER Figure 1. The schematic diagram of all fiber Raman laser based on FBGs for a long distance remote sensing system. The experimental scheme of the proposed fiber Raman laser based FBG sensor system is shown in Fig. 1. The fiber Raman laser consists of a tunable chirp FBG (TCFBG) acting as a cavity mirror, a 50-km long standard single mode fiber (SMF) acting as Raman gain medium and a single FBG embedded in the quartz plate acting operating as a cavity mirror and a sensing head. As shown in Fig. 1, the half of the uniform FBG was Proc. of SPIE Vol

3 embedded in the quarts plate to respond to temperature only. The quartz plate has a dimension of 1 cm length, 3 cm width, and 1 mm height. The half-length was firmly attached on the quartz plate by using epoxy glue in the one end and middle point of the FBG. In the bonding process, the great care was taken due to the deformation of the grating period in the fiber. The rest part of the grating was free to respond to both temperature and strain. Consequently, the uniform FBG has two segments to response to temperature and strain differently. Since two sections have the same temperature sensitivity, the resonant peak shift occurs as the temperature increases. However, the peak separation can be induced by the strain change due to their different the different strain dependence. This can directly affect the Raman fiber laser output. The FBG was fabricated by exposing the photosensitivity fiber to KrF excimer laser through the phase mask. The grating length was 2 cm and the Bragg wavelength was nm at room temperature. To generate the Raman laser, the proposed sensor head was connected to one end of 50-km long standard SMF and the TCFBG with the broad bandwidth more than ~5 nm was connected in the other end to build Raman fiber laser cavity. The Raman pump for this laser consists of four laser diodes operating at the wavelength of 1425, 1435, 1455, 1465 nm respectively. These laser diodes are multiplexed and launched in the laser cavity by uisng an passive 14XX WDM coupler. A total pump power and Raman gain were up to 1 W and 15 db, respectively. This level of Raman gain can compensate total loss in the cavity and generate the lasing signal. The lasing wavelength is determined by FBG sensing head. Since we utilized a single uniform FBG and a TCFBG, we achieved a single lasing Raman laser output with high extinction ratio more than ~50 db as shown in Fig. 2. After generating a single Raman laser output based on a single uniform FBG, we measured the temperature and strain sensitivity. 10 <0.15 nm 0 Output power [dbm] > 50dB Figure 2. Output spectrum of the Raman fiber laser with high extinction ratio more than 50 db. Proc. of SPIE Vol

4 (a) 0 Temperature 0 to 90 0 C Output power [dbm] (b) Temperature [ 0 C] Figure 3. (a) Output spectra of the Raman laser with the temperature change. (b) The shift of the Raman lasing wavelength as a function of the applied temperature (8.75 pm/ o C). Proc. of SPIE Vol

5 Fig. 3(a) shows the Raman laser output spectra with the temperature change in the range from 30 to 90 o C. Since two segments in the uniform grating have the same temperature sensitivity, the Raman lasing wavelength shifted into the longer wavelength. Fig. 3(b) shows the lasing wavelength change of the Raman laser output as a function of the applied temperature. After fitting the measured data, temperature sensitivity of the lasing wavelength shift was estimated to be 8.75 pm/ o C. After stretching FBG sensor head using two translation stages, we examined the strain sensitivity of the proposed sensor system at room temperature (25 o C). Output power [A.U.] (10dB/div.) λ (a) 1 λ (b) Strain [µε] λ 1 λ 2 Wavelength separation ( λ) [nm] (c) Strain [µε] Figure 4. (a) Output spectra of the Raman laser output with the strain change, (b) Lasing wavelength shift with the applied strain change, and (c) The lasing wavelength separation as a function of the applied strain (1.47 pm/µε). Fig. 4(a) shows the output spectra of the Raman laser as the applied strain increases. Between two lasing wavelength, one (λ 1 ) corresponding to the fixed section of the FBG into the quartz plate was the same with the pristine lasing wavelength of the Raman lasing output as the applied strain increased. However, when the strain was applied, the new lasing wavelength (λ 2 ) could be included and shifted into the longer wavelength since the rest section of the FBG without the quarts plate was readily affected by the strain. Accordingly, the lasing wavelength was separated by the applied strain due to the different strain sensitivity in two sections of the FBG head. Fig. 4(b) shows the Raman lasing wavelength changes corresponding to the two different segments of the FBG as a function of the applied strain. Fig. 4(c) shows the lasing wavelength separation with Proc. of SPIE Vol

6 the applied strain change. The strain sensitivity of the lasing wavelength separation ( λ=λ 2 λ 1 ) was estimated to be 1.47 pm/µε in the range from 0 to 1500 µε. Based on the shift and separation of the Raman lasing wavelength, it is possible to distinguish two sensitivities between temperature and strain, respectively at a long distance more than 50 km. 3. CONCLUSION In conclusion, we proposed and experimentally demonstrated the simple and practical, long-distance remote sensing scheme based on the Raman fiber laser for simultaneous measurement of strain and temperature at a long distance more than 50 km. The proposed Raman laser was composed of two FBGs for a laser cavity and a conventional single mode fiber with the length of 50 km. We achieved a single channel Raman laser output with high extinction ratio more than ~ 50 db. In the uniform FBG as a sensing head, its half segment was firmly embedded on the quartz plate to suppress the strain sensitivity and to respond to temperature only. The rest section was free to respond to both temperature and strain. Eventually, the Raman lasing wavelength was shifted or separated by the applied temperature or strain, respectively. The temperature sensitivity of the lasing wavelength shift was estimated to be 8.75 pm/ o C. The strain sensitivity of the lasing wavelength separation was measured to be 1.47 pm/µε. This allows simultaneous measurement of unambiguous strain and temperature at a long distance more than 50 km. 4. REFERENCES [1] A.D. Kersey, M. A. Davis, H. J. Patrick, M. LeBlank, K. P. Koo, C. G. Askins, M. A. Putnam, and E. J. Friebele, Fiber Grating sensors, J. Lightwave Technol., vol. 15, , [2] S. W. James, M. L. Dockney and R. P. Tatam, Simultaneous independent temperature and strain measurement using in-fiber Bragg grating sensors, Electron. Lett., vol. 32, pp , [3] Y. G. Han, S. B. Lee, C. S. Kim, Jin U. Kang, U. C. Paek, and Y. Chung, Simultaneous measurement of temperature and strain using dual long-period fiber gratings with controlled temperature and strain sensitivity, Opt. Express, vol. 11, pp , [4] Y. Nakajima, Y. Shindo, and T. Yoshikawa, Novel concept as long-distance transmission FBG sensor system using distributed Raman amplification, Proc. OFS-16, pp , [5] J. H. Lee, J. H. Kim, Y. G. Han, S. H. Kim, and S. B. Lee, Investigation of Raman fiber laser temperature probe based on fiber Bragg gratings for long distance remote sensing applications, Opt. Express, vol. 12, pp , [6] Y. G. Han, T. V. A. Tran, S. H. Kim, and S. B. Lee, Development of multiwavelength Raman fiber laser based on phase-shifted fiber Bragg gratings for long-distance remote sensing applications, Opt. Lett., vol. 30, pp , Proc. of SPIE Vol

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