High-Resolution Strain and Temperature Sensor Based ... - IEEE Xplore

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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 20, OCTOBER 15, 2007

High-Resolution Strain and Temperature Sensor Based on Distributed Bragg Reflector Fiber Laser Li-Yang Shao, Xinyong Dong, A. Ping Zhang, Member, IEEE, Hwa-Yaw Tam, Senior Member, IEEE, and Sailing He, Senior Member, IEEE

Abstract—We present a high-resolution strain and temperature sensor by using a polarimetric distributed Bragg reflector fiber laser. The mean wavelength and polarization beat frequency of the laser output are utilized to determine the strain and temperature of the sensor. Experimental results show that the sensor has a capability of sensing strain and temperature simultaneously, with root and 0.05 C, respectively. mean square deviations of 9.3 Index Terms—Fiber Bragg grating (FBG), fiber laser, fiber-optic sensor, strain sensor, temperature sensor.

I. INTRODUCTION

F

IBER BRAGG grating (FBG) sensors have attracted significant interest for a wide variety of industrial applications, particularly in structural health monitoring for buildings, bridges, trains, and composite materials [1], [2]. Broadband light sources or wavelength-tunable lasers are commonly used to interrogate passive FBG sensors. The light reflected from FBGs can be quite weak particularly with a broadband light source which has a much smaller optical spectral density. In applications where FBG sensors are located at long distance from the interrogator, this method could limit the signal-to-noise ratio (SNR) of the FBG sensors and thus reduces the accuracy of the sensor system. Active FBG sensor systems based on fiber lasers have the potential to alleviate this problem due to their high output power and narrow linewidth [3]–[5]. Two measurement schemes based on fiber lasers are reported. The first one measures the shift of the lasing wavelength. For example, Peng et al. reported a high resolution sensor using a linear cavity fiber laser scheme [3]. Mandal et al. presented a grating-based fiber laser for temperature sensing and the reported repeatability is 1.7 C [4]. The second scheme measured the birefringence of a fiber laser. A fiber laser exhibits two orthogonal polarization modes, external perturbations change the birefringence and consequently its polarization beat frequency. Kim et al. demonstrated a polarimetric fiber laser sensor for monitoring either

Manuscript received March 27, 2007; revised June 18, 2007. This work was supported in part by the Natural Science Foundation of China under Grant 60607011 and in part by the Hong Kong Polytechnic University-Zhejiang University Joint Research Project G-U224. L.-Y. Shao, X. Dong, and H.-Y. Tam are with the Department of Electrical Engineering, Hong Kong Polytechnic University, Kowloon, China. A. P. Zhang and S. He are with the Centre for Optical and Electromagnetic Research, Department of Optical Engineering, Zhejiang University, Hangzhou 310058, China. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2007.903535

strain or temperature [5]. It was a multimode laser due to the long-cavity length and limited the performance of the sensor. Hadeler et al. demonstrated a single-frequency polarimetric distributed feedback fiber (DFB) laser sensor for simultaneous measurement of strain and temperature [6]. However, fabrication of a DFB laser requires precision control of the phase shift during the inscription of grating and optical powers are produced at both ends of the DFB laser. In order to achieve adequate pump absorption, the reported sensor has a length of 45 mm which may not be short enough for some practical applications. In this letter, we propose to use a dual-polarization DBR fiber laser sensor with a short cavity for simultaneous strain and temperature measurement. The laser cavity is only 16.5-mm long (same as our previous works [7]), ensuring single-frequency operation, i.e., two orthogonal polarization modes and consequently only one polarization beat tone is generated. By using a high reflection FBG at one end and a lower reflectivity FBG at the output end, virtually all the lasing power can be easily made to concentrate at the output end, thus allowing more efficient use of pump power. On the contrary, DFB fiber laser generally emits optical power from both ends and to maximize optical power emitting from one end would involve precise control of the location of the phase shift [8]. By measuring both the mean wavelength and polarization beat frequency of the DBR laser output, simultaneous measurement of strain and temperature with a high accuracy is demonstrated in our experiments. II. PRINCIPLE The DBR fiber laser sensor consists of a pair of wavelengthmatched Bragg gratings written in an active fiber with appropriate separation. Spatial-hole burning is the main source of mode competition in linear cavity fiber laser [9]. The Bragg gratings here act as feedback mirrors as well as mode discriminators introducing different losses (by different reflectivities) to different modes, and only allow the dominant mode to operate above the lasing threshold [10]. Single-frequency lasing can be achieved by carefully choosing the reflectivity and length of the gratings. However, due to the fiber birefringence introduced by, e.g., fiber fabrication and grating inscription, the laser always operates in two orthogonal polarization modes. For a low bire, where are the modal refracfringent fiber ( tive indexes), the beat frequency can be expressed as

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SHAO et al.: HIGH-RESOLUTION STRAIN AND TEMPERATURE SENSOR

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Fig. 1. Experimental setup: WDM is wavelength division multiplexer; ISO is isolator; PC is polarization controller; OSA is optical spectrum analyzer; RFSA is RF spectrum analyzer.

where is the induced birefringence, and is the mean of lasing wavelengths. When the strain or temperature changes, the cavity length and the birefringence of a DBR fiber laser will be changed. Then, the lasing wavelength and the polarization beat frequency will shift and can be considered as an effective signal output. In general, their responses to the strain and temperature change can be written as

(2) and

(3) , the thermal exwhere the strain-optic coefficient , and the thermo-optic pansion coefficient . Equations (2) and (3) can be coefficient written in a matrix form as

(4) The coefficient matrix can be defined by separately measuring the strain and temperature responses of the lasing wavelength and the polarization beat frequency of the fiber laser. If , one can quickly dematrix is well conditioned termine the strain and temperature simultaneously by measuring and and taking the inverse operation of matrix . III. EXPERIMENT AND RESULTS Fig. 1 shows our experimental setup. The DBR fiber laser is constructed by using two 1548-nm Bragg gratings written in an Er–Yb codoped fiber with a separation of 10 mm. The length of one grating is 10 mm with a reflectivity of 99%, while the reflectivity of another grating with length of 3 mm is 90%. The 980-nm pump light is launched to the DBR laser through a wavelength-division multiplexing (WDM) coupler. The output of the laser is split into the two arms by a 3-dB coupler. One

Fig. 2. (a) Measured spectra. (b) Measured polarization beat frequency of proposed fiber laser sensor.

arm is used to measure the wavelength of the laser with an optical spectrum analyzer (OSA). At the other arm, the polarization beat frequency is measured with a radio-frequency (RF) spectrum analyzer. Fig. 2 shows the measured spectra. At room temperature (20 C), the wavelength and polarization beat frequency of the unstrained DBR fiber laser were measured to be 1548.39 nm and 1094.4 MHz, respectively. In order to test the sensor responses to strain change, one end of the DBR fiber laser was fixed, whereas the other end was stretched by using a manual translation stage. Strain was calculated from the elongation of the stretched fiber divided by the original length. Fig. 3 shows measured sensor responses to ap. It can be plied strain in the range of to seen that the laser wavelength and polarization beat frequency increase with strain. By using a linear fitting of the experimental and , are obtained as data, the strain sensitivities, i.e., and 8.76 kHz respectively. 1.13 pm The temperature responses of the lasing wavelength and beat frequency of the fiber laser were measured by immersing the fiber laser in water and changing the temperature of the water. A thermometer with a resolution of 0.1 C was employed to monitor the temperature. Fig. 4 shows the measured wavelength

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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 20, OCTOBER 15, 2007

Fig. 3. Wavelength and polarization beat frequency as a function of strain.

Fig. 5. Comparison between simultaneously applied strain-temperature and measured data.

and temperature were 9.3 ment ranges of

and 0.05 C over the measureand 10 C to 52 C, respectively.

IV. CONCLUSION

Fig. 4. Wavelength and polarization beat frequency as a function of temperature.

and polarization beat frequency as the temperature changes from and , 10 C to 52 C. The temperature sensitivities, i.e., are 8.88 pm C and MHz C, respectively. It was observed that the polarization beat frequency increased with applied strain and decreased with temperature. The temperature coefficient of beat frequency is much larger than strain one, which gives rise to a higher temperature resolution in the measurement. By taking the inverse matrix of K and the measured coefficient, (4) can be rewritten as kHz C kHz

pm C pm (5)

One can thus employ the coefficient matrix above to simultaneously determine strain and temperature by measuring the sensor’s wavelength and beat frequency. In order to evaluate the capability of simultaneous measurement, we heated the DBR fiber laser up to 52 C while strain was randomly applied in . The resolutions of measurement the range of in the wavelength and polarization beat frequency were 10 pm and 10 kHz, respectively. Fig. 5 shows the comparison between measured and applied parameters with strain and temperature. The root mean square (rms) deviations of the measured strain

Based on a short-cavity dual polarization DBR fiber laser, simultaneous measurement of strain and temperature has been demonstrated by measuring the laser wavelength and polarization beat frequency. The DBR fiber laser operated robustly in two orthogonal polarizations. The DBR fiber laser is a promising candidate for embedded sensing, particularly suitable for smart structure applications. REFERENCES [1] H. Y. Tam, S. Y. Liu, B. O. Guan, W. H. Chung, T. H. Chan, and L. K. P. M. Cheng, “Fiber Bragg grating sensors for structural and railway applications,” in Proc. SPIE, 2005, vol. 5634, pp. 85–97. [2] T. H. T. Chan, L. Yu, H. Y. Tam, Y. Q. Ni, S. Y. Liu, W. H. Chung, and L. K. Cheng, “Fiber Bragg grating sensors for structural health monitoring of Tsing Ma bridge: Background and experimental observation,” Eng. Strut., vol. 28, pp. 648–659, 2006. [3] P.-C. Peng, H.-Y. Tseng, and S. Chi, “High-resolution fiber Bragg grating sensor system using linear-cavity fiber laser scheme,” Microw. Opt. Technol. Lett., vol. 34, pp. 323–325, 2002. [4] J. Mandal, S. Pal, T. Sun, K. T. V. Grattan, A. T. Augousti, and S. A. Wade, “Bragg grating-based fiber-optic laser probe for temperature sensing,” IEEE Photon. Technol. Lett., vol. 16, no. 2, pp. 218–220, Feb. 2004. [5] H. K. Kim, S. K. Kim, and B. Y. Kim, “Polarization control of polarimetric fiber-laser sensors,” Opt. Lett., vol. 18, pp. 1465–1467, 1993. [6] O. Hadeler, E. Rnnekleiv, M. Ibsen, and R. I. Laming, “Polarimetric distributed feedback fiber laser sensor for simultaneous strain and temperature measurements,” Appl. Opt., vol. 38, pp. 1953–1958, 1999. [7] B. O. Guan, H. Y. Tam, S. T. Lau, and H. L. W. Chan, “Ultrasonic hydrophone based on distributed Bragg reflector fiber laser,” IEEE Photon. Technol. Lett., vol. 16, no. 2, pp. 221–223, Feb. 2004. [8] V. C. Lauridsen, T. Ssndergaard, P. Varming, and J. H. Povlsen, “Design of distributed feedback fibre lasers,” in Proc. ECOC, 1997, vol. 3, pp. 39–42. [9] G. A. Ball and W. H. Glenn, “Design of a single-mode linear-cavity erbium fiber laser utilizing Bragg reflectors,” J. Lightw. Technol., vol. 10, no. 11, pp. 1338–1343, Nov. 1992. [10] G. A. Ball, W. H. Glenn, W. W. Morey, and P. K. Chan, “Modeling of short, single frequency, fiber lasers in high-gain fiber,” IEEE Photon. Technol. Lett., vol. 5, no. 5, pp. 649–651, May 1993.