1148 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 15, AUGUST 1, 2007 A Dual-Wavelength Fiber Laser Sensor System for Measurement of Temperature and Strain Duan Liu, Nam Quoc Ngo, Member, IEEE, Swee Chuan Tjin, and Xinyong Dong Abstract—A strain and temperature-sensing scheme based on a dual-wavelength fiber laser is demonstrated experimentally. By measuring the change in the wavelength separation of the two lasing wavelengths and the shift of any of two lasing wavelengths, the measured strain and temperature are very linear and have a strain sensitivity of 0.15 pm/ over a range of 0–2400 and a temperature sensitivity of 14.3 pm/ C over a range of 22 C–230 C, respectively. Index Terms—Erbium-doped fiber (EDF), fiber Bragg grating (FBG), fiber-optic sensors, temperature and strain measurement. I. INTRODUCTION O VER THE past several years, fiber Bragg grating (FBG)- based sensor systems have been demonstrated for various sensing applications [1], [2], due to their mechanical robustness, inherent immunity to electromagnetic noise, compactness, sim- plicity in fabrication, and especially their wavelength-encoded feature of measurand information. In most passive FBG-based sensor systems, an expensive broadband light source is nor- mally required to cover the whole bandwidth of the measureand. Due to the low power of the light reflected by the gratings in these sensors, the signal-to-noise ratio is usually poor, which reduces the system’s sensing accuracy and the sensing range due to the limited number of fiber gratings that can be mul- tiplexed in the interrogation system. Because of the required high optical power and the narrow linewidth of the light source associated with the measureand, fiber-laser-based sensor sys- tems have been developed for both strain [3], [4] and temper- ature [5] sensing applications. Most of the previously reported fiber-laser-based sensor systems used single-wavelength fiber lasers, which can only interrogate one measureand with one sensor system. Although parallel multiplexed fiber-laser-based sensor schemes [6] can interrogate two or more measureands at the same time, the system complexity (and hence cost) and the insertion loss also increase. In this letter, a strain and temperature sensing scheme is experimentally demonstrated. Both the strain and temperature sensing were achieved with only one fiber laser sensor system. The measured strain and temperature are very linear and have a strain sensitivity of 0.15 pm/ over a range of 0–2400 and a temperature sensitivity of 14.3 pm/ C over a range of 22 C–230 C, respectively. The advantages of the sensing Manuscript received November 1, 2006; revised March 31, 2007. The authors are with Photonics Laboratory 1, School of EEE, Nanyang Technological University, Singapore 639798, Singapore (e-mail: liuduan@pmail.ntu.edu.sg). 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.2007.901128 Fig. 1. Schematic diagram of the dual-wavelength fiber laser sensor system for temperature and strain measurements. scheme include simple structure, low cost, and the ability to measure two measureands with only one sensor head. II. PRINCIPLE AND SYSTEM CONFIGURATION The experimental setup of the proposed dual-wavelength fiber laser sensor system is schematically shown in Fig. 1. The linear cavity of the dual-wavelength fiber laser system was formed by a uniform polarization-maintaining fiber Bragg grating (PM-FBG), a broadband polarization-maintaining lin- early chirped fiber Bragg grating (PM-LCFBG), and a segment of polarization-maintaining erbium-doped fiber (PM-EDF) as the active gain medium [7]. The 8-m-long PM-EDF is pumped by a 980-nm laser diode through a 980/1550-nm wavelength-division-multiplexing (WDM) coupler to obtain lasing, and the output spectrum is monitored by an optical spectrum analyzer (OSA) with a resolution of 0.01 nm. The PM-FBG, which has two reflection peaks within the stopband of the PM-LCFBG, functions as both the wavelength-selective component of the fiber laser and the sensor head in the fiber laser sensor system. Due to the birefringence of the polarization-maintaining (PM) fiber used in the grating fab- rication, the two reflection peaks of the uniform PM-FBG correspond to the two lasing wavelengths generated by the fiber laser. Making use of the birefringence’s dependence on the applied strain, strain sensing is realized by measuring the wavelength separation of the two lasing wavelengths. Using the Bragg wavelength’s temperature dependence of the uni- form PM-FBG, temperature sensing can also be achieved by monitoring the shift of any of the two lasing wavelengths. The uniform PM-FBG and the PM-LCFBG were both fab- ricated on a segment of high-birefringence (Hi-Bi) fiber [7]. The elliptical-core Hi-Bi fiber used in the fabrication of the PM-FBG has a beat length of 4.3 mm at 1550 nm. The Hi-Bi fiber used in the fabrication of the PM-LCFBG has a cut-off wavelength of over 1420 nm, a mode-field diameter of 10 m, and a birefringence of . Both of these two Hi-Bi fibers were hydrogen loaded for two weeks at 80 C to enhance their photosensitivities before grating fabrication. The PM-FBG and PM-LCFBG were both fabricated with the phase mask method using an argon ion laser. The PM-FBG was fabricated with a uniform phase mask (with a grating pitch of 1045 nm), and has two reflection peaks around 1545 nm. The 1041-1135/$25.00 © 2007 IEEE