IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 59, NO. 11, NOVEMBER 2010 2997 A Temperature Self-Compensated LPFG Sensor for Large Strain Measurements at High Temperature Ying Huang, Zhi Zhou, Yinan Zhang, Genda Chen, and Hai Xiao Abstract—In this paper, a CO 2 laser-induced long-period fiber- grating (LPFG) optic sensor was packaged with a hybrid mecha- nism of elastic attachment and gauge length change for large strain measurements in a high-temperature environment. An emphasis was placed on the use of two cladding modes (LP 06 and LP 07 ) of a single LPFG sensor for simultaneous strain and temperature evaluations so that exact temperature was used to compensate strain measurements. Both strain and temperature sensitivities of the LPFG sensor, as well as the strain transfer ratio due to a com- bined effect of elastic attachment and gauge length change, were analytically derived and validated with tension tests at elevated temperatures. The strain sensitivity of the LPFG sensor switched sign from negative for LP 06 or lower modes to positive for LP 07 or higher modes, whereas its temperature sensitivity remained positive. The sign switch for the strain sensitivity resulted from two competing changes of grating period and effective refractive index as the gratings are subjected to an axial strain. The LPFG sensor was demonstrated to be operational up to 700 ◦ C for a strain measurement of up to 1.5%. Index Terms—High temperature, large strain, long-period fiber grating (LPFG), simultaneous strain and temperature mea- surement, strain sensitivity, strain transfer ratio, temperature sensitivity. I. I NTRODUCTION B UILDINGS are exposed to increasing fire hazards dur- ing recent extreme events such as earthquake-induced gasoline ruptures and terrorist threats. Their behavior in a high-temperature environment (e.g., progressive collapse of steel buildings) has thus become a fundamentally important subject that will continue to receive growing interests in the research community. To the best of our knowledge, sensors are presently unavailable for deployment in fire environments even for laboratory experiments. For example, to understand the fundamental physics involved in a fire–structure interaction process, two insulated steel trusses were tested in jet fuel fires [1]. However, no sensor was actually instrumented on Manuscript received September 2, 2009; revised February 25, 2010; accepted March 11, 2010. Date of publication May 3, 2010; date of current version October 13, 2010. This work was supported in part by the U.S. National Science Foundation under Award CMMI-0825942 and in part by the Mid- America Transportation Center under Award 0018358. The Associate Editor coordinating the review process for this paper was Dr. George Xiao. Y. Huang, Z. Zhou, and G. Chen are with the Department of Civil, Ar- chitectural, and Environmental Engineering, Missouri University of Science and Technology, Rolla, MO 65401-0030 USA (e-mail: yhgn6@mst.edu; zhouzhi@mst.edu; gchen@mst.edu). Y. Zhang and H. Xiao are with the Department of Electrical and Computer Engineering, Missouri University of Science and Technology, Rolla, MO 65401 USA (e-mail: yzfyc@mst.edu; xiaoha@mst.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIM.2010.2047065 the structural members to directly measure temperature and strain inside fire flames, although having these parameters was highly desirable. The fire–structure interaction would never be fully understood without sensors that can directly measure large strains at high temperature. In reference to the collapse investigation of the former World Trade Center towers [1], none of the steel samples recovered from ground zero showed evidence of exposure to temperatures above 600 ◦ C for as long as 15 min. Only three of the recovered samples of exterior panels reached temperatures in excess of 250 ◦ C during the fire or after the collapse. Therefore, a temperature range of up to 700 ◦ C seems appropriate for building research in fire environments. Strain measurements at high temperature have been at- tempted by several researchers with fiber Bragg grating sen- sors [2] and Fabry–Pérot (F–P) sensors [3]. For simultaneous strain and temperature measurements, long-period fiber-grating (LPFG) sensors fabricated with a birefringence fiber [4] and compact LPFG pairs [5] have been investigated. Due to limited deformability of the optical fiber, these sensors can only sustain a strain of less than 4000 με. Han et al. [6] reported that a dual-LPFG sensing system with a cladding mode stripper in between can simultaneously measure temperature up to 180 ◦ C and strain up to 8000 με. However, the weak stripper between the two LPFGs limits the temperature range of the dual sensor within 200 ◦ C. Rao et al. [7] presented a hybrid LPFG/micro extrinsic F–P interferometric sensor for a simultaneous mea- surement of strain and temperature up to 650 ◦ C. However, its strain dynamic range is very small. The first long-period grating was successfully inscribed on an optical fiber in 1996 [8], and the modulation of a relative effective index change between the core and the cladding of an LPFG sensor can be achieved by UV irradiation [9] and CO 2 laser irradiation [10]. With different fabrication methods, LPFG sensors have different properties for strain and temper- ature measurements. The strain and temperature properties of UV-induced LPFG sensors have widely been investigated in the past few years. UV-induced LPFG strain sensors largely depend on the types of fibers due to their diverse strain-optic coefficients [8]. They exhibited positive strain sensitivity and negative temperature sensitivity with cladding modes lower than LP11 [11], [12]. On the other hand, the properties of LPFG sensors induced by CO 2 lasers have not been investigated systematically. The effects of various interrelated physical pa- rameters such as strain and temperature on the sensitivity of LPFG sensors remain unclear in various applications. The objectives of this paper are to design, fabricate, and characterize a CO 2 laser-induced LPFG optic sensor that is 0018-9456/$26.00 © 2010 IEEE