Fiber Bragg grating (FBG) sensors are one of the most exciting developments in the fields of fiber-optic sensors in recent years. One of the problems in using grating sensors is the discrimination of temperature and strain responses (cross-sensitivity effect between temperature and strain), because in most real-world applications both of these quantities will be acting on the sensor elements. Another problem is how to measure a small Bragg wavelength shift accurately. This article presents a comprehensive and systematic overview of discrimination measurement methods and demodulation techniques for FBG sensors. It is anticipated that FBG sensors will be commercialized and widely applied in practice in the near future due to the maturity of economical production of FBGs and the availability of cost effective measurement and demodulation techniques. Recent research work about a differential FBG sensor for simultaneous measurement of down-hole high pressure and temperature and the corresponding demodulation techniques are also introduced in this paper.

A novel fiber-optic sensor used for small intenal curved surface measurement is presented. A measurement principle based on beam reflection is described. Theoretically calculated curves and measured values are given. The results provide an essential theoretical and design basis for the measurement of curved surfaces with fiber-optic sensor. This sensor, which is composed of eight symmetrically cross-arranged receiving fibers and one centered emitting fiber, can accomplish distance measurement with high vertical resolution of 0.1

A double-arched-beam-based fiber Bragg grating (FBG) sensor for displacement measurement is presented. Unlike most FBG sensors that measure the resonance wavelength shifts to detect the measurand, this kind of FBG sensor is proposed to measure the displacement based on the demodulation of the FBG's reflective spectra bandwidth. According to the strain distribution on the novel double-arched beam surface, the positive and negative strain will act on only one FBG, which is stuck on the beam surface. Thus, the positive and negative strain will make the spectra broaden as the displacement increases. What is more, the problem of cross-sensitivity in the FBG sensor is solved because temperature only affects the wavelength shift, but not the spectra bandwidth. Simulation and preliminary experimental results indicate the feasibility of the proposed idea and a fairly good linearity of the measurement characteristic.