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.

A novel target type flow sensor has been proposed for liquid flow measurement in pipeline. The force created by the fluid on the target results in the distortion of a cantilever structure. A couple of fiber Bragg gratings (FBGs) are mounted at either side of the cantilever. By monitoring the wavelength shift difference of the two FBGs, the fluid flow-rate can be obtained and the cross-sensitive problem of the FBG sensor can be solved by compensating the temperature effect. Measurement theory has been described. Simulation and preliminary experiments have been carried out, and the results verify the feasibility of the proposed sensor with a measurement range from 0 to 1000 cm3/s.

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.

This paper describes an optical-fiber sensor developed for temperature measurement under the offshore oil well conditions. The sensor exploits the displacement of the optical absorption edge occurring in semiconductors under the influence of temperature variation as a result of temperature-induced energy shifting of conduction band extrema. The structure of the sensor and the measurement principle are described. The common-path reference measurement and node type error compensation technologies are developed. And the detailed theoretical analysis indicates that the proposed sensor system can effectively improve measurement errors caused by light fluctuation, difference and variation of the detector responsiveness, circuit magnification times, and so on. It approves that this sensor system can be applied under the long-term formidable conditions with fairly good measurement stability.

The refractive index of salt water changes corresponding to the variation of salinity. Based on a differential refractometer, the salinity canbe measured due to beam deviation, and the influence of temperature on the measurement results is reduced effectively. Beam deviation caused by salinity change is detected by a fiber-optic array and recorded by a charge-coupled device. According to the Gauss distribution theory, the light-spot center can be determined by fitting a Gauss curve using each received light spot intensity and the position of each receiving fiber. With this method, measurement errors caused by light power fluctuation and ambient light disturbance can be avoided effectively. Variation in transmission power loss of each receiving fiber reduces the measurement accuracy of salinity. Therefore, a method of correction for each fiber transmission power loss in the array can effectively improve the salinity measurement accuracy. Experimental results indicate the feasibility of the developed system, and the measurement repeatability error is better than ±0.25‰.