A free active element bulk-modulus-based fiber Bragg grating (FBG) sensor for down-hole pressure and temperature measurement is presented. With a differential measurement method and an isosceles triangle cantilever structure, problems of cross-sensitivity and chirped-signal in FBG sensors are solved. Simulation results indicate that the measurement sensitivity in measuring pressure and temperature is estimated to be ～8. 5 pm/MPa in a range from standard atmosphere pressure to 100 MPa and～27.5 pm/℃ from 20 to 200℃, respectively.

High-pressure measurement is made by demodulating Bragg grating peak splitting caused by transverse stress differences in the core of a high-birefringence (HB) single-mode fiber, which is bonded on the exterior surface of a free active element bulk modulus. The two orthogonally polarized signals reflected from the HB fiber Bragg grating (FBG) can be independently detected with a simple FBG interrogation system, which is based on the light intensity measurement method. In addition, the cross-sensitivity of the FBG sensor can be self-compensated by this method. The measurement principle is introduced, and preliminary experimental results indicate that the measurement sensitivity is estimated to be～12 pm/N in a linear measurement range from 0 to 120 N.

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‰.

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.