Multi-wire steel strands have been widely used in various prestressed concrete structures. In this study, experimental evaluation of fiber Bragg grating (FBG) sensors for strain measurements in a seven-wire prestressed steel strand has been carried out. An installation technique of FBG sensors has been developed to fulfill the special requirements of the prestressed steel strand. The experiment results show that fiber Bragg gratings can represent the overall stress of the prestressed steel strand without being affected by the specific structure of the strand when it is only fixed on one wire. It is also demonstrated that the maximum strain that the FBG sensor can measure is 6260 με, while the prestressed steel strand usually endures the strain greater than 10000 με. This means that an offset of about 4000 με is necessary to measure the maximum strain that the strand could experience in its applications.
EFPI(extrinsic Fabry-Perot interferometer) and FBG(fiber Bragg grating) sensors are two kinds of the most promising sensors for structure health monitoring and can resolve the problem of cross sensitivity of sensing well when they are used simultaneously for strain and temperature measurement. A parallel demodulation method for the two types of sensors is developed for the goal, which based on a set of Michelson interferometer and combined the methods of low coherence interference and Fourier transform spectrum. The signals from EFPI and FBG sensors are obtained at the same time by scanning one arm of Michelson interferometer, and an algorithm model is established to retrieve the information of wavelength of FBG and cavity length of EFPI, which then be used to determine the information of strain and temperature. The experiment shows the demodulation precision of FBG is 14.4 pm and that of EFPI is 0.203μm.
Optical coherence tomography (OCT) is a new biomedical imaging technique in resent year. It has some good qualities, such as non-ionizing radiation, non-invasive, high resolution and sensitivity. High-speed OCT imaging is very important for obtaining the cross-sectional images of the internal microstructure of living tissue. Increasing the imaging speed can produce imaging real time. To study high speed OCT, a new method of OCT imaging technique has been designed in this paper--replacing the point-focus mode with line-focus mode in the sample arm. Cylindrical lens can be used for focusing the incident light into a line in the sample. And a 2D OCT imaging can be obtained in one dimension scanning. In the paper we analyze the interference principle of line-focus imaging mode.
The optical fiber EFPI sensor with cavity length shorter than 20 µm is suited for structure health monitoring. The cavity length of EFPI is crucial measurand and usually obtained by low coherence interference method, but the coherence lengths of most laser source are greater than 20 µm, which lead to the overlapping of multiple interference fringes and failing to obtain cavity length. In this paper, a new demodulation method is proposed. The EFPI sensor of system is made with multimode fiber, the other part is monomode fiber, and a low coherence interference experiment setup with ASE light source, which avoids overlapping of different orders of low coherence interference fringes wave packet through reflectivity match, is established. A algorithm is developed for retrieving cavity length and a new method for multiplexing of EFPI sensors is presented. Experiments show that the cavity length of EFPI can be obtained efficiently and multiplexing is easily realized.
Timely, accurate and reliable pressure information about how the reservoir is performing is an important component to optimizing oil yield and production rates. This paper reviews the use of fiber optical pressure sensor for downhole monitoring in the oil industry. Several types of pressure transducer with different characteristics have been introduced. Due to their multiplexing capabilities and versatility ,the use of Bragg grating sensors appears to be particularly suited for this application. A sensor for accurate and long term fluid pressure monitoring based on optical fiber Bragg gratings(FBGs) is developed. The sensor converts fluid pressure into optical fiber strain by means of a mechanical transducer to enhance its sensitivity to pressure. It can also implement distributed or multiplexed sensing. The sensor operation is studied at pressure up to 100 Mpa(1000bar) and the tested temperature to ~175°. It is possible to be used in the well.