Jingyun Dai

Applications of fiber optic sensors in concrete structural health monitoring

The research of fiber optic extrinsic Fabry-Perot interferometer (EFPI) sensors and their applications in concrete structural health monitoring are presented in this paper. Different types of fiber optic EFPI sensors are designed and fabricated. Experiments are carried out to test the performance of the sensors. The results show that the sensors have good linearity and stability. The applications of the fiber optic EFPI sensors in concrete structural health monitoring are also introduced. Ten fiber optic sensors are embedded into one section of the Liaohe Bridge in Qinghuangdao-Shenyang Railway. Field test demonstrates that the results of fiber optic sensors agree well with conventional strain gauges.

A novel fiber optic Fabry-Perot temperature sensor

Fiber optic Fabry-Perot sensor has been shown to possess high sensitivity among phase-modulated fiber optic sensors. In the last decade, they have found numerous applications in industry, military, and civil engineering. But they are not easy to manufacture. To avoid the difficult in fabricating the temperature sensor by embedding the gauge in to temperature-sensitive materials, a novel Fabry-Perot sensor is designed and fabricated. The most influential factors of the thermal sensitivity of the sensor are analyzed, which gives the guidelines in design of different sensitivity sensors. Experiments were carried out to verify the theoretical analysis. And the results show that the sensors have good performance and by changing the gauge length the sensitivity of the sensor can be easily controlled.

Distributed fiber optic crack sensor for concrete structures

In this paper a novel distributed fiber optic crack sensor with polymer encapsulation based on optical time domain reflectometer (OTDR) is presented. The sensor head includes two layers of polymer and a bare fiber embedded in them. The crack induced deformation in the sensor head will result in the bend loss in the fiber. Utilizing the OTDR measurement, the position and the size of the crack can be determined. Two types of fiber arrangement are introduced in this paper. Experiment was carried out to evaluate the performance of the distributed fiber optic crack sensor. The primary results show that the fiber optic crack sensor has good linearity between the bend loss and the crack open size. The crack size resolution is better than 1mm.

FBG sensor network in Qinghai-Tibet railway monitoring system

A fiber Bragg grating (FBG) sensors network in the roadbed temperature monitoring system in Qinghai-Tibet railway is presented. This sensor network is composed of FBG sensor chains embedded in the roadbed, slave optical cables, work stations which include the FBG sensor interrogator and the optical router, master optical cables, and center workstation. We give the primary experimental results of this sensor network in monitoring the roadbed temperature of the railway at DK0985+000 near Chumar River. The temperature data was collected after a month when the temperature field in the monitoring hole became stable. This experiment had been carried out for more than one year. The FBG sensor network has successfully measured the temperature in the roadbed with an accuracy of 0.1 centigrade. Furthermore, the experimental result shows the stability and reliability of the FBG sensor network in the harsh environment of Qinghai-Tibet plateau. The prospect of the FBG sensor network in monitoring other parameters in the Qinghai-Tibet plateau is also presented.

Research on integrated fibre Bragg grating/extrinsic Fabry-Perot sensor

This paper reports on a novel integrated FBG/EFPI sensor. This sensor consists of an in-fiber Bragg grating (FBG) and an optical fiber based extrinsic Fabry-Perot interferometer (EFPI): FBG to measure the strain and EFPI to measure the temperature. The accuracy of the sensor in measuring temperature and strain is estimated to be plusmn0.1degC in a range from 20degC to 70degC and plusmn2.5muepsiv from - 1000muepsiv to +1000muepsiv, respectively

Study on demodulation algorithm of fiber optic Fabry-Perot sensors based on spectrum adjusting

This paper aims at proposing a modified algorithm based on spectrum adjusting. The theoretical analysis of the error between the real light source spectrum and the Gaussian spectrum is presented. The hardware of the demodulation system is introduced, including the coupler, collimation lens, volume phase grating, focus lens, CCD array, A/D card, and PC. The modified algorithm will adjust the interferometric spectrum of the sensor by getting rid of the effect of the light source spectrum. Experiment was carried out to test the performance of the demodulation system. It can be found from the result that the algorithm has improved the accuracy of the demodulation system significantly. The demodulation accuracy for the strain sensor is better than 0.5 microstrain.

FBG sensor network in Qinghai-Tibet Railway

In this paper, fiber Bragg grating (FBG) sensors network in the subgrade temperature monitoring system in Qinghai- Tibet railway is presented. The principle of the FBG temperature sensor is introduced and the feasibility of the FBG sensors in the permafrost regions of Qinghai-Tibet plateau is analyzed. The FBG sensor network architecture is introduced. This sensor network includes the sensor chains embedded in the roadbed, slave optical cables, work stations including the FBG sensor interrogator and the optical router, master optical cables, and center workstation. We give the primary experimental result of this sensor network in monitoring the roadbed temperature of the railway at DK0985+000 near Chumar River. Fifteen FBG temperature sensors were embedded into the roadbed monitoring hole. The temperature data was collected after a month when the temperature field in the monitoring hole is stable. The result of the experiment shows that the FBG temperature sensors have an accuracy of 0.1 centigrade. Furthermore, the experimental result shows the stability and reliability of the FBG sensor network in the harsh environment of Qinghai-Tibet plateau. In the end, the conclusion is drawn and the prospect of the FBG sensor network in monitoring other parameters in the Qinghai-Tibet plateau is also presented.

A novel fiber optic concrete sensor based on EFPI

In this paper, a novel fiber optic concrete sensor based on extrinsic fiber Fabry-Perot interferometer (EFPI) is designed and analyzed. Two fiber ends are inserted into a glass capillary and encapsulated into a cement cylinder to act as the sensor head. In this way, the cement cylinder itself is the sensor head instead of the traditional steel tube, which makes it very convenient to embed the sensor head into the concrete, because the cement consists with the concrete well. Based on the theory of white light interferometry and the theory of elasticity, the wavelength modulation method and the strain transfer are analyzed theoretically. The demodulation system is also introduced in this paper. The experiment being made by our research group is aimed at testing the consistency, stability, reliability and the sensitivity of the fiber optic sensor. The sensor head of the cement cylinder is embedded into a model ferroconcrete beam together with traditional strain gauges. The experiment is carried out using the PEM-500A hydraulic pulsation fatigue test machine after 2 million stress circles. The readout of the fibre optic sensor and the strain gauges is recorded and made a contrast. It can be found from the result that the fibre optic sensors have good stability and reliability, the accuracy for the fibre optic sensor is better than 0.5 micro-strain, which shows that the sensor can meet the demand of the long-term monitoring of large-size concrete structure.

Investigation on simultaneous measurement of strain and temperature based on hybrid FBG/EFPI sensor

In this paper a novel fiber Bragg grating and extrinsic fiber Fabry-Perot interferometer (FBG/EFPI) sensor which can measure the strain and temperature simultaneously in the smart materials is designed and analyzed. The FBG and EFPI are encapsulated into a special stainless steel structure. The structure makes the FBG strain-sensitive. On the other hand, the EFPI is insensitive to the strain but sensitive to the temperature. The strain sensitivity and the temperature sensitivity are derived in details. Experiments show that the sensor has good performance and high accuracy (0.1°C for temperature and 1.3pm/με for strain). In conclusion, the FBG/EFPI sensor is applicable for simultaneous measurement of strain and temperature in smart materials and structure health monitoring.

A novel fiber-mercury temperature sensor

Fiber optic sensors are widely used in structure monitoring and smart materials to measure the temperature. In this paper, a novel fiber-mercury temperature sensor (FMTS) is designed and tested. The sensor is made using the surface of the mercury to act as the reflector. The intensity of the reflective light is the function of the distance between the surface of the mercury and the fiber end, which is determined by the thermal extension of the mercury. And different length of the mercury in the capillary defines different sensitivity of the sensor. The modulation function of the sensor is analyzed. Experiment was carried out to evaluate the performance of the sensor, and the results present a good correlation between the theoretical and experimental result.