Objective Spatial, oceanic and geological exploration technologies have increasingly become important directions in our country development. In order to meet the demand of accurate detections, it is necessary to obtain basic physical information (e.g. temperature and strain). However, the requirement for sensors in these complex environments is extremely stringent. Compared with traditional electromagnetic sensors, optical fiber sensors have the characteristics of small size, light weight, corrosion resistance, anti-electromagnetic interference, etc. For optical fiber mechanical and thermal sensing, there are two common methods. One is using the fiber-Bragg-grating-based sensing structure which obtains the mechanical and thermal parameters through the spectral information of the reflected light. The other is using a combination of different sensing structures such as the combination of long-period fiber gratings (LPFG) with photonic crystal fiber (PCF). The above two methods both have shortcomings such as small measurement range, large measurement error, complex structure, and high cost. In response to the above problems, a fiber Bragg grating for temperature and strain multi-parameter sensing system based on a tunable laser is theoretically proposed and environmentally tested.
Methods In this study, the system consists of a tunable laser, a Fabry-Perot etalon, a driving circuit, a beam splitter, fiber Bragg grating temperature and strain sensors, photodetectors and a data acquisition card. After passing through the coupler, the light output from the tunable laser is divided into two paths, which respectively enter the multi-channel sensor and the etalon. The light entering the multi-channel sensor passes through the beam splitter and enters the 16 sensing channels. The light reflected by the sensor and the light passed through F-P etalon are transmitted to the photodetector and converted into electrical signals. The driving circuit generates a square wave signal synchronized with the triangular wave, which is used as the trigger signal of data acquisition. It controls the acquisition card and the field programmable gate array (FPGA) data processing module to process the electrical signal and demodulate the temperature and strain value according to the relationship between the wavelength and the mechanical and thermal parameters. In the aspect of sensor, we utilize a special packaging structure. The fiber optic temperature sensor is packaged with a ceramic tube. The grating coated with a high temperature resistant polyimide is used as the sensing element. The fiber grating is bonded with the outer layer of an alumina ceramic tube by using low melting point glass. This method avoids the problem of adhesive aging at low temperatures, so the temperature sensor can be applied to a wider temperature range, and the stability of the sensor is improved. The fiber optic strain sensor is composed of a metal substrate, a fiber Bragg grating and a sleeve. A desensitized substrate is used to protect the strain sensing grating. In addition, the 316L stainless steel is used as the base material of the strain sensor. The material has corrosion resistance. Besides, the thermal expansion coefficient of the 316L stainless steel is close to that of the measured structure in engineering applications.These characteristics can further improve the accuracy of strain measurements. In the manufacturing process, the metal substrate should be polished and wiped with ethanol to remove foreign matters on the surface. The fiber Bragg grating is welded with the metal substrate by using low melting point glass in the pre-stretched state. After the substrate is cooled, the epoxy adhesive with good temperature adaptability is used for further fixation. This method improves the temperature adaptability of the sensor while ensuring the accuracy of strain measurements.
Results and Discussions It can be seen that in the range of 0-200 ℃, the relationship between temperature and wavelength has a good linear relationship (Fig. 6). The sensitivity of the temperature sensor is about 11.60 pm/℃. The wavelength resolution of the optical fiber mechanical and thermal sensing instrument is 1 pm, so the demodulation sensitivity of the corresponding temperature sensing system is about 0.09 ℃. In the range of temperature below zero, temperature and wavelength quadratic polynomial fitting can improve the demodulation accuracy of the temperature sensing system. At each temperature node, the measurement error of the temperature sensing system is less than ±0.8 ℃ (Fig. 8). For the strain sensing, the FBG strain sensor can still work normally in the whole temperature range of -252.75--200.94 ℃. The sensitivity of the strain sensor is 1.66 pm/με, and the average measurement error is less than 2.9 με (Fig. 7). The experimental results show that the proposed sensor system has good accuracy and stability.
Conclusions This study describes a wide range and high-precision optical fiber temperature and strain sensing system based on a tunable laser. Besides, the fabrication method of an optical fiber temperature and strain sensor is improved to enhance the temperature adaptation range. Finally, the instrument development is carried out, and the performances in high and low temperature environments are tested and analyzed. The experimental results show that the system can realize accurate temperature and strain measurements in the temperature range of -252.75--200.94 ℃. The temperature measurement accuracy is less than ±0.80 ℃ and the strain measurement accuracy is less than ±2.90 με. In addition, the system can realize a multi-channel and multi-parameter measurement at the same time, which is suitable for an engineering application in special environments with high and low temperatures.
