In geophysical, marine science, oil and gas well detection, aircraft structure health monitoring, and other application scenarios, the sensor is required to have small size, high resolution, and robust against harsh environments and electromagnetic interference. Especially in underground and deep-sea observation scenarios, the temperature sensor should be equipped with remote monitoring ability and temperature resolution of milli Celsius level. In these scenarios, electronic temperature sensors are difficult to meet the requirements due to their limitations, and fiber grating-based temperature sensors have the advantages of high resolution, large dynamic measurement range, and multiplexing sensing capability. This paper proposes a high-resolution multiplexed temperature sensing system based on optical fiber grating, which adopts the phase-shifted fiber Bragg gratings with different center wavelengths utilizing wavelength division multiplexing technology as the temperature sensing unit. The resonant wavelength of each optical fiber grating is detected by sweeping laser wavelength, and a hydrogen cyanide absorption chamber is introduced as the wavelength reference. An unbalanced Mach-Zehnder interferometer is employed to compensate for the nonlinearity in the wavelength sweeping of the laser to improve the wavelength measurement accuracy. In the experiment, the simultaneous detection of ten temperature sensing heads is achieved with a temperature resolution of 10^-4 ℃ and measurement range of 0-100 ℃. This fiber grating temperature sensing system has a broad application prospect in the fields which require high-resolution temperature measurement.

This study puts forward a high-precision multiplexed temperature sensing system based on fiber grating. The swept laser is divided into four paths after the coupler. The first path is the probe light for sensor heads, which enters the sensing grating array by wavelength division multiplexer. The second path is directly connected to the detector, which is utilized to compensate for the power fluctuation of the swept light source. The third path is connected to the unbalanced MZI interferometer, which compensates for the sweeping nonlinearity of the light source. The fourth path passes through an HCN gas absorption chamber, which provides an absolute frequency reference for the laser. The spectrum of the phase-shifted fiber Bragg grating is recorded, and then a cross-correlation operation is carried out to detect the spectrum movement of the grating. The offset of the correction peak indicates the frequency movement of the fiber grating caused by the temperature change, and then the current temperature of the environment is obtained based on the temperature sensitivity coefficient and the initial frequency of the fiber grating. The sensing fiber grating is packaged with metallization to improve the temperature sensitivity in this study. The fiber grating is enclosed in a capillary copper tube. The thermal expansion coefficient of the capillary copper tube is larger than the fiber grating, so the fiber grating is subjected to additional strain caused by the thermal expansion of the copper tube, which increases the shift of the resonant frequency. Finally, a handle structure is designed to reduce external vibration interference and protect the fiber grating.

The linearity between wavelength and temperature is verified, and the temperature sensitivity coefficient of the encapsulated grating is calculated to be 22.335 pm/℃ (Fig. 3), about twice the original sensitivity coefficient of unpackaged fiber grating. The sensor array is placed in an oil bath for temperature measurement, and the whole system can realize a temperature measurement range from 0 ℃ to 100 ℃. The sensing probe multiplexing scale reaches ten (Fig. 6). The system employs a comparison with the theoretical value to verify its temperature resolution. The sensing probe is placed in water to measure the natural cooling of water for 3000 s. The measured temperature basically follows the exponential decay form. The time window of 60 s is chosen to perform the first-order exponential fitting, and the standard deviation of the residuals is (Fig. 8), indicating that the temperature resolution of the system reaches10^-4 ℃.

This paper proposes a temperature sensing system of high-resolution wavelength division multiplexed fiber grating based on a swept laser, which contains four main optical paths. One path is to probe the sensing probe array, and the other three paths are employed for the power compensation of the swept laser, the compensation of the laser swept nonlinearity, and the calibration of the absolute frequency of the laser. The encapsulation of fiber grating is studied to improve the temperature sensitivity, and a metalized encapsulation structure for the fiber grating is designed to increase the temperature sensitivity of fiber grating. In the demonstrational experiment, a temperature measurement range of 0-100 ℃ is achieved through an oil bath, and the number of sensor multiplexing scales reaches ten. Based on the comparison between the theoretical and measured temperatures of water, a temperature resolution better than 10^-4 ℃ is verified.