Objective  Currently distributed fiber optic temperature sensor has become an important tool for oil and gas storage tank seal ring fires, high temperature furnace crack detection, and thermal pipeline leakage detection. Most detection methods are judged by the temperature threshold and temperature rise rate. And when the leak area is too small and the system spatial resolution is insufficient, the temperature and temperature rise rate will be much lower than the actual value. As one of the main factors of the spatial resolution of the distributed optical fiber temperature measurement system, the pulse width of the laser determines whether the sensor can detect the occurrence of accidents in time. Therefore, it is necessary to use a method to reduce the impact of laser pulse width on the spatial resolution of the system.

  Methods  A Raman signal segmentation and reconstruction method is proposed. By analyzing the signal characteristics of the laser pulse in the optical fiber at the region to be measured, the relationship between the laser pulse width and the temperature deviation is obtained. According to this relationship, the original Raman signal at different temperature regions is segmented into known temperature segments and temperature segments to be measured, and the Raman signal intensity at the region to be measured is reconstructed from the signal of the temperature segment to be measured with the help of a fitted signal to eliminate the Raman signal offset at the known temperature segment.

  Results and Discussions   The method was tested using a laser source with 20 ns pulse width (the theoretical spatial resolution is about 2 m) and 0.72 m test fiber. The results show that within the test temperature range of 40-90 ℃, the temperature error is reduced from a maximum of 33.9 ℃ to below 5.8 ℃, and the spatial resolution of the system was improved from 2.27 m to 1.13 m.

  Conclusions  By analyzing the signal characteristics of the laser pulses passing through the high-temperature region, a Raman signal segmentation reconstruction method is proposed. By dividing the high temperature area covered by the laser pulse into the known temperature region and the temperature region to be measured, the signal offset of the known temperature segment is removed and the signal of the high temperature area is reconstructed according to the signal intensity of the temperature segment to be measured. A test is carried out using 20 ns laser pulses and 0.72 m test fiber. The results show that the temperature error is reduced from 33.9 ℃ to 5.8 ℃ at 90 ℃, and the spatial resolution of the system is improved from 2.27 m to 1.13 m. This method mainly solves the temperature measurement error when the length of the optical fiber in the area to be measured is less than the laser pulse width, and has outstanding advantages for small-scale leakage monitoring. The remaining temperature error mainly comes from insufficient APD bandwidth. This method can gradually shorten the measurable temperature region length and temperature accuracy with the increase of APD bandwidth.