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Study on length calibration method of coherent optical time domain reflectometer

In order to meet the length calibration requirements of coherent optical time domain reflectometer(COTDR) in state monitoring of long-distance communication, a new coherent optical time domain reflectometer length calibration method was proposed. The method utilized the principle that light circulated continuously in the fiber ring. And based on the principle, the COTDR length calibration device was developed. The device used the pulse frequency and pulse width emitted by the function generator to control the breaking time of the optical switch. Thereby it could realize the control of the propagation time of the light in the fiber loop. If a fiber amplifier was added to the fiber loop, simulation of fiber links of different lengths could be realized. Finally, the length calibration of the coherent optical time domain reflectometer was achieved. The experimental results showed that the length of the fiber used in the device is 150km, and the optical pulse could be rotated up to 7 times in the fiber loop. Therefore, the maximum range of the coherent optical time domain reflectometer length calibration was 1050km. Its extended uncertainty was 7.0m, and the containing factor was 2. This method could not only promote the development of coherent optical time domain reflectometry technology, but also provided a good idea for COTDR length calibration.