Research On Distributed Fiber Sensing Technology Based On Multi-frequency ?-OTDR

?-OTDR,as a vital technique in fiber sensing,is outstanding for its sensitivity,simple and robust system scheme and availabilty of distributed sensing.The main research focus of the ?-OTDR system lies in the phase demodulation technology.At this stage,phase demodulation of ?-OTDR is mainly achieved through frequency sweeping,chirped pulse,and coherent detection combined with algorithms.These method can suffer from limited response range,poor spatial resolution or complexity of system.It is of great importance to study a simple and reliable dynamic distributed measurement ?-OTDR system.The multi-frequency ?-OTDR is proposed in this paper utilizeing the frequency difference between detection pulses to compensate the phase change of the scattered signal caused by the temperature change.Measurement of temperature change rate is realized by selecting the appropriate frequency pair.This method has the advantages of large dynamic measurement range,immunity for fading noise and low requirement of device bandwidth.The main research work is as follows:(1)A simulation model of a multi-frequency ?-OTDR scheme was established to perform simulation research.Firstly,the relationship between the temperature change rate and the delay of the dual-frequency pulse-scattering waveform is simulated and analyzed.Then the relationship between the frame error(reflecting the time delay)and the measurement error is studied,and the frequency selection criterion of the multi-frequency ?-OTDR technology is determined: The measurement results are more accurate when the difference is within the range of 30-50.(2)A temperature-sensing experimental study of the dual-frequency ?-OTDR was carried out and real-time monitoring of dynamic temperature signals with a temperature range of 80 °C-55.4 °C was achieved with 200 MHz acousto-optic modulation.The program can correctly measure the sign of temperature changes.The result is consistent with the measured temperature change of the thermocouple.(3)Based on the dual-frequency experiment,a multi-frequency ?-OTDR experiment was carried out.Several pairs of pulses with frequency differences as 3 MHz,4 MHz,8 MHz,and 16 MHz are used to monitor temperature changes.The results of the research show that when the pulse pair with the frame difference of about 40 is selected,the error of the calculation and measurement is the minimum,which verifies the correctness of the frequency selection basis derived from the simulation study.