Long-distance High-resolution Distributed Fiber Optic Temperature Sensing Technology Based On Coherent Optical Time Domain Reflectometer With Optical Frequency-shift Scanning

Distributed optical fiber sensing is a unique sensing technology which has a wide prospect in engineering applications.It measures the common physical information such as temperature,strain and vibration.Generally,distributed optical fiber sensors are based on nonlinear optical scattering(such as Raman,brillouin scattering phenomena).Limited by the scattering intensity and temperature sensitivity,it is hard to achieve a long range(over 100km),high resolution(less than 0.1 ?)distributed temperature sensing.Coherent optical time-domain reflectometer(COTDR)with frequency-shift controlled light source can achieve distributed temperature sensing with very high measurement resolution over long range.The sensing temperature can be obtained by locating the peak of cross-correlation function of the Rayleigh scattering light interference pattern,which changes with the environmental temperature.As for COTDR with optical frequency-shift scanning,the rapid reduction in intensity due to high fiber attenuation,lowering the signal to noise ratio at the end of fiber.Effective temperature measurement is hard to realize.To solve this problem,on the one hand,a repeater for distributed amplification sensing is designed and analyzed.On the other hand,a couple of optical pulse modulator cascade is designed to improve the extinction ratio and a noise-reduction signal processing method is adopted to improve the signal to noise ratio.Using these technique,we realized temperature measurements in 110-km-long fiber with the temperature change 0.0075 ?.In the implementation of COTDR with optical frequency-shift scanning,there is a limit of the range of optical frequency sweep,the temperature detection range and minimum detectable temperature restrict each other.Based on Rayleigh backscatter shift correlation,an optical frequency extension method is adopted to improve the temperature detection range.Both theoretical and experiment results show that the proposed method effectively increases the temperature detection range.Based on the statistical properties of rayleigh backscattering,a COTDR with optical frequency-shift scanning cross-correlation fitting model is derived to optimized the detectable temperature.