Compensation Of Frequency Sweep Nonlinearity In Optical Frequency Domain Reflectometry

Optical frequency domain reflectometry(OFDR)has been widely used in structural health detection due to its advantages of high spatial resolution,high sensitivity and high sensing accuracy.Tunable laser(TLS)is a key component of OFDR,and its sweep linearity and sweep range affect the spatial resolution of the system.Generally,TLS has some frequency sweep nonlinearity,which leads to the decrease of the spatial resolution.Solving this problem is the key to improve the performance of OFDR system.So the frequency sweep nonlinearity of TLS and its compensation methods were studied to improve the spatial resolution of OFDR.The specific work of this paper are as follows:Firstly,the non-uniform Fourier transform(NUFFT)and zero-crossing resampling method were used for nonlinearity compensation.In the NUFFT method,the nonlinearity compensation is resampling based on the frequency tuning curve of TLS,so the real-time nonlinearity compensation cannot be realized in the process of data acquisition.In the experiment,a spatial resolution of 0.44 mm over a measurable range of 155 m with 60 m delay fiber was realized.In the zero-crossing resampling method,the length of the fiber under test(FUT)is at most half of the length of the delay fiber in Mach-Zehnder auxiliary interferometer.In the experiment,200 m delay fiber was used to compensate the nonlinearity in 57 m FUT,and the spatial resolution of 0.156 mm was achieved.In order to solve the above problems,we proposed a method based on time-scale factor for compensating the frequency sweep nonlinearity in OFDR.The method calibrates the positions of all the data points by the time-scale factor determined by the zero-crossing points of the auxiliary interferometer.Then the signal of the main interferometer is interpolated with the calibrated results.This method can solve the problem that measurable range is limited by the length of delay fiber in the auxiliary interferometer.And it provides the possibility of real-time nonlinearity compensation in the process of data acquisition.In the experiment,200 m delay fiber was used to compensate the nonlinearity in 155 m FUT,and the spatial resolution of 0.13 mm was realized.Finally,a nonlinearity compensation method based on acoustic optical modulator(AOM)was proposed.We introduced AOM into the auxiliary interferometer to shift the frequency,and achieved the large beat signal with short delay fiber.The measured length can be greater than that of the delay fiber of the auxiliary interferometer with high resolution.In the experiment,the 105 m delay fiber and 40 MHz AOM was set in the auxiliary interferometer.A spatial resolution of 0.3mm over a measurable range of 155 m was obtained.When using 200 m delay fiber and 40 MHz AOM to compensate the nonlinearity in 155 m FUT,the spatial resolution of 0.13 mm was achieved,and the amplitude of stray peaks at the end was small.In this paper,we studied the compensation of frequency sweep nonlinearity in OFDR.Because our methods can significantly improve the spatial resolution and the performance of OFDR,they have a broad application prospect in the field of distributed optical fiber sensing.