Fiber Transfer Delay Measurement System Based On Phase Shift Method

Fiber delay line（FDL）has attracted great attention for its large bandwidth,low attenuation,anti-interference and good confidentiality.Up to now,FDL has been widely used in phased array radar,optical fiber communication systems,and electronic countermeasures and other fields.The continuous development of technology has made an increasingly accuracy requirements of FDL.And the development of high-precision FDL is inseparable from the high-precision measurement of fiber transfer delay.But there are few measuring instruments on the market can meet the measurement requirements of fiber transfer delay now.Therefore,it is of great significance to design a high-precision fiber transfer delay measurement system.In this paper,a fiber transfer delay measurement system based on phase detection is designed and implemented.The main principle is to convert the fiber transfer delay in time domain into the phase detection,and eliminating the ambiguity by frequency scanning,followed with data process to get the result.A fiber delay measurement scheme based on phase peak detection is proposed.The relative phase difference between two measurement frequencies is accumulated by phase peak detection.It is no longer limited to the measurement frequency in the same transmission period,thereby increasing the interval between the two measurement frequencies,effectively reducing the measurement error of the phase shift method,thereby improving the delay measurement accuracy.In this paper,fiber of different lengths is selected for fiber transfer delay measurement experiments.Under the same experimental conditions,the measurement system and the Agilent N5244A vector network analyzer are used to measure the delay of the fiber under test,and the results obtained by the two methods are compared and analyzed.Experiments show that the delay measurement accuracy of the system is up to picosecond,the dynamic range is up to 40 km,together with a good stability,time deviation of the system inherent delay is below 0.9 ps.Experiments have also shown that due to the inherent delay of the system,the system has advantages in long fiber measurement.For a 40km long fiber,the optical fiber delay measurement accuracy is up to 8 ps with a time deviation below 4 ps,attaining a relative measurement accuracy of 4.04×10^-8,and the corresponding fiber length measurement accuracy is 1.615 cm.