Theoretical Analysis And Algorithm Study On Optical Fiber Refractive Index Profile Measurement By Transmitted-light DIC Microscope

Optical fibers is applied frequently in communication, sensors and gyroscopes. The refractive index profile (RIP) determines the loss ratio and transmission function of optical fibers, thus the measurement of RIP is an important task. In this paper, theoretical analysis and experimental study about the measurement of optical fibers is done. A novel transmitted-light Differential Interference Contrast (DIC) system is applied to nondestructively measure the refractive index profile of an optical fiber. Successful experiments of different optical fibers by this system is conducted and the results prove that we have provided a brand new technique in the field of optical fiber RIP measurents. In this paper, the principle and theory of DIC system in quantity measurement is introduced. The effect of changing the parameters of Nomarski prism is introduced. A simple and appropriate procedure of system adjustment is also established according to the theory. The new method of retrieving the measurement result in one measurement step is also investigated. In order to apply the DIC system in the field of RIP measurement, a new mathematical model of the optical fibers is presented. By means of dividing fibers into layers, a new algorithm to calculate the RIP of fibers from the results of DIC system is generated. The error, precision and scope of application of this algorithm is also discussed by mathematical analysis and computer simulation. The result shows that the number of layers should be larger than 25 and the uncertainty by this algorithm will better than 10-3.Utilizing this system, the RIP of different types of optical fibers is measured. System calibration is performed by comparing the results of this system to those by a commercial instrument utilizing refractive near field method. Experiments for determining system uncertainty, error and stability are also conducted. The experimental results prove that our multi-layer model and RIP retrieving algorithm are correct and stable.Theoretical analysis and experimental results show that the system can be applied in RIP measurement of optical fibers. The uncertainty is better than 10-3 and the system has the advantage of anti-disturbing ability, easy for real-time monitoring ,non-destructive measurement and very good prospect in the field of RIP measurement.