Design And Experimental Study Of Glass Refractive Index Measurement Device Based On FPGA

How to accurately measure the refractive index of glass materials has always been an important issue in the field of optical materials.The refractive index of glass materials plays an important role in the system used by the materials.In the past,some refractive index measurement methods not only have complicated steps,but also have special requirements for the shape and size of materials.All these limitations will lead to measurement errors and waste of materials.In this paper,we will calculate the refractive index of the material according to the distance of laser propagation in glass materials based on the technology of FMCW laser measurement.However,a tunable laser with a highly linear modulation frequency is needed in theory for the measurement of FMCW technology.The tunable laser must have inherent frequency modulation nonlinearity in the fact,leads to the result of measurement has a large error.Therefore,in order to improve the measurement accuracy,a digital phase-locked loop(DPLL)is designed by using phase-locked loop(PLL)technology to solve the problem of frequency modulation nonlinearity of tunable laser based on the field programmable gate array(FPGA).This works will solves the problem of tunable laser frequency modulation nonlinearity.In this topic,the method of measuring glass refractive index and the improvement of the precision of glass refractive index measurement system are studied.The goal is to design a set of device which can measure the refractive index of glass automatically,and test the measuring precision of the device through experiment.The refractive index measurement device based on FPGA is composed of two parts,one is the refractive index measurement system using FMCW technology.The other is the laser frequency modulation nonlinear correction system using PLL technology.The principle of FMCW technology is introduced firstly,and explain the correction method of DFB laser frequency modulation nonlinearity.Then,the laser frequency modulation nonlinearity correction system based on DPLL is designed.The FPGA circuit design and the function simulation of DPLL are completed.The glass refractive index measurement device based on FPGA is designed and the system test is completed.At last,the effect of measuring multi-group glass samples with glass refractive index measuring device and the factors affecting the accuracy of the device are studied experimentally.The test results show that the designed nonlinear correction system is stable and can correct the output nonlinearity of DFB laser,the output nonlinearity of the laser is corrected from 5.46% to 0.0066% about the uncorrected up sweep and 2.32% to 0.0102% about the down sweep respectively,the relative uncertainty of BK7 glass is 0.0028,SK2 glass is 0.0023 and PMMA glass is 0.0017.