Nonlinear Correction Of Photoelectric Phase-locked Semiconductor Laser Based On FPGA

Laser Continuous Wave Scanning(FMCW)absolute distance measurement technology has the advantages of derailment,measurement without touching target,independence from cooperative target,and no manual operation.It is an important means to obtain the topographic information of large three-dimensional objects.In this paper,a digital-analog hybrid photoelectric phase-locked loop(DPPLL)system based on FPGA and a pre-correction experiment based on secant iteration algorithm are designed for the tuning nonlinearity of current-tuned semiconductor lasers.Firstly,the principle of FMCW absolute distance measurement system using current tuned semiconductor lasers as measuring light source is introduced.The influence of nonlinearity in sweeping frequency of current tuned semiconductor lasers on ranging resolution and accuracy is analyzed.Then,the non-linearity correction method of frequency modulation of semiconductor lasers is analyzed and introduced emphatically.A closed-loop feedback correction method based on secant iteration algorithm and digital-analog hybrid photoelectric phase-locked loop(DPPLL)is established,which is supplemented by open-loop pre-correction and based on FPGA.By comparing with classical electrical phase-locked loop system,the mathematical model and transfer function of digital-analog hybrid photoelectric phase-locked loop based on FPGA are analyzed in detail.Then,the measurement methods of important parameters of digital-analog hybrid photoelectric phase-locked loop based on FPGA and the specific implementation methods of each module are analyzed.Secondly,the correction effect of the frequency modulation non-linear correction system of current tuned semiconductor lasers is tested and evaluated.At the same time,an experimental system based on iterative algorithm pre-correction is built and the correction effect of open-loop pre-correction is verified.On the basis of open-loop pre-correction,the circuit and optical system with digital-analog hybrid photoelectric phase-locked loop based on FPGA are designed,manufactured and debugged.Through closed-loop correction,a stable semiconductor laser lock-in is achieved,that is,the frequency modulation non-linearity correction of the conductor laser is realized.Finally,the closed-loop locking effect is tested and evaluated.Three important parameters,including the spectrum width of beat signal before and after the closed-loop,the repeatability of time-frequency curve and the nonlinearity of laser sweep frequency,are obtained,which verify the closed-loop locking effect.