Detecting And Processing Technology Of Differential Astigmatic Focus-error Signal Based On FPGA

The defocus detection technology is one of the key technique in the laser direct writingsystem and play a crucial role overall situation. In this paper, the differential astigmaticmethod is selected in the focus-error signal’s collection and processing system. The wholethesis can be divided into three parts: basic principle introduction, hardware design andsoftware design.In the basic principle introduction, it set forth the fundamental of the differentialastigmatic method, which the total optical design plan establishes on. Meanwhile, it picks upthe suitable optical elements and makes the parameter simulation and comparison to completethe optical system.The hardware part keeps the FPGA as the core, combines the signal collection andpre-process module, analog-to-digital conversion module and the data transmission modulethree parts. The signal collection and pre-process module includes the conversions of L-E(light to electronic) and I-V (current to voltage) and lowpass filter circuit; Analog-to-digitalconversion module and the data transmission module are both controlled by the FPGA toachieve the purpose. The AD7865with four channels is used in the A-D module, data in it canbe sampled simultaneously and converted successively, and this collection not only saves thespace but also improves the accuracy of the data conversion. The data transmission moduleuses the USB method to speed up the transmission.In software processing, this paper designs the controlling program of the A-D conversionand the data transmission module and their combination. Besides, it also makes theprogramming and modification in the firmware code of the USB. In the computer, it mainlycontains the data posttreatment included of data capture, digital filter and related operation.Based on the content above, the paper builds the optical experiment platform to text thetheory results. The experiment calibrates the sensitivity, checkout the linearity and stability ofthe system. The experiment’s effect is good, meets the design expectations basically.