Design Of Signal Processing Board For Dual-frequency Laser Interference System

Dual-frequency laser interferometry is widely used in the displacement measurement of lithography moving table.With the improvement of lithography process,the demand for the performance of laser interferometry is also increasing.The dual-frequency laser interferometry technology modulates the motion information of the measured object into the optical signal,and the accurate non-contact displacement measurement is realized by detecting the phase change of the optical signal based on the wavelength of the light.The accuracy of signal phase detection determines the accuracy of displacement measurement.In this paper,the signal processing requirements of dual-frequency laser interferometry for 20 MHz frequency difference are investigated in detail,and the specific design is carried out: the photoelectric signal conversion circuit with high bandwidth,high gain and low noise and the negative high voltage APD drive circuit with controllable voltage are designed to realize the acquisition of weak high speed optical signal,and the fifth order Chebyshev bandpass filter is designed to accurately limit the bandwidth of 20 MHz ±16MHz signal.The high dynamic range automatic gain compensation circuit of 60 d B is designed to realize the automatic dynamic gain compensation of the signal in the signal band;the ADC circuit with high sampling rate and the FPGA driver and communication circuit are designed to realize the real-time signal processing of dual-frequency laser interference signal;and combined with the design parameters,the noise and error produced in the signal processing process are analyzed.In the design process,combined with the simulation analysis of board-level signal integrity and power integrity based on the actual PCB model and device parameters,reasonably plan the current flow direction of the signal loop and power supply,optimize the layout and wiring of the signal processing board,reduce the signal interference on the board as much as possible,and realize the development of the signal processing board with low noise and high stability.Based on the above work results,the performance of the signal processing board is tested in the actual measurement environment combined with the high-precision motion table equipped with four-subdivision dual-frequency laser interferometer.The test results show that after the weak high-speed optical signal is collected and compensated by the signal processing board designed in this paper,the SNR of the reference signal can reach 28.88 d B and the SNR of the measurement signal can reach 23.86 d B.For the signal processing test of the actual dual-frequency laser interferometry signal,in a data cycle(409.6μs),the maximum phase increment deviation of the dual-frequency laser interferometry signal measured by the moving table is 1.5798 mrad,the equivalent displacement is 39.79 pm,the phase increment standard deviation is 0.2615 mrad,and the equivalent displacement standard deviation is6.586 pm.The maximum phase increment deviation of the signal measured by dualfrequency laser interferometry during the motion of the moving table is 3.2475 mrad,which is about 1/1935 of the single-period phase of the signal,which achieves the 10-bit(1024times)subdivision of the single-period phase of the signal measured by the dual-frequency laser interferometer.