Research On Frequency Sweeping Absolute Distance Measurement Method Based On Dynamic Sideband

Distance measurement has always been an indispensable link in frontier technology exploration,production and life.With the development of high-end manufacturing industry and aerospace industry of our country,increasing demands are placed on the reliability and applicability of distance measurement.The method of laser interference to achieve distance measurement has the advantages of high measurement accuracy and large measurement range.The absolute distance measurement method based on frequency sweeping interference has a simple light source system and is not limited by fuzzy distance,so it is widely used in cutting-edge fields such as large-scale parts processing,space satellite formation flight,etc.,but the laser frequency sweeping range needs to be calibrated with a laser frequency monitoring device.In order to solve the problem that the laser frequency sweeping range needs to be calibrated with additional devices in the laser frequency sweeping absolute distance measurement,this thesis proposes a frequency sweeping absolute distance measurement method based on dynamic sidebands,which uses an electro-optical phase modulator to generate optical frequency sidebands.The frequency sweeping is performed by sweeping the modulation frequency of the electro-optical phase modulator to control the optical frequency sidebands,and the absolute distance measurement is realized by the heterodyne optical path.Since the sidebands sweeping range can be precisely controlled by the RF signal source,no additional laser frequency measurement device is required.In order to realize the phase demodulation of multiple frequency components in the multi-heterodyne interference signal,a phase-demodulation algorithm of the multi-heterodyne interference signal based on lock-in amplifier is designed.The algorithm extracts the phase information of each frequency component by quadrature down-mixing and low-pass filtering on the basis of under-sampling.In order to solve the problem of nonlinear error in laser heterodyne interference optical path due to frequency mixing,a nonlinear error compensation algorithm based on differential operation is proposed in this thesis.The differential operation is used to eliminate the DC offset contained in the signal,so as to realize nonlinear error compensation.The algorithm design was implemented on FPGA,in order to test the performance of the proposed nonlinear error compensation algorithm,a classical heterodyne interference optical path was built to measure the displacement of the object to be measured with sinusoidal motion.The signal to noise and distortion of the displacement measurement result reached 54.65.d B,the amplitudes of all harmonics are in the sub-nanometer order.In order to ensure the reliability of the proposed frequency sweeping absolute distance measurement method based on dynamic sidebands,test the key components of the system.By testing the change of the output power of the signal source during the frequency sweeping process,the upper computer is used to adjust the output power of the signal source in real time,and the power fluctuation of the signal source during frequency sweeping in the range of 6～11 GHz is controlled less than 2 d Bm.Two tunable semiconductor lasers were used to test the sideband orders generated by the electro-optic phase modulator under different modulation powers.When the modulation frequency was 6 GHz,the effective sideband orders reached 11 and 23 orders under the modulation power of 18 d Bm and 23 d Bm,respectively.In order to test the performance of the designed absolute distance measurement system,the absolute distance repeatability experiment and 0.04 m step experiment within the measurement range of 1 m were carried out respectively.The fluctuation range of the repeatability measurement results was L_ave±30μm,and the relative stability was 1.6×10^-5.The measurement results of the stepping experiment are compared with the indication value of the guide rail,and the deviation range is±150μm.Finally,the uncertainty of the absolute distance measurement system is analyzed,and the final synthetic uncertainty is obtained by calculating the uncertainty of each factor affecting the distance measurement result.