Research On Low Frequency Small Displacement Measurement Based On Higher Order Mode

The development of science cannot be separated from measurement,and the improvement of measurement accuracy will promote new physical discoveries,thus promoting the development of physics^[1].Precision measurement technology has been applied more and more in many military,industrial production and life.For example the atomic force microscopy^[2][3],biometrics^[4],and even laser eavesdropping^[5].During the Gulf War,the United States once fired lasers into the windshield of enemy commanders'cars.The sound caused the glass to vibrate so that the laser beam carried relevant information back to the receiving device.Finally,the reflected light beam could be detected and demodulated to recover the sound signal.The information carried by the change of the transverse position of the beam caused by the sound signal is obtained.According to the Heisenberg uncertainty principle in quantum mechanics,for any pair of conjugate observable measurements,if the measurement accuracy of one of the physical quantities is improved,the measurement accuracy of the conjugate quantity will be reduced.This concept can provide a wide range of application prospects for quantum precision measurement.In the previous experiments,in order to avoid the influence of laser noise and various low-frequency noises on the experiment,the experiments were mainly carried out in the MHz segment^[53].However,with the development of technology,the precision measurement in the acoustic frequency band(20 Hz-20 kHz)also gradually highlights the important role.This paper mainly studies the related program design and experimental measurement of low frequency lateral small displacement measurement based on LabVIEW.The main contents are as follows:(1)By using LabVIEW to design the program of signal acquisition and analysis.? is designed based on NI PXIE-4464,and DAQ acquisition assistant is used to improve the program block diagram.Analyze the experimental requirements,carry on the overall structure of the program,and then to the program writing and the final software testing,complete the spectrum analysis and other functions.(2)The influence of noise in low frequency band on experimental measurement results is analyzed.The effects of particle noise,directional noise,laser intensity noise and electronic noise on the minimum measurable displacement in low frequency measurement are analyzed,and the noise in the laboratory is measured Through analysis and comparison,it is concluded that the directivity noise has the greatest influence on the measurement accuracy.(3)Based on the high order Hermite-Gaussian model,a balanced homodyne detection device is used to measure the transverse small displacement of the beam.The theoretical minimum displacement measurable limits of balanced homodyne detection,beam-split detection and multi-quadrant detection are studied.Through comparison,it is found that balanced zero-beat detection has higher minimum measurable displacement than beam-split detection and multi-quadrant detection.In the experiment,the balanced homodyne detection method is adopted,which is used as the signal light and the local light respectively,and the low-frequency balanced homodyne detector is used for detection.The final experimental results were calculated and the minimum measurable displacement at 15kHz was 6.81×10^-12 m.