Research On Laser Self-mixing Grating Interferometric Three-dimensional Dynamic Displacement Measurement Technology

As an indispensable part of modern technology and industry,precision measurement technology plays an important role in promoting the development of science and society.Laser interferometer is often used in the field of precision measurement due to its advantages of non-contact measurement,high accuracy and resolution.When the light is incident on the surface of the external target during measurement,it will be reflected or scattered.Part of the light will be fed back into the laser cavity,mixed with the light in the cavity and interfered,thereby affecting the output frequency and output power of the laser.Since this phenomenon is similar to the traditional two-beam interference,it is named the laser self-mixing interference effect.With the research on self-mixing interference becoming more and more in-depth,it is found that self-mixing interferometers have the advantages of high measurement sensitivity,simple and compact structure,easy collimation,and convenient operation when compared with traditional laser interferometers.As a result,many applications based on self-mixing interference have been created,which can be used to measure displacement,velocity,absolute distance,vibration,refractive index and many other physical quantities.However,self-mixing interferometer cannot avoid the inherent shortcomings of the laser interferometer.The laser wavelength is very sensitive to environmental disturbance such as air flow,variation of temperature and refractive index,which will affect the measurement accuracy of the system.Regardless of whether the measurement environment is strictly controlled or the corresponding compensation for environmental noise is made,the cost of the system will be greatly increased.As a result,we turn to another popular technology in the field of precision measurement,which is grating interferometry.By combining interferometry with laser self-mixing interference,we proposed an innovative laser self-mixing grating interferometer.The basic principles of laser self-mixing interferometer and laser self-mixing grating interferometer is analyzed in the paper.On this basis,a laser self-mixing grating interferometer with Littrow structure is designed for the measurement of three-dimensional dynamic displacement.Introducing a grating into self-mixing interferometer can bring many benefits.Firstly,the measurement standard is changed from the laser wavelength to the relatively stable grating pitch,which effectively improves the measurement stability of the system;Secondly,the Littrow structure of the grating makes it possible to achieve the multiple degrees of freedom measurement of the target,and the optical path is simpler,more compact,and easier to operate.In general,only one surface of the target needs to be irradiated with a laser to realize the measurement of two-dimensional or even three-dimensional displacement with the use of grating.In order to improve the measurement accuracy,an electro-optic modulator is added to the optical path to apply sinusoidal phase modulation to the self-mixing interference signal,and FFT is used to demodulate the phase.In order to increase the reliability of the experimental measurement,the precision multi-axis PZT produced by PI is used to control the movement of the grating,and the measurement result of the system is compared with the displacement given by the internal sensor of PZT to verify that the system has the capability of measuring three-dimensional dynamic displacement.The measurement resolution can reach the nanometer level.Based on laser interferometers or grating interferometers,many methods have managed to measure one-dimensional and two-dimensional displacement.However,in regard to three-dimensional displacement measurement,there are not many studies in this area having been reported.The laser self-mixing grating interferometer proposed in this paper provides a brand-new scheme for the measurement of precise three-dimensional displacement,which is of positive significance to a certain extent.