Research On Laser Self-mixing Grating Interference Displacement Measurement Technology

Laser self-mixing interference occurs when a portion of light emitted from a laser source is reflected or scattered back into the laser cavity by a remote target,the reflected light mixes with the light inside the cavity,causing a modulation of both the amplitude and the frequency of the lasing field.Laser self-mixing interference has the same phase sensitivity as traditional two-beam interference.Similar to the traditional two-beam interference,the measurement standard of laser self-mixing interference is the laser wavelength.However,the laser wavelength is influenced by the variations in temperature,humidity,air pressure and air flow in the environment.In this paper,a novel laser self-mixing grating interference technique is proposed to measure micro-displacement by combining the grating technology and the laser-self-mixing interference technology.The proposed technique adopts the pitch of the grating as the measurement standard which provides better immunity against environmental disturbances and the laser feedback.First,a self-mixing grating interferometry with current modulation is proposed to measure one-dimensional displacement.To improve measurement accuracy,through sinusoidally modulating driving current of a semiconductor laser,the output wavelength is also changed sinusoidally.The phase of the interference signal is also modulated,then Fourier analysis method is introduced to demodulate the phase.The system adopts Littrow conguration design which has the advantages of simple and compact conguration and easy operation.The principle of the displacement measurement and the modulation technique are explained in detail.The simulation results are given.The experimental results show that the displacement noise level of the measurement system is within 50 nm.Then a self-mxing grating interferometry which can measure two-dimensional in plane-displacement is proposed in this paper.The system also adopts Littrow configuration.The displacement measurement accuracy is improved by introducing an electro-optic crystal to modulate the interference signal.The measurement principle and the modulation technique is explained in detail and the simulation results are given.The experimental results show that the displacement noise level of the measurement system for the 2-D in plane displacement measurement are better than 20 nm.