Research On Real-time Precision Measurement Technology Of Phase-shift Laser Self-mixing Interference For Large-range Displacement

In the field of modern industrial technology such as microelectronics,superfinishing,the demand of precision displacement measurement is becoming increasingly rigorous.Some applications require the technology with the resolution of nanometer level in the long range displacement measurement.In this paper,it focuses on the phase-shift laser self-mixing interference principle applied in the long range displacement precision measurement in real-time,including its measurement theory and key technology.Laser self-mixing interference,refers to a portion of light emitting from a laser source is reflected or scattered by an external object and coupled into the laser cavity.The feedback light which carries information of the external object then mixes with the light inside the cavity,and modulates the output signal's power and frequency.The signal characteristic of this system is similar to that of the conventional two-beam interference,and thus it is called laser self-mixing interference(SMI).And the SMI effect is the new rise technology in the displacement precision measurement field.In order to improve the measurement accuracy and realize the real-time measurement speed of the laser self-mixing interferometer in the large range displacement,advanced sinusoidal phase-shifting technique and the time-domain phase demodulation method are adopted.The electro-optic crystal modulator is used to realize the sinusoidal phase-shifting on the laser beam in the external cavity and the interference signal will be demodulated by the time-domain phase demodulation method.By combining the two together,it can meet the speed requirement in a wide displacement measurement range and implement the interferometer's real-time measurement requirement at the same time.Experimentally,PI's high-precision commercial electric displacement platform's calibration results verified the sinusoidal phase modulation laser self-mixing interferometer's displacement measurement can reach less than 0.5 ?m error in the hundred mm large-scale displacement measurement process.In addition,the factors affecting the interferometer's measurement speed in the real-time displacement measurement process is analyzed and the maximum speed of our system is obtained.In addition,the calibration experiment with the commercial displacement sensor illustrates that the interferometer prototype has reached a practical level.In addition,this article studies another new measurement technique for the laser self-mixing interferometer.Through the metal-dielectric film attenuating the laser intensity and introducing ?/2 phase-shift on the reflected self-mixing interference signal in the electronic domain,it is easy to make the laser self-mixing interferometer system work under weak feedback conditions and to produce two orthogonal interference signals which are used to measure the displacement.The simulation results indicates high measurement accuracy.In order to improve the interferometer's measurement accuracy and resolution in the hundred mm level displacement,the collected self-mixing interference signal is dealt with electronic subdivision in the digital domain,improving the noise performance and measurement reliability.PI's high precision commercial electric displacement platform is used for the calibration experiments and 0.4um error in 100mm displacement measurement process is achieved.As the self-mixing interference signal is detected directly,the system measurement speed can be high.But subject to sampling rate of the data collection and processing system,the speed can reach close to 100mm/s in the hundred mm displacement measurement range theoretically.