Study On High Speed Heterodyne Interferometer With Spatially Separated Beams

With the development of the ultra precision engineering and nanotec hnologyin recent years, there are more and more requirements on the measurementprecision, velocity and anti jamming capability of heterodyne interferometer.High measurement speed, high measurement precision and high measurementresolution are the development tendencies of the heterodyne interferometer. Themeasurement speed is required to reach several meters per second. Themeasurement resolution is required to reach subnanometer. The measurementuncertainty is required to reach10-8~10-9. This is a challenge and an opportunityfor the heterodyne interferometer.In this paper “Study on high speed ultra precision heterodyne interferometerwith spatially separated beams”, the influence factors of the heterodyneinterferometer are investigated. This paper aims to put forward methods forimproving the measurement speed, precision and resolution of the heterodyneinterferometer. The main works are as follows:In order to solve the problem of low frequency reproducibility of thetraditional laser frequency stabilized methods, the influence factors of thestabilized laser are discussed. The frequency property is analyzed. Therelationship between the laser frequency and the stabilized temperature is putforward. Then a laser frequency stabilized method based on mode order changefeedback is proposed. The mode order change is used to precisely monitor thethermal condition of the laser tube which is insensitive to the environment. Thestabilized He-Ne laser based on the relationship between mode change numberand the thermal condition can rapidly achieve a reproducible temperature underdifferent ambient temperatures, resulting in a high frequency reproducibility,high efficiency and strong anti jamming capability.In order to solve the problem that the frequency difference of the lasersource limits the measurement speed of the heterodyne interferometer, therelationship between the frequency difference and measurement speed is furtheranalyzed. Then a heterodyne interferometer with two spatially separated beams isproposed. The proposed heterodyne interferometer has two measurement signalswith opposite Doppler shift. The two signals are selected used according to thedirection of the target movement, ensuring the selected measurement signal witha positive Doppler shift. As a result, the measurement speed of the heterodyneinterferometer is no longer limited by the laser frequency difference.In order to solve the problem of the periodic nonlinearity in the heterodyne interferometer, the periodic nonlinearity and its compensation methods arefurther analyzed based on the principle research of the heterodyne interferometer.It is discovered that the traditional nonlinearity model is not perfect. Thedynamic nonlinearity in high speed measurements is analyzed to perfec t thetraditional nonlinearity model. The proposed heterodyne interferometer uses twospatially separated beams to avoid optical mixing which essentially eliminatesthe periodic nonlinearity.Based on the above research, a high speed ultra precision heterodyneinterferometer is designed and developed, including the stabilized laser withspatially separated beams, receiver, measurement electronics and so on.Experiments are made to validate the performance of all the parts and the wholesystem. Experimental show that the frequency stability of the designed lasersource is better than2×10-10, the frequency reproducibility reaches1×10-9. Thebandwidth of the receiver is15MHz. the resolution of the measurementselectronics is0.62nm. The measurement speed is no longer limited by the laserfrequency difference, the proposed heterodyne interferometer can measure highspeed target using a laser source with small frequency difference. The first orderperiodic nonlinearity is about0.06nm, the second order periodi c nonlinearity isabout0.025nm, the dynamic periodic nonlinearity is about0.055nm.