Laser Autocollimation Technology Based On Common-path Beam Drift Measurement And Synchronous Compensation

Laser autocollimation is an important method for precision small anglemeasurement with high angle resolution, long measurement distance, andcharacteristics of noncontact, convenient to use, etc. It is widely used inmanufacture and testing of precision equipment, processing and testing of optics,assembling and testing of large structures, aiming and calibration of aerospaceequipment, and scientific research, etc. However, it is commonly experienced thatlaser beam drift due to laser generation mechanism and instability of beamtransmission medium cause a poor measurement stability in laser autocollimation.Laser beam drift is the main obstacle of improving the measurmement stability oflaser autocollimation.In this dissertation of “Laser autocollimation technology based oncommon-path beam drift measurement and synchronous compensation”, the modelof autocollimation in Gaussion optics is researched and used to analyze the effect oflaser beam drift on laser autocollimation to find methods to eliminate the drift oflaser autocollimation foundamentally. Common-path beam drift measurementmethod in laser autocollimation is also proposed. The beam drift is synchronouslycompensated by beam steering technology with high frequency response and highsensitivity, to achieve high stability of laser autocollimation small anglemeasurement. The main researches of this dissertation are described as followings:An autocollimation model based on Gaussion-optics is established by usingray-tracing method to solve the problem that the conventional autocollimationmodel can not be used to analyze the affects of autocollimation system effectively.The established model describes the relationship of all the parameters ofautocollimation system. Simulations have been done to analyze the effects oftwo-dimensional tilt-angle couple of the mirror, defocus of object and image planes,and angular drift of illuminating light source. The simulation results show that themaximum deviation caused by two-dimensional tilt-angle couple of the mirror isabout0.7170″in a measurement range of±2000″when an object point locates at therange of±5mm relative to the optical axis. The maximum diviation is about4.3540″in a measurement range of±2000″when object and image planes defocus in a rangeof±2mm. The drift of autocollimation small angle measurement can be0.0182″when the angular drift of illuminating light source ranges in±10″. The influencefactors of the resolution of laser autocollimation are also analyzed. The collimatinglens with long focal length and optical amplification method are studied to improvethe resolution of autocollimation small angle measurement. Common-path beam drift measurement methods based on combination reflectorand reference mirror reflector in laser autocollimation are proposed to solve theproblem that laser beam drift and the small angle being measured are alwayscoupled and hard to be separated in laser datum based small angle measurement.The combination reflector and reference mirror reflector are used to generate areference beam in autocollimation. The reference and measurement beams arederived from the same illuminating light source and transmitted on common-path, sothey have the same angular drift. In addition, the reference beam is insensitive to theangular rotation of the reflectors due to their unique characteristics. So the referencebeam can be used to accurately obtain the angular drift of measurement beam forsynchronous compensation to improve the small angle measurement stability oflaser autocollimation.Two beam steering methods based on flexible hinge are proposed to solve thecollision of high frequency response and high angular sensitivity of beam steeringmechanism in close-loop control of laser beam drift. The dynamical models of thetwo methods are established to analyze the variations of natural frequencies withtheir structure parameters. The models provide theoretical optimization of thestructure parameters to increase natural frequencies along with high angularsensitivities. Another transmission mirror based beam steering method is alsoproposed. The beam steering is realized by the transmission of a mirror which isactuated by PZT actuators. There is no mechanical friction or motion transmissionloss, so the method has a very high angular sensitivity and high frequency response.Experimental results show that angular sensitivity of the two beam steering methodsbased on flexible hinge can be0.003″and the transmission mirror based method canbe better than0.002″. All the frequency responses of the three methods can be up to2000Hz.An experimental laser autocollimation system based on common-path beamdrift measurement and synchronous compensation has been set up. The performanceof constitution units has been tested, including effectiveness of optical amplificationmethod, resolution of the photoelectrical detection units, consistency of the drifts ofreference and measurement beams, performance of the beam steering mechanisms,the effective resolution and measurement stability of the autocollimation system.Experimental results show that the effective resolution is0.0003″, and the2-hourstability can reach up to0.0026″, which demonstrates that a high stability is realizedon the base of high effective resolution.