Small-angle Measurement Based On Laser Self-mixing Interferometry

Angle measurement is one of the most important parts in geometry measurement fields, which is used broadly in optical engineer, machinery, astronautics, and military affairs. With the further development of science and technology, a higher requirement to the angle measurement method and its precision is claimed. Therefore many methods and theories of angle measurement appear increasingly, many domestic and foreign institutes have developed various equipments and facilities for angle measurement, the technical level and precision of angle measurement improve continuously. At the same time the testing process to be automatic has been realized.This paper presents a method for small-angle measurement based on laser self-mixing interferometry, It's found that the output laser power changes a fringe cycle when feedback light path changes every half-wavelength laser, which caused by the change of external cavity prism angle. Through repeated experiments, it is found that the different density degrees are gained from the self-mixing interferometry fringe by changing the condition of experiment, so the rotation angle measurement of the detected objects is achieved. And theoretically the mathematic relationship between feedback light path and angle measurement of the detected object is developed.In order to verify the method of small-angle measurement based on laser self-mixing interferometry, we established a rotation angle-measurement system for the rotation stage. By using the software development platform of LabVIEW , virtual instrumentation is designed in this measurement system to achieve the control, signal acquisition and processing functions. small-angle measurement based on laser self-mixing interferometry is also studied with this experment system.The stripe numbers of self-mixing interferometry we got from the experiment and the ones we obtained from theoretical calculation are totally coincident. self-mixing interferometry signal are shifted by one half of the resonance period from the experiment. Fourier-transform method is used to analysis the self-mixing interference signal, accuracy of analysis can be improved. Experimentally, rotation angles were measured with an accuracy of 10^-6 radian and the angle measurement range of approximately -0.0007 to +0.0007 radian. This method can also gain the absolute angle measurement by determining the absolute zero base angle.Repeation position error measurement for rotation stage drived by stepper motor can be achieved by using the method of small-angle measurement based on self-mixing interferometry. Test data and the curve was measured with different drivie segments and measurement methods.Repeation position error is less than 0.005°. And the factors effecting the repeation position error are analyzed in the end.