A polarized lateral shearing interferometer and application for on-machine form error measurement of engineering surfaces
by Liu Xiaojun
Ph.D. Mechanical Engineering
xvii, 107 leaves : ill. (some col.) ; 30 cm
As technology in the semiconductor and optics industries advances very rapidly, demands for high quality surfaces such as optical mold surface and wafer surface are steadily increasing. This has posed greater challenges than ever before to precision machining in making precision objects....[ Read more ]
As technology in the semiconductor and optics industries advances very rapidly, demands for high quality surfaces such as optical mold surface and wafer surface are steadily increasing. This has posed greater challenges than ever before to precision machining in making precision objects.
To meet the challenge, direct on-machine surface measurement has become very important. However, there exist little practical application examples for on-machine precision surface measurement, in particular for form error measurement, due to the difficulties in handling vibrations and disturbances, commonly experienced during a machining process, without compromising measurement precision.
To deal with the problems, a novel optical interferometer is proposed based on a shearing interference principle. The proposed interferometer is simple and compact in structure, features a complete common optical path with polarized phase shifting interference, and is less sensitive to vibrations and environmental disturbances to have good stability and reliability, while high precision is maintained by the optical approach.
For accurate, stable, and convenient interferogram analysis in the new interferometer, a new phase unwrapping algorithm is developed based on wrapped phase statistics and phase correction. For reliable, accurate, and convenient wave front reconstruction from shearing interferometric phases, the existing Zernike polynomial fitting algorithm is improved. A new point-to-point mapping based least square algorithm and a B-Spline based fitting algorithm have been proposed.
A prototype of the new interferometer has been developed. The accuracy and characteristics of the proposed device for on-machine measurement are analyzed. Experimental tests have been conducted to validate the proposed design and the optical data processing algorithms. The results indicated that the developed prototype device has an error less than 1/80λ under general experimental condition, where λ=0.6328 μm. Under the on-machine condition of 0.4 μm and 12.22 μrad vibrations commonly found in precision machining, the error was less than 1/60λ. The results show that the proposed interferometer is quite suitable for on-machine form error measurement for machining many precision objects.