A bidirectional shearing interferometeric system for collimation testing is presented. It consists of a wedge plate and two plane mirrors. Two lateral shearings of same amount but opposite in direction take place in both sides of a wedge plate, and the two interference fringes can be seen simultaneously on a screen if one of them is reflected back by a mirror. By rotating the wedge direction through 90˚ or overlapping the two interference patterns, the change of fringe patterns show the degree of collimation independently. This system has its own reference mark to indicate collimation of the light beam and provides a twofold increase in sensitivity compared to Murty's single wedge plate shearing interferometer. The experimental equipment and precision analyses for various configurations are presented, and the usefulness for practical applications is demonstrated. We used this technique in measuring the optical parameters, such as focal length, radius of curvature, and refractive index.
Improved methods for measuring the long focal length and radius of curvature by using a bidirectional shearing interferometer in noncollimated illumination method are given. By defocusing the point source, the converged or diverged beam is normally incident on the test lens or surface placed on the optical axis of the collimating lens. The collimation of the transmitted light beam for a lens and reflecterd light beam for a spherical surface is determined by the parallelism of two interference fringes that are oppositely sheared at the same amount. Detailed analyses for optical arrangements and achieable accuracy are presented. This method can be used both lenses and surfaces which are positive and negative.
Nondestructive technique for measuring the refractive index of a simple lens is developed. The Fourier transform spectra of a grating have been employed with a liquid immersion method. The test lens itself acts as an autocollimator and a decollimator. The bidirectional shearing interferometric technique is used for collimation testing of the expanded laser beam and for determining the exact focal plane of a lens by immersing it in various liquids. The slit attached to micro X-Y translator has been used to measure the distance between two sucessive diffraction orders in the back focal plane of the test lens. The refractive index of an unknown liquid can also be determined by using a known lens, and a quick and accurate identification of a lens or the liquid can be made. An equation for the defocusing error has been theoretically deduced and experimentally verified. The experimental equipments are described and results are presented.
