Setting sensitivity in collimation testing using Lau interferometry

An improved method for testing the collimation of an incoherent optical beam using Lau interferometry is presented. The experimental setup consists of a white light source and a set of three identical gratings. A source grating G1 is placed in front of a collimating lens. Beyond the collimating lens gratings G2 and G3 are so aligned that the lines of the gratings make a small but equal and opposite angle with the vertical. The self-image of grating G2 is superimposed on the grating G3, resulting in the generation of moire fringes. Decollimation of the optical beam results in a change in the dimensions of the self-image. This change is detected using the magnification effect of the moire method. The horizontal moire fringes are indicative of collimation of the optical beam. Deviation from the horizontal inclination of the moire fringes indicates setting in of decollimation. Theoretical and experimental investigations to determine the sensitivity achievable in this collimation testing technique are undertaken and results of the investigations are reported. Experimental results are in close agreement with the theoretical predictions.

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