On Collimation Of A Laser Beam

ON COLLIMATION OF A LASER BEAMR.S. SIROHI and M.P. KOTHIYALEngineering Design CentreIndian Institute of Technology,Madras-600 036, India.1. INTRODUCTION, ~. Several schemes for testing the collimation of a laser beam based on Talbot imag­ ing and Murty shearing interferometer have been reported. We present here two variations of these techniques.2. TALBOT IMAGING TECHNIQUESFig.l(a) shows the basic arrangement of generating Moire fringes using Talbot imagi ng. If a grating G, is illuminated by a collimated beam, it produces self images at distances given by Z = m/^/<^2 where /-6 is the spatial frequency of the grating, m an integer andMhe wavelength of light. If a second grating G- identical to G, is placed in the plane of one of the self images, a Moire pattern can be seen. A departure from collimation of the beam results in a change in the frequency of the self image of the grating G,. This change in frequency produces a rotation of the Moire fringes, which is used as a test of collimation of the illuminating beam. In practice the lines of the gratings G, and G^ make small equal angles on either side of vertical so that the Moire fringes are horizontal. In otherwords, if gratings G, and G- in Fig. l(a) are identical, a horizontal Moire pattern is indicative of collimation of the illuminating beam. Any rotation from this reference orientation shows a departure from collimation. It is rather difficult to establish this reference orientation of the Moire fringes as it would require precise rotation mechanism for gratings. We also need to establish vertical and horizontal (two orthogonal) directions.L G,