Koester's interferometer modification for gauge blocks calibration Marko G. Nikolić * , Aleksandar J. Krmpot, Bratimir Panić, Zoran Grujić, Dejan V. Pantelić Institute of Physics, 118 Pregrevica, Belgrade, Serbia ABSTRACT Modification of Koester's interferometer is described in this paper, which is used for calibration of gauge blocks up to 100mm in length. Higher precision was obtained by using a camera (for measurement of fringe fractions) and lasers as the source of light. The process of digitalization and processing of the interference pattern images is also described, as well as rejection of noise caused by transmission of laser beams through the optical fiber. Keywords: Koester's interferometer, gauge blocks, lasers 1. INTRODUCTION Determining of gauge blocks length is performed by interferometric comparators, which are actually modification of Michelson or Fabri-Perot interferometer. Karl Zeis interferometer was previously used with spectral lamps as light source 1 . The fringe fraction, which is actually phase shift between two sinusoidal fringe patterns originated from the top and from the bottom of the gauge block, expressed in percents, was determined by operator eye. Maximum accuracy of this method is λ /4. We replaced the spectral lamps by two He-Ne lasers, the red one (633nm) and the green one (543nm), so process of measuring is faster and more accurate now. Fig 1. Modified Koester’s interferometer The beam from the laser is fed into the interferometer by the optical fiber and it is collimated by a lens at the entrance (fig 1.). At the Koester’s prism (monochromator) the beam is refracted towards semitransparent mirror where it is *nikolic@phy.bg.ac.yu 14th International School on Quantum Electronics: Laser Physics and Applications, edited by Peter A. Atanasov, Tanja N. Dreischuh, Sanka V. Gateva, Lubomir M. Kovachev, Proc. of SPIE Vol. 6604, 66040P, (2007) 0277-786X/07/$15 doi: 10.1117/12.726899 Proc. of SPIE Vol. 6604 66040P-1