Optical Metrology

1. Fizeau interferometer for testing optical flatness

         Fizeau interferometer is one of the most important optical metrology instruments at optics development facilities and laboratories for non-contact surface measurements of polished optical surfaces. A polarization phase shifting Fizeau interferometer has been developed for quantitative evaluation of surface flatness and 3-D surface form of polished optical surfaces. The developed interferometer measures Peak-Valley deviation, aberrations and displays 3-D surface form of the polished optical surfaces. The interferometer can measure surface form of the test optics up to a maximum diameter of 100 mm and with an accuracy of λ/10.
   

   
   Photograph of the developed Fizeau interferometer

2. Schlieren imaging for glass homogeneity testing

   Setup for schlieren imaging is developed which is being used to detect refractive index inhomogeneity in large size Nd-doped phosphate glass slabs. Imperfections in glass, generated during the melting and casting process results in inhomogeneities in the refractive index of the medium. When light rays pass through regions of imperfections, light rays deflect and scatter. This fractional deflection of the light ray results in distortion of the wave-front of the light beam. The resultant deflection of the light rays produce intensity change in corresponding portions of the image of the light beam and therefore can be used to identify the inhomogeneity of the glass medium. Schlieren image provides a good qualitative estimate of the inhomogeneity of the glass medium if it is of the order of 10-3 which is otherwise not observable through visual/naked eye inspection.

   
   Photograph of the experimental setup              Stitched shadow graphic image of the test Nd: Glass slab.
   
   
   High contrast stitched schlieren image of the test Nd-glass slab.