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Research

     
   

Surface Diagnostics

Supervisor: Dr. Larichev A.V.

 


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Our main achievement in this research direction is development of a non-contact laser sensor for reconstructing the shape of partially reflective surfaces and analyzing phase inhomogeneities. The sensor uses the Shack-Hartman method implying that a lenslet array focuses onto a CCD array a laser beam reflected from the surface under study or passed through an optically inhomogeneous medium. The location of each focal spot produced depends on the local slope of the wavefront of the beam passing through the input aperture of the sensor. The local slope matrix is transformed into a set of coefficients for Zernike polynomials see Zernike polynomials ] that fully represent low-order aberrations of the wavefront under study (up to 12-th order).

Thus, the sensor is capable of quantitatively reconstructing the shape of a reflective surface or an optical thickness distribution across an inhomogeneous medium. The transverse resolution is determined by the number of lenslets the array contains, which is 16x16 in our device. The exposure time can be as small as 10-4 s, assuming that the object under study reflects a sufficient fraction of the probing light.

The key parameters of the sensor are the following:

  maximum power of the probing laser beam 1 mW;
  diameter of the measurement area from 1 to 10 cm (zoom optics included);
  maximum radius of curvature of surface to be measured 10 km;
  minimum dynamic range for the amplitudes of measured aberrations 300:1;
  measurement accuracy λ/30;
  temporal performance 25 Hz.

The sensor can be used for:

  testing the flatness of semiconductor wafers;
  measuring the radii of curvature of the surfaces of optical elements;
  analyzing dynamical deformations of surfaces;
  studying convective and turbulent flows in gases and liquids;
  analyzing phase objects;
  measuring laser beam parameters.