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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.
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