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Fig.1.
Idea of wavefront curvature-based optical inspection
Fig.2.
Surface diagnostic system supplied to the International
Laser Center at Bratislava (Slovak Republic).
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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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;)
;)