LASER CONOSCOPY OF LARGE-SIZED OPTICAL CRYSTALS

1Tver State University, Tver, Russia 2Acousto-Optic Research Centre, MISIS, Moscow

A.I. Kolesnikov1, R.M. Grechishkin1, S.A. Tretiakov1, V.Ya. Molchanov2, A.I. Ivanova1, E.I. Kaplunova1, E.Yu. Vorontsova

* Corresponding author. E-mail address: rostislav.grechishkin@tversu.ru

MOTIVATION:MOTIVATION:Paratellurite (Paratellurite (-TeO2) large sized crystals for -TeO2) large sized crystals for

acoustoopticsacoustoopticstetragonal symmetry, class 422

THEORY (1)THEORY (1)

It is shown theoretically that in the method of conoscopy in the general case of arbitrary orientation of the optical axis and normal to the crystal surface the isochromes may be presented by the curves of fourth order and higher. Such type of isochromes was observed experimentally. Examples of the computer calculation of the system of equations derived without usual approximations adopted by other authors are presented.

On the calculation of the angle of refraction of the ray incident at an angle on the face of uniaxial crystal G, defined by the vector of the normal at an angle to the optical axis

On the calculation of the path difference of the rays in anisotropic plate cut from a uniaxial positive crystal; ordinary (o) and extraordinary (e) beam

THEORY (2)THEORY (2)

THEORY (3)THEORY (3)

Calculated effect of the sample thickness on the conoscopic picture and the number of isochromates in the field of view (FOV) Parameters: No=2.4, Ne=2.6, angular aperture 10о, wavelength λ=500 nm, lens focus length 350 mm, sample thickness 0.5, 1.0 and 2 mm

Calculated effect of angular aperture of the light cone on the conoscopic picture and the number of isochromates in FOV. Parameters: No=2.4, Ne=2.6, sample thickness 0.5 mm, wavelength λ=500 nm, lens focus length 350 mm, angular aperture 8°, 15° and 30°

Calculated effect of refractive indices values on the conoscopic picture and the number of isochromates in FOV. Parameters: angular aperture10°, sample thickness 0.5 mm, wavelength λ=500 nm, lens focus length 350 mm, a) No=2.1, Ne=2.0 b) No=2.2, Ne=2.05 c) No=2.2, Ne=2.12

EXPERIMENTAL EXPERIMENTAL

Experimental setup for conoscopic observation of large-sized crystals

Acoustooptic duct on the base of paratellurite crystal

Large-sized crystal of L iTaO3

Conoscopic pattern produced by a polar cut

of TeO2 crystal in normal orientation

Conoscopic pattern produced by a TeO2

crystal for an angle of 3° between the optical axis

and light cone axis

Conoscopic pattern distortion due to

mechanical stresses at the edge of the sample

EXPERIMENTAL CONOSCOPIC PATTERNS (1) EXPERIMENTAL CONOSCOPIC PATTERNS (1)

EXPERIMENTAL CONOSCOPIC PATTERNS (2) EXPERIMENTAL CONOSCOPIC PATTERNS (2)

Effect of striations on the distortion of isochromate shapes in paratellurite crystal

CONCLUSIONCONCLUSIONThe analytical results of the earlier performed first-principle calculations of isochromate patterns in birefringent media were applied to the conoscopic examination of large-sized crystal boules unusable for examination in conventional polarizing optical microscopes with Bertrand lens. In the present work we have shown that the usage of convergent or divergent laser beams in simple benchtop configuration makes it possible to explore big samples by the method of conoscopy including crystals elongated in the optical axis direction. As distinct from traditional optical microscopy the conoscopic figures obtained with the aid of the laser installment may contain tens and hundreds of isochromate fringes, thus increasing the informative capabilities of the method. Large-sized crystals of LiNbO3 (6595 mm), LiTaO3 (7040 mm), TeO2 prisms (505030 mm) were examined experimentally at different angles between the optical axis and normal to the crystal surface. The capabilities of conoscopic observations as a fast contactless method for characterizing the quality of large boules for their potential use in optical instrumentation were demonstrated. Among the advantages of using lasers to this end are their high intensity of illumination, monochromaticity and small angular aperture. However, a separate study of the ways of improving the laser beam uniformity and stability (pi-shapers, etc.) would increase the functional possibilities of the method. This would also be helpful for the development of computer-controlled scanning conoscope imaging system.