1. LASER CONOSCOPY OF LARGE-SIZED
OPTICAL CRYSTALS
1
Tver State University, Tver, Russia
2
Acousto-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

2. MOTIVATION:MOTIVATION:
Paratellurite (Paratellurite (αα-TeO2) large sized crystals for-TeO2) large sized crystals for
acoustoopticsacoustooptics
tetragonal symmetry, class 422

3. 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 β of the ray incident at an
angle α on the face of uniaxial crystal
G, defined by the vector of the normal
at an angle ψ 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

4. THEORY (2)THEORY (2)

5. 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

6. EXPERIMENTALEXPERIMENTAL
Experimental setup for conoscopic observation of large-sized crystals
Acoustooptic duct on the base
of paratellurite crystal
Large-sized crystal of L iTaO3

7. 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)

8. EXPERIMENTAL CONOSCOPIC PATTERNS (2)EXPERIMENTAL CONOSCOPIC PATTERNS (2)
Effect of striations on the distortion of isochromate shapes in
paratellurite crystal

9. CONCLUSIONCONCLUSION
The 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 (∅65×95 mm), LiTaO3 (∅70×40 mm), TeO2 prisms (50×50×30 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.

10. CONCLUSIONCONCLUSION
The 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 (∅65×95 mm), LiTaO3 (∅70×40 mm), TeO2 prisms (50×50×30 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.