SU958922A1 - Device for measuring non-uniformity of double refraction in crystals - Google Patents

Description

(54) DEVICE FOR MEASURING THE INHOMOGENEITIES OF TWO-APPEARANCE IN CRYSTALS

The invention relates to measurements in optics and can be used, for example, in studying the quality of optical elements made of crystals and used in nonlinear optics. A Twiman-Green interferometer can be used to measure irregularities in the refractive index of optical elements. containing a source of monochromatic light, a polarizer, an optical system of two mirrors and a translucent plate, as well as a hysteristor 1. The disadvantages of this interferometer are the increased requirements placed on the quality of the mirrors used in it, as well as the complexity of the design, due to the multiple elements of the optical system and the need to protect it from vibrations and the tempo of natural gradients. A device that is simpler in construction is known, which contains an optically coupled source of monochromatic light, a polarizer, a crystal wedge, an analyzer, and a recorder. The wedge is made of quartz, which is an optically active uniaxial crystal, and the optical axis of the wedge is parallel to one of its working faces 2. The disadvantage of this device is the low measurement accuracy due to the fixed orientation of the interference fringes relative to the sample under study. This makes it difficult to detect and estimate the magnitude of the birefringence irregularities. Closest to the invention is a device that contains an optically coupled monochromatic light source, a polarizer, a crystal wedge, an analyzer, and a recorder. The wedge is made of an optically inactive uniaxial crystal, and its optical axis is oriented to the face at an angle greater than the angle between the clinic face. This allows the rotation of the wedge around its optical axis to establish the most convenient for measuring the position of the interference pattern, which provides an increase in the accuracy of measurements 3. Or the disadvantage of the known device is that it can be used to investigate only crystalline samples whose optical axis is parallel to the working face. This disadvantage is associated with the divergence of the light beam, since it is practically impossible to form a parallel beam of light under real conditions. As a result, the various rays of the light beam enter the sample under study at different angles and have different refractive indices of unusual rays. This leads to a distortion of the interference pattern. If the optical axis is parallel to the working edges of the sample, the dependence of the refractive index on the direction of propagation is small and a slight distortion of the interference pattern does not interfere with the measurement of the birefringence inhomogeneities. In the case of an arbitrary orientation of the optical axis relative to the working faces of the sample under study, the divergence of the light beam leads to such a distortion of the interest pattern, which makes it impossible to measure birefringence inhomogeneities.

The purpose of the invention is to make it possible to measure birefringence irregularities in crystalline samples with an arbitrarily oriented optical axis.

The goal is achieved in that a device containing an optically coupled source of monochromatic light, a polarizer, a crystal wedge made of an optically inactive uniaxial crystal, and an optically coupled analyzer and recorder are additionally introduced a flat mirror, a beam-splitting plate and a wedge of isotropic material installed so that, together with the main wedge, it forms a plane-parallel plate, behind the coyur there is a mirror, while the analyzer is placed in the path of the light UČKA reflected from the beamsplitter plate mounted for Polizim torus.

The drawing shows the optical layout of the device.

The device contains successively located monochromatic light source 1, a polarizer 2, a beam-splitting plate 3, a plane-parallel plate 4 consisting of a crystalline wedge 5 and an isotropic wedge 6, a plane mirror 7, and an analyzer 8 and screen 9 as a recorder placed on the path reflected from the beam-splitting plate 3 light beam.

The crystal wedge 5 is made of an optically inactive uniaxial crystal. In the general case, the optical axis Z of the wedge 5 is oriented to the face at an angle exceeding the angle between the faces of the wedge. In the variant shown in the drawing, the Z axis is perpendicular to the working face of the plane-parallel plates 4. Isotropic wedge 6

made of a material with a refractive index Pi, where, P is the refractive index for the extraordinary beam and Pau is the refractive index for the ordinary beam in the wedge 5. The plane-parallel plate 4 / is rotatably mounted around the Z axis. The flat mirror 7 is installed perpendicular to the axis of the light beam formed by the source 1. Test sample 10 is placed between the beam splitter plate 3 and the plane-parallel plate 4, and the optical axis Z of the sample lies in the plane of oscillation of the wedge 5 and makes an angle of 0 with the axis of the light beam.

The device works as follows

Source 1 forms a monochromatic quasi-parallel beam of light, the individual rays of which due to the divergence of the light beam are inclined to its axis at different angles d ..

Pass through polarizer 2, bundle

light acquires linear polarization. The beam-splitting plate 3 transmits a linearly polarized beam of light, which then enters the crystal sample 10 under investigation. In sample 10

each beam of the beam is decomposed into two co-, gergent beams with mutually perpendicular polarization directions - an ordinary ray with a refractive index and extraordinary, the refractive index of which Pe (6f ° C) depends on the direction of the beam propagation and is determined from the ratio

Ke (e + --- lo-j v n2 sin (e -os) cos2 (in + -ot)

Vn sin2 () - nZ

5 where 0 is the angle between the axis of the light beam and the optical axis Z of the sample 10; P2 - the main index of refraction

for an extraordinary ray. The rays propagate in sample 10 at different speeds, as a result of which

between them there is a phase difference:

 25 t g yy

,

DU 1 Po

where L is the wavelength of light;

5L - length of sample 10 along the beam

Sveta.

Claims (3)

Since the refractive index of Pe (0+ of-) depends on the direction of propagation of the beam, the phase difference acquired by each pair of coherent rays is different when passing through the sample 10. The presence of birefringence inhomogeneities in sample 10 under study leads to an additional change in the phase difference between the rays. Wedge 5 provides an additional phase shift, linearly varying across the beam cross section, and wedge b, due to the fact that n PC n compensates for the refraction of the light beam in the wedge 5. As a result, the light beam passing through the plane-parallel plate 4 does not change its original direction, i.e., the beam axis is perpendicular to the mirror 7. Therefore, the beam propagating during the direct passage through the test sample 10 at an angle (+ + Å) to the optical axis Z of the sample 10, when reflected from the mirror 7, propagates in sample 10 at an angle (G- o ( -) to the Z axis. Since the angle ot is small (the light beam is quasi-parallel formed by the source 1), the relation (e) is satisfied. As a result, each pair of coherent rays with a double pass through the sample 10 has the same phase shift. The beam-splitting plate 3 guides the transmitted through the sample 10, the light beam to the analyzer 8, which extracts components from each beam with oscillations lying in the plane of its main cross section. These vibrations interfere with each other and an interference pattern is observed on screen 5, which represents istemu parallel fringes with local bends. Bands appear due to a phase shift introduced by the wedge 5, and local bends are due to a phase shift caused by the presence of irregularities in birefringence in sample 10. The magnitude of bends depends not only on the magnitude of irregularities in birefringence, but also in the mutual orientation of the sample 10 and the plane-parallel plate 4. By rotating the plane-parallel plate 4 around the Z axis, one finds a position of the interference fringes rotating on the screen 9 such that the local bending of the fringes has a maximum value. The magnitude of the birefringence inhomogeneities is determined by the formula dA .. ° 2L b where d is the magnitude of the heterogeneity of the birefringence, the refraction index; O. - the magnitude of the maximum bending of the strip; b is the spacing between the stripes. Thus, the introduction of a flat mirror, a wedge of an isotropic material and a beam-splitting plate into the device makes it possible to eliminate the influence of the divergence of the light beam and to ensure a positive effect of crystalline samples with an arbitrarily oriented optical axis. The advantages of the device also include an increase in the sensitivity of measurements due to the double passage of the light beam through the sample under study. An apparatus for measuring birefringence inhomogeneities in crystals, containing an optically coupled source of monochromatic light, a polarizer, a crystal wedge made of an optically inactive uniaxial crystal, an analyzer and a recorder, characterized in that, for the purpose of measuring non-uniformity of a birefringence crystalline samples with an arbitrarily oriented optical axis; a flat mirror was additionally introduced into it, a beam-splitting plate and a wedge of an isotropic material iala mounted so that together with the main wedge it forms a plane-parallel plate which is disposed behind the mirror, wherein the analyzer is placed in the path of the light beam reflected from the splitter plate placed behind the polarizer. Sources of information taken into account in the examination 1. M. Born, Wolf E. Fundamentals of Optics, D., Nauka, 1970, p. 333-337. 2. Grum-Grzhimailo S. V. Instruments and methods for optical crystal research. M., “Science, 1972, p. 104-106. 3. USSR author's certificate for application number 2792086, cl. G 01 N 21/40, 1979 (prototype). N h: a about