RU2819107C1 - Monoblock converter of light beam with circular polarization into beam with azimuthal polarization - Google Patents

Abstract

FIELD: polarization optics.

SUBSTANCE: invention relates to polarization optics, namely to conversion of polarization state of light radiation, and can be used to convert radiation with circular polarization into radiation with azimuthal polarization. Monoblock polarization converter is made of refracting material and has taper angle of the first conical surface β related to the Brewster angle (arctan(n)) by the ratio β=π/2-arctan(n), providing conversion of a beam with circular polarization, falling into the aperture of the cone, into a reflected beam with azimuthal polarization. Second conical surface with taper angle ω forms the outer surface of the element and serves for uniform distribution of polarized radiation energy over the circular cross-section of the output beam. Second conical surface reflects incident polarized radiation onto third surface with taper angle ψ. After refraction on the third surface, the polarized radiation beams propagate in the same direction as the light beams incident on the first surface, i.e. parallel to the optical axis.

EFFECT: converting a circularly polarized incident light beam into a collimated ring-shaped light beam with azimuthal polarization.

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Description

The invention relates to the field of polarization optics, namely to the transformation of the polarization state of light radiation, and can be used to convert radiation with circular polarization into radiation with azimuthal polarization.

Converters of polarization of laser radiation are known (see RF patents No. 2175450, IPC G02B 5/30, publ. 2000.01.12, No. 2087020 IPC G02B 27/28, G02B 27/44 publ. 1997.08.10) based on subwavelength diffraction gratings for conversion circular polarization to radial or azimuthal, as well as linear polarization to circular.

Such devices cannot be used to convert radiation from non-laser light sources due to high requirements for monochromatic radiation. In addition, the disadvantage of such devices is the technological complexity of manufacturing large-area diffraction gratings. Such gratings are produced, as a rule, on electronic lithographs with a small working area.

Another approach to converting circular polarization into radial or azimuthal is based on the polarizing properties of the process of refraction or reflection of light. For example, a device is known (see R.V. Skidanov, A.V. Morozov // Diffraction axicons for the formation of radially polarized light based on the use of Stoletov’s foot / Computer Optics. - 2014. - T. 38, No. 4. - P. 614-618), consisting of a high-aperture diffraction axicon, producing a conical beam of light with a divergence angle equal to the Brewster angle for a stack of glass plates (Stoletov’s stack) located behind it and optically connected to it. Due to the presence of a diffraction element, such a device also places high demands on the monochromaticity of the radiation used. In addition, due to the large values of the Brewster angle, collimation of the output radiation by a separate element is very difficult, and the output intensity distribution is obtained in the shape of a ring.

Some of the listed disadvantages are eliminated in a device based on an interference polarizer (see Sergey V Karpeev, Vyacheslav D Paranin and Svetlana N Khonina Generation of a controlled double-ring-shaped radially polarized spiral laser beam using a combination of a binary axicon with an interference polarizer 2017 J. Opt 19 055701). This device implements radiation collimation using a refractive axicon, since the angles of incidence on the polarizer are significantly smaller than the Brewster angle, and the degree of polarization of the radiation is much greater. However, a high degree of monochromaticity of the radiation is still required and the output intensity distribution is obtained in the form of rings. In addition, the device itself consists of several elements that are adjusted to each other with a high degree of accuracy, which makes operation difficult.

The closest in essence to the claimed device is a monoblock polarization converter for converting a beam with circular polarization into a beam with azimuthal polarization (see Degtyarev, S.A. Karpeev, S.V., Ivliev, N.A., Podlipnov, V.V., Khonina, S.N. Refractive Bi-Conic Axicon (Volcone) for Polarization Conversion of Monochromatic Radiation Photonics, 2022, 9(6), 421), which is a radially symmetrical element made of a transparent material with a refractive index n, formed by two conical surfaces, the inner of which has a cone angle (3, associated with the angle Brewster (arctan(n)) with the relation β=π/2-arctan(n), and the external one has a cone angle с, which ensures the reflection of rays passing through the first surface in the same direction as the rays of light incident on the first surface.

Such an element alone ensures the transformation of a beam of incident light with circular polarization into a collimated beam of light of a ring shape with azimuthal polarization. The basis of the invention is the task of obtaining a beam of light with azimuthal polarization of a circular shape with a given diameter.

This problem is solved due to the fact that a monoblock converter of a beam with circular polarization into a beam with azimuthal polarization, which is a radially symmetrical element made of a transparent material with a refractive index n, formed by two conical surfaces, the inner one of which has a cone angle β associated with Brewster angle (arctan(n)) with the ratio β=π/2-arctan(n), and the outer one has a cone angle ω and ensures reflection of rays in the direction of the third surface with a cone angle ψ, on which the energy of the polarized beam is uniformly distributed in a circular manner cross section, as well as collimation of the output beam, and ω and ψ are related to each other by the formula:

and the radius of the incoming beam r and the outgoing beam R are related by the formula:

In fig. 1 shows a general view of the laser radiation polarization converter, and FIG. 2 sectional design of the element.

Monoblock polarization converter 1 is made of refractive material and has a cone angle of the first conical surface β, related to the Brewster angle (arctan(n)) by the ratio β=π/2-arctan(n), providing conversion of a beam with circular polarization falling into the aperture of the cone , into a reflected beam with azimuthal polarization. The second conical surface with a cone angle ω forms the outer surface of the element and serves to uniformly distribute the energy of polarized radiation over the circular cross-section of the output beam. The second conical surface reflects the polarized radiation incident on it onto the third surface with a cone angle ψ. After refraction on the third surface, the rays of polarized radiation propagate in the same direction as the light rays incident on the first surface, that is, parallel to the optical axis. The third conical surface forms the back surface of the element and has a base radius ρ:

In this case, ρ is also the outer overall radius of the optical element.

The device operates as follows: optical radiation with circular polarization illuminates the opening of the first conical surface, and when incident on the surface at the Brewster angle, the beam reflected from the surface becomes plane-polarized with a polarization plane parallel to the reflection surface. Thus, due to the radial symmetry of the conical surface, all rays reflected from it will be polarized perpendicular to the radial coordinate. The refracted rays will be partially polarized and, when propagating, will be spatially separated from the reflected rays. Obviously, in order to obtain a reflected beam with azimuthal polarization, it is necessary to illuminate the surface of the cone with optical radiation with circular polarization, ensuring at each moment of time the desired polarization state of the rays incident on the surface of the cone. Next, the reflected rays fall on the opposite surface of the cone and pass through it with refraction (see the path of the rays in Fig. 2). The transmitted rays fall from the inside onto the second conical surface and, due to total internal reflection, are reflected in the direction to the third conical surface so that the rays are evenly distributed over the circular cross-section of the output beam without intersections and a circular void in the center, that is, so that the output beam has a circular cross-section, and not a roundabout. The rays reflected from the second surface fall on the third conical surface, the cone angle of which ψ, calculated by formula (1), ensures the parallelism of all emerging rays after refraction on the third conical surface. In addition, the rays incident on the extreme points of the first conical surface, subject to all dimensions in accordance with formulas (1), (2), (3), emerge from the third conical surface one on the optical axis, and the second at a distance R from the optical axes. It follows from this that an incident radially polarized beam of a circular shape is transformed into an azimuthally polarized beam of also a circular shape with a radius R, which can be calculated using formula (2).

As follows from the description of the proposed invention, in the optical system, in comparison with the prototype, a round-shaped beam is formed. A positive effect is achieved due to the fact that the output radiation is collimated by a third conical surface, the cone angle of which and its location give a uniform distribution of the output rays throughout the circle.

Claims (2)

Monoblock converter of a beam with circular polarization into a beam with azimuthal polarization, which is a radially symmetrical element made of a transparent material with a refractive index n, formed by two conical surfaces, the inner of which has a cone angle β related to the Brewster angle (arctan(n)) ratio β=π/2-arctan(n), and the outer one has a cone angle ω, ensuring reflection of rays passing through the first surface in the same direction as light rays incident on the first surface, characterized in that behind the second conical surface a third conical surface is added with a cone angle ψ, and ω and ψ are related by the formula: