RU162920U1 - HIGH-APERTURE LENS FOR FOCUSING OPTICAL RADIATION - Google Patents

Abstract

1. High aperture lens for focusing optical radiation, including a spherical mirror with a central hole, characterized in that an optical plate is additionally introduced in front of the spherical mirror along the rays, on one side of which a diffraction structure is made, which ensures focusing of the optical radiation within a narrow radial region a spherical mirror, the centers of the diffraction structure and the spherical mirror lying on the same axis, and the center of curvature of the spherical mirror is located m between the diffraction structure and the spherical mirror. 2. The high aperture lens according to claim 1, characterized in that the spherical mirror is made in the form of a through hole with a concave-spherical inner surface in a plate of optical material, one side of which is matte, and the other side directed toward the diffraction structure is polished.

Description

This technical solution relates to the field of optical instrumentation and can be used as a high-aperture lens for focusing optical (laser) radiation in optical information recording devices, laser technological installations, laser lithographs, confocal microscopes, optical measurement and control systems, etc.

A known technical solution is a high-aperture mirror lens, comprising a mirror aspherical surface and a lens system (US Patent No. 7643226. Maximal-aperture reflecting objective, Publ. 5.01.2010).

A disadvantage of the known technical solution is the complexity of the design and alignment due to the use of an aspherical mirror surface and a lens system, as well as large light losses.

A technical solution is also known - a high-aperture lens with an increased working length - consisting of a concave mirror with a central hole mounted along the optical axis and a convex mirror (see Corning information material: “Tropel µСАТ UV Micro-Objectives”, https: // www. corning.com/media/worldwide/csm/documents/Tropel%20MicroCAT%20UV%20Micro-obiectives_011613%20B.pdf.pdf).

The disadvantage of this technical solution is the large loss of light due to the presence of a central hole, the complexity of the design and alignment, the limited numerical aperture due to the presence of a convex mirror between the focus plane and the output concave mirror.

Closest to the proposed technical solution and selected as a prototype is a high-aperture mirror lens containing a spherical mirror with a central hole, as well as a number of optically coupled concave and convex spherical mirrors (RF Patent No. 2296353. “Mirror lens”, published March 27, 2007 )

The known mirror lens has several disadvantages: large losses of light energy due to the presence of central shielding, design complexity due to the presence of several optically coupled mirrors, alignment complexity due to the need to center all the mirrors relative to the optical axis and relative to each other with high accuracy, and also limited numerical aperture due to the presence of a convex mirror between the focus plane and the output concave mirror.

The authors set the task of developing a high-aperture lens designed to focus optical radiation and free from these drawbacks.

The solution of this problem consists in the fact that in a high-aperture lens for focusing optical radiation, which includes a spherical mirror with a central hole, an optical plate is additionally introduced in front of the spherical mirror along the rays, on one side of which a diffraction structure is made providing focusing of optical radiation in within a narrow radial region of a spherical mirror, and the centers of the diffraction structure and the spherical mirror lie on the same axis, and the center of curvature is spherical The first mirror is located between the diffraction structure and the spherical mirror. In addition, the spherical mirror is made in the form of a through hole with a concave-spherical inner surface in a plate of optical material, one side of which is matte, and the second side directed toward the diffraction structure is polished.

The technical result of the claimed utility model is to expand the arsenal of funds for this purpose. In addition, simplification of the design and alignment, an increase in the numerical aperture and an increase in the coefficient of utilization of light energy (in the absence of an input central shielding and the presence of an output central shielding) are achieved.

The utility model is illustrated by drawings. In FIG. 1 shows the meridional section of the optical scheme of a high-aperture lens with light ray tracing, where 1 is an optical plate, 2 is a diffraction structure, 3 is an intermediate focal ring, 4 is a spherical mirror, 5 is a plate of optical material, 6 is the matte side of the plate of optical material, 7 — polished side of a plate of optical material, 8 — focusing plane, 9 — optical radiation of an interferometer, 10 — optical radiation reflected from an optical plate 1, 11 — optical radiation, reflection other than a plate of optical material 5.

In FIG. Figure 2 shows an example of an interferogram when aligning a high-aperture lens using an interferometer.

The proposed high aperture lens operates as follows (Fig. 1). Optical radiation from a light source (for example, a laser) enters the optical plate 1, on one side of which is located a diffraction structure 2, made, for example, in the form of a thin-layer surface relief. The diffraction structure is made, for example, by photolithography and ion etching of the optical plate 1 (see, for example: “Diffraction computer optics.” Edited by V. A. Soifer, “FIZMATLIT”, 2007).

To ensure high diffraction efficiency, the relief is made with a sawtooth shape and depth

where m = 1, 2, 3 ... is the working diffraction order, λ ₀ is the working wavelength and n is the refractive index of the optical plate. For example, with m = 1, λ ₀ = 266 nm and n = 1.46, the relief depth will be h = 580 nm.

If the optical radiation from the light source has a flat wave front, then the optical plate can be made, by known methods, both plane-parallel and wedge, with an angle of 0.25 to 2 degrees. If the optical radiation from the light source has a spherical wavefront, then the optical plate must be made plane-parallel.

The optical radiation from the light source is deflected by the diffraction structure 2 from the optical axis Z, forming an intermediate focal ring 3 at a distance D (shown by a dashed line), and then it arrives at a spherical mirror 4 with a radius of curvature R and a center O lying on the optical axis Z. With the indicated course of light radiation, a highly efficient redistribution of the energy of the input radiation and radiation concentration is ensured within the narrow radial region of the spherical mirror 4, the center of curvature of which is located between the diffraction action structure 2 and the spherical mirror 4 itself.

The spherical mirror 4 has a through hole and is made on the concave-spherical inner surface of the plate 5 of optical material, one side 6 of which is matte, and the second side 7, directed to the diffraction structure 2, is polished.

The plate 5 of optical material can be made of fused silica, glass or other material with a small value of the temperature coefficient of linear expansion. The surface of the spherical mirror 4 is coated with a highly reflective coating, for example, metal, dielectric, or a combination thereof.

The optical radiation arriving at the spherical mirror is reflected and focused at the point O1 lying in the focusing plane 8 on the optical axis Z.

Diffraction structure 2 is calculated in such a way as to ensure either stigmatic focusing by a spherical mirror 4 at point O1 (within the framework of geometric optics, all rays intersect at the caustic point of one focus O1) or astigmatic focusing with the formation, for example, of caustics of rays in the form of an axial segment ( in the framework of geometric optics, the rays intersect at different focal points, in a small neighborhood of the central point O1). In the second case, the implementation of the so-called axicon focusing is possible, with the formation of spatially limited Bessel-like light beams and with the implementation of edge correction of the path of light rays, which reduces the oscillations of the axial light intensity in the focus area (see, AG Sedukhin, Beam-preshaping axicon focusing , J. Opt. Soc. Am. A, 1998, vol. 15, No. 12, pp. 3057-3066; AG Sedukhin, Marginal phase correction of truncated Bessel beams, J. Opt. Soc. Am. A, 2000, vol. 17, No. 6, pp. 1059-1066).

The diameter of the central focal spot when focusing with an ordinary lens with a full aperture (diameter 2r ₁ ) is determined by the first zero of the first-order Bessel function (see Honina S.N., Ustinov A.V. Reducing the size of the focal spot with radial polarization using a binary ring element / / Computer Optics, 2012, 36, No. 2, pp. 219-226, http://www.computeroptics.smr.ru/KO/PDF/KQ36-2/11.pdf):

where NA is the numerical aperture defined by the formula

where n _c is the refractive index of the medium in which focusing is performed, and α is the half aperture angle of the light beam (see Fig. 1).

The proposed technical solution uses a ring aperture function. The optical radiation arriving at the input of a high-aperture lens with the help of diffraction structure 2 is concentrated without a significant energy loss within a narrow radial region (r ₂ -r ₁ ), a spherical mirror 4, and then, after reflection, focuses into a focal spot in plane 8. The diameter of the central focal spots will be determined by the first zero of the zero order Bessel function:

As can be seen from a comparison of expressions (2) and (4), the annular aperture function allows us to reduce the size of the focal spot by 1.6 times.

The numerical aperture of the proposed high-aperture lens (Fig. 1) is found from the geometry of the beam path according to the formula:

where r1 is the radius of the output aperture of the spherical mirror, and WD is the free working segment.

Example 1

When focusing optical radiation with a wavelength of λ ₀ = 266 nm in air (n _c = 1) with a spherical mirror with R = 14 mm, r1 = 9 mm, WD = 2 mm, the numerical aperture of the high-aperture lens will be NA = 0.97, and the size of the central focal spot (zero in intensity level) from formula (4) will be D _KA ~ 0.2 μm. When using the thermochemical method of laser recording of micro and nanostructures, the recording process is nonlinear and recording is carried out at a level of 0.8-0.9 from the maximum intensity at the center of the central focal spot (see, V.P. Veiko, V.P. Korolkov, A.G. Poleschuk et al., Study of spatial resolution of laser thermochemical recording technology // Quantum Electronics, 2011, 41, No. 7, pp. 631-636). In this case, the width W of the recorded track will be 2–3 times smaller than the size D _{KA of the} central focal spot:

Thus, the use of the proposed high-aperture lens with the thermochemical method of recording provides the recording of microstructures with nanometer resolution.

To ensure sharp focusing of optical radiation into the central focal spot of a submicron size, it is necessary to accurately align the surface of the optical plate 1 containing the diffraction structure 2 relative to the input optical radiation and the spherical mirror 4. For this, the surface 7 of the plate 5 of the optical material is polished. When adjusting, for example, using an interferometer, its optical radiation 9 is partially reflected from the surface of the optical plate 1, which is free from the diffraction structure 2. This optical radiation 10 is fed back to the interferometer. The part of the optical radiation that has passed through the optical plate 1 is reflected from the surface 7 of the plate 5 of optical material in which a spherical mirror 4 is made. This optical radiation 11 also enters back into the interferometer and interferes with the optical radiation 10. An example of such an interference pattern is shown in FIG. 2. The inclination angle β of the optical plate 1 relative to the plate 5 of the optical material (alignment error) is calculated by the formula:

where λ _and is the wavelength of the optical radiation of the interferometer, T is the observed period of interference fringes.

Example 2

In the example of FIG. 2, the period of interference fringes T ~ 1 mm. From this it follows that at λ _and = 633 nm, the angle of inclination is β ~ 4 angles. min For the proposed high-aperture lens to work correctly, it is necessary to provide an inclination angle β <5 angles. sec This is quite simply provided when tuning to the “endless” band.

On the optical plate 1, outside the diffraction structure 2, if necessary, auxiliary diffraction structures can be applied, for example, diffraction lenses with a focal length equal to the distance D1 between the optical plate 1 and the plate 5 of optical material. In this case, when adjusting, for example, using an interferometer, interferograms with a small number of bands will be recorded only when the distance D1 between the optical plate 1 and the plate 5 of optical material is precisely set.

The advantages of the proposed technical solutions are:

- the implementation of a high-aperture lens with a mirror having a spherical surface shape makes it high-tech and makes it possible to produce a high-quality spherical surface capable of working in the UV and GUF regions of the spectrum;

- the use of the reflection effect from a spherical mirror allows to reduce the requirements for the quality of surface polishing and the inhomogeneity of the material of the mirror plate and to ensure the operation of the high-aperture lens in the UV and GUF regions of the spectrum;

- redistribution of energy of the input light flux, using a diffraction structure, into a narrow annular region located in front of the spherical mirror, provides the effect of central screening of the output flux focused by the mirror, as well as the absence of light energy losses;

- a high numerical aperture (close to the limiting one) is provided by a simple two-component optical design of a high aperture lens with a sufficiently large front working segment, which simplifies the design of the lens and makes it easier to obtain small aberrations when focusing radiation;

- the annular aperture function of the high-aperture lens makes it possible to reduce the size of the central focal spot by approximately 1.6 times and to ensure the recording of nanostructures, for example, by the thermochemical method;

- simple adjustment with an interferometer allows you to set the relative position of the components of the high-aperture lens with each other with high accuracy;

- a simple replacement of an optical plate with a diffractive structure allows using a high-aperture lens in the spectral regions from GUF to IR without changing the design (with the same spherical mirror).

Claims (2)

1. High aperture lens for focusing optical radiation, including a spherical mirror with a central hole, characterized in that an optical plate is additionally introduced in front of the spherical mirror along the rays, on one side of which a diffraction structure is made, which ensures focusing of the optical radiation within a narrow radial region a spherical mirror, the centers of the diffraction structure and the spherical mirror lying on the same axis, and the center of curvature of the spherical mirror is located m Between the diffraction structure and the spherical mirror. 2. The high-aperture lens according to claim 1, characterized in that the spherical mirror is made in the form of a through hole with a concave-spherical inner surface in a plate of optical material, one side of which is matte, and the other side directed toward the diffraction structure is polished.

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