EA007874B1 - Device for recording diffraction elements - Google Patents

Abstract

The invention relates to optical holography for recording diffraction optical elements and can be applied for manufacturing diffraction elements, rainbow holograms, for protecting goods and securities against counterfeit, in polygraphy. An apparatus for recording diffraction optical elements comprises an optically-coupled light source, a unit of beam deflection, the first lens, the unit of beam deflection comprises the first prism, a init of light beam separation, a focusing lens, a recording means, a drive for the recording medium movement and a control means electrically connected to the light source, the unit of beam deflection, the drive for the recording medium movement, the unit of beam deflection comprises the second lens and two reflex deflectors set sequentially along the optical axis, wherein the distance between the mirror plane of any of two deflectors and the light focusing point of the first lens is selected from the ratio:l<BTff/fdwherein B is o.1-0.5, T is a period of a diffraction element grating in the registered medium, fis the focus distance of the second lens, fis the focus distance of the first lens, fis the focus distance of the focusing lens, dis a light beam diameter at the inlet of the second lens.

Description

The invention relates to optical holography, is intended to record diffractive elements and can be used in the manufacture of diffractive elements, rainbow holograms, in the protection of goods and securities from fakes, in printing. The invention can also be used in optical memory systems and optical information processing.

A device for recording diffractive elements is known, comprising a laser, a modulator, a rotational drive diffraction grating, a diaphragm, a focusing lens, a two-coordinate photo-resistor plate moving device, a computer (computer patent US No. 5291317, IPC C02B 5/18 priority from 12.07, sequentially installed along the optical axis. .90, published 03/01/94).

The disadvantages of this device are the low recording rate of diffraction structures with different spatial orientation, due to the large time of rotation of the diffraction grating relative to the axis of the electric motor, as well as low light transmission, due to the limited diffraction efficiency of the diffraction grating.

It is also known a device containing a laser, an electromechanical shutter, a spatial light modulator, a polaroid, a projection lens, a photoresist plate, a two-coordinate plate displacement device with a photoresist, and a computer (Russian patent No. 2207611, IPC 603Н01 / 26, priority from 30.07 .2001, published 06/27/2003).

The disadvantages of this device are the limited speed of the spatial light modulator, its low light transmission caused by the absorption of light in the material of the spatial modulator and polaroid, the high cost and complexity of the entire device, the complexity and high cost of the high-resolution projection system. In addition, a significant disadvantage of this device is the limited radiation resistance of the spatial light and polaroid modulator, which prevents the use of this device with high-power pulsed lasers operating in the ultraviolet range.

The closest to the claimed device is the device with which the method of recording an array of point holograms is implemented (Russian patent No. 2194296, IPC 603Н01 / 26, priority from 04/18/2001, published on December 10, 2002). This device is selected as a prototype. A known device for recording diffractive elements comprises an optically coupled light source, a light beam deflection assembly, a first lens, a light beam separation assembly including a first prism, a focusing lens and a recording medium, a control assembly electrically connected to the light source, a light beam deflection assembly and drive movement of the recording medium.

The disadvantages of this device are low light transmission and, consequently, large light losses due to the low diffraction efficiency of the two-axis acousto-optic deflector (usually 30-50%), high cost and design complexity associated with the high cost of acousto-optic deflectors and their electronic drives, large dimensions and complexity and high power consumption, large distortions of the formed diffraction pattern associated with the large optical thickness of the acoustic duct acoustoopt deflectors. Acoustic-optic deviation devices are difficult to use in the ultraviolet range. Also in the known device, the light beam separation unit has a complex structure consisting of four separate prisms and low light transmission due to the use of two beam-splitting elements. In addition, a significant disadvantage of this device is the limited radiation resistance of acousto-optic modulators, which prevents the use of this device with high-power pulsed lasers operating in the ultraviolet (UV) range.

The authors were tasked to develop a device that has minimal loss of light energy, including in the UV range, high radiation resistance of all components, simplicity and compactness of the optical system, high speed recording of diffraction elements, low cost and compact design with the ability to use for direct recording ablation or thermostructural and thermochemical changes of the recording medium using high-power pulsed lasers generating light from teaching in the UV and deep UV ranges.

The task is solved by the fact that in a device for recording diffractive elements containing an optically coupled light source, a light beam deflection assembly, a first lens including a first prism, a light beam separation node, a focusing lens, a recording medium, a recording medium movement drive and a control node electrically connected with the light source, the light beam deflection node, the driving of the recording medium movement, the light beam deflection node is designed as installed in series along the optical axis and the second lens and two mirror deflectors, and the distance 1 between the plane of the mirror of either of the two deflectors and the focus point of the light by the first lens is selected from the relation

1 <VGSH · where the coefficient B = 0.1-0.5, T is the period of the grating of the diffractive element in the recording medium, 1 is the focal length of the second lens, 1 ₂ is the focal length of the first lens, 1 ₃ is the focal length of the focusing lens, f - beam diameter at the entrance of the second lens. The mirror of at least one of the deflectors is arranged to perform a rotation angle greater than half the angle aper.

- 1 007874 second lens tours. In addition, the mirrors of the deflectors are made with the possibility of rotation in orthogonal planes. The node separating the light beam is made additionally containing a Porro prism. The prism is installed in series along the optical axis with the first prism. Moreover, the first prism contains a beam-splitting coating on the diagonal face. It is glued to the diagonal facet of a Porro prism. In addition, the node is made with the possibility of rotation relative to the optical axis.

The technical effect is that the claimed device provides high-speed recording of diffractive elements (holograms) using a high-power pulsed laser, and an ultraviolet laser with a wavelength of 250-360 nm can be used, which makes it possible to implement a direct laser recording method - to directly form the holograms on the surface of the workpiece.

The inventive device is illustrated by the following graphics.

FIG. 1 shows a functional diagram of a device for recording diffractive elements.

FIG. 2 shows variants of the position of light beams in the input plane of the focusing lens and the corresponding interference gratings in the plane of the recording medium.

FIG. 3 shows the position of the interference grating formed by two interfering beams of light in the plane of the recording medium.

The inventive device consists of a light source 1, a second lens 2, two mirror deflectors 3 and 4, a first lens 5, a light beam separation unit 6 including a Porro 7 prism and a first prism 8 containing a beam-splitting coating 9 on a diagonal face glued with Porro 7 prism, focusing lens 10, recording medium 11 with displacement drive 12 and control unit 13.

The device works as follows. A beam of light from the light source 1 (for example, a pulsed laser) with a wavelength λ and a diameter of F _{1 is} fed to the input of the light beam deflection node, which consists of a second lens 2 and two mirror deflectors 3 and 4. The output of the light beam deflection node is set to first lens 5. The distance between lenses 2 and 5 along the optical axis is chosen so that the focal plane of lens 5 coincides with the point δ of focusing lens 2 of the light beam. The light beam A at the exit of the lens 5 is collimated. The deflector mirror 3 is set before the δ point at a distance of 1 ₁ , and the deflector mirror 4 - at a distance of 1 ₂ after the δ point. Under the action of a control electrical signal, and _{at the} mirror of the deflector 3, it rotates by an angle <p _y and deflects the light beam along the axis of the op-amp. Under the action of a control electrical signal and _{x, the} mirror of the deflector 4 rotates through an angle <р _y and deflects the light beam by a corresponding angle along the axis OX. Electrical signals are supplied from the control unit 12. Lens 5 converts the angular deviation of the light beam into its linear displacement along the coordinate axes X and Υ by

Χι = 2φ _{χ 2} and ν2φ, Ρ (1) where £ ₂ is the focal length of the lens 5, £ ₂ >>1; 1 ₂ , ф _{х х} , ф _у << 1.

The light beam And is fed to the input of the node 7 of the separation of the beam of light, made with one input and two exit pupils. The node separating the light beam is made in the form of a Porro 7 prism and the first prism 8 installed in series along the optical axis, the prism 8 being made containing the face with the beam-splitting coating 9 and glued to the diagonal face of the Porro prism 7. Porma Prism is known in the literature (Reference Designer Optical Mechanical devices. / / Edited by E. Panov, Mashinostroenie, L., 1985). Light beam A is divided into two beams on face 9 approximately in half. The first, straight beam (A ') passes this prism 8 without changing the direction. The second beam (A) experiences four reflections (two along the OX axis and two along the axis) from the faces of the Porro 7 prism and leaves mirror-oriented with respect to the first beam. The optical axes of the direct and mirror output beams are collinear with the original optical axis. To ensure the collinearity of the beams, the light beam separation unit, consisting of prisms 7 and 8, is rotatable about the optical axis. When rotating and tilting a node, beam A 'does not change direction, while beam A deviates according to the rotation and tilt of the entire node. Next, the beams A 'and A arrive at the focusing lens 10 and are focused in the plane of the recording medium 11 at a distance δ ₂ from the optical axis equal to δ _; ± 2 <p1M '_; χ · 1ί '_; / Ρ ^; (2) where Ф ~ 'У ^{1 +} Фу, £ ₃ is the focal length of the lens 10, 1 = 1 = 1 ^ The + and - signs indicate that the centers of the δ' and δ regions of the focusing of the beams A 'and A are shifted in different directions. If distance 1 is small, then the offset δ2 will also be small. As a result of the interference of two focused light beams in the plane of the recording medium, a sinusoidal interference grid is formed. This interference grating is sequentially fixed in time by the recording medium 11, which changes its properties under the action of light. The recording medium 11 is moved by the actuator 12 relative to the focusing region δ 'by commands from the control unit 13. When the recording medium moves relative to the objective 10, a series of grids is formed (recorded) on the surface of the latter. To eliminate the blurring of the interference grating laser pulse

- 007874 have a duration τ equal to τ = Τ / ν (3) where Τ is the period of the interference grating, ν is the speed of movement of the recording medium, c = 0.1-0.01 is a constant coefficient. At T = 1 μm and ν = 0.1 m / s, the pulse duration is τ ~ 0.01 μs. The pulse repetition rate G _lase is determined by the distance b between the individual gratings on the recording medium:

Γ _3Η .. = ν / Γ (4)

For example, at b = 20 μm, the pulse repetition rate of the laser is G _lase = 5 kHz, and in 1 second the proposed device allows you to record 5,000 interference gratings (or holograms). The most suitable light source is a solid-state pulsed Νά: ΥΑΟ Q-switched laser and tripling (or quadrupling) frequencies (I. Kondilenko, P. Korotkov, A. I. Khizhnyak, Laser Physics, Vishcha School-84g., A A. Ioltukhovsky, II Kuratev, Y. Tsvetkov, Solid-state lasers pumped by laser diodes for laboratory and industrial applications, Laser-Inform No. 1-2, January 2000). The light radiation of such a laser is a sequence of short (5-10 ns) and high-power light pulses with a repetition rate from hundreds of hertz to 10-50 kHz and a wavelength of 355 or 268 nm. When recording a diffractive element, the control unit 13 synchronizes the pulse switching on of the laser 1 with the movement of the recording medium 11 and controls the angle of deflection of the mirrors of the deflectors 3 and 4 in accordance with a given program.

The operation of switching on / off the laser light beam when recording gratings is performed by rotating the mirror at least one of the deflectors at an angle of more than half the angular aperture of the lens 5 or the focusing lens 10. The light beam is blocked by the aperture of the focusing lens and does not pass to the recording medium. This operation is necessary if you need to skip several arrays when recording. The switching frequency of high-speed mirror deflectors with a mirror size of 2x2 mm ² is about 5 kHz.

In the proposed device, each interference grating can have a specified orientation and period. As an example in FIG. 2 a, b, c show different positions of the light beams A 'and A in the input plane of the focusing lens 10 and the corresponding interference gratings in the plane of the recording medium (Fig. 2 d, e, f). In the initial state (and _x = and _y = 0), the beam of light A 'passes through the intersection line of the second and third quadrants at a distance of y! from the optical axis (Fig. 2a). The mirror beam A in the initial state lies at the intersection of the I and IV quadrants at a distance of x ₁ from the optical axis. The interference pattern has the form of a lattice, as shown in FIG. 2g. When applied to deflectors 3 and 4 of nonzero control signals and _x and _y, beams A 'and A are moved, respectively, in II and III and I and IV quadrants, as shown in FIG. 2 b, c. In this case, either the period of the interference grating (Fig. 2e) is reduced, or the angle of inclination 1d (0) = <p,. / <P _y (Fig. 2e) changes.

The main condition for the correct operation of the claimed device is the exact combination of focused beams A 'and A in the plane of the recording medium, since the interference grating is formed only at the point of their intersection. When either of the two mirrors is tilted, the light beam at the output of the lens 5, moving along one of the coordinates, acquires a slight tilt, which leads to an error of position (see expression (2)) of the interference grating in the plane of the recording medium 11, as shown in FIG. 3. The focused beams do not overlap completely, and the interference grid arises only at the point of their intersection. The diameter of the light spot (Airy mugs) in the plane of the recording medium is defined as

Α «2.44λΓ ₃ / ά2 (5) where b ₂ = b | Γ _; / Y | - diameter of the light beam A 'or A at the input of the focusing lens 10, G ₁ - focal length of the lens 2 (GS Landsberg. Optics, M, Nauka, 1976).

The ratio of the magnitude of the displacement δ = 2δ ₂ light spot to its diameter

Ε = δ / Α = 1Χ161 / (1,22λΓ1Γ2) (6) is a relative error and characterizes the influence of the inclination of the mirror on the relative position of the spot.

From the expression (6) it follows that in order to reduce the error E it is necessary to reduce the distance 1 and increase the focal lengths £ and T ₂ . The value of 1 is determined by the dimensions of the deflector mirrors and the distance between them and cannot be chosen arbitrarily. For a given value of the error E from expression (6), it follows that the distance between one of the mirrors and point 8 should not exceed 1 <1.22ΕλΓ1Γ2 / Χ1ά1 (7)

If the permissible value of the relative displacement lies within пределах = 0.04-0.2, then at λ = 0.35 μm, Γι = 150 mm, G ₂ = 100 mm, x ₁ = 1 mm, b ₁ = 1 mm distance 1 should not exceed 0.32-1.3 mm, which is constructively feasible.

Given the fact that the period T of the interference grating (Fig. 2d) in the recording spot is approximately equal to T ^ T _{3/2 s} expression (7) for the correct choice of the distance 1 can finally be written as

1 <VTG1G2 / G ₃ 61 (8)

- 3 007874 where B = 2.44E = 0.1-0.5.

Thus, the proposed device provides high-speed recording of diffraction elements (holograms) using a pulsed laser. A distinctive feature of the proposed device is the simplicity of the optical scheme and the absence of expensive and inefficient acousto-optical deflectors. Since there are no restrictions on light transmission, an ultraviolet laser with a wavelength shorter than 360 nm can be used as a radiation source. The proposed device is distinguished by very small losses of light energy in the optical path, which makes it possible to use a powerful source of pulsed radiation and to implement the direct laser recording method to form the relief structure of holograms directly on the surface of the workpiece. Since the mirror deflectors are small in size, weight and low consumption of electrical energy, the proposed device can be made small-sized, in the form of a compact optical head. Deflectors can be made using micromechanics technology in film design.

The inventive device was tested experimentally. A pulsed laser with tripling of the frequency (O-5) 11 of the type LSC Pt-374SV with an average power of 20 mW, a wavelength of 354 nm, and a pulse duration of about 5 ns was used as a radiation source. High-speed mirror deflectors were used such as the 6220M (SatbtbdeTes11po1odu. 1ps.) With a maximum frequency of up to 5 kHz. The focal lengths of the lenses were selected in accordance with the parameters given earlier in the text. Thin films of chromium and silicon deposited on glass substrates by magnetron sputtering were used as a recording medium. Were recorded images containing diffraction elements (microholograms) with a diameter of about 10 microns and a grating period from 0.8 to 1.4 microns and with an angular orientation in the range θ = 0-90 °. Recording was carried out by direct laser recording (evaporation and thermochemistry). The recording speed ranged from 500 to 4000 micro-holograms per second.

Claims (5)

CLAIM 1. A device for recording diffraction elements containing an optically coupled light source, a light beam deflection unit, a first lens including a first prism, a light beam separation unit, a focusing lens, a recording medium, a recording medium moving drive, and a control unit electrically connected to a light source, a node deflecting the light beam, a drive moving the recording medium, characterized in that the node deflecting the light beam is made in the form of sequentially installed along the optical axis in one lens and two mirror deflectors, the distance 1 between the mirror plane of either of the two deflectors and the light focusing point of the first lens is selected from the relation 1'ВТ1'М3с1 · where the coefficient В = 0.1-0.5, Т is the grating period of the diffraction element in the recording medium, ί the focal length of the second lens, ί ₂ - the focal length of the first lens, ί ₃ - the focal length of the focusing lens, f is the diameter of the beam at the entrance of the second lens. 2. The device according to claim 1, characterized in that the mirror of at least one of the deflectors is configured to perform an angle of rotation greater than half the angular aperture of the second lens. 3. The device according to claim 1, characterized in that the deflector mirrors are rotatable in orthogonal planes. 4. The device according to claim 1, characterized in that the unit for separating the light beam is further comprising a Porro prism mounted in series along the optical axis with the first prism, the first prism being made comprising a beam splitting coating on a diagonal face and glued to the diagonal face of the Porro prism. 5. The device according to claims 1 and 4, characterized in that the light beam splitting unit is rotatable relative to the optical axis.