SU1124244A1 - Method and device for recording transmission hologram - Google Patents

Abstract

1. A method for recording a transmission hologram — I consisting in exposing a photographic material to an interference pattern formed by mixing two beams of coherent light. It is distinguished by the fact that, in order to improve the recording quality, the wavefront of one of the interfering beams is formed after it passes through: a photo material using a mirror reflector, and the double optical path length from the substrate with the photo layer to the mirror reflector is greater than the coherence length of the radiation using: a regional laser . V. . 2. A transmissive hologram recording device, including a laser, optical systems for the formation of interfering beams and photographic material, characterized in that (O photo material is installed between optical systems for the formation of interfering beams, in the focal plane of one of which a reflecting surface is inserted, having an opening in the center beam.: 4 e 4ik 4

Description

The exemption refers to devices for focusing a laser beam and can be used to construct holographic lenses. There is a known method of recording holographic lenses, which includes the formation of spherical beams and the recording of the pattern of their interference in convergent beams. It is also known a device for implementing the method, consisting of a coherent radiation source and channels for the formation of interfering beams tl. A disadvantage of the known method and device for its implementation is the necessity of introducing spedial optical elements to ensure convergence of interfering beams, which causes the appearance of the resulting hollowness aberrations. These claims of the bride have to be corrected in one way or another. With other methods of converging beams, shading occurs on the surface of the hologram, as a result of which part of the surface is not used to focus the beam, which negatively affects the size of the spot being formed. The closest in technical essence to the present invention is a method of recording a transmissive hologram, which consists in exposing the photographic material to an interference pattern formed by mixing two beams of coherent light. Devices for implementing such a method include optical systems for forming interfering beams and photographic material 21. For information (object and reference) beams in the plane of the photographic material, an inclined light separator plate is used. Such a solution is practically unsuitable for recording hologo lenses with a relative aperture of more than 1: 5 and higher due to a significant distortion of the front of one of the beams, and, therefore, it is practically impossible to obtain a focal spot of small dimensions. The aim of the invention is to improve the recording quality of holographic lenses. The goal is achieved by the fact that according to the method of recording transmission holograms, consisting in exposing a photographic material to an interference pattern formed by mixing two beams of coherent light, the wavefront of one of the interfering beams is formed after it passes through the photographic material with a reflector, and the double optical path length from the substrate to the reflector is longer than the coherence length of the radiation. In a device for carrying out a method comprising a laser, optical systems for forming interfering beams and photographic material, the latter is installed between optical systems for forming interfering beams, in the focal plane of one of which a reflecting surface is inserted having a hole in the center of the beam. FIG. Figure 1 shows the recording scheme of a transmissive holo lens that converts a diverging beam into a converging one; in fig. 2 shows a part of a circuit for recording a holo-lens that converts a diverging beam into a parallel one (the circuits in Figs. 1 and 2 are intended for recording holo-lenses with rear trimming, comparable to or greater than the coherence length of the laser beam); in fig. 3 is a diagram of recording a holo-lens that converts a diverging beam into a convergent when the focal length of the holo-lens is small, i.e. several times smaller than for the circuits in FIG. 2 and 1. A device for recording a transmissive hologram contains a laser 1, a beam splitter 2, dividing the light beam into parallel light beams 3 and 4, flat mirrors 5-9, micro-lenses 10 and 11, spherical mirror 12, substrate with photo layer 13, hole 14 in the mirror, reference 15 and object 16 beams (Fig. 1); a flat mirror 17 with a hole 14 and a telescopic system 18 for expanding the object beam (Fig. 2); semi-transparent mirror 19 (fig. 3). A device for recording a transmissive hologram works as follows. The beam of light from the laser is separated by a beam splitter into beams 3 and 4, falling into micro lenses 11 and 10, respectively. From microobjective 10, the beam passes through an orifice with a diameter of a few micrometers in the substrate of mirror 12 and falls onto the substrate with photo layer 13, forming a reference diverging beam. The expendable beam of microobjective 11 passes. through the substrate, and is reflected by the mirror 12. After reflection from the mirror 12, an object beam arises. If the coherence length of the radiation were infinitely large, then two interference patterns would be recorded on photo layer 13; the first is from colliding beams emanating from lenses 11 and 10, and the second from a beam from micro-lens 10 and a beam reflected from a mirror (in accompanying beams). The first interference pattern is undesirable and reduces the diffraction efficiency of the hologram of the second pattern formed by the second interference pattern: In order to prevent the formation of the first interference pattern, the optical path of the ABECD + DG is chosen equal to the optical path AEZHDGG. laser coherence length. Under these conditions, the first interference pattern is not recorded on the photo layer, and the beam effect from the micro-lens is 11 s. odits only in the uniform illumination of the photographic layer 13, thus on the surface of photosensitive layer consisting um luminous flux from d.c., from the possible conditional mikroobek tive flare 10 and a variable component along the plane caused by the second interference pattern. To record holograms with a noticeable constant component of the exposure light flux, photographic material with a sufficiently large dynamic range is necessary. This is the photographic material that is chromic gelatin. Usually, in the manufacture of a layer of chrome-plated gelatin, the pre-tanning of photollo is performed. It is performed either with the help of tanning agents injected into the photo layer, or with the help of exposure to actinic (in the blue region of the spectrum) radiation. If this operation is absent, the hologram after the appearance has a noticeable light scattering, or noise, i.e., is unsuitable for use as a holographic optical element. In the proposed method, this operation can be eliminated and the said preliminary tanning of the photo chromed gelatin is carried out directly in the process of exposure due to the presence of a constant component in the exposure beam. As can be seen from the I2 scheme of recording a holo-lens on a substrate with photolayer 13, the reference and object beams in their propagation path do not have obstacles and, therefore, beams with a large divergence can be used to record the transmissive hologram, which is necessary to obtain a tolo-lens with a small spot size. Fig. 2 shows a recording scheme of a holo-lens, which converts a diverging beam of light (writing) from micro-lens 10 and a parallel beam. To form a parallel beam, a telescopic system 18 is used instead of micro-lens -11. A parallel beam of light passes through Photo Layer 13 and then reflects from a flat mirror 17. To form a parallel object beam. The optical stroke length is chosen similarly to the above, I Schemes in Figs 1 and 2 are suitable for obtaining a holo-lens with large linear dimensions, up to tens of centimeters, provided that a sufficiently powerful laser is used. To focus a laser beam in optical, video player systems, lenses with smaller linear dimensions and a small (up to a few millimeters) focal length are required. In the diagrams of Figs 1 and 2, the DG segment must be at least two times smaller than the laser coherence length , and this value ranges from tens (for a helium-neon laser) to units of centimeters (for argon). This difficulty can be circumvented using the circuit in FIG. 3, Here the beam from the micro-lens 10 passes through the hole in the mirror 12, falls on the surface of the translucent mirror and reflects onto the surface of the mirror 12. The beam reflected from the mirror 12 forms an object beam. The part of the beam that passes through the mirror 19 from the micro-lens 10 interferes with the beam 12, forming an interference pattern and, therefore, a hologram; 511 on the photo layer 13, Here the optical path length / is 2 DG, should be as much as two times shorter than the coherence length of the laser beam used. As seen in FIG. 1, the focus of the micro-lens 11 coincides with the double focal length: the distance (center of curvature) of the mirror 12. This condition is strict if there are no high demands on the quality of the wavefront of the object beam. In the considered schemes, an important characteristic is the radiation coherence length of the laser used. This characteristic depends on both the laser working medium and the parameters of the resonator. The coherence length of each laser is usually established experimentally using an interferometer. By changing the optical length of the stroke in one of its branches, it is established at what difference of the stroke the visibility of the interference pattern drops to zero. Other methods are known for determining the coherence length of laser radiation. Note that for known industrial lasers, the coherence length in the single-mode mode is in the range of 1-30 cm. Thus, the formation of the object and reference beams is achieved using the property of a limited coherence length of the laser beam. If a high diffraction efficiency is required for a holo lens, then a photo layer with a sufficiently large dynamical range, for example, chroming of gelatin, should be used as a photo material. Experimentally selecting the intensity ratio of the object and reference beams, one achieves such a degree of precautionary tanning of the layer of chrome gelatin that the noise of the layer can be considered insignificant.

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Claims (2)

METHOD FOR RECORDING A TRANSMISSION VOICE - - GRAMS AND DEVICE FOR ITS IMPLEMENTATION. . (57) 1. The method of "recording transmissive— 'Holograms, which consists in: exponing.poBiaHHH photographic material with an interference pattern formed by mixing two beams of coherent light (characterized in that, [in order to improve the quality of recording, the formation of a wavefront one of the interfering beams is produced after it passes through; the photographic material is using a specular reflector, and the double optical path length from the substrate with the photo layer to the specular reflector is greater than the coherence length of the radiation of the laser used. 2. A transmission device for transmitting a hologram, including a laser, optical systems for the formation of interfering beams and photographic material ®, characterized in that the photographic material is installed between optical systems for forming interfering beams, in the focal plane of one of which a reflective surface is introduced, having a hole in the center to enter the beam.