JPS63262614A - Hologram disk - Google Patents

Abstract

PURPOSE:To decrease the fluctuation in the diffraction efficiency between the facets of a hologram disk disposed with plural pieces of the hologram facets on one face of a substrate by adhering a wavelength plate to at least a part of the other face of the substrate. CONSTITUTION:The wavelength plate 15 is integrally formed over the entire rear face of the substrate 5 of the hologram disk. The plate 15 is formed of the hologram 15, i.e., the hologram 15 having a function as the wavelength plate. Namely, the hologram disk 10 is formed with the hologram 3 having the intrinsic functions of deflecting (scanning), imaging, and condensing beams on the front face and the hologram 15 having the function of the wavelength plate on the rear face of the substrate 5. The hologram (wavelength plate) 15 for phase shifting is a quarter wave plate (lambda/4 plate) for converting linearly polarized light to circularly polarized light. The incident linearly polarized light from the rear face side is circularly polarized by forming the lambda/4 plate 15 and the circularly polarized light is eventually entered to the hologram 3. The substantially specified diffraction efficiency by the circularly polarized light is thereby obtd. at all times and the approximately uniform diffraction efficiency is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary] By forming a hologram that functions as a wavelength plate on the back surface of a hologram disk, warping of the substrate is prevented and polarization characteristics specific to the hologram are effectively absorbed.

[Industrial application field]

The present invention relates to a hologram disk in which a plurality of hologram facets are formed on a single substrate, and particularly to a hologram disk in which polarization characteristics inherent to the hologram disk are prevented.

[Conventional technology]

BACKGROUND ART In information input/output devices such as PO3 (Point of 5ales) scanners and laser printers, holograms are utilized to polarize, scan, image, or focus laser light. In such an information input/output device, in order to enable multidirectional scanning of a laser beam, a plurality of holograms with different grating angles or shapes are generally placed on the surface of the same substrate 5, as shown in FIG. 12.13. 3
A disk-shaped hologram disk 1 is used in which holograms are arranged concentrically. That is, the hologram is divided into a plurality of hologram facets 3 on the substrate, and the polarization directions (scanning range) as wide as possible are ensured by rotating the hologram around its axis.

During hologram playback, linearly polarized light 1 is emitted from the back of the hologram.
0 is irradiated and the beam is scanned at a polarization angle corresponding to the hologram 3, so that, for example, a bar code 7 can be read.

[Problem that the invention seeks to solve]

However, conventional hologram disks have the following problems.

Holograms have an inherent property (polarization property) that the diffraction efficiency of the hologram differs depending on the polarization state of the incident light, so as shown in Figure 14, the diffraction efficiency (beam intensity) varies depending on whether the incident linearly polarized light is P-polarized light or S-polarized light. There is a large difference in the efficiency, and it is impossible to achieve uniformity of efficiency. That is, for example, the diffraction efficiency of P-polarized light is 80
%, that of S-polarized light is 40%, or conversely, P-polarized light is 40%.
, 80% S polarization. This is S/N
This results in a decrease in the ratio, which not only places a burden on the signal processing circuit system but also deteriorates information detection accuracy. Therefore, in the hologram disk as described above, if the grating direction varies from facet to facet, the diffraction efficiency differs from facet to facet, resulting in the above-mentioned disadvantages. Such fluctuations in the beam intensity after passing through the hologram disk (non-uniform diffraction efficiency) occur not only when the hologram is reproduced with a normal linearly polarized laser, but also when reproduced with a so-called random polarized laser that has two orthogonal linearly polarized beams. The same thing happens if you do.

Figure 15 shows the beam in a conventional hologram disk.
It shows the amount of variation ΔI in intensity ■ (actual measured value), and the variation rate (ΔI/I) reached as much as 20%. The goal of the present invention is to suppress this fluctuation rate to 5% or less.

Another problem with conventional hologram disks is that the substrate warps.

That is, when a hologram pattern is transferred by the photopolymer method to a photopolymer, which is a hologram recording material formed on a disk substrate 5 (generally glass or acrylic), when the photopolymer is exposed to light, the photopolymer hardens and contracts, and the substrate becomes the 16th layer. As shown in the figure, it is slightly warped. This warping of the substrate shifts the focal position of the scanning beam, significantly reducing information detection (reading) accuracy.

SUMMARY OF THE INVENTION An object of the present invention is to provide a hologram disk that can solve the above-mentioned problems, minimize fluctuations in diffraction efficiency, and prevent warping of the substrate.

[Means for solving problems]

In order to achieve the above object, the present invention provides a hologram disk in which a plurality of hologram facets are arranged on one surface of a substrate, in which a wavelength plate is attached to at least a part of the other surface of the substrate. The characteristics of

[For production]

The wave plate attached to the back side of the substrate (the side opposite to the hologram) not only functions as a reinforcing material to prevent the substrate from warping, but also has polarization properties (beam phase shift), i.e. conversion between P and S polarizations, Alternatively, since the polarization angle can be controlled, variations in diffraction efficiency between facets can be reduced. Depending on the purpose, the wave plate may be attached to the entire back surface of the substrate or only to the back surface portion of a specific facet.

〔Example〕

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 schematically shows an embodiment of a hologram disk 10 according to the present invention. According to this embodiment, the back surface (the surface opposite to the hologram 3) of the substrate 5 (made of acrylic, for example) of the hologram disk. ) A wavelength plate 15 is integrally formed throughout. □In the embodiment shown in FIG.
5 is formed by a hologram 15, that is, a hologram 15 having a function as a wave plate. That is, the hologram disk 10 has a hologram 3 formed on its front surface that has the original beam deflection (scanning), imaging, and focusing functions, and a hologram 15 that functions as a wavelength plate on the back surface of the substrate 5. . The hologram 3.15 may be either a volume hologram or a surface relief type hologram, but since the polarization characteristic unique to the hologram described above is more pronounced in the latter, that is, a surface relief type hologram, a surface relief type hologram (No. 5 Figure) is assumed.

The phase shift hologram (wave plate) 15 of the embodiment shown in FIG.
λ/4 plate). In FIG. 1, the wavelength of the reproducing laser (e.g., random polarization laser) is λR, and the acrylic substrate 5
Let n be the refractive index of 2 and 3)) is required to be shortened. That is, dl<λR/n... (1) Of course, since the wavelength λR of the reproduction beam must be smaller than the grating period d2 of the hologram 3, λR<d'2... (2) It is.

As an example, assuming that a He-Ne laser is used as a randomly polarized laser, its wavelength λR is λR
= 0.633 μm, so di <0.42 μm,
0.633/7m<d2. However, the refractive index of the acrylic substrate was n=1.50.

Generally, in order to construct a wave plate, that is, in order to cause a phase shift, it is sufficient that birefringence Δn exists. Usually, a wave plate is formed by a crystal with optical anisotropy, which is caused by the electrical anisotropy of the molecules that make up the crystal, but birefringence can also be caused by anisotropy on a scale much larger than that of the molecules. It is known. That is, it is an optically isotropic substance, and this corresponds to a case where particles whose size is sufficiently larger than a molecule and sufficiently smaller than the wavelength of light are regularly arranged. This kind of case is called structural birefringence, and it is known that surface relief holograms have the above-mentioned structural birefringence, which is equivalent to birefringence. obtain. The cause of structural birefringence in surface relief holograms is not directly related to the present invention, so a detailed explanation will be omitted.
It is described in detail in "Principles of Optics" co-authored by &E and Wolf, co-translated by Toru Kusayo and Hidetsugu Yokota, Tokai University Press, pp. 1030-1033.

FIG. 4 shows, for reference, the relationship between the birefringence Δn and the shape of the hologram, when the refractive index n' of the medium constituting the hologram is used as a parameter. Since the refractive index of the photopolymer is n'-1.5, the grating period d and the grating width W
Appropriately select the ratio q (-w/d) (for example, q =
0.5), it is possible to certainly obtain the birefringence. As shown in FIG. 4, the birefringence Δn is expressed by the difference between the refractive index Δn in the direction parallel to the lattice axis and the refractive index Δn in the direction orthogonal thereto.

In order to use such a surface relief type hologram as a quarter-wave plate, linearly polarized light may be incident on the hologram at an angle of 45° to the optical axis (FIG. 8(b)).

The amount of phase shift at this time is determined by the grating height (groove depth) h of the hologram. For example, λR=0.633μm
, when the refractive index of the photopolymer is n'' = 1.5, it is approximately 1.
．． A grating height of 6 μm is required (h#1.6 μm).

As described above, by forming the λ/4 plate 15 on the back surface of the hologram disk, the linearly polarized light incident from the back surface side becomes circularly polarized light, and the circularly polarized light enters the hologram 3. Therefore, it is no longer P-polarized light or S-polarized light. Differences in diffraction efficiency due to polarization do not matter, and the diffraction efficiency is always substantially constant due to circularly polarized light.

In addition, since similar holograms 3.15 are formed on both sides of the substrate 5, the warping of the substrate due to curing shrinkage of the photopolymer, which would otherwise occur during transfer of the hologram by the photopolymer method, is canceled out in both directions. , are canceled out. Therefore, warping of the substrate is also effectively prevented.

For the purpose of completely canceling out the warpage of the substrate, it is preferable that the holograms 3.15 formed on both sides of the substrate 5 be made of the same material (for example, photopolymer) and have the same shape; Depending on the function, the shapes may not necessarily be the same. However, even in that case, it is clear that the warping of the substrate is much more suppressed than in the conventional case in which the hologram 15 is not present on the back surface of the substrate.

FIG. 2.3 shows two examples of patterns of the phase shifting hologram 15 used in the present invention. Figure 2 shows a planar lattice in which the hologram grating in the facet has linear stripes parallel to the center of rotation of the disk, and Figure 3 shows a curved lattice in which the hologram lattice has concentric stripes arranged in parallel. show.

In Figure 2, when linearly polarized light is incident on a plane grating, even if a predetermined phase shift is obtained at the center of the facet (
Generally, the beam incidence angle is designed to be 45° with respect to the optical axis at the center of the facet), but as the hologram disk rotates and the beam irradiation point moves to both edges of the facet, the angle will no longer be 45°. Therefore, the amount of shift is insufficient and the deviation from circularly polarized light becomes large. This problem can be solved to some extent by increasing the number of facets 1, that is, by increasing the number of hologram divisions on the hologram disk.

However, even the planar grating shown in FIG. 2 has no disadvantage in achieving the object of the present invention.

In the curved grating shown in FIG. 3, the beam incidence direction does not change at any part within the facet, so the above-mentioned problem as in the flat grating does not occur at all. Therefore, a curved lattice is preferable. Incidentally, in both the case of a plane grating and a curved grating, fluctuations in the intensity (diffraction efficiency) of the scanning beam can be reduced by reducing the degree of polarization of randomly polarized light.

As shown in FIGS. 3(B) and 3(C), according to the present invention, the incident laser is converted into circularly polarized light regardless of whether it is linearly polarized light or random polarized light, so the effect is the same.

Furthermore, in the above embodiments, instead of making the wavelength plate 15 with a hologram, if a normal wavelength plate made of optically anisotropic crystal is attached to the substrate 5, the effect of preventing warpage of the substrate will be lower than in the above embodiments, but It goes without saying that similar objectives can be achieved.

Figure 6.7 shows another embodiment of the invention.

In this embodiment as well, the wavelength plate 25 formed on the back surface of the substrate 5 of the hologram disk is a hologram, but it differs from the first embodiment in that it is formed as a 1/2 wavelength plate (λ/2 plate). When linearly polarized light is incident on the λ/2 plate 25 at an orientation of 45° with respect to the optical axis of the grating (Fig. 8 (B)), it becomes 90".
Since the polarization direction is different from that of P polarization, P polarization becomes S
The S-polarized light is converted into the P-polarized light, and the S-polarized light is converted into the P-polarized light. That is,
The polarization direction can be controlled by the λ/2 plate 25. The orientation of the grating of the hologram 3 formed on the hologram disk is designed according to the scanning direction of the beam (the direction in which diffraction is desired). Here, for example, the hologram 3 is assumed to be a four-part facet having a grating direction as shown in FIG. In this case, if facets 3A and 30 are P-polarized light as shown by the arrows, then facets 3B and 3D are almost S-polarized light.

Therefore, if this hologram has a higher diffraction efficiency for S-polarized light than for P-polarized light, as shown in FIG. 25 by forming facet 3A.

S-polarized light can be incident on 3C. That is, S-polarized light can be incident on all facets 3A to 3D.

Using exactly the same idea, for a hologram that shows high diffraction efficiency for P-polarized light, P-polarized light can be incident on all facets, and in any case, the linearly polarized light with higher diffraction efficiency is used. I can do it.

The second embodiment not only eliminates variations in diffraction efficiency between facets, but also has the advantage that linearly polarized light with higher efficiency can always be used.

Separately, as shown in FIG. 11, a hologram 25 is formed on the entire back surface of the substrate, and the direction of the lattice stripes on the λ/2 plate 25 is gradually changed according to the direction of the lattice stripes on the hologram 3 on the front surface of the substrate. It is also possible to make the diffraction efficiency of the disk as a whole always substantially constant (not necessarily at the maximum value).

In the second embodiment as well, the effect of preventing warpage of the substrate can be expected, as in the case of the first embodiment.

However, if the hologram 25 is partially applied to the back surface of the substrate, the warpage prevention effect will be smaller than if it is applied to the entire back surface.

〔Effect of the invention〕

As described above, according to the present invention, variations in diffraction efficiency between facets in a hologram disk can be minimized to obtain substantially uniform diffraction efficiency for the entire hologram disk, and warpage of the substrate can be effectively prevented. This contributes to improving information detection accuracy.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view of a hologram disk according to the present invention, FIGS. 2(A) and (B) are diagrams showing an example of the pattern of a phase shift hologram used in the present invention, and FIG. (A) and (B) are diagrams showing examples of hologram patterns different from those in Figure 2, Figure 4 is a diagram showing the relationship between birefringence and hologram shape, and Figure 5 is a surface used in the present invention. FIG. 6 is a partially enlarged view of a relief type grating. FIG. 6 is a diagram showing the second embodiment of the present invention. FIGS. The top view and bottom view, FIGS. 8(A) and 8(B) are diagrams showing the state of polarization of the hologram constituting the half-wave plate shown in FIG. 6, and FIG. 9 is the implementation shown in FIG. 6. FIG. 10 is a plan view showing an example of the checkered pattern of the front hologram of the hologram disk in this example; FIG. 10 is a plan view showing an example of the checkered pattern of the hologram on the back side; FIG. 13 is a cross-sectional view of the hologram disk shown in FIG. 12, FIG. 14 is a diagram showing the polarization characteristics of the hologram disk shown in FIG. 13, and FIG. 15 is a diagram schematically showing a conventional hologram disk. 16 is a diagram showing variations in diffraction efficiency of a conventional hologram disk, and FIG. 16 is a diagram showing warpage of a conventional hologram disk. 3... Hologram, 5... Substrate, 10... Hologram disk, 15... Wave plate (hologram). (A) (B) Plane lattice/guturn #! Figure 3: Relationship between birefringence and hologram shape $4@5afr mmnm Seventh factor (-A) (B) Polarization state of λ/2 plate Procedural correction document (method) Showa February 8, 1962 Director General of the Japan Patent Office Kunio Ogawa1, Indication of the case Patent Application No. 096134 of 19882, Name of the invention Hologram disk3, Person making the amendment Relationship to the case Name of the patent applicant ( 522) Fujitsu Ltd. 4, agent address: 8-10-5 Toranomon-chome, Minato-ku, Tokyo 105
Date of amendment order 6, subject of amendment (11 "Brief explanation of drawings" column (2) of specification) Drawing (
(3 times) 7. Contents of the amendment (11 Figure 3 (A) stated on page 15, line 14 of the specification
), (B) is ``rFigure 3 (A), (B), (C
) is corrected to ``. (2) Correct Figure 3 as shown in the attached sheet. 8. List of attached documents

Claims (1)

[Claims] A hologram disk (10) having a plurality of hologram facets arranged on one surface of a substrate (5), characterized in that a wavelength plate (15) is attached to at least a part of the other surface of the substrate.