JPS6131465B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for creating and reproducing a hologram, and more particularly to a method for creating and reproducing a hologram that prevents a decrease in the signal strength of reproduced information due to variations in the film thickness of the hologram. A spherical object wave and a reference plane wave are irradiated onto the peripheral edge of the rotating hologram disk to form interference fringes on the hologram disk, and when reproducing information, a reproduction plane wave is irradiated onto the hologram to reproduce the spherical wave. Conventionally, information is reproduced using

FIG. 1 is an explanatory diagram of the above-mentioned hologram creation and reproduction method, in which 1 denotes a hologram disk 2 a rotation axis of the hologram disk, 3
is a laser light source, 4a and 4b are collimator lenses,
5 is a focusing lens, and B ₁ , B ₂ , and B ₃ are axes having a direction perpendicular to the wavefront of the spherical wave that is focused to a point S by the focusing lens 5 and diverges from this point S again. 6
1 is a laser light source, and 7a and 7b are collimator lenses, which enlarge the diameter of the laser spot from the laser light source 6 and create laser beams whose optical axes are parallel to each other. P ₁ , P ₂ , and P ₃ indicate the traveling direction of the plane wave and are optical axes having a direction perpendicular to the wave front. In this way, when the hologram disk is irradiated with the object spherical wave and the reference plane wave, interference fringes are generated. Figure 2 shows
2 is a diagram showing the appearance of interference fringes formed on a hologram, and parts equivalent to those in FIG. 1 are given the same reference numerals. _K1 to Kn are interference fringes formed by irradiation with a spherical object wave and a reference plane wave.As is well known, when the same part of a hologram is irradiated with light from two directions, the optical axis of these irradiated lights is Interference fringes are formed having an inclination angle in a direction that bisects the intersection angle. In Fig. 2, the intersecting angle between the optical axis B ₁ of the spherical wave and the optical axis P ₁ of the reference plane wave is α, so the interference fringe K ₁ is
It has an angle of inclination that divides it into equal parts. Also, the optical axis B ₃ of the spherical object wave
Since the intersecting angle between the optical axis _P3 and the reference plane wave is β, the interference fringe Kn has an inclination angle that equally divides this intersecting angle β into two. Other interference fringes formed on the hologram disk
The same is true for K ₂ and K ₃ , and as is clear from FIG. 2, they have respective inclinations with respect to the normal N to the hologram disk surface. Therefore, when reproducing information, when a plane wave P for reproduction is irradiated onto the hologram disk 1 shown in FIG. . In this way, information is reproduced. When the intensity of the reproduction plane wave irradiated onto the hologram is constant, the intensity of the convergent spherical wave increases as the thickness d of the hologram increases, and the so-called diffraction efficiency improves. However, as the thickness of the hologram increases,
The permissible range of incident angles of reproduced light that satisfies Bragg's diffraction conditions becomes narrower. Figure 3 shows 1 for the object spherical wave.
It shows the angular dependence of the intensity of the second-order diffracted light, where the vertical axis shows the intensity of the first-order diffracted light on an arbitrary scale, and the horizontal axis shows the angular deviation from the optical axis of the incident light that satisfies Bragg's diffraction conditions. In the figure, the solid line indicates the case where the hologram film thickness is thick, and the chain line indicates the case where the hologram film thickness is thin. As the hologram thickness increases, the angle dependence of the intensity of the first-order diffracted light becomes steeper.
In other words, if there is no angular deviation, that is, if the reproduction plane wave is irradiated perpendicularly to the hologram disk 1 in Figures 1 and 2, the intensity of the first-order diffracted light (convergent spherical wave) will be I.
However, if the optical axis of the reproduced plane wave has a tendency with respect to the normal line of the hologram disk and an angular deviation occurs, the intensity of the convergent spherical wave decreases, and when the angular deviation becomes large, it rapidly decreases to zero level. I can see it getting closer. Incidentally, the thickness of the hologram changes due to temperature changes and light irradiation, and the inclination angle of the interference fringes changes with this thickness change. The dashed line in Figure 2 shows the interference fringes K' ₁ to K'n when the hologram thickness reaches d', and before the thickness changes,
In other words, the inclination angle changes by ε ₁ , ε ₂ , . . . εn compared to the interference fringes when the hologram film thickness is d. Therefore, even if the reproduction light is irradiated onto the hologram under the same conditions, that is, with the same intensity and at the same angle of incidence, the inclination of the interference fringes will change as the hologram thickness changes, so the intensity of the convergent spherical reproduction light will be What is desired is not obtained and the reproduced information is unclear, thus resulting in poor signal-to-noise ratio and poor quality of the reproduced information.

The present invention has been made in view of the above, and provides an information recording and reproducing method that can obtain high-quality reproduced information even when the thickness of a hologram changes. In an information recording and reproducing method in which information is recorded by irradiating a hologram onto a disk to form a hologram, and information is reproduced by irradiating a reference wave to the hologram forming area, the former object wave is a spherical wave. , the normal to the hologram disk surface in the area where the hologram is to be formed.
With respect to N ₁ , we create a spherical wave whose optical axis passes through a spatial point S'' that is axially symmetrical with the center point S of the previous spherical object wave and reaches the hologram formation area, and whose center point S' is on the hologram disk rotation axis. irradiate as a reference wave to form hologram interference fringes in the normal direction of the disk surface, and irradiate a reproduction spherical wave having a convergence point at the center point S' of the reference spherical wave on the rotation axis of the disk. Fig. 4 is a specific configuration diagram of an embodiment of the present invention, and the same parts as in Fig. 1 are given the same reference numerals. In the figure, 8 is a laser light source, 9a,
9b is a collimator lens for enlarging the spot diameter of the laser beam of the laser light source 8 and aligning the optical axes of the laser beams parallel to each other; 10 is a focusing lens. After the laser beam from the focusing lens 10 is focused at a point S' on the center line of the rotation axis, it diverges from the point S' as a center point to become a spherical wave, which is used as a reference spherical wave. _B4 , _B5 , _B6 ,
is the optical axis of the spherical wave from the point S'. The reference spherical wave and the spherical object wave are provided so as to have substantially the same incident angle with respect to the hologram formation area. B ₅ is the optical axis of the reference spherical wave, and is a point at the intersection of the optical axis B ₅ and the surface of the hologram disk 1, which is axially symmetrical to the center point S of the spherical object wave with respect to the normal N ₁ to the hologram disk surface.
Since the optical axis B ₅ passes through S'', the optical axis B ₅ and the optical axis B ₂ of the object spherical wave have the same inclination angle with respect to the normal to the hologram disk surface. Since the center points S and S' of the object spherical wave and the reference spherical wave are located away from the hologram disk surface, the optical axis of the spherical object wave is B ₁
The optical axis B ₄ of the reference plane wave and the optical axis B 4 of the hologram disk have almost the same inclination with respect to the normal to the hologram disk, and similarly the optical axis B ₃ of the spherical object wave and the optical axis B ₆ of the reference spherical wave are also inclined to the hologram disk. has almost the same inclination angle with respect to the normal. Therefore, the interference fringes formed on the hologram by the irradiation of the object spherical wave and the reference spherical wave are parallel to the thickness direction of the hologram, that is, substantially coincide with the direction of the normal to the hologram disk surface. In the case of reproduction, the hologram disk 1 shown in FIG. 4 is irradiated with a reproduction spherical wave from the diagonally lower right side of the drawing. FIG. 5 shows a sectional view of the main part of the hologram. Since the optical axis B ₂ of the object spherical wave and the optical axis B ₅ of the reference spherical wave have the same inclination angle θ ₁ with respect to the normal N ₁ of the hologram disk, the interference fringes K ₁ ″ are formed on these optical axes B ₂ , B This corresponds to the bisecting direction of the angle formed by ₅ , that is, the direction of the normal line N _1. Also, the optical axis of the object spherical wave B ₁ and the optical axis of the reference spherical wave B ₄ are relative to the normal line N ₂ of the hologram disk. Almost equal inclination angles θ ₂ and θ′ ₂
Therefore, the interference fringe K″ ₂ has _an inclination angle in the direction that bisects the angle formed by these optical axes B ₁ and B ₄ .
Since the inclination angles of B ₄ with respect to the normal N ₂ are almost equal, the inclination angle of the interference fringe K ₂ ″ with respect to the normal N ₂ is ε ₁ ′, and the direction of the interference fringe K ₂ ″ almost coincides with the normal direction. . Similarly, since the optical axis B ₃ of the object spherical wave and the optical axis B ₆ of the reference spherical wave have almost equal inclination angles θ ₃ and θ ₃ ' with respect to the normal N ₃ of the hologram disk, interference fringes occur.
K ₃ ″ is the normal to the inclination angle that bisects the angle formed by these optical axes.
N ₃ , but the normal N ₃ of these optical axes to B ₃ and B ₆
Since the angles formed with respect to the normal line N 3 are almost equal, the angle of inclination of the interference fringe K ₃ ′ with respect to the normal line N ₃ is ε ₂ ′, and the direction of the interference fringe K ₃ ″ almost coincides with the direction of the normal line N ₃ . By irradiating the hologram so that the spherical object wave and the reference spherical wave have approximately the same inclination angle with respect to the normal to the hologram disk, the direction of the interference fringes formed on the hologram is set to the normal to the hologram disk surface. The direction can be roughly matched.
Therefore, even if the hologram thickness changes due to temperature changes or light irradiation, the change in the inclination angle of the interference fringes will be small.
FIG. 6 is a diagram showing the variation in the inclination angle due to the change in the hologram thickness of the hologram interference fringes according to this embodiment. When the thickness decreases to d′ as shown by the broken line, the interference fringes become as shown by the dashed line, but as is clear from the figure, the variation in the inclination angle of the interference fringes is △ε
₁ , Δε ₂ , which is very small. For this reason, the spherical wave for reproduction is almost satisfied, and there is no significant decrease in the diffracted light intensity due to changes in hologram thickness as in the conventional case, so the signal strength of the reproduced information is high and the quality is low with less influence of noise etc. You can obtain playback information.

[Brief explanation of the drawing]

1 and 2 are diagrams for explaining the conventional information recording method and information reproducing method, FIG. 3 is a diagram for explaining hologram diffraction conditions, and FIG. 4 is a configuration diagram of an embodiment of the present invention. , FIG. 5, and FIG. 6 are diagrams for explaining the appearance of hologram interference fringes. 1; Hologram disk, 2; Rotation shaft, 3; Laser light source, 4a, 4b; Collimator lens, 5;
Focusing lens, 6; Laser light source, 7a, 7b; Collimator lens, _K1 ~Kn, _K'1 ~ _K'3 , _K''1 ~
K″ ₃ ; interference fringes.

Claims (1)

[Claims] 1. An information recording and reproducing method in which an object wave and a reference wave from an information carrier are irradiated onto a disk to form a hologram to record information, and a reference wave is irradiated to a hologram forming area to reproduce information. In , the first object wave is a spherical wave, and the normal N ₁ of the hologram disk surface in the hologram formation area is
The reference wave is a spherical wave that has an optical axis that passes through a spatial point S'' that is axially symmetrical to the center point S of the spherical object wave and reaches the hologram formation area, and that has a center point S' on the hologram disk rotation axis. to form hologram interference fringes in the normal direction of the disk surface, and irradiate a reproduction spherical wave having a convergence point at the center point S' of the reference spherical wave on the rotation axis of the first disk. An information recording and reproducing method characterized in that information is reproduced.