1. Field of the Invention
The present invention relates to a tilt detection apparatus, a hologram apparatus, a tilt correction method for a medium, and a tilt correction method for a hologram medium.
2. Description of the Related Art
Among hologram media adapted to record digital data as holograms is a photosensitive resin (e.g., photopolymer) sealed between glass substrates.
To record digital data on a hologram medium as a hologram, a laser beam from a laser device is first split into two beams by a PBS (Polarization Beam Splitter). Then, one of the two beams (hereinafter referred to as reference beam) and a laser beam (hereinafter referred to as data beam) produced by the other beam irradiating an SLM (Spatial Light Modulator) having digital data in the form of a two-dimensional contrast image pattern, which beam reflects information of the two-dimensional contrast image pattern, are irradiated onto a hologram medium at a given angle, and thereby the digital data is recorded onto the hologram medium.
More specifically, the photosensitive resin making up the hologram medium has a finite number of monomers. When the laser beam (hereinafter referred to as laser beam) made up of the reference and data beams is irradiated thereonto, the monomers change into polymers correspondingly with the energy determined by the light intensity of the laser beam and the irradiation time. As a result of the transformation of the monomers into polymers, interference fringes, made up of polymers, are formed correspondingly with the laser beam energy. Therefore, as a result of the formation of such interference fringes in the hologram medium, digital data is recorded as a hologram. Later, remaining monomers migrate (diffuse) to those locations that have had monomers consumed. Further, by irradiating the laser beams again, changes of such monomers into polymers are iterated. FIG. 2 schematically illustrates how monomers transform into polymers in response to the laser beam energy in the hologram medium.
When there is a large amount of digital data to be recorded in the hologram medium, so-called angle-multiplexed recording may be performed which forms a number of holograms by varying the incidence angle of the reference beam onto the hologram medium. For example, a hologram formed on the hologram medium is called a page, whereas a multiplexed hologram made up of a number of pages is called a book. FIG. 3 schematically illustrates a book and pages in the angle-multiplexed recording. As shown in FIG. 3, for a single book in the angle-multiplexed recording, e.g., ten pages of holograms are formed by varying the incidence angle of the reference beam. Thus, the recording of digital data onto a hologram medium by the angle-multiplexed recording allows for the recording of a large amount of digital data.
Meanwhile, to reproduce digital data from the hologram medium, the reference beam is irradiated onto the interference fringes representing the digital data at the same incidence angle as when the interference fringes were formed, and the reference beam diffracted by the interference fringe (hereinafter referred to as reproduction beam) is received by an image sensor or other means. The reproduction beam received by the image sensor or other means produces a two-dimensional contrast image pattern representing the above-mentioned digital data. Then, the digital data can be reproduced by demodulating this two-dimensional contrast image pattern by a decoder or other means.
As such, when digital data is reproduced from a hologram medium, a two-dimensional contrast image pattern is reproduced from the reproduction beam. Hence, the reproduction beam at an image sensor or other means must have a light intensity equal to or above a given level to allow the reproduction of the two-dimensional contrast image pattern. To have the reproduction beam have at least the given level of light intensity, therefore, the interference fringes diffracting the reference beam must have a specified or higher value of diffraction efficiency, which indicates the ratio of the reproduction beam light intensity to that of the incident reference beam. It is to be noted that the specified value of diffraction efficiency is such a value that the reproduction beam has the given level of light intensity.
FIG. 4 is a diagram showing an incidence angle θ between the reference beam (hereinafter called a recording reference beam) and the data beam when forming the interference fringes in a hologram medium. FIG. 5 is a diagram showing an angle θ between a reference beam (hereinafter called a reproducing reference beam) and the reproduction beam (hereinafter called a reproducing angle θ) when reproducing digital data from the interference fringes of FIG. 4.
When reproducing digital data from the hologram medium, letting Δθ, n, t, and λ be the angle difference between the incidence angle θ and the reproducing angle θ, the refraction index of the hologram medium, the thickness of the hologram medium, and the wavelength of the laser beam emitted from a laser device respectively, Δθ is expressed as (λ×√(n2−sin2θ))/t×sin θ×cos θ. If n=1 and sin θ=1, then Δθ=λ/t, which is found to be no greater than the wavelength of the laser beam. If the laser beam is, e.g., a helium neon laser, its wavelength λ is 633 nm, and Δθ is very small. As a result, it is understood that the reproducing angle θ for reproducing digital data from the hologram medium needs to be within such an angle difference Δθ of the incidence angle θ for when the interference fringes representing the digital data are formed. That is, the reproducing reference beam needs to be incident on the hologram medium at a very accurate reproducing angle θ. And the hologram medium needs to be placed in such a position (hereinafter called a reference position) that the reproducing reference beam is made incident thereon at the very accurate reproducing angle θ.
Meanwhile, it is required for the sake of convenience, flexibility, and the like that the hologram medium is configured to be attachable to and detachable from a hologram apparatus able to record or reproduce holograms. For example, when a hologram medium on which a hologram apparatus recorded digital data as a hologram is required to be played back by another hologram apparatus, if the hologram medium is attachable, the requirement can be easily fulfilled (refer to, e.g., Japanese Patent Application Laid-Open Publication Nos. 2004-177958 and 2004-272268).
However, in the case of an attachable hologram medium, the hologram medium may be tilted with respect to the above reference position depending on the mechanical configuration and the like of a body on which the hologram medium is mounted. The tilt of the hologram medium may cause the angle difference between the incidence angle θ and the reproducing angle θ to be greater than the angle difference Δθ thereby making the reproducing reference beam not be accurately incident and disabling the reproduction of digital data. Or, the reproducing reference beam is not accurately incident on a desired hologram but on another hologram, and false digital data may be reproduced. Also, when recording digital data onto the hologram medium, the digital data may not be recorded as a hologram at a desired position in the hologram medium.
In particular, where the hologram medium is shaped like a disk, since being thin, it is difficult to hold the hologram medium horizontal with respect to the reference position, and the hologram medium may be tilted. Moreover, the disk-shaped hologram medium, in order to make it attachable to a hologram apparatus, may be mechanically processed to have, e.g., a central opening therein. And the hologram apparatus is provided with a support that fits detachably the central opening and with a table to rotate hologram media to form a number of holograms. As a result, due to distortion that occurs in mounting hologram media on the hologram apparatus or centrifugal force exerted on the hologram media when rotating, the hologram media may be tilted with respect to the reference position.