1. Field of the Invention
The present invention relates to a hologram recording medium where a hologram is recorded by overlapping illumination with a recording beam and a reference beam.
2. Description of the Related Art
An example of the conventional hologram recording medium is disclosed in JP-A-H09-305978. The hologram recording medium described in this reference has a configuration in which a transparent dielectric layer is laminated over the entire one surface of a disk-like substrate and a hologram recording layer is laminated thereon. The hologram recording medium as a whole has a disk-like shape. Emboss pits for the track servo are formed at a ratio of 3 pits per 1 track on the boundary surface of the transparent dielectric layer serving as a boundary with the substrate. One emboss pit is positioned exactly on the track, whereas the remaining two emboss pits are located in the positions offset in the opposite directions (radial direction) with respect to the track so as to sandwich the track therebetween. For example, during recording, the boundary surface of the transparent dielectric layer where the emboss pits have been formed is illuminated from the substrate side with a first illumination beam for recording (reference beam) so that the beam is converged on the boundary surface, and a second illumination beam for recording (recording beam) is illuminated from the side of the hologram recording layer so as to overlap the first illumination beam for recording, while being converged in the location on this side of the hologram recording layer. As a result, an interference pattern generated due to overlapping of the first illumination beam for recording and second illumination beam for recording is recorded as a hologram on the hologram recording layer. At this time, the reflected light created by the first illumination beam for recording reflected at the boundary surface of the transparent dielectric layer is converted with a photodetector into a reproduction signal, the optical system is displaced based on this reproduction signal so that the amplitudes corresponding to the two aforementioned offset emboss pits become equal to each other and the track servo is thereby conducted.
However, in the above-described conventional hologram recording medium, the offset quantity of the emboss pits for the track servo is comparatively small. Therefore, the first illumination beam for recording undergoes diffraction in the gap between the emboss pits, and the diffracted beam generated thereby reaches the hologram material layer. As a result, the unnecessary interference pattern created by the diffracted beam is also recorded in the hologram material layer. Another problem is that the first illumination beam for recording serving as the reference beam is intensity modulated by the emboss pits and, therefore, the SN ratio is at a low level during recording and reproduction.
With the foregoing in view, it is an object of the present invention to provide a hologram recording medium having a structure in which the recording beam and reference beam are guided in the prescribed direction and making it possible to realize a high SN ratio.