1. Field of the Invention
This invention relates to an optical disc obtained on laminating two optical discs, each having an information recording surface on a substrate, and a method for manufacturing the optical disc.
2. Related Art
In the field of information recording, researches into the optical information recording system are proceeding briskly. This optical information recording system has a number of advantages, such as non-contact recording/reproduction, recording density higher by not less than one digit than with the magnetic recording medium recording system, or compatibility to memory configurations of the read-only, write-once and overwrite type memories. Thus, the optical information recording system is finding a wide field of application in both the industrial and domestic usages as a
As an optical disc capable of recording/reproducing the information by optical means, there are a read-only optical disc, a phase-change type optical disc and a magneto-optical disc.
In keeping up with the demand for high recording density, a laminated optical disc of the sole surface readout type, that is an optical disc in which a laser light beam is adapted to fall on one disc side and the focal point position of the laser light beam is changed to selectively read and write the information from or on the two optical discs. This increases the volume of the information per optical disc. There is also no necessity of providing two pickups or a mechanism for moving the sole pickup to both sides. Therefore, the laminated disc is convenient in promptly having access to disc data or in reducing the size of the recording/reproducing apparatus.
For producing the sole surface readout type laminated disc, two manufacturing methods have been proposed. In the first one of the manufacturing methods, a substrate is molded by injection molding and a first information recording portion is formed on one substrate surface. On the first information recording portion is formed a reflective surface, for example, for forming an optical disc. On the surface of the information recording portion is applied a UV curable resin onto which a stamper carrying the embossed information is pressed in order to mold a second information recording portion. At this time, the thickness of the UV curable resin needs to be controlled precisely. Moreover, since the first and second information recording portions are formed by respective different methods, it is difficult to perform control so that the same signals can be recorded on the two recording surfaces.
In the second manufacturing method, a disc substrate is prepared by injection molding and an information recording portion is formed on one substrate surface for preparing an optical disc. This operation is repeated once to complete two optical discs. These two discs are laminated by bonding the surface of information recording portion of one of the discs to the surface of the other disc opposite to its information recording portion by a UV curable resin to complete a sole optical disc. In this case, the thickness of the UV curable resin is easier to control than with the first manufacturing method. Moreover, since the substrates and the information recording portions are prepared by the same method, the signals on the respective surfaces are of uniform quality.
However, the optical disc prepared by laminating two optical discs with the above-described second method has a drawback that focal point of the laser light of the reproducing apparatus is more susceptible to blurring than with the conventional single disc thus leading to unstable playback signals and deteriorated playback properties.