| 摘要 |
<P>PROBLEM TO BE SOLVED: To obtain compatibility in two different optical recording media by forming good spots at both of optical recording media having at least two substrates different in thickness each other using a single objective lens. <P>SOLUTION: The phase difference of phase shifter surface is limited in a certain range in order not to degrade the signal level by maintaining a spot for a thickness t1 requiring no aberration correction in good condition by an aberration correction element (phase shifter surface), and a spot for condensing on a substrate with a thickness t2 in the range is aberration-corrected. As for the spot for the thickness t1, it is limited to a range of level difference height H of the phase shifter surface for obtaining the phase difference capable of securing signal level of about 50 percent or more. The level difference structure is designed in order to correct aberration in the spot of optical recording medium having thickness t2. Consequently, with a single objective lens, a good spot can be formed in both optical recording mediums having thickness t1 and t2, respectively, and high reproduced signal level can be obtained. <P>COPYRIGHT: (C)2010,JPO&INPIT |
