High density optical disk system using a new two-element lens and a thin substrate disk

The areal density of an optical storage disk can be increased by improving the transverse resolution of the optical head which is proportional to λ/NA. In order to improve the resolution, a lot of research has been carried out to develop shorter wavelength light sources such as green or blue SHG lasers1,2) and recently ZnSe or GaN semiconductor blue lasers3). These lasers will soon become commercially available. On the other hand, it is difficult to make a high NA objective using a single molded aspherical element. Furthermore the higher the numerical aperture, the less the tolerance for variations in disk thickness, disk tilt, and focusing which are inversely proportional to the fourth, third, and second power of the numerical aperture, respectively. Other experiments have been carried out to improve the resolution using near-field optics4) such as scanning probes which can improve the areal density drastically, however, such probes seem to be tolerated only for working distances of around 100 nm.