The present invention relates to an optical pickup device, and more particularly, to an optical pickup device which can record/reproduce information, at high density, employing a solid immersion lens (SIL).
FIG. 1 is a diagram of a conventional optical pickup device for recording/reproducing information at high density. Light emitted from a light module 1 is reflected onto a SIL 10 by a reflection member 5. The SIL 10 focuses the light to form a beam spot on a recording plane of a disk 19. The light module 1 includes a light source, a device for converting a traveling path of incident light and a photodetector for receiving the light reflected from the recording plane of the disk 19. The reflection member 5 performs minute tracking by making fine changes to the incidence angle of incident light by moving the position of the beam spot formed on the disk 19 little by little. The SIL 10 is supported by a slider 15 separated several tens of nanometers from the disk 19 by an air bearing effect when the disk 19 rotates. An incident surface 10a of the SIL 10 is curved to focus incident light. The opposite surface 10b of the SIL 10, facing the disk 19, is planar.
The size of a beam spot formed on a disk can be substantially represented as: ##EQU1## A where .lambda. is the wavelength of light emitted from a light source and NA is the numerical aperture of the light focusing device. Thus, to reduce the size of a beam spot to enable higher density recording/reproduction, the wavelength of the light must be reduced or the numerical aperture must be increased. However, the maximum numerical aperture theoretically available in air is approximately 1.
Since the surface 10b of the SIL 10 is very close to the disk 19, when the refractive index of the SIL 10 is .eta..sub.SIL, the wavelength of light in the SIL 10 and the disk 19 equals ##EQU2##
When compared to, ##EQU3## the numerical aperture of the SIL 10, with respect to the wavelength of light emitted from a light source, i.e., .lambda., is greater than or equal to 1, thereby reducing the size of the beam spot. Here, the SIL 10 has a refractive index which is substantially the same as that of a protective film for protecting the recording plane of the disk 19. Since the distance between the SIL 10 and the disk 19 is in the range of several tens of nanometers, the beam spot is not exposed to the air and thus the numerical aperture is greater than or equal to 1, thereby reducing the size of the beam spot.
FIG. 2 is a detailed diagram of the SIL 10 shown in FIG. 1. The SIL 10, having one curved surface cannot remove both spherical aberration and coma aberration. On the other hand, if the incident surface 10a of the SIL 10 is elliptically curved, as shown in FIG. 2, the spherical aberration can be removed. However, the elliptically curved surface 10a does not satisfy Abbe's sine condition and becomes very sensitive to the tilt of incident light. Thus, coma aberration is generated in the light S1 (which is incident obliquely) and is focused by the SIL 10, as shown in FIGS. 2 and 3. If the incident surface 10a of the SIL 10 was curved hemispherically, the coma aberration is removed but defocusing and spherical aberration still exist. Since an optical recording/reproducing apparatus employing the SIL 10 is very sensitive to tilt, which generates coma aberration, it is very difficult to fabricate.
FIG. 4 is a diagram of a proposed optical pickup device. The proposed optical pickup device has a focusing objective lens 7 installed between the light module 1 and the SIL 10. In this case, spherical aberration and coma aberration are removed by the objective lens 7 and the SIL 10 serves to increase the numerical aperture. However, since the proposed optical pickup device employs two lenses, that is, the objective lens 7 and the SIL 10, the system is complex, bulky and costly.