The present invention relates to a lens that is capable of correcting chromatic aberration by itself. Also, the present invention relates to a device for correcting the chromatic aberration inherent in optical system. More particularly, the present invention relates to a chromatic aberration correcting device that is intended for use in combination with a single aspheric lens that is corrected for aberrations other than chromatic ones.
The use of a single objective lens having an aspheric surface on both sides has expanded these days in the art of optical disks and one of the principal reasons for this use is its contribution to weight reduction. However, the single lens in conventional use has been incapable of effective correction of chromatic aberration. A laser diode which is used as a light source for optical disks has the disadvantage that its emission wavelength is shifted on account of the change either in the output power of the laser or in the temperature. Hence, if the objective lens is not corrected for chromatic aberration, the focusing position of light rays will change in response to the shift in wavelength and this can cause errors when reading or writing information.
To solve this problem, the present inventors previously proposed chromatic aberration correcting devices that had two or three glass lens elements cemented together (see Japanese Patent Public Disclosure Nos. Hei 3-155514 and 3-155515). By combining either one of these chromatic correcting devices with a single aspheric lens, one could offer a lens system that was immune to the effect of wavelength variations, requiring less lens elements than the conventional system that is effectively corrected for chromatic aberration.
However, the techniques proposed in the two patents cited above suffer from the disadvantage that in order to correct chromatic aberration, it is necessary to provide a device that is not directly concerned with the focusing action inherent in the objective lens. Therefore, optical system that is properly corrected for chromatic aberration weighs more and requires more parts than optical system that is not corrected.
The conventional chromatic aberration correcting device has had the problem that its manufacturing cost is so high as to cancel the advantage of lower cost that results from the use of a single aspheric lens, whereby the net benefit is reduced to nil.
The present invention has been accomplished under these circumstances and has as an object providing a lens that utilizes diffraction effect so as to correct chromatic aberration effectively without unduly increasing the number of lens elements.
The present invention has been accomplished under these circumstances and has as an object providing a chromatic aberration correcting device that can be manufactured at a lower cost than devices that consist of two or three glass plates cemented together.