The present invention relates to an objective lens for an optical disc drive, which is capable of recording data to and/or reproducing data from two types of optical discs having different thicknesses of cover layers and/or different recording densities. 
In general, an optical pick-up for an optical disc drive is provided with a laser source such as a laser diode, an objective lens which converges the laser beam emitted by the laser source on a recording surface of the optical disc to form a beam spot on the recording surface, and a signal detecting system that receives the laser beam which is reflected from the recording surface and passes through the objective lens.  The signal detecting system produces various signals based on the received laser beam. 
There are various types of optical discs having different thicknesses of cover layers and/or different recording densities.  For example, a DVD (digital versatile disc) has a recording density higher that that of a CD (compact disc) or a CD-R (CD Recordable) and has a cover layer thinner than that of the CD or CD-R. DVD-R, DVD−RW and DVD+RW are classified as a DVD standard because DVD-R, DVD−RW and DVD+RW have substantially the same thickness of the cover layer and a required numerical aperture as those of the DVD.  Also, the CD-R and CD-RW are classified as a CD standard because the CD-R and CD-RW have substantially the same thickness of the cover layer and a required numerical aperture as those of the CD. 
The optical disc drive is required to correct a spherical aberration caused by differences between the thicknesses of cover layers of the various types of optical discs.  Each of Japanese Patent Provisional Publications No.  HEI 9-54973 (hereafter, referred to as a document 1), 2000-81566 (hereafter, referred to as a document 2), and 2001-249273 (hereafter, referred to as a document 3) discloses an optical system of an optical disc drive configured to correct the spherical aberration caused by the differences of the thicknesses of cover layers of optical discs. 
According to the publications, the optical system is provided with an optical surface having a special structure (called a phase shift surface, a diffracting surface, or a hologram surface) formed on an optical element located adjacent to an objective lens or formed on the objective lens so as to correct (cancel) the spherical aberration caused by the change of the thickness of the cover layer of the optical disc by utilizing a phenomenon that a condition of the spherical aberration changes in the optical surface having the special structure depending on a wavelength of a light beam. 
Each of the phase shift surface, the diffracting surface and the hologram surface represents a surface structure configured such that a plurality of minute steps are formed on a base shape of an optical element.  That is, the phase shift surface, the diffracting surface and the hologram surface have the function of giving a phase shift (or an optical path length difference) at each step so that light beams passed through adjacent zones lying at both sides of each step are interfere with each other and thereby an optical effect other than refraction acts on the light beams. 
More specifically, the optical system disclosed in the document 1 is configured to have a hologram surface formed on an optical element located adjacently to an objective lens.  It is also disclosed in the document 1 that the hologram surface is formed by etching.  In order to simplify a manufacturing process of the hologram surface and to improve the precision of the manufacturing process, the hologram surface is designed to have a plurality of step-like structures each of which has approximately four steps.  However, the optical system can not correct the change of the spherical aberration caused depending on a variation of ambient temperature if the objective lens is made of resin. 
In the document 2, an objective lens having a surface on which a diffracting structure having the function of phase shifting the light beams passing therethrough is formed.  In contrast to the optical system of the document 1, the objective lens of the document 2 can correct the spherical aberration caused depending on the variation of ambient temperature even if the objective lens is made of resin.  The objective lens of the document 2 is configured to correct the spherical aberration caused in the cover layer of the optical disc being used through the use of a fact that a condition of the spherical aberration is changed by the diffracting structure depending on a wavelength of the light beam passing through the diffracting structure. 
However, the optical system of the document 2 has a drawback that the change of the spherical aberration due to the ambient temperature is overcorrected by the change of the spherical aberration caused by the objective lens. 
As described above, the optical system of the document 2 corrects the spherical aberration through use of the wavelength dependence of the change of the spherical aberration in the objective lens.  For this reason, the optical system of the document 2 is required to use a light source having a property of a relatively small individual difference (i.e. variation of wavelength). 
The optical system of the document 3 includes an objective lens having a surface on which a diffracting structure is formed for phase shifting the light beams passing therethrough.  The objective lens of the document 3 has a low NA (numerical aperture) area contributing to the formation of a beam spot for each of the light beam for the optical disc having a relatively low recording density (e.g. CD) and the light beam for the optical disc having a relatively high recording density (e.g. DVD), and has a high NA area contributing to the formation of a beam spot only for the light beam for the optical disc having a relatively high density (e.g. DVD). 
The diffracting structure formed in the low NA area has substantially the same structure as the diffracting structure shown in the document 2.  The high NA area is formed such that the change of the spherical aberration is reduced to a low level even if the ambient temperature changes by a relatively large amount so that the change of the spherical aberration in the entire optical system due to the temperature variation is reduced. 
It is generally required to increase a NA (numerical aperture) to further enhance a recordation efficiency of the optical disc having a relatively low recording density.  Therefore, in order to increase NA for the optical disc having the relatively low recording density, it is required to broaden the low NA area.  However, if the low NA area of the objective lens is broadened, a disadvantage of the objective lens (i.e. overcorrecting the spherical aberration) is intensified.