The present invention relates to an optical pickup device, an optical information recording and reproducing apparatus and an objective lens.
Following upon recent high density of an optical disc, a demand for a smaller light-converged spot makes a numerical aperture (NA) of an objective lens used for recording/reproducing for these high density optical discs to be high.
For example, in the case of a high density optical disc employing a violet semiconductor laser light source having wavelength λ of 405 nm, an objective lens having image-side numerical aperture NA of about 0.85 is needed for achieving high density.
Further, for objective lenses such as CD (compact disc), MO (magneto-optical disc) and DVD (digital video disc), a plastic lens is mainly used for the reasons that the plastic lens is light in weight, and it can be manufactured at low cost on a mass production basis through injection molding that employs a mould. Therefore, even in the case of an optical pickup device for a high density optical disc, it is preferable to use a plastic lens as an objective lens for the same reasons.
However, for realizing an objective lens wherein a high numerical aperture NA of 0.85 has been realized while securing sufficient manufacturing tolerance that allows mass production, it is necessary to use a lens structure of at least two groups, for the reason to divide refracting power to two lenses and thereby to relax manufacturing tolerance for each lens, and there are proposed high NA plastic lenses each being of a two-group structure in, for example, Patent Documents 1 and 2.
(Patent Document 1)
TOKKAI No. 2002-236252
(Patent Document 2)
TOKKAI No. 2003-6908
(Problems to be Solved by the Invention)
Incidentally, in the optical pickup device, an objective lens is generally moved by an actuator in the direction of an optical axis and in the radial direction of a disc for conducting focusing and tracking. In this case, an electric current is made to flow through a focusing coil and a tracking coil which institute an actuator, and electromagnetic force thus generated between a magnet and the coil is used to move the objective lens.
However, it was found that a new problem which is not experienced in the conventional structure is caused when the high NA plastic lens with a two-group structure is moved in the aforesaid way. Contents of the problem are as follows; when an electric current is made to flow through a focusing coil and a tracking coil, uneven temperature distribution tends to be caused in the objective lens due to an influence of generation of heat in these coils, because a volume tends to be greater than that of a plastic lens with a one-lens structure which has been used, and in the optical pickup device, on the other hand, temperatures are changed by an influence of environmental temperature and by an influence of heat generation in the focusing coil and the tracking coil.
If a temperature in the optical pickup device is changed under the condition that uneven temperature distribution is caused in the objective lens, spherical aberration of the high NA plastic lens is changed greatly, and recording/reproducing of information for the high density optical disc is obstructed. The reason for the foregoing is that deterioration of spherical aberration caused by temperature is extreme even when compared with a lens wherein temperature distribution is substantially even, and a difference of refractive index further grows greater in proportion to the fourth power of NA, because the refractive index is as great as 10 times in comparison especially with a glass lens.
In particular, the larger an amount of electric current flowing through a focusing coil and a tracking coil is, the larger an amount of heat generation is, resulting in larger unevenness of temperature distribution in an objective lens, thus, a problem of a change (deterioration) of spherical aberration of a high NA plastic lens in the course of operation of the optical pickup device becomes more obvious.
It is difficult, on the point of a mechanism of the actuator, to design and arrange a focusing coil and a tracking coil so that they may be rotational symmetry about an optical axis of the objective lens. Therefore, during operations of the actuator, temperature distribution that is non-rotational-symmetry about an optical axis is caused in the objective lens, resulting in changes of astigmatism of the objective lens. In particular, when the objective lens is made to be a plastic lens of a two-group structure, the refracting power for the ray of light having NA of 0.85 is divided to two lenses, and therefore, a change in a height of passing ray of light in each lens is small, and the lens is easily affected by changes in refractive index following upon the temperature distribution. Therefore, changes of the astigmatism grow great, which adversely affects characteristics of recording/reproducing for optical discs.
However, in the technologies disclosed in the Patent Documents 1 and 2, there is no description of deterioration of spherical aberration and deterioration of astigmatism caused by the change of temperature in the optical pickup device, under the condition that uneven temperature distribution is caused in the objective lens by an influence of heat in the course of driving an actuator as stated above, and any actions for the problem are not taken accordingly, though the aforesaid technologies are those to correct spherical aberration caused by changes in the form of lenses and in refractive index, when temperature of two plastic lenses forming an objective lens is changed evenly.