1. Field of the Invention
The present invention relates to an optical pickup unit and an optical disc apparatus, capable of reproducing data recorded on an optical disc and of recording data onto the optical disc.
2. Description of the Related Art
Of late years, proposed in an optical pickup unit not shown, a so-called optical head whose lens holder is mounted with two objective lenses (see, e.g., Japanese Patent Application Laid-open Publication No. 9-120573 (p. 2, FIGS. 1-3).
However, with respect to a conventional optical pickup unit having a lens holder with an objective lens, if the distances are unequal between the coils around the lens holder and the objective lens, it has been feared that heat transfers to the objective lens may become non-uniform when heat is generated in the coils. It is a problem that non-uniform heat transfers to the objective lens may induce uneven temperature gradients in the objective lens. It is feared, in the case of non-uniform heat transfers to the objective lens, that thermal expansions of the objective lens may not become uniform, resulting in occurrence of aberrations on a spot of laser light focused by the objective lens to be applied to the optical disc.
Since the conventional optical pickup unit is of a structure where the objective lens is warmed up gradually from its portions closer to the coils, the objective lens is expected to have large temperature gradients when checking the temperature characteristics of the objective lens. It is difficult for the objective lens with large temperature gradients to have substantially uniform thermal expansions or thermal contractions, as a result of which aberrations may occur on a spot of laser light focused by the objective lens to be applied to the optical disc.
Furthermore, with respect to the conventional optical pickup unit having a lens holder with two objective lenses, occurrence of higher-order resonance has been feared. The higher-order resonance of the optical pickup unit means that the lens holder with lenses is subjected to significant vibrations in frequency regions of about 10 KHz or more for example. In the case of the occurrence of a higher-order resonance in the lens holder with lenses, when the optical disc apparatus having the optical pickup unit makes random access to an optical disc, it is feared that the optical pickup unit may not be able to perform a normal seeking operation to the optical disc due to its runaway. The seek means moving the optical pickup unit to a designated position on the disc. Required is an optical pickup unit capable of preventing the occurrence of runaway.
It is also feared, in the case of the occurrence of a significant higher-order resonance in the lens holder with lenses of the optical pickup unit, that moving members such as the lens holder may be deformed. To suppress the higher-order resonance occurring in the lens holder with lenses of the optical pickup unit, an increase in rigidity of the lens holder, etc., is required.
The optical pickup unit is also facing the needs for a reduction in weight, an improvement in responsivity and a lowering of the price.