1. Field of the Invention
The present invention relates to an optical head for an optical disk device which records and reproduces information by using light, and more particularly to an optical head which includes a plurality of light sources having different wavelengths.
2. Description of the Related Art
To protect the information recording layer of an optical disk, exit light from an objective lens is converged onto a minute spot when recording or reproducing information on or from the optical disk. The objective lens is designed to form the smallest possible spot for an optical disk having a substrate of a particular thickness.
Recently, to improve recording density etc., optical disks having different substrate thicknesses have been standardized. However, conventional single-light source optical systems cannot accommodate such different substrate thicknesses. Therefore, optical systems for accommodating two different substrate thicknesses have been developed.
FIG. 6 shows an example of such a two-light-source optical system. This configuration accommodates two different substrate thicknesses by using two light sources having different wavelengths.
Specifically, exit light from a first light source 1 is shaped by a collimator lens 13 and an objective lens 6, to form a minute spot on an optical disk 7 having a first substrate thickness. Reflection light from the optical disk is guided by a beam splitter 14 to a first detection system 3, which detects necessary information.
On the other hand, an integrated module 17 incorporates a second light source whose wavelength is different from that of the first light source. Exit light from the integrated module 17 is superimposed on an optical path extending from the first light source by a wavelength combining element 10. In this case, a minute spot can be formed on a second optical disk 8 having a second substrate thickness different from that of the first optical disk. Reflection light from the optical disk is separated by the wavelength combining element 10, and a necessary signal is detected by a second detection system that is provided in the integrated module 17.
Thus, the configuration shown in FIG. 6 accommodates optical disks having different substrate thicknesses by employing two light sources having different wavelengths and establishing different objective lens incident states.
However, the optical head shown in FIG. 6, is larger than the conventional single-light source optical head because the number of parts required is much greater than that of the conventional single-light source optical head. As a result, such a larger structure is more difficult to move, and hence high-speed access of the entire optical disk with a two-light-source optical head, is difficult to achieve.
To remedy the slow movement of large optical heads, a separation-type optical head is conventionally used. A conventional separation-type optical head has a structure in which the optical system is divided at a position between the objective lens and a collimator lens, so that typically only the objective lens is moved in the optical axis direction. The separation-type optical head takes into account that a variation in distance between the collimated light portion and the objective lens does not affect the optical characteristics of the head.
However, a separation-type optical head cannot be used with the head shown in FIG. 6, because the second light source is not collimated. That is, since exit light from the second light source (which is indicated by broken lines) incorporated into module 17, is not collimated between the collimating lens 13 and the objective lens 6, the optical characteristics of the second light would be changed by a variation in the distance, which would prevent a minute spot from being formed on the second optical disk. Thus, the reproduced or recorded signal from/on the optical disk would deteriorate.