In the conventional technology, in order to perform recording information onto, reproducing it from, or both recording onto and reproducing from DVD (digital versatile disc) and CD (compact disc), a two-light-source-type optical pickup device has been used that has a DVD laser output unit whose output wavelength is approximately 650 nm, and a CD laser output unit whose output wavelength is approximately 780 nm. Moreover, for the purpose of downsizing each of these light sources, a two-wavelength integrated laser output unit by which beams of two-kind wavelengths can be outputted using a single package has also been practically used. There are known as the two-wavelength integrated laser output unit, for example, a monolithic-type laser output unit that is obtained by forming two laser diodes on a monolithic-type semiconductor substrate, and a hybrid-type laser output unit that is obtained by bonding closely together two semiconductor substrates on each of which a laser diode is formed.
In the two-wavelength integrated laser output unit, beam-emitting portions of the two laser diodes (for DVD and CD) are slightly apart from each other, and the distance therebetween is generally approximately 110 μm. Therefore, when the optical axis of one of the laser diodes coincides with the system optical axis passing through the centers of the objective lens and the collimating lens of the optical pickup device, the optical axis of the laser beam emitted from the other laser diode is displaced from the system optical axis. In the as-is state, each of the return light beams having been outputted from the DVD laser diode and the CD laser diode and then reflected by the optical recording medium cannot be received by a common photo-detector. Accordingly, a method is proposed, in which both of the return light beams are guided to the common photo-detector by diffracting one of the return light beams or both of them, using a diffraction grating, etc., either or both of the return light beams (for example, refer to Patent Documents 1 and 2).
Moreover, in recent years, mass storage of optical recording media has been required, and an optical recording medium such as an optical recording disc, compatible with the blue-violet laser system, whose capacity is several times larger than that of DVD and CD has been practically used. Resultantly, from a viewpoint of device downsizing and cost reduction, a method has been required that information can be recorded onto and reproduced from, for example, DVD, CD, and an optical recording medium compatible with the blue-violet laser system by a single optical pickup device. Consequently, a three-light-source type pickup device having a blue-violet laser diode, in addition to a DVD laser diode and a CD laser diode, has been developed.
As an example of the three-light-source type optical pickup device, the following configuration has been proposed. That is, each of the optical axes of three-kind-wavelength optical beams whose output wavelengths are different from each other, using prisms corresponding to respective wavelengths, made coincident with the system optical axis of the optical pickup device; thereby, light beams of each wavelength are guided to an optical recording medium. Three-kind-wavelength return light beams of the light beams of each wavelength that are reflected by the optical recording medium pass through the respective prisms to be guided to a common photo-detector, and then detected by the photo-detector (for example, refer to Non-Patent Document 1).
[Patent Document 1] Japanese Laid-Open Publication No. 2001-143312
[Patent Document 2] Japanese Laid-Open Publication No. 2001-256670
[Non-Patent Document 1] “Philips, Holland, has developed optical head that enables recording onto and reproducing from CD, DVD, and Blu-ray Disc”, [ONLINE], Jul. 16, 2004, Nikkei Business Publications, Inc., [accessed on Feb. 20, 2005], Internet <http://TECHON.nikkeibp.co.jp/members/NEWS/20040716/104521/>