Abstract:
An optical pickup device compatible with two types of optical recording media in accordance with the present invention includes: a first light source emitting a first laser beam with a first wavelength; a second light source emitting a second laser beam with a second wavelength greater than the first wavelength; an objective lens with parameters according with the first wavelength and adapted to focus the first and second laser beams on the at least two types of optical recording media; a collimating lens for collimating an incident beam of light and transmitting the collimated light beam to the objective lens; an optical path synthesizer/separator for receiving the first and second laser beams and transmitting the first and second laser beams to the collimating lens; and a compensator for correcting the second laser beams and transmitting the corrected second laser beams to the optical path synthesizer/separator.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to optical pickup devices used in optical disk recording and/or reproducing apparatuses, and more particularly to an optical pickup device enabling recording and/or reproducing with two or more types of recording media. 
   2. Description of Prior Art 
   An optical writing and/or reading system carries out recording and/or reproducing of information such as video, audio or other data to/from a recording medium. In such system, a semiconductor laser is used for generating a laser beam, and an objective lens is used for converging the laser beam and forming a focused spot on the recording medium. The recording density of the recording medium is determined by the size of the focused spot. In general, the size of the focused spot (S) is proportional to the wavelength (λ), and inversely proportional to the numerical aperture (NA), as expressed by formula (1):
 
S∝λ/NA  (1)
 
   Therefore, to increase the recording density, the size of the spot being focused on the optical disk must be reduced. To reduce the spot size, as can be inferred from formula (1), the wavelength (λ) of the laser beam must be reduced and/or the numerical aperture (NA) of the objective lens must be increased. This has been demonstrated by the ongoing development of optical recording media. For example, the wavelength of read beams for compact disks (CDs) is about 780 nm, the wavelength of read beams for digital versatile disks (DVDs) is about 650 nm, and the wavelength of read beams for high-definition DVDs (HD-DVDs) is about 405 nm. Furthermore, the numerical aperture for CDs is 0.45, the numerical aperture for DVDs is 0.6, and the numerical aperture for HD-DVDs is 0.65-0.8. 
   On the other hand, coma aberration, which occurs due to a tilting of the optical disk, is associated with the tilt angle of the disk, the refractive index of the disk substrate, the thickness of the disk substrate, and the numerical aperture of the objective lens. To ensure an acceptable level of coma aberration with respect to the tilt of a disk for high-density recording, the thickness of the disk substrate is in general reduced accordingly. For example, CDs have a thickness of 1.2 mm, and DVDs have a thickness of 0.6 mm. Further, the thickness of many HD-DVDs is 0.6 mm or less. 
   In an apparatus for high-density recording onto or playing from a medium such as a HD-DVD, a primary consideration is the compatibility of the apparatus with existing disks including CDs and DVDs. Conventionally, there are two kinds of optical writing and/or reading systems that are used in multi-compatible home entertainment players. In the first kind of optical writing and/or reading system, an independent optical system is provided therein for each type of disk. That is, generally, the optical writing and/or reading system has at least three light sources and three objective lenses for these disks. This kind of writing and/or reading system needs too many optical elements, and is unduly large and costly. In the second kind of writing and/or reading system, there are some common optical elements that function for both existing disks and for HD-DVDs; for example, a common objective lens. This kind of writing and/or reading system reduces the total number of optical elements and simplifies the overall configuration. However, the optical performance of the optical pickup head is limited. In respect of the common objective lens, chromatic aberration occurs because each kind of disk operates according to different wavelengths. Further, spherical aberration occurs because the disks have different thicknesses. 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the present invention is to provide an optical pickup device for high-density recording/reproduction which has a single objective lens, and in which chromatic aberration related to different wavelengths of light and/or spherical aberration due to thickness variations of optical disks is corrected. 
   To achieve the above object, an optical pickup device compatible with at least two types of optical recording media in accordance with the present invention includes: a first light source emitting a first laser beam with a first wavelength; a second light source emitting a second laser beam with a second wavelength greater than the first wavelength; an objective lens with parameters according with the first wavelength and adapted to focus the first and second laser beams on the at least two types of optical recording media; a collimating lens for collimating an incident beam of light and transmitting the collimated light beam to the objective lens; an optical path synthesizer/separator for receiving the first and second laser beams and transmitting the first and second laser beams to the collimating lens; and a compensator for correcting the second laser beams and transmitting the corrected second laser beams to the optical path synthesizer/separator. 
   Other objects, advantages and novel features of the present invention will be drawn from the following detailed description of preferred embodiments of the present invention with the attached drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates an optical arrangement of an optical pickup device for high-density recording/reproduction according to a first embodiment of the present invention; and 
       FIG. 2  illustrates an optical arrangement of an optical pickup device for high-density recording/reproduction according to a second embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , an optical pickup device  100  compatible with recording media having different formats according to a first embodiment of the present invention is illustrated. The optical pickup device  100  includes first and second semiconductor modules  11  and  12  which handle laser beams having different wavelengths, and an objective lens  8  facing an optical disk  9  for focusing an incident beam on the optical disk  9 . The first semiconductor  11  integrates a first light source  110  generating a laser beam (not labeled) with a first photo detector  111 . The first photo detector  111  is for receiving the laser beam generated by the first light source  110  and reflected from the optical disk  9 . The second semiconductor  12  integrates a second light source  120  generating a laser beam (not labeled) with a second photo detector  121 . The second photo detector  121  is for receiving the laser beam generated by the second light source  110  and reflected from the optical disk  9 . An optical path synthesizer/separator  4  is positioned on a first optical path from the first light source  110  to the objective lens  8 , and on a second optical path from the second light source  120  to the objective lens  8 . The optical path synthesizer/separator  4  transmits the laser beam from the first light source  110  toward the optical disk  9 , and reflects the laser beam from the second light source  120  toward the optical disk  9 . Thus, a common optical path shared by the first and second optical paths is formed between the optical path synthesizer/separator  4  and the objective lens  8 . 
   A collimating lens  5 , an optical path changer  6  and a wavelength selector  7  are sequentially arranged in the common optical path. The optical path changer  6  can be a mirror. The wavelength selector  7  has different transmissivities according to the different wavelengths. A first diffraction grating  21  is positioned on the first optical path between the first light source  110  and the optical path synthesizer/separator  4 . A second diffraction grating  22  and a compensator  3  are positioned on the second optical path between the second light source  120  and the optical path synthesizer/separator  4 . 
   In the present embodiment, the optical disk  9  is either a first optical disk or a second optical disk. The first and second optical disks have different formats. The first optical disk may be a future generation digital versatile disk which has a great numerical aperture and corresponds to a short wavelength; for example, an HD-DVD. The second optical disk may be a DVD, which has a small numerical aperture and corresponds to a long wavelength. The first optical path is used for recording an information signal on and/or reproducing an information signal from the first optical disk. The laser beam generated by the first light source  110  has a relatively short wavelength of about 405 nm, which is suitable for the first optical disk. The laser beam generated by the second light source  120  has a relatively long wavelength of about 650 nm, which is suitable for the second optical disk. Further, both the collimating lens  5  and the objective lens  8  have optical parameters according with the short wavelength for the first optical disk, and the objective lens  8  also has a great numerical aperture according with the first optical disk. 
   When recording an information signal on and/or reproducing an information signal from the first optical disk, the first light source  110  emits first laser beams having a wavelength of about 405 nm. Then, after passing through the first diffraction grating  21  along the original direction thereof, the first laser beams are transmitted to the optical path synthesizer/separator  4 . The optical path synthesizer/separator  4  transmits the first laser beams from the first light source  110  directly therethrough, such that the first laser beams maintain their original direction. After passing through the optical path synthesizer/separator  4 , the first laser beams are condensed by the collimating lens  5  and transformed into a first luminous flux of parallel light beams. Because the collimating lens  5  accords with the first optical disk, the beams of the first luminous flux are fully parallel to each other. The first luminous flux is transmitted to the optical path changer  6 , which changes the transmitting direction toward the optical disk  9 . After being reflected by the optical path changer  6 , the first luminous flux illuminates the wavelength selector  7 . The wavelength selector  7  does not block any of the first luminous flux, so that the first luminous flux completely passes through the wavelength selector  7  and is incident on the objective lens  8 . The objective lens  8  converges the first luminous flux to a light spot (not labeled) on the first optical disk. 
   After forming the light spot on the first optical disk, the first optical disk reflects the incident beams as first return beams (not labeled). The first return beams sequentially pass through/from the objective lens  8 , the wavelength selector  7 , the optical path changer  6 , the collimating lens  5 , and the optical path synthesizer/separator  4 , and reach the first diffraction grating  21 . The first diffraction grating  21  diffracts the first return beams toward the first photo detector  111 . Then, the first photo detector  111  receives the first return beams and generates corresponding electrical signals. 
   In the first optical path, the parameters of all the components accord with the first optical disk. In particular, the objective lens  8  matches the parameters of the first optical disk, such as the wavelength, the numerical aperture and the thickness of the substrate of the first optical disk. Therefore, the objective lens  8  avoids chromatic aberration and spherical aberration in the first optical path. 
   When recording an information signal on and/or reproducing an information signal from the second optical disk, the second light source  120  emits second laser beams having a wavelength of about 650 nm. Then, after passing through the second diffraction grating  22  along an original direction thereof, the second laser beams are transmitted to the compensator  3 . The compensator  3  is a converging lens; for example, an aspherical lens. The second laser beams are converged a first time by the compensator  3  and transmitted to the optical path synthesizer/separator  4 . The optical path synthesizer/separator  4  reflects the second laser beams to the collimating lens  5 . The second laser beams are converged a second time by the collimating lens  5 , and are thus transformed into a second luminous flux of substantially parallel light beams. The second luminous flux is transmitted to the optical path changer  6 , which changes the transmitting direction toward the second optical disk. After being reflected by the optical path changer  6 , the second luminous flux illuminates the wavelength selector  7 . The wavelength selector  7  transmits a center part of the second luminous flux, and blocks a peripheral part of the second luminous flux. That is, only the center part of the second luminous flux passes through the wavelength selector  7  and is incident on the objective lens  8 . The objective lens  8  converges the center part of the second luminous flux to a light spot (not labeled) on the second optical disk. 
   After forming the light spot on the second optical disk, the second optical disk reflects the incident beams as second return beams (not labeled). The second return beams sequentially pass through/from the objective lens  8 , the wavelength selector  7 , the optical path changer  6 , the collimating lens  5 , the optical path synthesizer/separator  4 , and the compensator  3 , and reach the second diffraction grating  22 . The second diffraction grating  22  diffracts the second return beams toward the second photo detector  121 . Then, the second photo detector  121  receives the second return beams and generates corresponding electrical signals. 
   In the second optical path, three converging lenses (i.e., the compensator  3 , collimating lens  5  and objective lens  8 ) are used to focus the second laser beams on the second optical disk. Therefore, any spherical aberration caused by lack of matching between the second luminous flux and the collimating lens  5  and objective lens  8  is corrected. Furthermore, the wavelength selector  7  is used to control the second luminous flux incident on the objective lens  8 , so that the numerical aperture of the objective lens  8  accords with the second optical disk. 
   Hence, any aberrations of the first and second laser beams generated along the first and second optical paths are eliminated by the relatively simple configuration of the optical pickup device  100 . This enables high quality information signal recording and/or reproduction. Furthermore, the first light source  110  and the first photo detector  111  adjoin each other in a single unified package, and the second light source  120  and the second photo detector  121  adjoin each other in a single unified package. Therefore the emission path and the return path of the first optical path can share the same optical elements, and the emission path and the return path of the second optical path can share the same optical elements. This reduces the total number of optical elements needed, and further simplifies the configuration of the optical pickup device  100 . Thus the optical pickup device  100  has a reduced size and lowers costs. 
   Referring to  FIG. 2 , an optical pickup device  100 ′ compatible with recording media having different formats according to a second embodiment of the present invention is illustrated. Unlike in the optical pickup device  100  of the first embodiment, the optical pickup device  100 ′ includes an optical path synthesizer/separator  4 ′ and a compensator  3 ′ that are integrated together. The compensator  3 ′ is formed on a surface of the optical path synthesizer/separator  4 ′ which faces the second light source  120 , thereby giving the optical pickup device  100 ′ a further simplified configuration. 
   Although the present invention has been described with reference to specific embodiments, it should be noted that the described embodiments are not necessarily exclusive, and that various changes and modifications may be made to the described embodiments without departing from the scope of the invention as defined by the appended claims.