Compatible optical pickup apparatus

Abstract: A compatible optical pickup apparatus which can adopt two disks having different thickness is provided. The optical pickup apparatus having an optical source (31), a beam splitter (35) for changing the direction of an incident beam, an objective lens (39) which is located between the optical source (31) and a disk (50) for focusing the incident beam to form an optical spot on the disk (50), and a photodetector (44) for detecting information and error signals by receiving the beam which is reflected from the disk (50) and passed through the beam splitter (35), comprises a hologram optical element (HOE) (43) which is located between the beam splitter (35) and the photodetector (44) for diffracting the beam reflected from the disk into a zero-order beam (63) and a positive first-order beam (65), wherein the photodetector includes a first photodetector (45) and a second photodetector (47) which receive the zero-order beam (63) and the positive first-order beam 63), respectively.

Description: The present invention relates to an optical pickup apparatus, and more particularly, to a compatible optical pickup apparatus which can be adopted with disks having different formats. Such an apparatus having the features of the preamble of claim 1 is known from US-A-5 278 817. Generally, optical pickup apparatus is employed in a compact disk player (CDP), digital video disk player (DVDP), CD-ROM drive, DVD-ROM drive and the like, to record and reproduce information on and from a recording medium. The optical pickup apparatus employed in the DVDP and DVD-ROM which can record and reproduce high density video and audio signals should be able to use a CD as well as a DVD as a recording medium for compatibility. However, the standard thickness of a DVD is different from that of the CD family in consideration of an allowable error in the mechanical inclination of the disk and a numeric aperture of the objective lens in the optical pickup apparatus. That is, the thickness of the conventional CD family is 1.2mm while that of the DVD is 0.6mm. Due to the difference in thickness of the CD family and DVD, spherical aberration occurs when an optical pickup apparatus for the DVD is used for the CD family. Here, it is impossible to obtain a sufficient amount of light for recording information and a reproduced signal is deteriorated by the spherical aberration. A conventional compatible optical pickup apparatus developed for solving the above problems will be described with reference to Figure 1. Such an apparatus is disclosed in EP-A-0 610 055. As shown in Figure 1, an optical source 11 as a semiconductor laser emits a laser beam. An objective lens 19 converges an incident beam 1 to form a beam spot on the recording surface of a disk 10. A beam splitter 15 located on an optical path between the optical source 11 and the objective lens 19 passes the beam emitted from the optical source 11 straight to the objective lens 19 and reflects the beam being incident from the objective lens 19 toward a photodetector 23. A hologram optical element (HOE) 17, located on the optical path between the objective lens 19 and the beam splitter 15 diffracts the incident beam 1 into a zero-order beam 3 and positive first-order beam 5. The zero-order beam 3 continues straight with respect to the incident beam 1 and the positive first-order beam 5 is diverged. Thus, the zero-order beam 3 and the positive first-order beam 5 pass through different portions of the objective lens 19, forming spots on different portions of the disk 10. The zero-order beam 3 is used for a relatively thin disk such as a DVD and the positive first-order beam 5 is used for a relatively thick disk such as a CD. The photodetector 23 receives light reflected from the recording surface of the disk 10 to thereby detect a radio frequency information signal, a track error signal and a focus error signal. Also, the optical pickup apparatus further includes a grating 25 which is located between the optical source 11 and the beam splitter 15 for diffracting the incident beam 1, a collimating lens 13 which is located between the beam splitter 15 and the HOE 17 for collimating a divergent beam, and an astigmatism lens 21 for forming a circular or elliptical optical spot on the photodetector 23 according to the distance between the objective lens 19 and the disk 10. In the operation of the optical pickup apparatus having the above structure, a beam emitted from the optical source 11 is diffracted into the zero-order beam 3 and the positive first-order beam 5 by the HOE 17 and then converged by the objective lens 19, thereby forming an optical spot on the disk 10. When the disk 10 is relatively thin such as a DVD, information is recorded and reproduced using the zero-order beam 3, and when the disk 10 is relatively thick such as a CD, information is recorded and reproduced using the positive first-order beam 5. That is, either the zero-order beam 3 or the positive first-order beam 5 is focused on the disk 10, so that the amount of beam focused on the disk 10 is only half of the beam 1 emitted from the optical source 11. In addition, the amount of beam received by the photodetector 23 is decreased to about 25% of the incident beam 1. With a view to solving or reducing the above problems, it is an aim of embodiments of the present invention to provide a compatible optical pickup apparatus which has an improved optical efficiency. According to a first aspect of the invention, there is provided an optical pickup apparatus as defined in claim 1 including an optical source, a beam splitter for changing the direction of an incident beam, an objective lens which is located between the optical source and a disk for focusing the incident beam to form an optical spot on the disk, and a photodetector for detecting information and error signals by receiving the beam which is reflected from the disk and passed through the beam splitter, the optical pickup apparatus further comprising a hologram optical element (HOE) which is located between the beam splitter and the photodetector for diffracting the beam reflected from the disk into a zero-order beam and a positive first-order beam, wherein the photodetector includes a first photodetector and a second photodetector which receive the zero-order beam and the positive first-order beam, respectively wherein said first photodetector is used for reproducing a signal from a relatively thick disk and said second photodetector is used for reproducing a signal from a relatively thin disk. Said hologram optical element preferably has a pattern for forming the positive first-order beam as an abnormal spherical wave having an inverse spherical aberration with respect to the incident beam. Preferably, there is provided a grating which is located on an optical path between said optical source and said beam splitter for diffracting the incident beam. Preferably, the apparatus further comprises a collimating lens which is located on an optical path between said optical source and said objective lens for collimating an incident divergent beam. There may also be provided an astigmatism lens located between said beam splitter and said photodetector. Preferably, said optical source is a semiconductor laser emitting a beam having a wavelength of about 650nm and said objective lens has a numeric aperture of about 0.6. Said first and second photodetectors may have six divided plates, respectively. According to a second aspect of the invention, there is provided an optical pick-up apparatus comprising means for focusing an incident beam from a light source onto a disk and means for receiving a reflected beam from said disk, said means for receiving comprising means for splitting said reflected beam into a zero-order beam and a first-order beam, said zero-order beam being directed towards a first photodetector and said first order beam being directed towards a second photodetector. The apparatus of the second aspect may further comprise any one or more features from the accompanying claims, description, Figures or abstract in any combination. For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which: Figure 1 is a diagram showing the optical structure of a conventional compatible optical pickup apparatus; Figure 2 is a diagram showing the optical structure of an optical pickup apparatus according to an embodiment of the present invention; Figure 3 is a diagram showing the first and second photodetectors of the optical pickup apparatus shown in Figure 2. Figure 4 is a diagram showing the operational state of the optical pickup apparatus of Figures 2 and 3 when a comparatively thin disk is adopted; and Figure 5 is a diagram showing the operational state of the optical pickup apparatus of Figures 2 and 3 when a comparatively thick disk is adopted. As shown in Figure 2, a compatible optical pickup apparatus according to a preferred embodiment of the present invention includes an optical source 31, a beam splitter 35 for changing the path of an incident beam 61, an objective lens 39 for focusing the incident beam 61 to form an optical spot on a disk 50, a photodetector 44 for detecting information and error signals by receiving the beam which is reflected from the disk 50 and then passed through the beam splitter 35, and a hologram optical element (HOE) 43 disposed between the beam splitter 35 and the photodetector 44. The optical source 31 emits a laser beam having a short wavelength of about 650nm. The objective lens 39 has a numeric aperture of about 0.6 which is suitable to be used for a relatively thin disk such as a DVD. The HOE 43 diffracts the reflected light into a zero-order light 63 for a DVD and positive first-order beam 65 for a CD. As follows, the HOE 43 has a pattern for forming the positive first-order beam 65 as an abnormal spherical wave having an inverse spherical aberration when the incident beam is an ideal spherical wave, and for forming the positive first-order beam 65 as an ideal spherical wave when the incident beam is an abnormal spherical wave. Here, the pattern of the HOE 43 is formed using a well-known method. The photodetector 44 is constituted by a first photodetector 45 and a second photodetector 47 which are arranged near each other, wherein the first photodetector 45 receives the zero-order beam 63 and the second photodetector 47 receives the positive first-order beam 65. As shown in Figure 3, preferably, the first and second photodetectors 45 and 47 are formed of six divided plates, respectively, wherein four divided plates are for detecting an information signal and a focus error signal, and two divided plates, one located above and the other below the four divided plates, are for detecting a track error signal. Also, the compatible optical pickup apparatus of the present invention may further include a grating 33 and a collimating lens 37 which are located on an optical path between the optical source 31 and the objective lens 39, and an astigmatism lens 41 placed between the first and second photodetectors 45 and 47 and the beam splitter 35. The operation of the CD/DVD compatible optical pickup apparatus invention will be described in each case where a DVD and a CD are adopted as a disk, respectively. First, the operation of the compatible optical pickup apparatus when the DVD is adopted as the disk 50 will be described with reference to Figures 2 and 4. Since the optical source 31 is suitable for reproducing the signal recorded on the DVD, the beam reflected from the DVD becomes an ideal spherical wave. The beam reflected from the DVD is incident on the HOE 43 via the beam splitter 35. The HOE 43 diffracts the incident beam into the zero-order beam 63 and the positive first-order beam 65. Here, the zero-order beam 63 is maintained as an ideal spherical wave 51a which is the same as the incident beam and the positive first-order beam 65 is changed to an abnormal spherical wave 51b by the pattern of the HOE 43. In this case, the signal recorded on the DVD is reproduced using the first photodetector 45. Next, the operation of the compatible optical pickup apparatus when the CD is adopted as the disk 50 will be described with reference to Figures 2 and 5. Since the optical source 31 and the objective lens 39 are designed for the DVD, the beam reflected from the CD has a spherical aberration. That is, the beam incident on the HOE 43 is an abnormal spherical wave 53 as shown in Figure 5. The HOE 43 diffracts the incident abnormal spherical wave 53 into the zero-order beam 63 and the positive first-order beam 65. The zero-order beam 63 is a spherical wave 53a like the incident abnormal spherical wave 53, and the positive first-order beam 65 is a normal spherical wave 53b which is obtained by offsetting the spherical aberration of the incident spherical wave 53 using a pattern which is formed in the HOE 43 and generates an inverse spherical aberration. The positive first-order beam 65 is incident on the second photodetector 47, and the second photodetector 47 detects a reproduced signal and an error signal. Here, there is a defect in that an optical spot formation on the CD is not accurate. However, since the compatible optical pickup apparatus adopts an optical source which emits a short wavelength beam of 650nm and an objective lens having a numeric aperture of 0.6, an optical spot formed on the CD is smaller than that formed using the conventional optical pickup apparatus which adopts an optical source which emits a beam having a wavelength of 780nm and an objective lens having a numeric aperture of 0.45, so that there is no effect to the reproduction of a signal from the CD. Therefore, according to the compatible optical pickup apparatus of embodiments of the present invention wherein a hologram optical element is installed between the beam splitter and the photodetector, the amount of beam incident on the first and second photodetectors is maintained at about 50% of the initial beam emitted from the optical source. In addition, an operational current of the optical source can be reduced compared with that of the conventional optical pickup apparatus. Also, the hologram optical element has a pattern for offsetting the spherical aberration, so that information can be accurately reproduced from the CD.

Claims: An optical pickup apparatus including an optical source (31), a beam splitter (35) for changing the direction of an incident beam, an objective lens (39) which is located between said optical source and a disk (50) for focusing the incident beam to form an optical spot on the disk (50), and a photodetector (44) for detecting information and error signals by receiving the beam which is reflected from the disk (50) and passed through said beam splitter (35), said optical pickup apparatus further comprising a hologram optical element (HOE) (43) which is located between said beam splitter (35) and said photodetector (44) for diffracting the beam reflected from said disk (50) into a zero-order beam (63) and a positive first-order beam (65), wherein said photodetector (44) includes a first photodetector (45) and a second photodetector (47)which receive the zero-order beam (63) and the positive first-order beam (65), respectively, characterized in that said first photodetector (45) is used for reproducing a signal from a relatively thick disk and said second photodetector (47) is used for reproducing a signal from a relatively thin disk. An optical pickup apparatus as claimed in claim 1, wherein said hologram optical element (43) has a pattern for forming the positive first-order beam (65) as an abnormal spherical wave having an inverse spherical aberration with respect to the incident beam. An optical pickup apparatus as claimed in claim 1, further comprising a grating (33) which is located on an optical path between said optical source (31) and said beam splitter (35) for diffracting the incident beam. An optical pickup apparatus as claimed in claim 1, further comprising a collimating lens (37) which is located on an optical path between said optical source (31) and said objective lens (39) for collimating an incident divergent beam. An optical pickup apparatus as claimed in claim 1, further comprising an astigmatism lens (41) located between said beam splitter (35) and said photodetector (44). An optical pickup apparatus as claimed in claim 1, wherein said optical source (31) is a semiconductor laser emitting a beam having a wavelength of about 650nm and said objective lens (39) has a numeric aperture of about 0.6. An optical pickup apparatus as claimed in claim 1, wherein said first and second photodetectors (45, 47) have six divided plates, respectively.

Inventors: LEE YONG-JAE, LEE, YONG-JAE
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1996
Main classification: G11B7
IPC codes: G11B7