Title of Invention	"OPTICAL PICKUP COMPATIBLE WITH A DIGITAL VERSATILE DISK AND A RECORDABLE COMPUTER DISK USING A HOLOGRAPHIC RING LENS"
Abstract	An optical pickup apparatus compatible with at least two types of optical recording media, using light beams having respective different wavelengths for recording and reading information, the optical pickup apparatus including two laser light sources to emit light beams having the different wavelengths, respectively, a holographic lens, including a holographic ring, to transmittal of the light beams emitted from the laser light sources in an inner region of the holographic ring, and diffracting a specific light beam among the light beams emitted from the laser light sources in an outer region relative to the inner region, an objective lens to focus the light beams passed through the holographic ring lens on the respective information recording surfaces of the two types of the optical recording media, optical elements to alter optical paths of the light beams reflected from the information recording surfaces of the optical recording media; and two photodelectors to individually detect optical information from the light beams incident from the optical elements. The optical pickup apparatus is used compatibly with optical recording media by using the holographic lens to eliminate spherical aberration generated when an optical recording medium is changed to another medium having a different thickness, thereby providing advantages which include ease in construction of the holographic lens and good mass-production capabilities.

Title of Invention

"OPTICAL PICKUP COMPATIBLE WITH A DIGITAL VERSATILE DISK AND A RECORDABLE COMPUTER DISK USING A HOLOGRAPHIC RING LENS"

Abstract

An optical pickup apparatus compatible with at least two types of optical recording media, using light beams having respective different wavelengths for recording and reading information, the optical pickup apparatus including two laser light sources to emit light beams having the different wavelengths, respectively, a holographic lens, including a holographic ring, to transmittal of the light beams emitted from the laser light sources in an inner region of the holographic ring, and diffracting a specific light beam among the light beams emitted from the laser light sources in an outer region relative to the inner region, an objective lens to focus the light beams passed through the holographic ring lens on the respective information recording surfaces of the two types of the optical recording media, optical elements to alter optical paths of the light beams reflected from the information recording surfaces of the optical recording media; and two photodelectors to individually detect optical information from the light beams incident from the optical elements. The optical pickup apparatus is used compatibly with optical recording media by using the holographic lens to eliminate spherical aberration generated when an optical recording medium is changed to another medium having a different thickness, thereby providing advantages which include ease in construction of the holographic lens and good mass-production capabilities.

Full Text	BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup apparatus compatible with a digital video disk (DVD) and a recordable compact disk (CD-R), and more particularly, to an optical pickup apparatus which can compatibly record information on and read information from a digital video disk (DVD) and a recordable compact disk (CD-R), respectively, using a holographic lens. 2. Description of the Related Art An optical pickup apparatus records and reads the information such as video, audio or data at a high density, and various types of recording media are a disk, a card and a tape. Among them, the disk type is primarily used. Recently, in the field of the optical disk apparatus, a laser disk (LD), a compact disk (CD) and a digital video disk (DVD) have been developed. Such an optical disk includes a plastic or glass medium having a certain thickness along an axial direction to which light is incident, and a signal recording surface on which information is recorded and located on the plastic or glass medium. So far, a high-density optical disk system enlarges a numerical aperture of an objective lens to increase a recording density, and uses a short wavelength light source of 635 nm or 650 nm. Accordingly, the high-density optical disk system can record or read signals on or from a digital video disk, and can also read signals from a CD. However, to be compatible with a recent type of a CD, that is, a recordable CD (CD-R), light having a wavelength of 780 nm should be used, due to the recording characteristic of the CD-R recording medium. As a result, using the light beam wavelengths of 780 nm and 635 (or 650) nm in a single optical pickup becomes very important for compatibility of the DVD and the CD-R. A conventional optical pickup which is compatible with the DVD and the CD-R will be described below with reference to FIG. 1. FIG. 1 shows an optical pickup using two laser light diodes as light sources for a DVD and a CD-R and a single objective lens. The FIG. 1 optical pickup uses laser light having a wavelength of 835 nm when reproducing a DVD, and uses laser light having a wavelength of 780 nm when recording and reproducing a CD-R. Light having the 635 nm wavelength emitted from a first laser light source 11 is incident to a first collimating lens 12, in which the light is shown in a solid line. The first collimating lens 12 coilimates the incident light beam to be in a parallel light beam. The light beam passing through the first collimating lens 12 is reflected by a beam splitter 13 and then goes to an interference filter prism 14. Light having the 780 nm wavelength emitted from a second laser light source 21 passes through a second collimating lens 22, a beam splitter 23 and a converging lens 24, and then goes to the interference filter prism 14, in which the light is shown in a dotted line. Here, the light beam of the 780 nm wavelength is converged by the interference filter prism 14. An optical system having such a structure is called a "finite optical system." The interference filter prism 14 totally transmits the light beam of the 635 nm wavelength reflected from the beam splitter 13, and totally reflects the light beam of the 780 nm wavelength converged by the converging lens 24. As a result, the light beam outgoing from the first laser light source 11 is incident to a quarter-wave plate 15 in the form of a parallel beam by the collimating lens 12, while the light beam from the second laser light source 21 is incident to the quarter-wave plate 15 in the form of a divergent beam by the converging lens 24 and the interference filter prism 14. The light transmitted through the quarter-wave plate 15 passes through a variable aperture 16 having a thin film structure and then is incident to an objective lens 17. The light beam of the 635 nm wavelength emitted from the first laser light source 11 is focussed by the objective lens 17 on an information recording surface in the DVD 18 having a thickness of 0.6 mm. Therefore, the light reflected from the information recording surface of the DVD 18 contains information recorded on the information recording surface. The reflected light is transmitted by the beam splitter 13, and is then incident to a photodetector 19 for detecting optical information. If the finite optical system described above is not used, when the light beam of the 780 nm wavelength emitted from the second laser light source 21 is focused on an information recording surface in the CD-R 25 having a thickness of 1.2 mm using the above-described objective lens 17, spherical aberration is generated due to a difference in thickness between the DVD 18 and the CD-R 25. The spherical aberration is due to the fact that the distance between the information recording surface of the CD-R 25 and the objective lens 17 is farther than that between the information recording surface of the DVD 18 and the objective lens 17, along an optical axis. To reduce such a spherical aberration, a construction of a finite optical system including the converging lens 24 is required. By using the variable aperture 16 to be described later with reference to FIG. 2, the light beam of the 780 nm wavelength forms an optimized beam spot on the information recording surface of the CD-R 25. The light beam of the 780 nm wavelength reflected from the CD-R 25 is reflected by the beam splitter 23, and then detected in a photodetector 26. The thin-film type variable aperture 16 of FIG. 1, as shown in FIG. 2, has a structure which can selectively transmit the light beams incident to the regions whose numerical aperture (NA) is less than or equal to 0.6, which coincides with the diameter of the objective lens 17. That is, the variable aperture 16 is partitioned into two regions based on the numerical aperture (NA) of 0.45 with respect to an optical axis. Among the two regions, a first region 1 transmits both light beams of 635 nm wavelength and 780 nm wavelength. A second region 2 totally transmits the light beam of the 635 nm wavelength and totally reflects the light beam of the 780 nm wavelength. The region 1 is a region having a numerical aperture less than or equal to 0.45, and the region 2 is an outer region relative to the region 1 in which a dielectric thin film is coated. The region 1 is comprised of a quartz (Si02) thin film to remove any optical aberration generated by the dielectric thin film coated region 2. By using the variable aperture 16, the 780 nm wavelength light transmitted through the region 1 having the 0.45 NA or below forms a beam spot appropriate to the CD-R 25 on the information recording surface thereof. Thus, the FIG. 1 optical pickup uses an optimum beam spot when a disk mode is changed from the DVD 18 to the CD-R 25. Accordingly, the FIG. 1 optical pickup is compatible for use with the CD-R. However, the optical pickup shown in FIG. 1 and as described above should form a "finite optical system" with respect to the 780 nm wavelength light in order to remove any spherical aberration generated when changing a DVD compatibly with a CD-R. Also, due to the optical thin film, that is, the dielectric thin film, which is formed in the region 2 of the variable aperture 16 having the NA of 0.45 or above, an optical path difference between the light transmitted through the region 1 having the NA of 0.45 or below and that transmitted through the region 2 having the NA of 0.45 or above, is generated. To eradicate this difference, it is necessary to form an optical thin film in the region 1. Due to this reason, a quartz coating (SiO.sub.2) is formed in the region 1 and a multi-layer thin film is formed in the region 2. However, such a fabricating process does not only become complicated but also adjustment of the thickness of the thin film should be performed precisely in units of ".mu.m". Thus, it has been difficult to mass-produce the optical pickup. SUMMARY OF THE INVENTION An object of the present invention is to provide an optical pickup apparatus which is compatible with a digital video disk (DVD) and a recordable compact disk (CD-R), by adopting an infinite optical system and using a holographic lens to remove a spherical aberration generated due to a difference in thickness between optical disks. Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. To accomplish the above and other objects of the present invention, there is provided an optical pickup apparatus compatible with at least two types of optical recording media, using light beams having respective different wavelengths for recording and reading information, the optical pickup apparatus including two laser light sources to emit light beams having different wavelengths, respectively, a holographic lens, including a holographic ring, for transmitting both of the light beams emitted from the two laser light sources in an inner region of the holographic ring, and diffracting a specific light beam among the light beams emitted from the laser light sources in an outer region of the holographic ring, an objective lens to focus the light beams passed through the holographic ring lens on the respective information recording surfaces of the two types of the optical recording media, optical elements to alter optical paths of the light beams reflected from the information recording surfaces of the optical recording media, and two photodetectors to individually detector optical information from the light beams incident from the optical elements. The Present invention relates to a an optical pickup apparatus with at least two types of optical recording media, using light beams having respective different wavelength for recording and reading information, the optical pickup apparatus comprising: a plurality of laser light sources to emit light beams having different wavelengths; optical elements to alter optical paths of light beams reflected from an information recording surface of the optical recording media; a photodetector for detecting information of the light beams output from the optical elements; and an objective lens forming light spots on the optical recording media using first and second light beams of respectively different wavelengths, the objective lens comprising: an inner region including an optical center of the objective lens; a holographic region surrounding the inner region; and an outer region surrounding the holographic region; wherein the inner region transmits the first and second light beams, the holographic region diffracts the second light beam, and the outer region transmits the first light beam and a working distance becomes shorter in a relatively thick optical recording medium that in a relatively thin optical recording medium. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and advantages of the invention will become apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: FIG. 1 is a view showing the construction of a conventional optical pickup; FIG. 2 is a view for explaining the structure of a conventional variable aperture shown in FIG. 1; FIG. 3 is a view showing an optical system of an optical pickup according to an embodiment of the present invention; FIG. 4A is a view showing a positional relationship between a holographic ring lens and an objective lens according to the embodiment of the present invention, and FIG. 4B is a view showing the plane surface of the holographic ring lens; FIG. 5A is a view showing the plane surface of the holographic ring lens, and FIG. 5B is a graphical view showing that part of the FIG. 5A region which is enlarged; FIG. 6 is a graphical view showing transmissive efficiency according to the groove depth of the holographic ring lens with regard to two wavelengths; and FIG. 7 is a view showing that the holographic ring lens and the objective lens are integrally incorporated. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to the present preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures. FIG. 3 shows an optical system of an optical pickup according to an embodiment of the resent invention. Referring to FIG. 3, the optical pickup apparatus includes two laser light sources 31 and 39 for emitting light beams having different wavelengths, respectively, two holographic beam splitters 32 and 40 for altering optical paths of the light beams reflected from information recording surfaces of first and second types of optical disks, a beam splitter 33 for totally transmitting or reflecting the incident light beam according to the light wavelength, a collimating lens 34 for collimating the incident light beam to be in a parallel form, a holographic ring lens 35 for diffracting the incident light beam according to its wavelength, and an objective lens 36 for focusing the light beams on the respective information recording surfaces of optical disks 37 and 41. Two photodetectors 38 and 42 which detect the light beams reflected from the respective information recording surfaces of the optical disks 37 and 41 and the laser light sources 31 and 39 are integrally incorporated into single modules to form units 30 and 43, respectively. The operation of the optical pickup constructed above will be described below, in which a DVD and a CD-R are described as optical recording media. First, when recording and/or reading information on a DVD, a light beam having the 650 nm (or 635 nm) wavelength is emitted from the first laser light source 31 and is incident to the holographic beam splitter 32, in which the light is shown as a solid line. The incident light beam passes through the holographic beam splitter 32 and proceeds to the beam splitter 33. When recording and/or reading information about a CD-R, a light beam having the 780 nm wavelength is emitted from the second laser light source 39 and is incident to the holographic beam splitter 40, in which the light is shown as a dotted line. The incident light beam passes through the holographic beam splitter 40 and proceeds to the beam splitter 33. The beam splitter 33 totally transmits the incident light beam of the 650 nm wavelength and totally reflects the incident light beam of the 780 nm wavelength. The totally transmitted or reflected light beam goes to the holographic ring lens 35 in the form of a parallel beam after passing through the collimating lens 34. The holographic ring lens 35 selectively diffracts the incident light beam according to the wavelength thereof, to prevent the generation of spherical aberration with regard to the light beams focused on the information recording surfaces of the optical disks 37 and 41. FIG. 4A is a view showing a positional relationship between the holographic ring lens 35 and an optical surface of the holographic ring lens 35. As shown in FIG. 4A, the objective lens 36 is partitioned into regions A and B. The region A, being closer to an optical axis of the objective lens 36, has little effect on a spherical aberration and the region B, being farther from the optical axis, has a large effect on the spherical aberration. Also, the objective lens 36 is most appropriate for a disk having a thin thickness such as a DVD. Thus, when a DVD is exchanged with a thick disk such as a CD-R to operate the optical pickup, the holographic ring lens 35 is required. If the holographic ring lens 35 is not used when recording and/or reading information on the CD-R, the spherical aberration in the beam spot formed on the information recording surface of the disk becomes large, in which the size is more than 1.7 .mu.m. Generally, the size of the beam spot formed on the information recording surface of the CD-R is 1.4 .mu.m. The holographic ring lens 35 diffracts the 780 nm wavelength light beam passed through the region F of the holographic ring lens 35 so as to prevent the generation of spherical aberration, for which a hologram depicted with dots in FIG. 4B is disposed on the region F of the holographic ring lens 35. Accordingly, the light beam which is incident to the region of the holographic ring lens 35, passes through the objective lens 36 without any diffraction by the holographic ring lens 35, and then is directly focused on the disk. The region F of the light beam which is incident to the holographic ring lens 35, is wavelength-selectively diffracted by the holographic ring lens 35 and then proceeds to the objective lens 36. The diffracted light beam of 780 nm wavelength passing through the objective lens 36 makes the size of the beam spot focused on the disk smaller, and no spherical aberration is generated. A focal plane on which the diffracted 780 nm wavelength light beam passing through the region F is focused should coincide with an optimized surface of the disk on which the 780 nm wavelength light beam passing through the region A is focussed. By using the holographic ring lens 35, a working distance from the surface of the objective lens 36 to the information recording surfaces of the disks becomes shorter in the CD-R 41 rather than in the DVD 37. FIG. 5A is a view showing the structure of the holographic ring lens 35. A region D where a hologram in the holographic ring lens 35 shown in FIG. 5A is provided, corresponds to the numerical aperture of 0.3 - 0.5 which is intended to be appropriate for the CD-R. In FIG. 5A, a symbol E indicates the diameter of the objective lens for a DVD whose numerical aperture (NA) is 0.6. Also, the holographic ring lens 35 used in the present invention can selectively adjust the numerical aperture (NA) of the objective lens according to the wavelengths of the light beam, and requires no separate aperture. The holographic ring lens 35 has the same function as a general spherical lens which transmits a negative optical power and uses a phase shift hologram. An optimized depth of grooves of the hologram should be determined so that the holographic ring lens 35 selectively diffracts the incident light beam according to the wavelength thereof. The holographic rind lens 35 is constructed so that the light beam of the 780nm wavelength has a zero-order transmissive efficiency of 0% with respect to a non-diffracted light beam. For that, the phase variation by the groove depth of the holographic ring lens 35 should be 360°, the holographic ring lens 35 transmits the 650 nm wavelength light. The phase variation should be made by 180o with respect to the 780nm wavelength light, by which the 780nm wavelength light is all diffracted as first-order light. As a result, the holographic ring lens 35 is designed not to diffract the 650 wavelength light, but to diffract the 780nm wavelength light as first-order light. An optimized surface groove depth d of the holographic ring lens 35 for selectively diffracting 650nm and 780nm wavelength light beams is determined by the following equations (1) and (2). (Equation 1 Removed) (Equation 2 Removed) Here, λ. is the 650 nm wavelength,. λ is the 780 nm wavelength, and n and n' denote a reflective index (1.514520) in the 650 nm wavelength and a reflective index (1.511183) in the 780 nm wavelength, respectively. In the above equations (1) and (2), if m=3 and m'=2, the depth d becomes about 3.8µm. FIG. 5B is a graphical view showing an enlarged view of the hologram region D shown in FIG. 5A. The hologram which is formed on the holographic ring 353 has grooves of a constant depth by etching or can be manufactured by molding. Further, grooves of the hologram can be formed stepwisely, together with a ring pattern. The grooves of the hologram can also be formed in a blazed type so as to maximize the diffraction efficiency of a non-zeroth order diffracted light. FIG. 6 is a graphical view showing zero-order transmissive efficiency of the holographic ring according to the wavelengths of incident lights. When the surface groove depth d is 3.8µm, the 650 nm wavelength light is transmitted via the holographic ring 353 by 100% as shown in a solid line overlapped with the symbol"++", and the 780 nm wavelength light is transmitted via the holographic ring 353 by 0% as shown by a solid line overlapped with a circle. At this time, the holographic ring 353 diffracts the 780 nm wavelength light as the first-order light, in which diffraction efficiency thereof is 40%. All of the 650 nm wavelength light incident to the holographic ring lens 35 having the above characteristics is transmitted and then proceeds to the objective lens 36. The incident light beam passes through the objective lens 36 and forms a beam spot on the information recording surface of the DVD 37. The light beam reflected from the information recording surface of the DVD 37 is incident to the holographic ring lens 35. After passing through the holographic ring lens 35, the reflected light beam is incident to the collimating lens 34, the beam splitter 33 and then to the holographic beam splitter 32, wherein the holographic beam splitter 32 directs the reflected light beam to the photodetector 38. The 780 nm wavelength light incident to the holographic ring lens 35 is transmitted in the region A and then proceeds to the objective lens 36, but is diffracted in the holographic ring 353 and then proceeds to the objective lens 36, as shown in FIG. 4A. Therefore, the light beam passing through the objective lens 36 forms an optimized beam spot on the information recording surface of the CD-R 41. The light beam reflected from the information recording surface of the CD-R 41 is incident to the beam splitter 33 and then reflected. The reflected light proceeds to the holographic beam splitter 40 and then is incident to the photodetector 42 by altering the optical path. The holographic ring lens 35 having the above functions may be manufactured integrally with an objective lens by being etched or molded to a constant depth inwards/outwards from one optical surface of the objective lens. The integrally incorporated holographic ring lens has the same function as the holographic ring lens 35. FIG. 7 is a view showing that the holographic ring lens and the objective lens are integrally incorporated. As described above, the optical pickup apparatus according to the present invention is used compatibly with a DVD and a CD-R, by using a holographic ring lens to eliminate a spherical aberration generated in response to a disk being changed to another disk having a different thickness, in which a working distance is shorter in the case of the CD-R than the DVD. Also, the optical pickup apparatus provides advantages which include ease in construction of a holographic ring lens and good mass-production capabilities. While only certain embodiments of the invention have been specifically described herein, it will be apparent that numerous modifications may be made thereto without departing from the spirit and scope of the invention. We claim: 1. An optical pickup apparatus with at least two types of optical recording media, using light beams having respective different wavelength for recording and reading information, the optical pickup apparatus comprising: a plurality of laser light sources to emit light beams having different wavelengths; optical elements to alter optical paths of light beams reflected from an information recording surface of the optical recording media; a photodetector for detecting information of the light beams output from the optical elements; and an objective lens forming light spots on the optical recording media using first and second light beams of respectively different wavelengths, the objective lens comprising: an inner region including an optical center of the objective lens; a holographic region surrounding the inner region; and an outer region surrounding the holographic region; wherein the inner region transmits the first and second light beams, the holographic region diffracts the second light beam, and the outer region transmits the first light beam and a working distance becomes shorter in a relatively thick r, optical recording medium thatiin a relatively thin optical recording medium. 2. The optical pickup apparatus as claimed in claim 1, wherein the holographic region is phase-shifted. 3. The optical pickup apparatus as claimed in claim 1, wherein a focal surface of the second light beam passing through the holographic region that is focused on the optical recording media coincides with a focal surface that the second light beam passing through the inner region is focused on the optical recording media, when reproducing relatively thick optical recording media. 4. The optical pickup apparatus as claimed in claim 3, wherein the holographic region diffracts the second light beam as first order diffracted light and does not diffract the first light beam. 5. The optical pickup apparatus as claimed in claim 4, wherein the holographic region has a positive optical power with respect to the second light beam. 6. The optical pickup apparatus as claimed in claim 4, wherein the holographic region diffracts the second light beam as first order diffracted light. 7. The optical pickup apparatus as claimed in claim 1, wherein the holographic region has grooves of a constant depth in order to diffract the second light beam. 8. The optical pickup apparatus as claimed in claim 7, whereiri the holographic region diffracts the second light beam as first order diffracted light. 9. The optical pickup apparatus as claimed in claim 8, wherein the holographic region has a zero-order transmissive efficiency of approximately 100% for the first light beam, and has a first-order diffraction efficiency of approximately 40% for the second light beam. 10. The optical pickup apparatus as claimed in claim 1, whereiri the holographic region has grooves formed in a stepwise form. 11. The optical pickup apparatus as claimed in claim 1, wherein the objective lens does not diffract a relatively short wavelength light and diffract a relatively long wavelength light according to the wavelengths of light to be incident. 12. The optical pickup apparatus as claimed in claim 1, wherein the objective lens is optimized with respect to a thick optical recording medium in thickness. 13. The optical pickup apparatus as claimed in claim 1, wherein the holographic region has the shape of a ring. 14. The optical pickup apparatus as claimed in claim 7, wherein the depth of the groove causes phase variation according to the wavelengths of light emitted from the laser light sources. 15. The optical pickup apparatus as claimed in claim 14, wherein the phase variation is produced by 360° with respect to a relatively short wavelength of the incident light beam and by 180° with respect to a relatively long wavelength of the incident light beam, between the first light beam and the second light beam emitted from the laser light sources. 16. The optical pickup apparatus as claimed in claim 15, wherein the holographic region does not diffract the incident light beam when the phase variation of 360° is generated, while the holographic regions diffracts the incident light beam to reduce the spherical aberration of a beam spot focused on the optical recording medium when the phase variation of 180° is produced. 17. The optical pickup apparatus as claimed in claim 1, wherein the optical elements are disposed between the laser light sources and the objective lens. 18. The optical pickup apparatus as claimed in claim I, wherein the optical elements are holographic beam splitters. 19. The optical pickup apparatus as claimed in claim 1, wherein the photodetectors and the corresponding laser light sources are integrally incorporated into a single unit. 20. An optical pickup apparatus compatible with at least two types of optical recording media, substantially as herein described with reference to the accompanying drawings.

Full Text

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical pickup apparatus compatible with a digital video disk (DVD) and a recordable compact disk (CD-R), and more particularly, to an optical pickup apparatus which can compatibly record information on and read information from a digital video disk (DVD) and a recordable compact disk (CD-R), respectively, using a holographic lens.
2. Description of the Related Art
An optical pickup apparatus records and reads the information such as video, audio or data at a high density, and various types of recording media are a disk, a card and a tape. Among them, the disk type is primarily used. Recently, in the field of the optical disk apparatus, a laser disk (LD), a compact disk (CD) and a digital video disk (DVD) have been developed. Such an optical disk includes a plastic or glass medium having a certain thickness along an axial direction to which light is incident, and a signal recording surface on which information is recorded and located on the plastic or glass medium.
So far, a high-density optical disk system enlarges a numerical aperture of an objective lens to increase a recording density, and uses a short wavelength light source of 635 nm or 650 nm. Accordingly, the high-density optical disk system can record or read signals on or from a digital video disk, and can also read signals from a CD. However, to be compatible with a recent type of a CD, that is, a recordable CD (CD-R), light having a wavelength of 780 nm should be used, due to the recording characteristic of the CD-R recording medium. As a result, using the light beam wavelengths of 780 nm and 635 (or 650) nm in a single optical pickup becomes very important for compatibility of the DVD and the CD-R. A conventional optical pickup which is compatible with the DVD and the CD-R will be described below with reference to FIG. 1.

FIG. 1 shows an optical pickup using two laser light diodes as light sources for a DVD and a CD-R and a single objective lens. The FIG. 1 optical pickup uses laser light having a wavelength of 835 nm when reproducing a DVD, and uses laser light having a wavelength of 780 nm when recording and reproducing a CD-R.
Light having the 635 nm wavelength emitted from a first laser light source 11 is incident to a first collimating lens 12, in which the light is shown in a solid line. The first collimating lens 12 coilimates the incident light beam to be in a parallel light beam. The light beam passing through the first collimating lens 12 is reflected by a beam splitter 13 and then goes to an interference filter prism 14.
Light having the 780 nm wavelength emitted from a second laser light source 21 passes through a second collimating lens 22, a beam splitter 23 and a converging lens 24, and then goes to the interference filter prism 14, in which the light is shown in a dotted line. Here, the light beam of the 780 nm wavelength is converged by the interference filter prism 14. An optical system having such a structure is called a "finite optical system." The interference filter prism 14 totally transmits the light beam of the 635 nm wavelength reflected from the beam splitter 13, and totally reflects the light beam of the 780 nm wavelength converged by the converging lens 24. As a result, the light beam outgoing from the first laser light source 11 is incident to a quarter-wave plate 15 in the form of a parallel beam by the collimating lens 12, while the light beam from the second laser light source 21 is incident to the quarter-wave plate 15 in the form of a divergent beam by the converging lens 24 and the interference filter prism 14. The light transmitted through the quarter-wave plate 15 passes through a variable aperture 16 having a thin film structure and then is incident to an objective lens 17.
The light beam of the 635 nm wavelength emitted from the first laser light source 11 is focussed by the objective lens 17 on an information recording surface in the DVD 18 having a thickness of 0.6 mm. Therefore, the light reflected from the information recording surface of the DVD 18 contains information recorded on the information recording surface. The reflected light is transmitted by the beam splitter 13, and is then incident to a photodetector 19 for detecting optical information.

If the finite optical system described above is not used, when the light beam of the 780 nm wavelength emitted from the second laser light source 21 is focused on an information recording surface in the CD-R 25 having a thickness of 1.2 mm using the above-described objective lens 17, spherical aberration is generated due to a difference in thickness between the DVD 18 and the CD-R 25. The spherical aberration is due to the fact that the distance between the information recording surface of the CD-R 25 and the objective lens 17 is farther than that between the information recording surface of the DVD 18 and the objective lens 17, along an optical axis. To reduce such a spherical aberration, a construction of a finite optical system including the converging lens 24 is required. By using the variable aperture 16 to be described later with reference to FIG. 2, the light beam of the 780 nm wavelength forms an optimized beam spot on the information recording surface of the CD-R 25. The light beam of the 780 nm wavelength reflected from the CD-R 25 is reflected by the beam splitter 23, and then detected in a photodetector 26.
The thin-film type variable aperture 16 of FIG. 1, as shown in FIG. 2, has a structure which can selectively transmit the light beams incident to the regions whose numerical aperture (NA) is less than or equal to 0.6, which coincides with the diameter of the objective lens 17. That is, the variable aperture 16 is partitioned into two regions based on the numerical aperture (NA) of 0.45 with respect to an optical axis. Among the two regions, a first region 1 transmits both light beams of 635 nm wavelength and 780 nm wavelength. A second region 2 totally transmits the light beam of the 635 nm wavelength and totally reflects the light beam of the 780 nm wavelength. The region 1 is a region having a numerical aperture less than or equal to 0.45, and the region 2 is an outer region relative to the region 1 in which a dielectric thin film is coated. The region 1 is comprised of a quartz (Si02) thin film to remove any optical aberration generated by the dielectric thin film coated region 2.
By using the variable aperture 16, the 780 nm wavelength light transmitted through the region 1 having the 0.45 NA or below forms a beam spot appropriate to the CD-R 25 on the information recording surface thereof. Thus, the FIG. 1 optical pickup uses an optimum beam spot when a disk mode is changed from the DVD 18 to the CD-R 25. Accordingly, the FIG. 1 optical pickup is compatible for use with the CD-R.

However, the optical pickup shown in FIG. 1 and as described above should form a "finite optical system" with respect to the 780 nm wavelength light in order to remove any spherical aberration generated when changing a DVD compatibly with a CD-R. Also, due to the optical thin film, that is, the dielectric thin film, which is formed in the region 2 of the variable aperture 16 having the NA of 0.45 or above, an optical path difference between the light transmitted through the region 1 having the NA of 0.45 or below and that transmitted through the region 2 having the NA of 0.45 or above, is generated. To eradicate this difference, it is necessary to form an optical thin film in the region 1. Due to this reason, a quartz coating (SiO.sub.2) is formed in the region 1 and a multi-layer thin film is formed in the region 2. However, such a fabricating process does not only become complicated but also adjustment of the thickness of the thin film should be performed precisely in units of ".mu.m". Thus, it has been difficult to mass-produce the optical pickup.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an optical pickup apparatus which is compatible with a digital video disk (DVD) and a recordable compact disk (CD-R), by adopting an infinite optical system and using a holographic lens to remove a spherical aberration generated due to a difference in thickness between optical disks.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
To accomplish the above and other objects of the present invention, there is provided an optical pickup apparatus compatible with at least two types of optical recording media, using light beams having respective different wavelengths for recording and reading information, the optical pickup apparatus including two laser light sources to emit light beams having different wavelengths, respectively, a holographic lens, including a holographic ring, for transmitting both of the light beams emitted from the two laser light sources in an inner region of the holographic ring, and diffracting a specific light beam among the light beams emitted from the laser light sources in an outer region of the holographic ring, an objective lens to focus the light beams passed through the holographic ring lens on the

respective information recording surfaces of the two types of the optical recording media, optical elements to alter optical paths of the light beams reflected from the information recording surfaces of the optical recording media, and two photodetectors to individually detector optical information from the light beams incident from the optical elements.
The Present invention relates to a an optical pickup apparatus with at least two
types of optical recording media, using light beams having respective different
wavelength for recording and reading information, the optical pickup apparatus
comprising:
a plurality of laser light sources to emit light beams having different wavelengths;
optical elements to alter optical paths of light beams reflected from an information
recording surface of the optical recording media;
a photodetector for detecting information of the light beams output from the
optical elements; and
an objective lens forming light spots on the optical recording media using first
and second light beams of respectively different wavelengths, the objective lens
comprising:
an inner region including an optical center of the objective lens;
a holographic region surrounding the inner region; and
an outer region surrounding the holographic region;
wherein the inner region transmits the first and second light beams, the holographic region diffracts the second light beam, and the outer region transmits the first light beam and a working distance becomes shorter in a relatively thick optical recording medium that in a relatively thin optical recording medium.

BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects and advantages of the invention will become apparent
and more readily appreciated from the following description of the preferred
embodiments, taken in conjunction with the accompanying drawings of which:
FIG. 1 is a view showing the construction of a conventional optical pickup;
FIG. 2 is a view for explaining the structure of a conventional variable aperture
shown in FIG. 1;
FIG. 3 is a view showing an optical system of an optical pickup according to an
embodiment of the present invention;
FIG. 4A is a view showing a positional relationship between a holographic ring
lens and an objective lens according to the embodiment of the present invention,
and
FIG. 4B is a view showing the plane surface of the holographic ring lens;
FIG. 5A is a view showing the plane surface of the holographic ring lens, and
FIG. 5B is a graphical view showing that part of the FIG. 5A region which is
enlarged;
FIG. 6 is a graphical view showing transmissive efficiency according to the
groove depth of the holographic ring lens with regard to two wavelengths; and
FIG. 7 is a view showing that the holographic ring lens and the objective lens are
integrally incorporated.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Reference will now be made in detail to the present preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures.

FIG. 3 shows an optical system of an optical pickup according to an embodiment of the resent invention. Referring to FIG. 3, the optical pickup apparatus includes two laser light sources 31 and 39 for emitting light beams having different wavelengths, respectively, two holographic beam splitters 32 and 40 for altering optical paths of the light beams reflected from information recording surfaces of first and second types of optical disks, a beam splitter 33 for totally transmitting or reflecting the incident light beam according to the light wavelength, a collimating lens 34 for collimating the incident light beam to be in a parallel form, a holographic ring lens 35 for diffracting the incident light beam according to its wavelength, and an objective lens 36 for focusing the light beams on the respective information recording surfaces of optical disks 37 and 41. Two photodetectors 38 and 42 which detect the light beams reflected from the respective information recording surfaces of the optical disks 37 and 41 and the laser light sources 31 and 39 are integrally incorporated into single modules to form units 30 and 43, respectively. The operation of the optical pickup constructed above will be described below, in which a DVD and a CD-R are described as optical recording media.
First, when recording and/or reading information on a DVD, a light beam having the 650 nm (or 635 nm) wavelength is emitted from the first laser light source 31 and is incident to the holographic beam splitter 32, in which the light is shown as a solid line. The incident light beam passes through the holographic beam splitter 32 and proceeds to the beam splitter 33. When recording and/or reading information about a CD-R, a light beam having the 780 nm wavelength is emitted from the second laser light source 39 and is incident to the holographic beam splitter 40, in which the light is shown as a dotted line. The incident light beam passes through the holographic beam splitter 40 and proceeds to the beam splitter 33.
The beam splitter 33 totally transmits the incident light beam of the 650 nm wavelength and totally reflects the incident light beam of the 780 nm wavelength. The totally transmitted or reflected light beam goes to the holographic ring lens 35 in the form of a parallel beam after passing through the collimating lens 34. The holographic ring lens 35 selectively diffracts the incident light beam according to the wavelength thereof, to prevent the generation of spherical aberration with regard to the light beams focused on the information recording surfaces of the optical disks 37 and 41.

FIG. 4A is a view showing a positional relationship between the holographic ring
lens 35 and an optical surface of the holographic ring lens 35. As shown in FIG.
4A, the objective lens 36 is partitioned into regions A and B. The region A, being
closer to an optical axis of the objective lens 36, has little effect on a spherical
aberration and the region B, being farther from the optical axis, has a large effect
on the spherical aberration. Also, the objective lens 36 is most appropriate for a
disk having a thin thickness such as a DVD. Thus, when a DVD is exchanged
with a thick disk such as a CD-R to operate the optical pickup, the holographic
ring lens 35 is required. If the holographic ring lens 35 is not used when
recording and/or reading information on the CD-R, the spherical aberration in the
beam spot formed on the information recording surface of the disk becomes
large, in which the size is more than 1.7 .mu.m. Generally, the size of the beam
spot formed on the information recording surface of the CD-R is 1.4 .mu.m. The
holographic ring lens 35 diffracts the 780 nm wavelength light beam passed
through the region F of the holographic ring lens 35 so as to prevent the
generation of spherical aberration, for which a hologram depicted with dots in
FIG. 4B is disposed on the region F of the holographic ring lens 35. Accordingly,
the light beam which is incident to the region of the holographic ring lens 35,
passes through the objective lens 36 without any diffraction by the holographic
ring lens 35, and then is directly focused on the disk. The region F of the light
beam which is incident to the holographic ring lens 35, is wavelength-selectively
diffracted by the holographic ring lens 35 and then proceeds to the objective lens
36. The diffracted light beam of 780 nm wavelength passing through the
objective lens 36 makes the size of the beam spot focused on the disk smaller,
and no spherical aberration is generated. A focal plane on which the diffracted
780 nm wavelength light beam passing through the region F is focused should
coincide with an optimized surface of the disk on which the 780 nm wavelength
light beam passing through the region A is focussed. By using the holographic
ring lens 35, a working distance from the surface of the objective lens 36 to the
information recording surfaces of the disks becomes shorter in the CD-R 41
rather than in the DVD 37.
FIG. 5A is a view showing the structure of the holographic ring lens 35. A region D where a hologram in the holographic ring lens 35 shown in FIG. 5A is provided, corresponds to the numerical aperture of 0.3 - 0.5 which is intended to be appropriate for the

CD-R. In FIG. 5A, a symbol E indicates the diameter of the objective lens for a DVD whose numerical aperture (NA) is 0.6. Also, the holographic ring lens 35 used in the present invention can selectively adjust the numerical aperture (NA) of the objective lens according to the wavelengths of the light beam, and requires no separate aperture. The holographic ring lens 35 has the same function as a general spherical lens which transmits a negative optical power and uses a phase shift hologram. An optimized depth of grooves of the hologram should be determined so that the holographic ring lens 35 selectively diffracts the incident light beam according to the wavelength thereof. The holographic rind lens 35 is constructed so that the light beam of the 780nm wavelength has a zero-order transmissive efficiency of 0% with respect to a non-diffracted light beam. For that, the phase variation by the groove depth of the holographic ring lens 35 should be 360°, the holographic ring lens 35 transmits the 650 nm wavelength light. The phase variation should be made by 180o with respect to the 780nm wavelength light, by which the 780nm wavelength light is all diffracted as first-order light. As a result, the holographic ring lens 35 is designed not to diffract the 650 wavelength light, but to diffract the 780nm wavelength light as first-order light. An optimized surface groove depth d of the holographic ring lens 35 for selectively diffracting 650nm and 780nm wavelength light beams is determined by the following equations (1) and (2).
(Equation 1 Removed)
(Equation 2 Removed)
Here, λ. is the 650 nm wavelength,. λ is the 780 nm wavelength, and n and n' denote a reflective index (1.514520) in the 650 nm wavelength and a reflective index (1.511183) in the 780 nm wavelength, respectively. In the above equations (1) and (2), if m=3 and m'=2, the depth d becomes about 3.8µm.
FIG. 5B is a graphical view showing an enlarged view of the hologram region D shown in FIG. 5A. The hologram which is formed on the holographic ring 353 has grooves of a constant depth by etching or can be manufactured by molding. Further, grooves of the hologram can be formed stepwisely, together with a ring pattern. The grooves of the hologram can also be formed in a blazed type so as to maximize the diffraction efficiency of a non-zeroth order diffracted light.
FIG. 6 is a graphical view showing zero-order transmissive efficiency of the holographic ring according to the wavelengths of incident lights. When the surface groove depth d is 3.8µm, the 650 nm wavelength light is transmitted via the holographic ring 353 by 100% as shown in a solid line overlapped with the symbol"++", and the 780 nm wavelength light is transmitted via the holographic ring 353 by 0% as shown by a solid line overlapped with a circle. At this time, the holographic ring 353 diffracts the 780 nm wavelength light as the first-order light, in which diffraction efficiency thereof is 40%.
All of the 650 nm wavelength light incident to the holographic ring lens 35 having the above characteristics is transmitted and then proceeds to the objective lens 36. The incident light beam passes through the objective lens 36 and forms a beam spot on the information recording surface of the DVD 37. The light beam reflected from the information recording surface of the DVD 37 is incident to the holographic ring lens 35. After passing through the holographic ring lens 35, the reflected light beam is incident to the collimating lens 34, the beam splitter 33 and then to the holographic beam splitter 32, wherein the holographic beam splitter 32 directs the reflected light beam to the photodetector 38. The 780 nm wavelength light incident to the holographic ring lens 35 is transmitted in the region A and then proceeds to the objective lens 36, but is diffracted in the holographic ring 353 and then proceeds to the objective lens 36, as shown in FIG. 4A. Therefore, the light beam passing through the objective lens 36 forms an optimized beam spot on the information recording surface of the CD-R 41. The light beam reflected from the information recording surface of the CD-R 41 is incident to the beam
splitter 33 and then reflected. The reflected light proceeds to the holographic beam splitter 40 and then is incident to the photodetector 42 by altering the optical path.
The holographic ring lens 35 having the above functions may be manufactured integrally with an objective lens by being etched or molded to a constant depth inwards/outwards from one optical surface of the objective lens. The integrally incorporated holographic ring lens has the same function as the holographic ring lens 35. FIG. 7 is a view showing that the holographic ring lens and the objective lens are integrally incorporated.
As described above, the optical pickup apparatus according to the present invention is used compatibly with a DVD and a CD-R, by using a holographic ring lens to eliminate a spherical aberration generated in response to a disk being changed to another disk having a different thickness, in which a working distance is shorter in the case of the CD-R than the DVD. Also, the optical pickup apparatus provides advantages which include ease in construction of a holographic ring lens and good mass-production capabilities.
While only certain embodiments of the invention have been specifically described herein, it will be apparent that numerous modifications may be made thereto without departing from the spirit and scope of the invention.

We claim:
1. An optical pickup apparatus with at least two types of optical recording media, using light beams having respective different wavelength for recording and reading information, the optical pickup apparatus comprising:
a plurality of laser light sources to emit light beams having different wavelengths;
optical elements to alter optical paths of light beams reflected from an information recording surface of the optical recording media;
a photodetector for detecting information of the light beams output from the optical elements; and
an objective lens forming light spots on the optical recording media using first and second light beams of respectively different wavelengths, the objective lens comprising:
an inner region including an optical center of the objective lens;
a holographic region surrounding the inner region; and
an outer region surrounding the holographic region;
wherein the inner region transmits the first and second light beams, the holographic region diffracts the second light beam, and the outer region transmits
the first light beam and a working distance becomes shorter in a relatively thick
r, optical recording medium thatiin a relatively thin optical recording medium.
2. The optical pickup apparatus as claimed in claim 1, wherein the holographic
region is phase-shifted.
3. The optical pickup apparatus as claimed in claim 1, wherein a focal surface of the
second light beam passing through the holographic region that is focused on the
optical recording media coincides with a focal surface that the second light beam
passing through the inner region is focused on the optical recording media, when
reproducing relatively thick optical recording media.

4. The optical pickup apparatus as claimed in claim 3, wherein the holographic
region diffracts the second light beam as first order diffracted light and does not
diffract the first light beam.
5. The optical pickup apparatus as claimed in claim 4, wherein the holographic
region has a positive optical power with respect to the second light beam.
6. The optical pickup apparatus as claimed in claim 4, wherein the holographic
region diffracts the second light beam as first order diffracted light.
7. The optical pickup apparatus as claimed in claim 1, wherein the holographic
region has grooves of a constant depth in order to diffract the second light beam.
8. The optical pickup apparatus as claimed in claim 7, whereiri the holographic
region diffracts the second light beam as first order diffracted light.
9. The optical pickup apparatus as claimed in claim 8, wherein the holographic
region has a zero-order transmissive efficiency of approximately 100% for the
first light beam, and has a first-order diffraction efficiency of approximately 40%
for the second light beam.
10. The optical pickup apparatus as claimed in claim 1, whereiri the holographic
region has grooves formed in a stepwise form.
11. The optical pickup apparatus as claimed in claim 1, wherein the objective lens
does not diffract a relatively short wavelength light and diffract a relatively long
wavelength light according to the wavelengths of light to be incident.
12. The optical pickup apparatus as claimed in claim 1, wherein the objective lens is
optimized with respect to a thick optical recording medium in thickness.

13. The optical pickup apparatus as claimed in claim 1, wherein the holographic
region has the shape of a ring.
14. The optical pickup apparatus as claimed in claim 7, wherein the depth of the
groove causes phase variation according to the wavelengths of light emitted from
the laser light sources.
15. The optical pickup apparatus as claimed in claim 14, wherein the phase variation
is produced by 360° with respect to a relatively short wavelength of the incident
light beam and by 180° with respect to a relatively long wavelength of the
incident light beam, between the first light beam and the second light beam
emitted from the laser light sources.
16. The optical pickup apparatus as claimed in claim 15, wherein the holographic
region does not diffract the incident light beam when the phase variation of 360°
is generated, while the holographic regions diffracts the incident light beam to
reduce the spherical aberration of a beam spot focused on the optical recording
medium when the phase variation of 180° is produced.
17. The optical pickup apparatus as claimed in claim 1, wherein the optical elements
are disposed between the laser light sources and the objective lens.
18. The optical pickup apparatus as claimed in claim I, wherein the optical elements
are holographic beam splitters.
19. The optical pickup apparatus as claimed in claim 1, wherein the photodetectors
and the corresponding laser light sources are integrally incorporated into a single
unit.

20. An optical pickup apparatus compatible with at least two types of optical recording media, substantially as herein described with reference to the accompanying drawings.

Documents:

3109-DEL-1998-Abstract.pdf

3109-del-1998-claims.pdf

3109-del-1998-correspondence-others.pdf

3109-del-1998-correspondence-po.pdf

3109-DEL-1998-Description (Complete).pdf

3109-del-1998-drawings.pdf

3109-del-1998-form-1.pdf

3109-del-1998-form-13.pdf

3109-del-1998-form-19.pdf

3109-del-1998-form-2.pdf

3109-del-1998-form-3.pdf

3109-del-1998-form-4.pdf

3109-del-1998-pa.pdf

3109-del-1998-petition-124.pdf

3109-del-1998-petition-138.pdf

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Patent Number

232392

Indian Patent Application Number

3109/DEL/1998

PG Journal Number

13/2009

Publication Date

27-Mar-2009

Grant Date

16-Mar-2009

Date of Filing

23-Oct-1998

Name of Patentee

SAMSUNG ELECTRONICS CO., LTD.

Applicant Address

416, MAETAN-DONG, PALDAL-GU, SUWON-CITY, KYUNGKI-DO, REPUBLIC OF KOREA

Inventors:

#	Inventor's Name	Inventor's Address
1	JANG-HOON YOO	2-1002, SHINDONGAH APT., 777-1, DAERIM 3-DONG, YOUNGDUNGPO-GU, SEOUL, REPUBLIC OF KOREA
2	CHUL-WOO LEE	32-902, HYUNDAI APT., ICHON 1-DONG, YONGSAN-GU, SEOUL, REPUBLIC OF KOREA

PCT International Classification Number

G11B 7/085

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA