Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 10/458,321, filed Jun. 11, 2003 and now U.S. Pat. No. 6,985,293, which is a continuation of U.S. patent application Ser. No. 09/516,507, filed Mar. 1, 2000, which issued as U.S. Pat. No. 6,791,933, which is a divisional of U.S. patent application Ser. No. 08/921,386, filed Aug. 29, 1997 and which issued as U.S. Pat. No. 6,222,812, which claims the benefit of U.S. Provisional Patent Application No. 60/025,100, filed Sep. 3, 1996 in the U.S. Patent and Trademark Office and of Korean Application No. 96-37946, filed Aug. 29, 1996 in the Korean Intellectual Property Office, the disclosures of which are incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical pickup apparatus which can record information on and read information from a digital video disk (DVD) and a recordable compact disk (CD-R), respectively. 
   2. Description of the Related Art 
   Recoding media for recording and reading the information such as using video, audio or data, are a disk, a card, or a tape. Among them, the disk type is chiefly 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 in order 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 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 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 635 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 laser diode light source  1  passes through a collimating lens  2  and a polarization beam splitter  3  and then goes to an interference filter type prism  4 . Light having the 780 nm wavelength emitted from a laser diode light source  11  passes through a collimating lens  12 , a beam splitter  13  and a converging lens  14  and then goes to the prism  4 , which converges the light having the 780 nm wavelength. An optical system having such a structure is called a “finite optical system.” The prism  4  transmits the light beam having a wavelength of 635 nm reflected from the polarization beam splitter  3 , and reflects the light beam converged by the converging lens  14 . As a result, the light beam from the light source  1  is incident to a quarter-wave plate  5  in the form of a parallel beam by the collimating lens  2 , while the light from the light source  11  is incident to the quarter-wave plate  5  in the form of a divergent beam by the convergent lens  14  and the prism  4 . The light transmitting through the quarter-wave plate  5  is incident to an objective lens  7 . 
   The light of the 635 nm wavelength emitted from the light source  1  is focused by an objective lens  7  on a signal recording surface in a DVD  8  having a thickness of 0.6 mm. Therefore, the light reflected from the signal recording surface of the DVD  8  contains information recorded on the signal recording surface. The reflected light transmits through the polarization beam splitter  3 , and is then incident to a light detector  10  for detecting optical information. 
   If the finite optical system described above is not used, when the 780 nm wavelength light emitted from the light source  11  is focused on a signal recording surface in the CD-R  9  having 1.2 mm thickness using the above-described objective lens  7 , spherical aberration is generated due to a difference in thickness between the DVD  8  and the CD-R  9 . In more detail, the spherical aberration is due to a fact that the distance between the signal recording surface of the CD-R  9  and the objective lens  7  is farther than that between the signal recording surface of the DVD  8  and the objective lens  7 , along an optical axis. To reduce such a spherical aberration, a construction of a finite optical system including a convergent lens  14  is required. By using a variable aperture  6  to be described later with reference to  FIG. 2 , the 780 nm wavelength light forms an optimized beam spot on the signal recording surface of the CD-R  9 . The 780 nm wavelength light reflected from the CD-R  9  is reflected by the prism  4  and then the beam splitter  13 , so as to be detected in the light detector  15 . 
   The variable aperture  6  of  FIG. 1  has a thin film structure as shown in  FIG. 2  which can selectively transmit the rays of the light incident to the region if not more than the numerical aperture (NA) of 0.6 which coincides with the diameter of the objective lens  7 . That is, the variable aperture  6  is partitioned into two regions based on the NA of 0.45 with respect to an optical axis. Among the two regions, a first region  1  transmits both 635 nm and 780 nm wavelength light and a second region  2  totally transmits the 635 nm wavelength light and totally reflects the 780 nm wavelength light. The region  1  has the numerical aperture of 0.45 or below and the region  2  is an outer region of the region  1  and is made by coating a dielectric thin film. The region  1  is comprised of a quartz (SiO 2 ) thin film in order to remove any optical aberration generated by the dielectric thin film coated region  2 . By using the variable aperture  6 , the 780 nm wavelength light transmitting the region  1  having the 0.45 NA or below forms a beam spot appropriate to the CD-R  9  on the signal recording surface thereof. Thus, the  FIG. 1  optical pickup uses an optimum light spot when a disk mode is changed from the DVD  8  to the CD-R  9 . Accordingly, the  FIG. 1  optical pickup is compatible for use with the CD-R. 
   However, the  FIG. 1  optical pickup 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  having the NA of 0.45 or above, an optical path difference between the light transmitting the region  1  having the NA of 0.45 or below and that transmitting 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 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 become only complicated but also adjustment of the thickness of the thin film should be performed precisely in units of “μm” Thus, it has been difficult in mass-producing 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 and a recordable compact disk by removing a spherical aberration using a phase plate. 
   To accomplish the above object of the present invention, there is provided an optical pickup apparatus for at least two optical recording media, which are different in distance from an optical pickup to information recording surfaces and uses light beams of different wavelengths for recoding and reading information, the optical pickup apparatus comprising: 
   a plurality of laser light sources for emitting a first light beam having a relatively shorter wavelength and a second light beam having a relatively longer wavelength, respectively; 
   an objective lens having a predetermined focal length in which the focal point of the objective lens according to the first light beam coincides with the position of the information recording surface in a first optical recording medium having the information recording surface closer to the objective lens; 
   optical detection means; 
   means for controlling an optical path so that the light beam emitted from the laser light sources is directed to the objective lens and the light output from the objective lens is directed to the optical detection means; and 
   phase shift means, coupled between the optical path control means and the objective lens, for shifting the phase of the second light beam proceeding from the optical path control means to the objective lens, thereby reducing the size of a beam spot which is formed on the position of information recording surface in the second optical recording medium having the information recording surface farther from the objective lens by the second light beam focused by the objective lens, 
   wherein a wavelength of one of the first light and second light beams is used according to the optical recording medium used. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments are described with reference to the drawings wherein: 
       FIG. 1  is a view of an conventional optical pickup using two laser diodes as light sources for a digital video disk (DVD) and a recordable compact disk (CD-R) and a single objective lens; 
       FIG. 2  is a view for explaining a variable aperture shown in  FIG. 1 ; 
       FIG. 3  is a view showing an optical system of an optical pickup according to a preferred embodiment of the 
       FIG. 4  shows a phase plate unit and an annular shield objective lens shown in  FIG. 3 ; 
       FIG. 5  is a view showing an optical system of an optical pickup according to another preferred embodiment of the present invention; 
       FIG. 6  shows an annular shield objective lens having a phase plate function as shown in  FIG. 5 ; 
       FIGS. 7A and 7B  are views showing the combined structure of a phase plate and a variable aperture according to the present invention; 
       FIG. 8  is a graphical diagram showing reduction effect of a spot size and a side lobe according to the present invention; 
       FIG. 9  is a graphical diagram showing characteristics of a focus servo signal during reproduction of a CD-R disk; 
       FIG. 10  is a graphical diagram showing phase variation of the light according to the depth of the groove on the phase plate; and 
       FIG. 11  is a graphical diagram showing variation of diffraction efficiency of zero-order diffracted light corresponding to the groove depth of the variable aperture according to the present invention. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   Preferred embodiments of the present invention will be described below in more detail with reference to the accompanying drawings. 
     FIG. 3  shows an optical system of an optical pickup according to a preferred embodiment of the present invention. Referring to  FIG. 3 , when a laser diode light source  31  operates, the 650 nm wavelength light emitted in the divergent form from the light source  31  is sequentially reflected and transmitted by a first polarization beam splitter  32  and a second polarization beam splitter  33 . The light transmitted by the second polarization beam splitter  33  is incident to a collimating lens  34 . When a laser diode light source  40  operates, the 780 nm wavelength light emitted in the divergent form from the light source  40  is reflected by the second polarization beam splitter  33  and then, is incident to the collimating lens  34 . The collimating lens  34  collimates the light beam incident from the second polarization beam splitter  33  to be parallel to an optical axis perpendicular to the surface of a variable aperture  35 , and the collimated light is selectively transmitted by wavelength by the variable aperture  35 . 
   Referring to  FIGS. 7A and 7B , the variable aperture  35  has a region  3  for transmitting both the 780 nm wavelength light and the 650 nm wavelength light and a region  4  for transmitting only the 650 nm wavelength light. The region  4  has a hologram structure. The hologram structure includes a diffraction grating portion whose diffraction efficiency is maximized with respect to the 780 nm wavelength light having a diffraction order of non-zero and whose diffraction efficiency is 100% with respect to the 650 nm wavelength light having the diffraction order of zero. Therefore, the 650 nm wavelength light can be transmitted without diffraction by the hologram structure. Referring to  FIG. 11  showing the diffraction efficiency of zero-order diffracted light corresponding to the groove depth of the diffraction grating portion, when the groove depth is 3.8 μm, the 650 nm wavelength light has the diffraction efficiency of 100% as shown in a solid line overlapped with the symbol “++”, and the 780 nm wavelength light has the diffraction efficiency of 0% as shown in a solid line overlapped with a circle. Therefore, the region  4  of the variable aperture  35  is designed with the diffraction grating portion having a groove depth of 3.8 μm. In this embodiment, a NA of 0.5 is used for partitioning the regions  3  and  4 . Therefore, the region  3  is the portion having a NA of 0.5 or below, and the region  4  is a portion having a NA more than 0.5. Thus, according to the embodiment of the present invention, the light beam transmitting the portion having a NA not more than 0.6 coinciding with the diameter of the objective lens  37  is selectively transmitted in the regions  3  and  4  of the variable aperture  35  according to the wavelengths. The variable aperture shown in  FIG. 7B  which is constructed with a hologram pattern of an asymmetric shape, eradicates a feedback noise produced by the light proceeding to an optical detection portion. 
   The light beam transmitting the variable aperture  35  transmits through a phase plate  36  (to be described later with reference to  FIG. 4 ), and then is incident to an annular shielding objective lens  37 . The objective lens  37  according to the present invention is designed to be focussed on an information recording surface of the DVD  8 . If the phase plate  36  of the present invention is not used, the size of the light spot formed in the information recording surface of the CD-R  9  becomes 1.8 μm or above when changing the disk currently in use from the DVD  8  to the CD-R  9 . However, since the conventional size of the light spot which is used in the CD-R  9  is generally 1.4 μm, information cannot be recorded on or read from the CD-R  9  via a light spot having a size of 1.8 μm. Therefore, the present invention uses the phase plate  36  in order to reduce the size of the light spot so that information can be recorded or read on or from the CD-R  9 . 
   The phase plate  36  is, as shown in  FIG. 3 , positioned between the variable aperture  35  and the objective lens  37 . The phase plate  36  includes an annular groove  361  (see  FIG. 4 ) which is concave inwards from the surface closer to the variable aperture  35  and has a predetermined width and depth. The annular groove  361  is manufactured by injection molding or conventional molding using an etch or metal mold, in which the depth D is determined by the following equations (1) and (2).
 
2π n′D/λ′− 2π D /λ′=(2 m ′)π  (1)
 
2π n′D/λ′− 2π D /λ′=(2 m+ 1)π  (2)
 
   Here, m is an integer, n′ and n denote a refractive index at wavelength λ′ (650 nm) and λ (780 nm), respectively. In the above equations (1) and (2), if m′=3 and m=2, the depth D of the annular groove  361  becomes about 3.9 μm. The phase plate  36  having the annular groove  361  of the depth D phases-shifts the 780 nm wavelength light by 180° and phase-shifts the 650 nm wavelength light by 360° when the two wavelengths proceed to the objective lens  37  from the variable aperture  35 .  FIG. 10  is a graphical diagram showing phase variation of the two wavelengths according to the depth D of the annular groove  361  on the phase plate  36 , in which a solid line represents the phase variation with represents that with respect to the 780 nm wavelength light. When D is 3.9 μm, the 780 nm wavelength light has the phase of 180° and the 650 nm light has the phase of 360°. 
   Thus, the 780 nm wavelength light which is phase-shifted by 180° has a substantially super-resolution effect and passes through an aperture compared with the case when the phase plate  36  is not used. By using the phase plate  36 , the size of the light spot formed on the information recording surface in the CD-R  9  is reduced to a degree such that information can be recorded or read on or from the CD-R  9 , to thereby remove any spherical aberration. 
   The phase plate  36  can be modified into a protrusion form having a predetermined width and height protruding outwards from the surface closer to the variable aperture  35 . Since such a modification is apparent to one having an ordinary skill in the art who knows the function of the phase plate, the detailed description thereof will be omitted. 
   The objective lens  37 , to which the light transmitting the phase plate  36  is incident, includes an annular shielding portion  371  as shown in  FIG. 4 . The annular shielding portion  371  shields part of the light transmitting the region  3 . Thus, the spherical aberration due to the changing of the DVD  8  to the CD-R  9  is reduced, and the sensitivity of the focus error signal in the focus servo system (not shown) is increased. 
   The light beam reflected from the information recording surface of the DVD  8  or CD-R  9  proceeds to a light detection lens  38  from the objective lens  37 , and is focussed in the light detector  39  by the light detection lens  38 . Thus, the  FIG. 3  apparatus can record or read information on or from both the DVD  8  and CD-R  9 . 
     FIG. 6  shows an objective lens  47  which is constructed by combining a phase plate  36  and an objective lens  37  of  FIG. 3  into a single unit.  FIG. 5  shows an optical system of an optical pickup having such an objective lens  47 . The  FIG. 6  objective lens  47  includes an annular groove  471  which is concave inwards from the surface closer to the variable aperture  35  and has a predetermined width and depth. The objective lens  47 , which is engraved with such an annular groove  471 , phase-shifts the 780 nm wavelength light by 180° as in the phase plate  36  and phase-shifts the 650 nm wavelength light by 360° . Thus, among the 780 nm wavelength light incident to the objective lens  47  from the variable aperture  35 , the light beam diffracted by the annular groove  471  serves to decrease the spherical aberration with respect to the CD-R  9 . The annular groove  471  removes the spherical aberration when the DVD  8  is exchanged with the CD-R  9 . Accordingly, a beam spot of a small size is formed on the information recording surface so that information can be recorded or read on or from the CD-R  9  with respect to the 780 nm wavelength light. The  FIG. 5  optical pickup includes a single unit  49  combining a light source  491  with a light detector  493  for the 780 nm wavelength light, in addition to a light source  31 , a light detection lens  51  and a light detector  53  for the 650 nm wavelength light. The  FIG. 5  optical pickup further includes a hologram type beam splitter  48  for the light output from the light source  491  of the unit  49  and the light incident to the light detector  493 . Since the construction and operation of the  FIG. 5  apparatus is apparent to a person skilled in the art who can fully understand the  FIG. 3  apparatus through the above-described explanation, the detailed description thereof will be omitted. 
   The annular groove  471  formed in objective lens  47  as shown in  FIG. 6  can be modified into a protrusion form which protrudes outwards from the surface of the objective lens  47  and has a predetermined width and depth. 
     FIGS. 7A and 7B  are views showing a single structure combining a phase plate with a variable aperture according to the present invention. Referring to  FIGS. 7A and 7B , a phase variation region contained in the region having a NA of 0.5 or below has a ring-shaped structure. Since the phase variation region performs the same function as that of the phase plate  36 , the detailed description thereof will be omitted. 
     FIG. 8  is a graphical diagram showing a reduction efficiency of a spot size and a side lobe. In  FIG. 8 , a curve (a) indicates when a conventional optical pickup optimized for a DVD is used for a CD-R, in which the spot size formed in the information recording surface of the CD-R is 1.53 μm. A curve (b) indicates when an optical pickup apparatus according to the present invention is used, in which the spot size is 1.33 μm. A curve (c) indicates when an conventional optical pickup is used for a CD-R, in which the spot size is 1.41 μm. It can be seen from  FIG. 8  that the optical pickup apparatus according to the present invention reduces the size of the spot by about 8% compared with the conventional optical pickup. Also, as the size of the side lobe is smaller at the time of the disk recording and reproduction, it can be seen that an amount of light in the peripheral portion of the spot which is called a side lobe, is reduced with respect to an optical pickup having a desirable optical characteristic.  FIG. 9  shows that the optical pickup apparatus according to the present invention has an excellent characteristic with respect to a focus servo signal during reproduction of the CD-R, when the optical pickup apparatus detects an optical signal in the astigmatism manner, as shown by a relatively lower graph. 
   The above-described embodiments have been described with the structure including a variable aperture, a phase plate and an annular shield objective lens. However, using only a phase plate, the spherical aberration due to a disk exchange is reduced and an optical spot appropriate for the CD-R can be formed on the information recording surface. 
   The above-described embodiments have been described in connection with a infinite optical system which is made by the collimating lens  34 . However, the present invention can be applied to a finite optical system which has no collimating lens located between a beam divider and an objective lens, as is apparent to one skilled in the art. 
   As described above, the optical pickup apparatus according to the present invention uses a phase plate. Accordingly, the present invention can provide an optical pickup which is used compatibly with a DVD and a CD-R with a single objective lens, without using a conventional optical apparatus which creates a problem in a manufacturing process. 
   While only certain embodiments of the invention have been specifically described herein, it will apparent that numerous modifications may be made thereto without departing from the spirit and scope of the invention.

Technology Category: 3