DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Preferred embodiments of the present invention will be described below in more detail with reference to the accompanying drawings.

[0030] 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 .

[0031] 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.

[0032] 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 .

[0033] 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 nd/λ− 2π d /λ=(2 m+ 1)π (2)

[0034] 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 respect to the 650 nm wavelength light, and a dotted line 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°.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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 .

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.