Optical pickup using an optical phase plate

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OPTICAL PICKUP USING AN OPTICAL PHASE PLATE

BACKGROUND OF THE INVENTION The present invention relates to an optical pickup apparatus which can record and read information on and from a digital video disk (DVD) and a 5 recordable compact disk (CD-R), respectively.

Recording media for recording and reading the information such as 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 635nm or 650nm. 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 780nm should be used. This is due to the recording characteristic of the CD-R recording medium. As a result, using the lights of 780nm and 650nm wavelengths in a single optical pickup becomes very important for compatibility of the DVD and the CD-R. An conventional optical pickup to be compatible with the DVD and the CD-R will be described below with reference to Fig. 1.

Kig. 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 635nm when reproducing a

DVD, and uses laser light having a wavelength of 780nm when recording and reproducing a CD-R. Light having the 635nm 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 780nm 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 780nm wavelength. An optical system having such a structure is called a finite optical system. The prism 4 transmits the light having a wavelength of 635nm reflected from the polarization beam splitter

3, and reflects the light converged by the converging lens 14. As a result, the light from the light source 1 is incident to a quarter- wove plate 5 in the form of parallel beam by the collimating lens 2, while the light from the light source 11 is incident to the quarter wave plate δ in the form of diverging beam by the converging lens 14 and the prism 4. The light transmitting through the quarter-wave plate δ is incident to an objective lens 7.

The objective lens 7 is designed to be focussed on a signal recording surface in a DVD 8 having a thickness of 0,6mm, by which the light of the 635nm wavelength emitted from the light source 1 is focussed on the signal recording surface in the DVD 8. 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

780nm wavelength light emitted from the light source 11 is focussed on a signal recording surface in the CD-R 9 having 1.2mm thickness using the above-described objective lens 7, spherical aberration is generated due to 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 the finite optical system including the converging lens 14 is required. By using the variable aperture 6 to be described later with reference to Fig. 2, the 780nm wavelength light forms an optimized beam spot on the signal recording surface of the CD-R 9. The 780nm 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 transmits the rays of the light incident to the region not more than the numerical aperture (NA) of 3.6 coinciding 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 635nm and 780nm wavelength lights and a second region 2 totally transmits the 635nm wavelength light and totally reflects the 780nrn wavelength light. The region 1 has the numerial 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 (SiOs) thin film in order to remove optical aberration generated by the dielectric thin film coated region 2, By using the variable aperture 6, the 780nm 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 to use the CD-R.

However, the Fig. 1 optical pickup as described above should form a ¹⁵ finite optical system with respect to 780nm wavelength light in order to remove spherical aberration generated when compatibly changing a DVD and a CD-R. As well, due to the optical thin film, that is, the dielectric thin film which is formed in the region 1 having the NA of 0.45 or above, an optical path difference between the light transmitting the region 2 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, 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 μπΥ’ 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 5 apparatus which is compatible to 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 of different wavelengths for recordingand reading information, the optical pickup apparatus comprising:

laser light sources for emitting first light having a relatively shorter wavelength and second light 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 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 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 proceeding from the optical path control means to the objective lens, to 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 focussed by the objective lens, wherein one of the first and second light is used according to the optical recording medium to be used.

Said phase shift means may be a phase plate including two regions having different thicknesses. The thinner of the two regions of said phase plate preferably comprises a portion for shifting the phase of the second light, and includes a groove, having a predetermined width and depth which is formed in a generally concave fashion inwards from a surface of said phase plate relatively closer to said optical path control means. Said thinner region may have a shape of concentric circles with respect to the optical axis of said objective lens. Said groove may have an optical axis of said objective lens. Said groove may have an optical depth for phase-shifting the first light by a degree of 360 and phase-shifting the second light by a degree of 180.

Preferably the relatively thicker region of said phase plate comprises a portion for shifting the phase of the second light, and includes a protrusion having a predetermined width and height which is protruded outwards from the surface of said phase plate relatively closer to said optical path control means. Said thicker region may have a shape of concentric circles with respect to the optical axis of said objective lens.

²⁵ Preferably said protrusion has an optical height for phase-shifting the first light by a degree of 360 and phase-shifting the second light by a degree of 180.

Said phase plate may be fabricated by one of etching, injection and molding operations.

Said phase shift means may be fabricated by etching a groove into the objective lens, the groove having a predetermined width and depth, which is concave and which extends inwards from a surface of said objective lens relatively closer to said optical path control means.

Said phase shift means may have a shape of a protrusion having a predetermined width and height, which is protruded outwards from the surface of said objective lens relatively closer to said optical path control means.

Said phase shift means may have the same curvature as that of said 10 objective lens and is to be coupled with a surface of said objective lens which is relatively closer to said optical path control means.

The apparatus may have a small size of structure, in which one of the first and second lights is appropriately selected and other components are consistently used whenever one optical recording medium is changed to the other optical recording medium.

Preferably one of said laser light sources and said optical detection means are integrated as a single unit.

Variable aperture means may be provided which is located between said optical path control means and said phase shift means, having a first 20 region for transmitting both of the first and second lights entered from said optical path control means and a second region for transmitting only the second light entered therefrom, wherein the first and the second regions have the same optical axis as that of said objective lens.

According to a second aspect of the invention, there is provided an optical pickup apparatus arranged for use with at least two optical recording media, which have different distances from an optical pickup to information recording surfaces and use lights having different wavelengths for recording and reading information, the optical pickup apparatus comprising:

laser light sources for emitting first light having a relatively shorter wavelength and second light having a relatively longer wavelength, respectively.

an objective lens having a predetermined focal length, in which the focal point of said objective lens according to the first light coincides, in use, with the position of an information recording surface in a first optical recording medium of a first type which has, in use, an information recording surface a first distance from the objective lens;

optical detection means;

means for controlling an optical path so that the light emitted from one of the laser light sources is directed to the objective lens and the light output from the objective lens is directed to said optical detection means; and variable aperture means, located between said optical path control means and said objective lens, having a first region for transmitting both of the first and second lights entered from said optical path control means and a second region for transmitting only the first light entered therefrom wherein the first and the second regions have the same optical axis as that of said objective lens, wherein one of the first and second lights is used according to an optical recording medium to be used, said objective lens comprises at least one annular shielding portion for shielding the part of light which is transmitted via said first region.

The second region of said variable aperture means may be constructed as a structure of diffraction grating pattern.

Phase shift means may be provided, located between said variable aperture means and said objective lens, for shifting the phase of a part of the second light proceeding from said variable aperture means to said objective lens, thereby reducing the size of a beam spot which is formed on the position of the information recording surface in the second optical recording medium having the information recording surface which is positioned farther from the objective lens by the second light focused with the objective lens.

Said phase shift means and said variable aperture means may be constructed as a single unit in which said phase shift means is formed within the first region of said variable aperture means.

BRIEF DESCRIPTION OF THE DRAWINGS The preferred embodiments are described with reference to the drawings whereinFig. 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 present invention;

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 5 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 PREFERRED 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 15 preferred embodiment of the present invention. Referring to Fig. 3, when a laser diode light source 31 operates, the 650nm wavelength light emitting in the diverging 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 transmitting the second polarization beam 20 splitter 33 is incident to a collimating lens 34. Wher. a laser diode light source 40 operates, the 780nm wavelength light emitting in the diverging 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 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 in wavelength by the variable aperture 35.

Referring to Figs. 7 A and 7B, the variable aperture 35 has a region 3 for transmitting both the 780nm wavelength light and the 650nm wavelength light and a region 4 for transmitting only the 650nm 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 780nm wavelength light having diffracticn order of non-zero and whose diffraction efficiency is 100% with respect to the 650nm wavelength light having the diffraction order of zero. Therefore, the 650nm 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 μ m, the 650nm wavelength light has the diffraction efficiency of 100% as shown in a solid line overlapping with symbol and the 780nm wavelength light has the diffraction efficiency of 0% as shown in a solid line overlapping with a circle. Therefore, the region 4 of the variable aperture 35 is designed with the diffraction grating portion having the groove depth of 3.8 μ m. In this embodiment, the NA of 0.5 is used for partitioning the regions 3 and 4. Therefore, the region 3 is a portion having the NA of C.5 or below, and the region 4 is a portion having the NA more than 0.5. Thus, according to the embodiment of the present invention, the light transmitting the portion having the 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 transmitting the variable aperture 35 transmits a phase plate to be descnbed 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 1.8pm 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 1.4pm, information cannot be recorded on or read from the CD-R 9 via the light spot having the size of 1.8 μ m. 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 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 or molding using an etch or metal mold, in which the depth D is determined by the following equations (1) and ¢2).

Tim'd/Λ '-2xd/J '=(2m')x π nd/' Λ -2 7Γ d/ Λ =(2111 + 1) π . . . (1) . . . (2)

Here, m is an integer, n' and n denote a refractive index at wavelength /1 ' (650nm) and λ (780nm), 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 μ m. The phase plate 36 having the annular groove 361 of the depth D phase-shifts the 780nm wavelength light by 180 ° and phase-shifts the 650nm light by 360 ° when the lights proceeds to the objective lens 37 from the variable aperture 35. Fig. 10 is a graphical io 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 650nm wavelength light and a dotted line represents that with respect to the 780nm wavelength light. When D is 3.9 μ m, the 780nm wavelength light has the phase of

180 ’ and the 650nm light has the phase of 360 ° .

Thus, the 780nm 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 the phase plate 36, the size of the light spot formed on the information recording surface in the CD-R 9 is reduced into a degree which can be recorded or read on or from the CD-R 9, to thereby remove 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 5 region 3. Thus, the spherical aberration due to the exchange of the DVD 8 to the CD-R 9 is reduced, and increase a sensitivity of a focus error signal in a focus servo system (not shown).

The light 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 10 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 15 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 predetermined width and depth. The objective lens 47 engraved with such an annular groove 471 phase-shifts the 780nm wavelength light by 180 ’ as 20 in the phase plate 36 and phase-shifts the 650nm wavelength light by 360 . Thus, among the 780nm wavelength light incident to the objective lens from the variable aperture 35, the light 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, the beam spot of the 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 780nm wavelength light. The Fig. 5 optical pickup includes a single unit 49 combining a light source 491 with a light detector 493 for the 780nm wavelength light, in addition to a light source 31, a light detection lens 51 and a light detector 53 for the 650nm 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.

Fig. 7 is a view showing a single structure combining a phase plate with a variable aperture according to the present invention. Referring to Fig. 7, a phase variation region contained in the region having the NA of 0.5 or below has a ring-shaped structure. Such a phase variation region ²⁰ 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 an 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 μ m.

A curve (b) indicates when an optical pickup apparatus according to the present invention is used, in which the spot size is 1.33 μπι. A curve (c) indicates when an conventional optical pickup is used for a CD-R, in which the spot size is 1.41 μ m. 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

1° 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, through a relatively lower graph.

I⁵ 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 the 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 a collimating lens which is located between a beam divider and an objective lens, which 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 compatibly used for a. DVD and a CD-R with a single objective lens, without using an conventional optical apparatus which creates a problem in a manufacturing process.

While only certain embodiments of the invention have been specifically described herein, it wall apparent that numerous modifications may be made thereto without departing from the scope of the invention.