KR100238069B1 - Optical pickup compatible with cd-r and dvd using hologram - Google Patents

Abstract

Optical pickup using one of the first light and the second light according to the optical recording medium currently used has different distances from the optical pickup to the information recording surfaces and uses light of different wavelengths for recording and reproducing information. Record or read information on at least two optical recording media. This optical pickup focuses the first laser light source, the second laser light source, and the objective lens emitted by the first light having a relatively short wavelength and the second light having a relatively long wavelength, respectively, closer to the objective lens. Refracting the first light emitted from the first laser light source to face an objective lens having a predetermined focal length that matches the position of the information recording surface of the first optical recording medium having the information recording surface, and toward the objective lens; And a hologram diffracting the second light emitted from the second laser light source.

Description

CD-R and DVD compatible optical pickup with hologram

The present invention relates to an optical pickup capable of recording and reproducing information on a digital video disc (DVD) and a recordable compact disc (CD-R).

The recording medium for recording and reproducing information, such as video, sound, or data with high density, is composed of a disk, a card, or a tape, but the disk type is mainstream.

Recently, products in the field of optical disc devices have been developed from laser discs (LDs), compact discs (CDs) to digital video discs (DVDs). Such an optical disc is composed of a plastic or glass medium having a constant thickness in the axial direction in which light is incident and a signal recording surface on which information is recorded.

High-density optical disc systems up to now have a large numerical aperture of the objective lens and a short wavelength light source of 635 nm or 650 nm to increase the recording density. It was developed to. However, in order to be compatible with the recordable compact disc (CD-R), which is a recent form of CD, light of 780 nm wavelength must be used. This is due to the recording characteristics of the CD-R recording medium, and making it possible to use both light of 780 nm wavelength and light of 650 nm wavelength in one optical pickup is a very important technology for compatibility of DVD and CD-R. It has emerged. Existing optical pickups compatible with DVD and CD-R will be described with reference to FIG.

1 shows an optical pickup using two laser diodes (LDs) and a single objective lens as a light source for a DVD and a CD-R. The optical pickup of FIG. 1 uses a laser wavelength of 635 nm for DVD reproduction and a laser light of 780 nm for recording and reproduction of CD-R. Light of 635 nm wavelength emitted from the light source 1, which is a laser diode, passes through a collimating lens 2 and a polarization beam splitter 3 and then proceeds to the interference filter prism 4. Light of 780 nm wavelength emitted from the light source 11, which is a laser diode, passes through the collimated lens 12, the polarized light splitter 13, and the condenser lens 14, and then proceeds to the prism 4. Here, light of 780 nm wavelength is converged in the prism 4, and the optical system of this structure is called a "finite optical system." The prism 4 transmits the light of 635 nm wavelength reflected by the polarized light splitter 3 and reflects the light collected by the condenser lens 14. As a result, the light from the light source 1 is incident on the quarter-wave plate 5 in a form paralleled by the quasi-lens 2 and the light from the light source 11 is focused on the condenser lens 14 and the prism ( It enters into the quarter wave plate 5 in the form diverging by 4). Light transmitted through the quarter-wave plate 5 is incident on the objective lens 7.

The objective lens 7 is designed to focus on the signal recording surface of the DVD disc 8 having a thickness of 0.6 mm, and focuses the light of the 635 mm wavelength emitted from the light source 1 on the signal recording surface of the DVD disc 8. To bear. Therefore, the light reflected from the signal recording surface of the DVD disc 8 contains the information recorded on the signal recording surface. The reflected light is incident on the photodetector 10 which passes through the polarized light splitter 3 and detects optical information.

When the light of 780 nm wavelength emitted from the light source 11 is focused on the signal recording surface of the CD-R disc 9 whose thickness is 1.2 mm using the objective lens 7 described above, the DVD disc 8 Spherical aberration occurs due to the difference in thickness and the thickness of the CD-R disc 9. More specifically, this spherical aberration is caused by the signal recording surface of the CD-R disc 9 farther from the optical recording axis of the signal recording surface of the DVD disc 8 with respect to the objective lens 7. In order to reduce such spherical aberration, the variable stop 6 is used at the position between the quarter wave plate 5 and the objective lens 7. By use of the variable stop 6, which will be described later in conjunction with FIG. 2, light of 780 nm wavelength accurately forms an optical spot on the signal recording surface of the CD-R disc 9, and in the CD-R disc 9 The reflected 780 nm wavelength light is reflected by the prism 4 and reflected by the polarized light splitter 13 so that it can be detected by the photodetector 15.

The variable stop 6 shown in FIG. 2 has a thin film structure that can selectively transmit light contained in an area equal to or smaller than the numerical aperture NA 0.6 corresponding to the diameter of the objective lens 7.

That is, the tunable 6 has a light of 780 nm wavelength and light of region 1 and 635 nm wavelengths that transmit both 635 nm and 780 nm wavelengths of light with respect to the numerical aperture (NA) 0.45 with respect to the optical axis. The total reflection is divided into two areas. The region 1 is an area below the numerical aperture NA 0.45, and the region 2 is an area outside the region 1, which is made by coating the dielectric thin film. Region 1 described above is composed of a quartz (SiO ₂ ) thin film to eliminate optical aberration caused by dielectric thin film coated region 2. By the use of the variable stop 6, light of 780 nm wavelength passing through area 1 with a numerical aperture NA of 0.45 or less forms an optical spot suitable for the CD-R disc 9 on its signal recording surface. Therefore, spherical aberration generated when changing from the DVD disc 8 to the CD-R disc 9 is eliminated.

However, the optical pickup described above with reference to FIG. 1 should constitute a finite optical system for light having a wavelength of 780 nm in order to eliminate spherical aberration generated when the DVD disc and the CD-R disc are compatible. Therefore, its optical structure is complicated and the use of additional prisms or interference filters is required. In addition, an optical path difference is generated between the light passing through the area 1 having a numerical aperture of 0.45 or less and the area 2 having a numerical aperture of 0.45 or more by the optical thin film formed in the area 2 having a numerical aperture of 0.45 or more of the variable aperture 6. Therefore, it was necessary to form a special optical thin film in the region 1 to remove it. For this reason, the quartz coating in the area 1 and the multilayer thin film were formed in the area 2, respectively, but the manufacturing process was not only complicated, but the film thickness had to be adjusted with an accuracy of 'μm unit', which was not suitable for mass production.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical pickup capable of constructing an optical system optimized for different optical recording media by using holograms showing different diffraction characteristics for different wavelengths of light. have.

1 is a view showing an optical structure of a conventional optical pickup using two laser diodes (LD) and a single objective lens as a light source of a DVD and a CD-R.

2 is a view for explaining the variable aperture of FIG.

3 is a view showing an optical structure of an optical pickup according to an embodiment of the present invention.

4 is a view showing an optical structure of an optical pickup according to another embodiment of the present invention.

5 is a view showing an optical structure of an optical pickup according to another embodiment of the present invention.

6 and 7 are views for explaining the area occupied by the diffraction grating.

* Explanation of symbols for main parts of the drawings

31, 34: laser light source 32, 35, 41, 51: hologram light splitter

33: hologram 36: objective lens

37, 38: photodetector

According to the present invention for achieving the above object, the optical pickup for at least two optical recording media having different distances from the optical pickup to the information recording surfaces and using light of different wavelengths for recording and reproducing the information are First and second laser light sources for emitting first light having a relatively short wavelength and second light having a relatively long wavelength, respectively; An objective lens having a predetermined focal length for matching the focal point of the objective lens by the first light to the position of the information recording surface of the first optical recording medium having the information recording surface closer to the objective lens; And a diffraction grating region having grooves of a specific depth for transmitting the first light emitted from the first laser light source and diffracting the second light emitted from the second laser light source so as to face toward the objective lens. And a hologram disposed at a position for removing optical aberration caused by the first light and the second light, and using one of the first light and the second light according to the optical recording medium currently used.

According to the present invention, an optical pickup for at least two optical recording media having different distances from the optical pickup to the information recording surfaces and using different wavelengths of light for recording and reproducing the information is provided with a relatively short wavelength first. First and second laser light sources that emit one light and a second light having a relatively long wavelength, respectively; An objective lens having a predetermined focal length for matching the focal point of the objective lens by the first light to the position of the information recording surface of the first optical recording medium having the information recording surface closer to the objective lens; And a diffraction grating region having grooves of a specific depth for reflecting the first light exiting from the first laser light source and diffracting the second light exiting from the second laser light source, towards the objective lens. It comprises a hologram mirror and uses one of the first light and the second light depending on the optical recording medium currently used.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

3 shows the structure of an optical pickup in the form of a finite optical system. Referring to FIG. 3, when the laser diode light source 31 operates, laser light having a wavelength of 650 nm emitted in the form emitted from the light source 31 passes through the first hologram splitter 32 and is transmitted. The received light is incident on the hologram 33. When the laser diode light source 34 operates, the 780 nm wavelength laser light emitted in a form emitted from the light source 34 passes through the second hologram splitter 35, and the transmitted light is transmitted to the hologram 33. Enter.

The hologram 33, by its structure, which will be described later, simply transmits the light of 650 nm wavelength to the objective lens 36 while diffracting the light of the 780 nm wavelength to the objective lens 36. The hologram 33 transmits the light from the light source 31 of 650 nm wavelength to the objective lens 36 and corrects the aberration with respect to the light from the light source 34 of 780 nm wavelength. The optical pickup of FIG. 3 forms a finite optical system.

Light of 650 nm wavelength incident on the objective lens 36 is focused on the information recording surface of the DVD disc 8 in the form of a light spot, and light of 780 nm wavelength and aberration on the information recording surface of the CD-R disc 9. Condensed into the removed light spot. The light reflected from the DVD disc 8 is split by the first hologram splitter 32 and transmitted to the first photodetector 37 for the 650 nm wavelength light shown in FIG. 3, and the CD-R disc. The light reflected by (9) is split by the second hologram splitter 35 and transmitted to the second photodetector 38 for light of 780 nm wavelength.

Typically, CD-R and DVD compatible optical pickups using two laser beams and one objective lens generally have an objective lens optimized for a DVD disc, so that the objective lens 36 also has information on the DVD disc 8. It has a focal length corresponding to the recording surface. The CD-R disc 9 has an information recording surface at a position farther from the objective lens 36 than the information recording surface of the DVD disk 8, as shown in FIG. For this reason, when a disc currently used is exchanged from a DVD disc 8 to a CD-R disc 9, a light spot formed on the information recording surface of the CD-R disc 9 is a CD-R disc. It has a size of 1.8 µm, which is larger than 1.4 µm, which is a suitable size for a CD disc.

Therefore, an optical structure capable of preventing spherical aberration occurring in the CD-R disc 9 is required. The hologram 33 meeting these needs will be described in more detail as follows.

The hologram 33 proposed by the present invention is located on an optical path traveling from the light source 31 or 34 to the objective lens 36. The hologram 33 also has a transmission diffraction grating that allows simple transmission of light of 650 nm wavelength and diffraction of light of 780 nm wavelength. The diffraction grating is formed on the surface of the light source side or the surface of the objective lens 36 side, and has grooves of depth such that the optical path difference becomes an integer multiple of the wavelength. If these grooves have a depth that satisfies the following equation (1), light of 650 nm wavelength is not diffracted in the hologram 33.

[Equation 1]

Where d is the depth of the groove, m is any integer, λ is the wavelength of the first light, and n is the refractive index of the hologram 33 with respect to the first light. Therefore, when the refractive index of the hologram 33 is 1.5, if the depth of the groove is 1.3 mu m, 2.6 mu m, 3.9 mu m, or the like, light of 650 nm wavelength is transmitted 100% of the hologram 33 without diffraction.

On the other hand, the light transmission efficiency of the hologram 33 is determined by the following equation 2 for the diffraction order of the 0th and 1st order,

[Equation 2]

Where E ₀ is the transmission efficiency for the 0th order diffraction, E ₁ is the transmission efficiency for the first order diffraction, and θ = πd (n−1) / λ.

Therefore, when the depth d of the groove of the diffraction grating formed in the hologram 33 is 3.9 µm, which is an optimized depth, the hologram 33 having a refractive index n of 1.5 is used for the zero-order diffraction of light of 780 µm wavelength. It has a transmission efficiency of 0% and a transmission efficiency of 40% for first order diffraction.

Therefore, only the light of 780 nm wavelength can be diffracted without affecting the light of 650 nm wavelength, and the size of the light spot formed on the information recording surface of the CD-R disc 9 can be determined by the CD-R disc. The recording and reproducing of the information about (9) can be reduced to such an extent that the above-described spherical aberration can be eliminated. In addition, by using the first diffracted light, light from the 650 nm wavelength light source 31 and the 780 nm wavelength light source 34 placed at different positions is transmitted to the objective lens 36 and the objective lens 36 Light can be transmitted to the light source 31 or 34. In design, it is preferable that the light source 31 of 650 nm wavelength is placed on the optical axis of the objective lens 36 and the light source 34 of 780 nm wavelength is disposed away from the optical axis of the objective lens 36. This arrangement makes the optical pickup more stable in terms of recording or reproducing the signal.

The hologram 33 described above can be fabricated to form a diffraction grating over the entire surface, as shown in FIG. 6, to include an acceptable numerical aperture (NA) when using the DVD disc 8. . In Fig. 6, NA = 0.6 is the numerical aperture corresponding to the aperture of the objective lens, and NA = 0.45 is the numerical aperture in the hologram 33 for the 780 nm wavelength when the DVD disc 8 is used. The numerical aperture of the optimized diffraction grating when using the CD-R disc 9 is determined to be around 0.45 to 0.5 around the axis of the light passing through the hologram 33 as shown in FIG. In this way, since the 780 nm wavelength zero-order diffracted light has a light efficiency of 0%, only the first diffracted light can form an optimized light spot on the information recording surface of the CD-R disc 9.

The optical pickup in which the optical pickup of the finite optical system type shown in FIG. 3 is transformed into an infinite optical system and the light pickup 31 and the first light detector 37 having a wavelength of 650 nm as one module is shown in FIG. lost. Since the optical elements in FIG. 4 having the same reference numerals as in FIG. 3 have the same structure and optical characteristics as the corresponding elements in FIG. 3, the description thereof is omitted. The unit 40 of FIG. 4 integrally forms the light source 31 and the photodetector 37 corresponding to the 650 nm wavelength of FIG. Therefore, the third hologram splitter 41 is manufactured to perform both light emission and light detection in the unit 40. A collimation lens 42 inserted into the optical path between the hologram 33 and the objective lens 36 makes the light incident from the hologram 33 into parallel light. That is, the light of 780 nm diffracted in the hologram 33 is allowed to travel in parallel toward the objective lens 36. This parallel plate passes through the wave plate 43 and proceeds to the objective lens 36. Therefore, the optical pickup of FIG. 4 forms an infinite optical system.

The optical pickup of FIG. 5 according to another embodiment of the present invention uses a hologram mirror 52 in which reflective diffraction gratings, also called reflection gratings, are formed instead of the transmission diffraction grating of the hologram 33 described above. The hologram mirror 52 serves to advance the light emitted from the light source or the light source 34 of the unit 50 toward the objective lens. The diffraction grating of this hologram mirror 52 is also designed to satisfy both equations (1) and (2) described in relation to the hologram (33). However, since the hologram mirror 52 is a reflection type, the depth of the diffraction grating is 1/2 of the transmission type. Therefore, the above expression 1 is changed to 4π d (n-1) / λ = 2mπ. And it is produced so that it may have the area demonstrated with respect to FIG. 6 or FIG. Therefore, this reflective diffraction grating has grooves having a depth of 1.95 占 퐉, reflects 0% diffracted light 0% and 40% of the first diffracted light to 780 nm wavelength light. And 650 nm wavelength light is reflected only without diffraction. On the other hand, the unit 50 is a laser light source and a corresponding photodetector are integrally formed as in the case of FIG. 4, the fifth hologram light splitter 51 is also the light and unit 50 emitted by the unit 50 It is designed to be optimized for the incident light. The hologram mirror 52 described above also, like the hologram 33, allows the light to which the aberration is removed toward the objective lens 36.

In the above description, the laser light source and the photodetector related to the light of 650 nm wavelength have been described as a single unit, but the formation of the laser light source and the photodetector related to the light of 780 nm wavelength as one unit is also described in this technique. It will be apparent to those skilled in the art. And, not limited by the above-mentioned specific wavelength of the present invention is also apparent to those skilled in the art.

As described above, the optical pickup according to the present invention uses a hologram that can selectively give diffraction characteristics to light of different wavelengths from different laser light sources, so that the thickness of the information recording surface may be changed due to the different thicknesses of the disks. Positions are compatible with different optical discs. Thus, it is possible to record or reproduce signals for existing DVD discs or CD-R discs.

In addition, the production cost of the optical pickup can be lowered due to the excellent mass productivity of the hologram, and it is also suitable for mass production. In addition to miniaturization of the optical pickup, the optical pickup can be further miniaturized by using a hologram as a light splitter used for a light source and a photodetector corresponding to each light.

Claims (20)

An optical pickup for at least two optical recording media having different distances from the optical pickup to the information recording surfaces and using different wavelengths of light for recording and reproducing information, the optical pickup being longer than the first light and the first light. First and second laser light sources respectively emitting a second light of a wavelength; An objective lens having a predetermined focal length for matching the focal point of the objective lens by the first light to a position of the information recording surface of the optical recording medium having an information recording surface closer to the objective lens; And a diffraction grating region having grooves of a specific depth for transmitting the first light emitted from the first laser light source and diffracting the second light emitted from the second laser light source to face toward the objective lens. And a hologram disposed at a position for removing optical aberration caused by the first light and the second light, wherein the optical pickup uses one of the first light and the second light according to the optical recording medium currently used. The optical pickup of claim 1, wherein the first laser light source lies on an optical axis of the objective lens and the second laser light source deviates from an optical axis of the objective lens. The optical pickup of claim 1, wherein the diffraction grating region has grooves of a depth to maximize the light efficiency of a specific diffraction order other than "0" of the second light. 4. The optical pickup of claim 3, wherein the depth of the grooves maximizes the light efficiency of the first order of the second light. 5. The method of claim 4, wherein the depth d of the groove is determined by the following equation: 2π d (n-1) / λ = 2mπ, where m is any integer, λ is the wavelength of the first light, and n is the refractive index of the hologram for the first light. The optical pickup of claim 3, wherein the diffraction grating region is a region having a numerical aperture of at least 0.45 about an optical axis of the objective lens. The optical pickup of claim 3, wherein the diffraction grating area occupies an area substantially the same as the entire area where the first and second lights pass through the objective lens. The optical pickup of claim 1, further comprising a collimating lens collimating a first light or a second light traveling from the hologram toward the objective lens. The optical pickup according to claim 1, further comprising holographic light splitting means for causing the light reflected from the currently used optical recording medium to pass through the hologram to reach a position different from the light source corresponding thereto. 10. The apparatus of claim 9, wherein the hologram light splitting means comprises: a first hologram light splitter for first light; And a second hologram light splitter for second light. The optical pickup of claim 1, further comprising photodetectors corresponding to each of the lights, wherein the light source and the photodetector corresponding to each light are formed in a single unit. In optical pickup for at least two optical recording media having different distances from the optical pickup to the information recording surfaces and using different wavelengths of light for recording and reproducing information, the first light and the relatively short wavelength First and second laser light sources respectively emitting second light having a long wavelength; An objective lens having a predetermined focal length for matching the focal point of the objective lens by the first light to the position of the information recording surface of the first optical recording medium having the information recording surface closer to the objective lens; And a diffraction grating region having grooves of a specific depth for reflecting the first light exiting from the first laser light source and diffracting the second light exiting from the second laser light source, towards the objective lens. An optical pickup that includes a hologram and uses one of the first light and the second light according to an optical recording medium currently used. 13. The optical pickup of claim 12, wherein the diffraction grating region has grooves of depth to maximize the light efficiency of a particular diffraction order other than "0" of the second light. The optical pickup of claim 13, wherein the depth of the grooves maximizes the light efficiency of the first order of the second light. 15. The method of claim 14, wherein the depth d of the groove is determined by the following equation: 2π d (n-1) / λ = 2mπ, where m is any integer, λ is the wavelength of the first light, and n is the optical pickup of the hologram mirror for the first light. The optical pickup of claim 13, wherein the diffraction grating is an area of a numerical aperture of at least 0.45 about an optical axis of the objective lens. The optical pickup of claim 13, wherein the diffraction grating occupies an area substantially the same as the entire area where the light passes through the objective lens. The optical pickup according to claim 13, further comprising holographic light splitting means for causing the light reflected from the optical recording medium to be used to reflect the holographic mirror to reach a position different from the corresponding light source. 19. The apparatus of claim 18, wherein the hologram light splitting means comprises: a first hologram light splitter for first light; And a second hologram light splitter for second light. The optical pickup according to claim 13, comprising photodetectors corresponding to each of the lights, wherein the light source and photodetector corresponding to the respective spot lights are formed in a single unit.