KR19990050002A - Dual Focus Optical Pickup - Google Patents

Abstract

The present invention relates to a dual focus optical pickup device, comprising: a phase hologram prism disposed obliquely with a recording surface at a lower portion of an optical disc to reflect, transmit, and diffract laser light having a constant polarization component, and to provide an error detection signal, and a phase A plurality of first and second laser diodes formed on one side and a lower side of the hologram prism to radiate laser light, respectively, and a plurality of beams of optical information which are provided on both sides of the second laser diode and diffracted to ± 1st order light and are incident It consists of two photodetectors and the like, so that the incident light intensity distribution of the light beam incident on the objective lens is varied so that the numerical aperture (NA) of the objective lens is actively converted so that spherical aberration is significantly reduced, so that digital audio and digital video discs can be reproduced interchangeably. Miniaturized picks that can be made mechanically and lightly There is a device easy to effect the production of

Description

Dual Focus Optical Pickup

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dual focus optical pickup device. In particular, a polarizing state of beams respectively incident from a laser diode is formed by forming a combination of a selective transmission plate and a λ / 4 wave plate selectively transmitting according to the wavelength of an incident beam. Phase hologram lattice for selective reflection and diffraction and phase hologram prism formed with astigmatism prisms in multiple layers to enable compatible playback of digital audio and digital video discs to achieve light and small size reduction of optical pickup devices. A dual focus optical pickup device is provided.

In general, an optical data recording medium, that is, an optical disk, is divided into a digital audio disk (DAD) for music playback having a thickness of 1.2 mm and a digital video disk (DVD) having a thickness of 0.6 mm, and has a thickness of 1.2 mm. Digital audio discs record data in multiple layers on one side, and 0.6 mm thick digital video discs record data in multiple layers in the middle to record a large amount of data on a single disc.

An optical pickup apparatus for reproducing data recorded on the above two types of discs reads information recorded on a 1.2 mm thick digital audio disc and a 0.6 mm thick digital video disc. Since the track pitch of the image is 0.74 µm and the shortest length between the pit, which is the recording signal, is 0.4 µm, the diameter of the optical spot must be different during playback because the track pitch is different from that of a digital audio disc having 1.6 µm and the shortest length between the feet is 0.834 µm. Because spherical aberrations do not match, simultaneous playback is impossible, and optical aberration is increased due to the difference of 0.6mm thickness of the discs. Only the recorded information on one of the 1.2mm discs could be read.

Therefore, an objective lens having a numerical aperture NA of 0.6 designed for a 0.6 mm disk as shown in the detailed view of FIG. 1 to selectively read out information recorded on a 1.2 mm disk and a 0.6 mm disk in recent years. The holographic optical device 1130 employs a dual focus lens 1125 in which the holographic optical device 1120 having the multi-stage diffraction layers w1, w2, and h0 formed by etching is formed. Since the zero-order light diffracted by 1120 goes straight and the first-diffracted laser light becomes divergent light, a dual focus optical pickup device for controlling the diffraction efficiency and appropriately distributing the amount of light has been developed.

As shown in FIG. 1, the dual focus optical pickup device employing the holographic optical device 1120 includes a laser diode 1100 that scans laser light having a linearly polarized constant wavelength, and the laser diode 1100. A diffraction grating 1105 which separates the laser beam scanned by the laser beam into a zero-order diffraction light and a ± first-order diffraction light for detecting a tracking error signal, that is, a three beam, and at one side of the diffraction grating 1105 A beam splitter 1110 which reflects and transmits the scanned laser light at a predetermined ratio with a predetermined slope, and a collimator lens 1115 which converts the laser light via the beam splitter 1110 into parallel light; The holographic optical element 1120 for distributing the amount of light by adjusting the diffraction efficiency of the parallel light via the collimator lens 1115 and the laser light via the holographic optical element 1120 for 0.6 mm digital discs. An objective lens 1130 that reads the recorded information by focusing on a disk 1150 (hereinafter referred to as "first disk") and a 1.2 mm disk (hereinafter referred to as "second disk") that is a digital audio disk; Astigmatism generating lens 1140 for generating a focusing error signal by the astigmatism method for detecting an error signal in the laser light with recorded information, and optical information passing through the astigmatism generating lens 1140 is detected. And a photodetector 1145 for converting the signal into a current signal.

On the other hand, the operation of the conventional optical pickup device having such a configuration is as follows.

First, laser light having a predetermined oscillation wavelength is irradiated from the laser diode 1100 and incident on the diffraction grating 1105, and the incident laser light passes through the diffraction grating 1105, and the 0th and ± 1st order light is transmitted. That is, it is split into three beams and radiated. At this time, the three beams are used for tracking errors, and are transmitted to the beam splitter 1110 through the diffraction grating 1105, and the three beams are reflected and transmitted at a constant rate by the beam splitter 1110. The laser light reflected from the reflected and transmitted laser light is incident from the beam splitter 1110 to the collimator lens 1115, and the laser light passes through the collimator lens 1115, thereby providing linearity. The laser light, which is thus provided with linearity and becomes parallel light, is incident from the collimator lens 1115 into the holographic optical element 1120, and the laser light is diffracted by the holographic optical lens 1120. Since the zero-order diffracted light goes straight among the diffracted laser beams, the light is collected in a spot having a diameter of 1.6 μm on the first disk 1150 of 0.6 mm via the aperture of the objective lens 1130 having a numerical aperture of 0.6. Since the light is converted into divergent light and condensed narrowly by the objective lens, the light is condensed on the second disk 1155 in an airy shape having a diameter of 0.8 μm, so that the laser light is reflected almost as it is in the absence of the pit 1155 on the disk. To return to the objective lens 1130, but where the pit 1155 is present, the laser light is diffracted by the pit 1155 and emitted outside the range of the objective lens 1130, thereby causing only a part of the incident light to return. Thereby generating a light quantity difference in the photodetector 1145. This is because the depth of the pit 1155 is set to λ / 4 of the wavelength, and the reflected light is canceled by interference due to different half wavelengths above and below the pit 1155, thereby reducing the amount of light returned to the photodetector 1145.

The modulated reflected light reflected by the first disk 1150 or the second disk 1160 is returned to the beam splitter 1110 via the holographic optical device 1120 and the collimator lens 1115. The reflected light is reflected and transmitted again at a constant rate, and the laser light transmitted by the beam splitter 1110 is moved straight toward the photodetector 1145. The modulated reflected light is irradiated from the beam splitter 1110 to the astigmatism generating lens 1140, and the reflected light is generated by the astigmatism generating lens 1140 to detect a focus error, and thus the photodetector 1145. The reflected light, which is modulated by the disk, is converted into RF (RF), focus error detection, tracking control, and information into a current by a photodetector, which is converted into an original signal by a control circuit (not shown). Demodulation is performed.

Therefore, as described above, the laser light emitted from the laser diode 1100 is collected in different airy shapes on the disk via the holographic optical device 1120, so that the beam focus is formed at different positions so that the thickness is 1.2 mm. The disc and the information recorded on the 0.6 mm disc can be selectively read.

However, such a conventional dual focus optical pickup device has a processing effect due to the complex arrangement of the holographic optical element 1120 and the objective lens 1130 using diffraction of light in order to focus the optical disk with a double focus according to the thickness of the disk. Since a high level of technical skill is required, manufacturing is difficult and at the same time, the holographic optical device 1120 is fixed to the mover 1125 and thus adversely affects the dynamic characteristics of the actuator, and from the first disk 1150 to the second disk 1160. In the case of change, there is a problem that the obstacle of miniaturization of the optical pickup due to the enlargement of the objective lens by increasing the focal length of the objective lens due to the aberration change and the increase of the design dimension due to maintaining a predetermined distance between the optical elements.

Accordingly, the present invention has been in view of the above problems, and solves the manufacturing difficulties due to the precise interlayer arrangement of the holographic optical elements by etching, and solves the complexity of the shape to compensate for the aberration change, thereby improving the dynamic characteristics of the actuator. The dual focus optical pickup device can reduce the size of the optical pickup device by eliminating the problem of adjusting the focal length of the objective lens due to aberration change during reproduction caused by a plurality of disc thickness differences. The purpose is to provide.

In the present invention for achieving the above object, in the dual optical pickup device for receiving a laser beam reflected from the optical disk by condensing the laser beam having a predetermined wavelength with an objective lens and irradiated with the optical disk,

A phase hologram prism which is installed at a lower portion of the optical disk to be inclined with the recording surface and reflects and transmits the incident laser light at a constant ratio, and has a diffraction region different according to the polarization state, and is provided on one side of the phase hologram prism and is horizontal to the incident surface A first laser diode that emits laser light polarized by a component, a second laser diode that is disposed below the phase hologram prism and emits laser light having a wavelength perpendicular to the incident plane, and on both sides of the second laser diode A plurality of photodetectors provided to detect beams of incident light information diffracted with ± 1st order light, and converting the polarization of the laser light incident from the upper side of the phase hologram prism, and transmitting and reflecting at different widths according to wavelengths It characterized by consisting of a selective transmission wave plate.

1 is a block diagram of a conventional optical pickup device;

2 is a block diagram of the present invention,

3 is a schematic diagram of an operating state in which the present invention is shown for playing a digital video disc;

4 is a schematic diagram of an operating state in which the present invention is shown for playing a digital audio disc.

<Explanation of symbols for main parts of drawing>

10, 15: laser diode 20: phase hologram prism

22: astigmatism prism 24: phase hologram grid

30,31,32,33: photodetector 50: selective transmission wavelength plate

52: selective transmission plate 54: λ / 4 wavelength plate

56: inner circular plate 57: outer plate

60, 70: optical disk 100: objective lens

The present invention selectively transmits and reflects the laser light incident on the objective lens according to the wavelength so as to be focused on the optical disc, so that the numerical aperture (NA) of the objective lens is actively converted to enable compatible playback of digital audio and digital video discs. It is.

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

In the present invention, as shown in Fig. 2, a phase hologram element prism 20 which diffracts laser light having a constant polarization component and imparts an error detection signal is inclined with the recording surfaces of the optical disks 60 and 70.

Here, the phase hologram prism 20 reflects and transmits an astigmatism prism 22 that generates astigmatism according to incident light, and a laser beam that is formed to have a predetermined thickness on the upper side of the astigmatism prism 22 and is incident. A diffractive phase hologram lattice plate 24 is formed in combination, and the astigmatism prism 22 has a rectangular rectangular prism inner side acting as a lens only in one direction with respect to the focal point so that astigmatism occurs in the incident light from the inner side.

In addition, the phase hologram grid 20 only includes the inner circular diffraction plate 21a that diffracts only the laser light of the component (S wave) perpendicular to the incident surface and the laser light of the component (P wave) that is horizontal to the incident surface. It consists of the outer diffraction plate 21b to diffract.

In addition, a second laser diode 10 is arranged below the phase hologram prism 20 to emit a laser light having a phase perpendicular to the traveling direction of the light, that is, a beam polarized by the abnormal light. A first laser diode 15 is disposed on one side of the phase hologram prism 20 that is installed perpendicular to the installation surface 10 and faces the installation vertical surface of the phase hologram prism 20 to emit polarized laser light having a horizontal phase with respect to the traveling direction of the light. .

On both sides of the second laser diode 10, a plurality of photodetectors 30, 31, 32, and 33 which receive beams of optical information diffracted from the inner circular diffraction plate 21a and the outer diffraction plate 21b are provided. Each of the phase hologram prisms 20 and the optical disks 60 and 70 is provided with a selective transmission wavelength plate 50 to convert the polarization of the laser light and to transmit and reflect the wavelength according to the wavelength of the laser light. Let's do it. The selective transmission wavelength plate 50 separates and transmits the phase of the incident light at different widths on the basis of the optical axis according to the wavelength of the incident light, and delays the phase of the incident light to the selective transmission plate 52 and the λ / 4 wavelength plate 54. The dual-selective transmission plate 52 includes an inner circular plate 56 that transmits the laser light emitted from the first laser diode 15 and reflects the laser light emitted from the second laser diode 10. The outer plate 57 includes both the laser light emitted from the first laser diode 15 and the laser light emitted from the second laser diode 10. Thus, since the selective transmission wave plate 50 is formed of an annular light-transmitting film, it adjusts the outermost incident angles θ1 and θ2 that are collected on the optical discs 60 and 70 based on the optical axis. Since the outermost incident angles θ1 and θ2 are proportional to the numerical aperture, the transmissive reflections are differently transmitted along the optical axis, so that the numerical aperture (NA: Numerical Aperture) can be adjusted when the light is collected at different incidence angles on the optical discs 60 and 70. It is.

On the other hand, the numerical aperture adjusted by the selective transmission wave plate 50 is arranged as follows.

According to the present invention, the diameters of the light spots focused on the optical discs 60 and 70 are differently determined according to the outermost incident angles θ1 and θ2. When the outermost incident angle irradiated to the digital video disk 60 having a thickness of 0.6 mm is θ2, and the outermost incident angle irradiated to the digital audio disk 70 having a thickness of 1.2 mm is θ1, and the refractive index of the medium is η, The determination of the numerical aperture NA for is determined by the following equation.

N.A.1 = ηsinθ1 N.A.2 = ηsinθ2

According to Equation 1, when the refractive indices are the same, the numerical aperture (rate) is proportional to the outermost incident angles θ1 and θ2, so that θ2> θ1 is determined as N.A.2> N.A.1.

Then, the size of the light spots due to the different numerical apertures is determined by the following expression (2). The radius W of the beam (K is a constant determined by the intensity distribution of the light),

Therefore, the diameter of the light spot is in inverse proportion to the numerical aperture.

Therefore, in the case of the optical disc 60 having a thickness of 0.6 mm, since the track interval and the size of the pit 65 as the signal surface are small, a small optical spot diameter is required, and thus an opening larger than the optical disc 70 having a thickness of 1.2 mm. You will need a number of objectives. At this time, the beam spot diameter can read an optical signal even if it is somewhat larger on the optical disk 70 having a thickness of 1.2 mm, so that the objective lens 100 having a numerical aperture (NA = 0.6) dedicated to the digital video disk 60 can be obtained. To try compatible playback.

The relationship between the numerical aperture and the light width incident on the objective lens is determined by the following equation. If D and f are the focal lengths of the objective lenses,

D = 2 × f × (N.A.)

That is, by controlling the incident light width to the objective lens having the same focal length in Equation 3, it is possible to adjust the effective N.A.

In the present invention, since a single objective lens, i.e., an objective lens 100 for digital video disc, is adopted based on the same focal length, the optical signal is read out on the optical disc 60 having a thickness of 0.6 mm without generating spherical aberration.

The following describes the operation of the dual optical pickup apparatus employing the selective transmission wavelength plate 50 capable of adjusting the numerical aperture.

First, when reproducing the digital video disk 60 having a thickness of 0.6 mm among the optical disks 60 and 70, as shown in FIG. 3, the laser light having a constant wavelength λ 2 is emitted from the second laser diode 10. In this case, the second laser diode 10 emits a beam perpendicular to the plane of incidence with respect to the advancing direction of the laser light. Therefore, this laser light is irradiated from the second laser diode 10 to the phase hologram prism 20, and the laser light is reflected and transmitted at a constant rate as it passes through the phase hologram prism 20.

That is, the phase hologram prism 20 reflects and transmits at a predetermined ratio irrespective of the laser beam of components horizontal and vertical to the incident surface.

Thus, the laser light of the S-wave, which is a component perpendicular to the incident plane passing through the phase hologram prism 20 at a constant rate, is irradiated onto the selective transmission wavelength plate 50, and at this time, the selective transmission wavelength plate 50 The linearly polarized light is converted into circularly polarized light by the λ / 4 wavelength plate 54 and the width of transmitted light is adjusted by the selective transmission plate 52. The control of the transmission width is selectively transmitted while varying the light distribution of the incident light intensity, and the predetermined wavelength passing through the inner circular plate 56 and the outer plate 57 in the selective transmission plate 52 of the selective transmission plate 50. All the laser light of (λ2) is transmitted.

As a result, the laser beam transmitted through the selective transmission wavelength plate 50 is condensed by the objective lens 100 through the selective transmission wavelength plate 50 at a wide width. do. That is, the laser light is collected by the objective lens 100 at a numerical aperture of 0.6 with respect to the optical axis to form an optical spot having a size of about 0.8 μm on the optical disc 60, and thus interferes with diffraction on the recording surface 65. Optical information is read out based on the amount of light and the presence or absence of light. At this time, since the objective lens 100 dedicated to the digital video disc 60 is used in the present invention, it is condensed accurately without generating spherical aberration in the recording surface.

On the other hand, the operation during reproduction of the digital audio disc 70 having a thickness of 1.2 mm is as follows.

That is, as shown in FIG. 4, laser light having a predetermined wavelength λ 1 is emitted from the first laser diode 15 installed at one side of the selective transmission wavelength plate 50, which is the first laser diode 15. The polarization is adjusted so that P polarized light is emitted to emit laser light. At this time, the polarized laser light is a laser light having a wavelength larger than a predetermined wavelength λ 2 of the second laser diode 10, and is irradiated from the first laser diode 15 to the phase hologram prism 20. Light is reflected and transmitted at a constant rate by the phase hologram prism 20 and is not diffracted at the same time.

Thus, the laser light transmitted at a predetermined rate through the phase hologram prism 20 is irradiated to the selective transmission wavelength plate 50, and the laser light is λ / 4 wavelength plate 54 of the selective transmission wavelength plate 50. By converting linearly polarized light into circularly polarized light, laser light having a predetermined wavelength λ1 is transmitted from the inner circular plate 56 in the reflective transmission plate 52 of the selective transmission plate 50, and the outer plate 57 Since it is reflected, it passes through the selective transmission wave plate 50 with a narrower width than the laser light emitted from the second laser diode 10. Then, the laser light of a constant wavelength λ 1, which has been battled by the inner circular plate 56 and reflected by the outer plate 57, is focused by the objective lens 100 at a numerical aperture of 0.3 and is θ 1 with respect to the optical axis 70. Is condensed on the

Therefore, the laser beams passing through the selective transmissive plate 52 composed of the inner circular plate 56 and the outer plate 57 have different outermost incident angles θ1 and θ2, respectively, The different numerical apertures NA pass through the? / 4 wavelength plate 54 at the same time. The laser light polarized by circularly polarized light and whose width is controlled is irradiated from the selective transmission wavelength plate 50 to the objective lens 100 and focused on the optical disks 60 and 70.

By the way, the laser light focused by the objective lens 100 via the selective transmission wave plate 50 with the numerical apertures different from each other when the incident light on the optical disks 60 and 70 is incident to the outermost incident angles θ1 and θ2 of the laser beam. As described above, the difference in the diameter of the light spot is because the beam width emitted from the second laser diode 10 and condensed is emitted from the first laser diode 15 to form the inner circular plate of the selective transmission plate 52. It becomes smaller than the beam width transmitted and collected through the 56 and the outer plate 57.

Thus, the laser light, which reads the optical information on each of the optical discs 60 and 70 in the above-described process, is irradiated to the selective transmission wave plate 50 via the objective lens 100. At this time, the circularly polarized light is polarized by linearly polarized light as the laser beam passes through the λ / 4 wavelength plate 54 of the selective transmission wavelength plate 50. At this time, since the linearly polarized laser light is reflected on the optical disks 60 and 70, the inverted circularly polarized light is incident on the λ / 4 wave plate 54, and thus is orthogonal to when it is incident on the initial λ / 4 wave plate 54. It becomes linearly polarized light which has a polarization direction. The inverted linearly polarized laser light is incident on the phase hologram prism 20 via the selective transmission wave plate 50.

In the case where the beam incident to the phase hologram prism 20 is optical information by the beam emitted from the first laser diode 15, the shape of the beam is incident through the selective transmission wave plate 50. The beam is thus polarized with P waves in circularly polarized light. Therefore, the beam of polarized light information is diffracted into 0th order and ± 1st order through the inner circular diffraction plate 21a of the phase hologram prism 20, where ± 1st order light is irradiated to the respective photodetectors 31 and 33. do.

At this time, astigmatism is generated while the beam of light information incident on the photodetectors 31 and 33 passes through the astigmatism prism 22. Therefore, the beam of the optical information causes the focus error to be detected in the imaging plane, i.e., the photodetectors 31 and 33 by the generation of aberration, and calculates the amount of light diffracted and formed by the photodetectors 31 and 33 on both sides. Tracking error is detected.

In addition, when the beam incident through the phase hologram prism 20 is optical information by the beam radiated from the second laser diode 10, the beam has a predetermined width and has an outer diffraction plate 21b of the phase hologram lattice 24. ) Is diffracted to ± 1st light and irradiated to the photodetectors 30 and 32.

That is, the laser light, which reads the optical information through the respective paths, is diffracted, interfered and reflected and transmitted to output the digital audio signal and the digital video signal as original electrical signals, and the output optical signal is a digital signal processor ( (Not shown) to output an RF (RF) signal which is an audio signal recorded on an optical disc, a focus signal which is an error detection signal, and a tracking signal.

In addition, according to the present invention, since the laser light passes through the inside of the selective transmission plate 50 having different transmission regions depending on the wavelength, the laser beam having a constant wavelength lambda 2 is generated when the digital video disc is reproduced. Since both the outer plate 57 and the inner circular plate 56 pass through and are condensed by the objective lens 100, an optical spot is formed on the optical disc 60 with a diameter of about 0.8 μm to reproduce the digital video disc, and the digital audio When the disc is reproduced, laser light having a predetermined wavelength λ1 passes through the inner circular plate 56 and is reflected by the outer plate 57 to be focused on the optical disc 70 by the objective lens 100 and at the same time, the mutual optical disc 60, The spherical aberration caused by the thickness difference 70 is greatly reduced, so that compatible playback can be performed.

As described above, the focus optical pickup apparatus according to the present invention changes the spherical aberration generated due to the thickness of the optical disk and the recording surface difference of the signal information, and varies the incident light intensity distribution of the light beam incident on the objective lens. Since the spherical aberration is significantly reduced, the spherical aberration is significantly reduced, so that the digital audio and digital video discs can be reproduced interchangeably and can be manufactured in a light and simple manner mechanically.

Claims (3)

In the dual optical pickup apparatus for receiving a laser beam reflected from the optical disk (60, 70) by condensing the laser beam incident with a predetermined wavelength to the objective lens 100 and irradiated to the optical disk (60, 70), A λ / 4 wave plate 54 for converting the phase of the laser light at a predetermined distance below the optical disks 60 and 70, and formed on an upper surface of the λ / 4 wave plate 54 in accordance with the wavelength of the incident light. A selective transmission wavelength plate 50 formed of a selective transmission plate 52 for selectively performing transmission and reflection at different widths; Astigmatism prism 22 which is installed at an inclined distance below the selective transmission wave plate 50 to generate astigmatism by incident light, reflects and transmits incident light on the upper surface of the astigmatism prism 22, and ± A phase hologram prism 20 having a phase hologram lattice plate 24 diffracted by primary light; A first laser diode 15 formed on one side of the phase hologram prism 20 so as to face a mounting vertical plane and emitting laser light polarized with a horizontal component in a direction of light propagation; A second laser diode (10) disposed below the phase hologram prism (20) to emit laser light polarized with a component perpendicular to the traveling direction of the light; A plurality of photodetectors 30 and 31 provided on both sides of the second laser diode 10 to detect beams of light information that are diffracted by ± 1st order light through each diffraction region of the phase hologram prism 20 32, 33, characterized in that the dual focus optical pickup device. The inner circular plate 56 according to claim 1, wherein the selective transmission plate 52 reflects the laser light emitted from the first laser diode 10 and transmits the laser light emitted from the second laser diode 15. And an outer plate 57 which transmits all of the laser light emitted from the first laser diode 10 and the second laser diode 15. 2. The phase hologram lattice plate (24) according to claim 1, wherein the phase hologram lattice plate (24) is an inner circular diffraction plate (21a) which diffracts only laser light of a component (S wave) perpendicular to the incident surface and a component (P wave) horizontal to the incident surface. The dual focus optical pickup device, characterized in that the outer diffraction plate (21b) for diffracting only the laser light.