KR19990050012A - Dual focus optical pickup device using liquid crystal device - Google Patents

Abstract

The present invention relates to a dual focus optical pickup apparatus using a liquid crystal device. It is provided below the optical disk 50, and the liquid crystal hologram wavelength plate 20 for selectively reflecting and diffracting the laser light and converting polarization, and the laser having a constant wavelength below the liquid crystal hologram wave plate 20 Between the laser diode 10 emitting light, a plurality of photodetectors 40 and 42 for receiving reflected light from both sides of the laser diode 10, and between the liquid crystal hologram wave plate 20 and the optical disk 50. It comprises an objective lens 30 for focusing the laser light. Therefore, the spherical aberration generated due to the thickness of the optical disk and the recording surface difference of the signal information is varied by varying the incident light intensity distribution of the light beam incident on the objective lens, so that the numerical aperture NA of the objective lens is actively converted to reduce the spherical aberration significantly. As a result, the digital audio and digital video discs can be interchangeably played.

Description

Dual focus optical pickup device using liquid crystal device

The present invention relates to a dual focus optical pickup device using a liquid crystal device, and in particular, the spherical aberration caused by the difference in the thickness of the optical disk and the recording surface of the signal information varies the incident light intensity distribution of the light beam incident on the objective lens. The present invention relates to a dual focus optical pickup device using a liquid crystal element capable of actively reproducing digital audio and digital video discs since the number NA is actively converted and spherical aberration is significantly reduced.

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. A thick digital audio disc records data in multiple layers on one side, and a 0.6 mm thick digital video disc records 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. 1130 and a holographic optical element 1120 having a multi-stage diffraction layer w1, w2, h0 formed by etching, adopt a dual focus lens in which a composite array is arranged, and the holographic optical element 1120 The zero-diffracted light is linearly diverged and the first-diffracted laser light is divergent light. Therefore, 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 detecting the optical information passing through the astigmatism generating lens 1140 And a photodetector 1145 for converting the signal into a current signal.

In operation of the conventional optical pickup apparatus having such a configuration, first, the laser light having a predetermined oscillation wavelength is scanned by the laser diode 1100 and incident on the diffraction grating 1105, and the incident laser light is incident on the diffraction grating 1105. ) And radiated into 0th and ± 1st light, that is, three beams. The three beams are used for tracking errors. The three beams penetrate the diffraction grating 1105 and enter the beam splitter 1110. 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 diffraction 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 disc 1150 having a diameter of 0.6 mm through 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.

The present invention has been made in view of the above problems, and improves the dynamic characteristics of the actuator by eliminating the complexity of the shape due to the manufacturing difficulties and aberration changes due to the precise interlayer alignment of the holographic optical element by etching, Dual focus optical pickup using a liquid crystal device that 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 playback caused by a plurality of disc thickness differences. The object is to provide a device.

The present invention to achieve the above object,

A phase hologram lattice provided below the optical disk and selectively diffracting according to the polarization of the laser light;

It is formed on the upper side of the phase hologram lattice and is installed opposite to the upper and lower sides, the transparent electrode is applied to the power supply, the hollow electrode is formed between the transparent electrode and formed in a predetermined radius inside, and formed outside the hollow park portion Light control liquid crystal plate consisting of a hollow liquid crystal plate consisting of a liquid crystal layer that transmits laser light when voltage is applied, and

A wavelength plate for converting a phase of a laser beam above the light control liquid crystal plate,

A liquid crystal hologram wave plate in which the wave plate, the light control liquid crystal plate and the phase hologram grid are integrally formed;

A laser diode which emits laser light of a predetermined wavelength under the liquid crystal hologram wave plate;

A plurality of photodetectors for receiving reflected light from both sides of the laser diode;

An objective lens for condensing laser light between the liquid crystal hologram wave plate and the optical disk;

Provided is a dual focus optical pickup apparatus using a liquid crystal device including the above components.

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

2 is a block diagram of the present invention,

3 is a partial detailed view of the present invention;

4 is an operating state in which the present invention shows a digital video disc playback;

5 is an operational state in which the present invention shows digital audio disc playback;

<Explanation of symbols for main parts of drawing>

10: laser diode 20: liquid crystal hologram wave plate

30: objective lens 40, 42: photodetector

50: optical disc

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

As shown in FIG. 2, the present invention provides a liquid crystal hologram wave plate 20 disposed below the optical disc 50 to selectively reflect and diffract laser light and convert polarization, and the liquid crystal hologram wave plate ( 20, a laser diode 10 that emits laser light of a predetermined wavelength from below, a plurality of photodetectors 40 and 42 for receiving reflected light from both sides of the laser diode 10, and the liquid crystal hologram wave plate ( 20) and an objective lens 30 for condensing laser light between the optical disk 50.

Since the photodetectors 40 and 42 on both sides are spaced apart from each other to form an image at an arbitrary point between the light receiving sections, the optical signals are condensed on each of the photodetectors 40 and 42, and the optical signals are output as electrical signals.

As shown in FIG. 3, the liquid crystal hologram wave plate 20 includes a phase hologram lattice 22 that selectively diffracts according to the polarization of the laser light, and whether voltage is applied above the phase hologram lattice 22. The light adjusting liquid crystal plate 24 which transmits laser light with different widths according to the above, and the wavelength plate 28 for converting the phase of the laser light on the upper side of the light adjusting liquid crystal plate 24.

Of these, the light control liquid crystal plate 24 is disposed opposite to the upper and lower sides and is provided with a transparent electrode 26 to which power is applied, and the hollow cone-shaped portion 25b formed between the transparent electrodes 26 and having a predetermined radius inside. And a hollow liquid crystal plate 25 formed on the outside of the hollow cone-shaped portion 25b and configured of a liquid crystal layer 25a that transmits laser light when voltage is applied thereto.

Therefore, the liquid crystal hologram wave plate 20 adjusts the outermost incident angles θ1 and θ2 of the laser light focused on the optical disc 50.

Since the outermost incident angles θ1 and θ2 are proportional to the numerical aperture, the light quantity is controlled by the transmittance according to the width, and the numerical aperture (N.A .: Numerical Aperture) can be adjusted when the light is focused at different incident angles on the optical disc 50.

The numerical aperture N.A. controlled by the liquid crystal hologram wave plate 20 is summarized as follows.

According to the present invention, the diameter of the light spot focused on the optical disc 50 is differently determined according to the outermost incident angles θ1 and θ2. When the outermost incident angle irradiated to the digital video disk 52 having a thickness of 0.6 mm is θ2 and the outermost incident angle irradiated to the digital audio disk 54 having a thickness of 1.2 mm is θ1, and the refractive index of the medium is η, Determination of the numerical aperture (NA) for is determined by the following equation (1).

N.A.1 = ηsinθ1 N.A.2 = ηsinθ2

When the refractive index is the same according to Equation 1, the numerical aperture is proportional to the outermost incidence angles θ1 and θ2. When θ2> θ1, the numerical aperture is determined to be 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 disk 52 having a thickness of 0.6 mm, since the track interval and the size of the pit 55 as the signal surface are small, a small optical spot diameter is required, and thus an opening larger than the optical disk 54 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 the beam spot diameter is slightly increased on the optical disk 54 having a thickness of 1.2 mm, so that the objective lens 40 having a numerical aperture (NA = 0.6) dedicated to the digital video disk 52 can be obtained. As a result, compatible playback will be attempted.

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, that is, an objective lens 30 exclusively for digital video discs, is adopted based on the same focal length, the optical signal is read out without spherical aberration on the optical disc 52 having a thickness of 0.6 mm.

However, in this case, when the optical disk 54 having a thickness of 1.2 mm is read out, first, when focus compensation is not performed, the focusing accuracy is not accurate due to defocusing, and thus the error occurrence width is increased. The spherical aberration causes the increase in the distribution light intensity of the side lobe of the laser beam and the decrease in the central intensity distribution, thereby increasing the crosstalk between adjacent signals and decreasing the signal-to-noise ratio (S / N ratio). The optical performance is significantly lowered, making it impossible to read optical information. Therefore, the effective N.A. must be adjusted by Equation 3 above. The range of these effective openings is determined as follows.

First, the amount of spherical aberration caused by the difference in disk thickness is determined by the following equation (4). If the refractive index is η and the disk thickness change is Δd,

On the other hand, if the amount of aberration generated by defocus is ΔZ, Equation 4 is modified as follows.

The cumulative wave front aberration of the optical system of the present invention can be expressed by the aberration system from the relationship between the center intensity distribution and the wave aberration by matching the spherical aberration by equating the aberration generation amount of Equation 4 to a value compensated for defocusing. According to marechal's criterion that should be less than 0.07λ, the effect on the defocus can be eliminated, thus approaching the anhydrometer.

Therefore, when the numerical aperture is compensated for in the above equations, the maximum value of spherical aberration value 0.5 is obtained. That is, the effective number of openings should be 0.5 or less, and the minimum value is limited to Equation 2, thereby increasing the diameter of the light spot. The increase in the light spot diameter causes a crosstalk caused by an increase in the light spot size rather than the crosstalk due to the change in the intensity distribution due to the generated aberration, thereby lowering the signal-to-noise ratio. Therefore, since the minimum value is limited to the intensity distribution of the generated aberration, when the optical disk having a thickness of 0.6 mm is read out by the objective lens having a numerical aperture of 0.6 used in this embodiment, the effective number of openings is summarized as follows.

0.27 <N.A.≤0.5

In this way, the numerical aperture can be determined by selecting an appropriate numerical value from among the values calculated for the change of the crosstalk according to the numerical aperture change and the sensitivity size of the reproduction signal. In the present invention, the numerical aperture is determined to be 0.3.

Therefore, when the outermost incident angles θ1 and θ2 from the liquid crystal hologram wave plate 20 onto the optical disc 50 are adjusted by Equations 1 to 6, the numerical aperture is adjusted so that only the optical disc 52 dedicated to the thickness of 0.6 mm may be used. The objective lens 30 can reproduce the optical disk 54 having a thickness of 1.2 mm, and the spherical aberration generated during the compatible reproduction is reproduced by fighting the narrow laser light, that is, the laser light around the optical axis. Regeneration can be excellent.

Next, a description will be given of the operation of the present invention by adopting the liquid crystal hologram wave plate 30 capable of adjusting the numerical aperture as described above.

First, as shown in FIG. 4, a laser light having a predetermined wavelength is emitted from the laser diode 10, which is a light component perpendicular to the incident surface with respect to the advancing direction of the laser light. For example, S-polarized light is emitted. The polarization is adjusted so as to emit laser light. The polarized laser light is irradiated from the laser diode 10 to the liquid crystal hologram wave plate 20, and the laser light is not diffracted while passing through the phase hologram lattice 22 of the liquid crystal hologram wave plate 20. This does not diffract the S-polarized laser light, but rather diffracts only the laser light parallel to the optical axis direction, that is, the P-polarized laser light, so that incident light is transmitted without diffraction.

Thus, the laser light passing through the phase hologram lattice 22 is irradiated to the light adjusting liquid crystal plate 24 of the liquid crystal hologram wave plate 20, and the laser beam is controlled to be transmitted by the light adjusting liquid crystal plate 24. will be. The control of the amount of transmission is to selectively transmit while varying the light intensity distribution of the incident light intensity, when the power is applied to the light control liquid crystal panel 30 from the transparent electrode 26 of the light control liquid crystal panel 30 from the liquid crystal layer 25a Since the liquid crystal molecules of the liquid crystal layer 25a are arranged in the vertical direction in the electric field direction, the laser light of S polarization can be transmitted, and power is applied to the light regulating liquid crystal plate 30. If no voltage is applied to the liquid crystal layer 25a from the transparent electrode 26 of the light control liquid crystal panel 30, the liquid crystal molecules are twisted and oriented in the same direction as the polarization plane of the laser light. It will not be transmitted.

Therefore, the laser beam is transmitted separately from the liquid crystal layer 25a of the hollow liquid crystal plate 25 depending on whether voltage is applied, and the liquid crystal layer 25a and the hollow cone 25b of the hollow liquid crystal plate 25 are applied when voltage is applied. ) Is transmitted through the objective lens 30, the outermost incident angle is focused at θ2 with respect to the optical axis, and when no voltage is applied, the laser beam does not pass through the liquid crystal layer 25a, The laser beam transmitted only through 25b) is focused on the outermost incident angle θ1 with respect to the optical axis via the objective lens 30. That is, the laser beams passing through the light control liquid crystal plate 24 have different outermost incident angles θ1 and θ2, respectively, and the wavelength plates 28 have different numerical apertures NA according to Equations 1 to 6 described above. Will pass through at the same time. Thus, the laser light polarized by circularly polarized light through the wave plate 28 and whose light amount is controlled is irradiated from the liquid crystal hologram wave plate 20 to the objective lens 30 to be focused onto the optical disc 50.

By the way, the laser light focused by the objective lens 30 via the light control liquid crystal plate 24 at the different numerical apertures is different from the outermost incident angles θ1 and θ2 of the laser light when it is incident on the optical disc 50. Due to the difference in the diameter of the light spot, the light collected through the liquid crystal layer 25a is smaller than the light collected through the hollow cone-shaped portion 25b of the light control liquid crystal plate 24 as described above. .

Thus, when reproducing each optical disk 50, for example, a digital video disk 52 having a thickness of 0.6 mm, the numerical aperture is transmitted by the wavelength plate 28 and the objective lens 30 through the liquid crystal layer 25a. 0.6, an optical spot having a size of about 0.8 占 퐉 is formed on the optical disk 52 at θ2 with respect to the optical axis, which interferes with diffraction on the recording surface, thereby reading optical information with an amount of light and the presence or absence of light. At this time, since the objective lens 30 dedicated to the digital video disk 52 is used in the present invention, it is condensed accurately without generating spherical aberration in the recording surface.

When the digital audio disc 54 having a thickness of 1.2 mm is reproduced, as shown in FIG. 5, since no voltage is applied to the light adjusting liquid crystal plate 24, the liquid crystal molecules are arranged to be deflected, and thus the hollow liquid crystal plate 25 is used. The laser light, which does not penetrate the liquid crystal layer 25a, but passes only the hollow cone-shaped portion 25b, is polarized by the wavelength plate 28, is focused by the objective lens 30 at a numerical aperture of 0.3, and is an optical disk at θ1 with respect to the optical axis. It is condensed on (50).

Thus, the laser light, which reads the optical information on each optical disc 50 in the above-described process, is irradiated to the liquid crystal hologram wave plate 20 via the objective lens 30. In the laser light, circularly polarized light is polarized by linearly polarized light when the wavelength plate 28 of the liquid crystal hologram wave plate 20 is transmitted. At this time, since the linearly polarized laser light is incident on the wavelength plate 28 by the inverted circular polarized light upon reflection on the optical disk 50, the linearly polarized laser light becomes a linearly polarized light having a polarization direction orthogonal to the time of incidence upon the initial wavelength plate 28. . The inverted linearly polarized laser light is incident on the phase hologram lattice 22 via the light control liquid crystal plate 24.

Thus, the reflected light is irradiated onto the phase hologram lattice 22 with a delayed phase of the linearly polarized light. At this time, the laser light, which is focused at θ2 on the digital video disk 52 and reflected at a broad width, is diffracted from the phase hologram lattice 22. .

Thus, the diffracted laser light is irradiated from the phase hologram lattice 22 to the photodetectors 40 and 42, and the reflected light is diffracted to cause the focus error to be detected in the photodetectors 40 and 42 and diffracted to both the photodetectors 40 , A tracking error is detected by calculating the amount of light to be imaged.

In this way, the laser light, which reads the optical information via the respective paths, is diffracted, interfered and reflected and transmitted, and outputs 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). Demodulated to the original signal to output an RF (RF) signal which is an audio signal recorded on the optical disc, a focus signal which is an error detection signal, and a tracking signal.

As described above, the present invention passes through the laser beam to the light control liquid crystal plate 24 having different transmittances depending on the application of the power source, so that the hollow cone-shaped portion 25b of the light control liquid crystal plate 24 is used during the reproduction of the digital video disc. Since all of the liquid crystal layer 25a is transmitted and condensed by the objective lens 30, an optical spot is formed on the optical disc 52 with a diameter of about 0.8 占 퐉 and received by the photodetectors 40 and 42, thereby reproducing a digital video disc. When the digital audio disc is reproduced, laser light is transmitted through the hollow cone 25b and does not penetrate the liquid crystal layer 25a, so that the objective lens 30 causes the light on the optical disc 54. Since the light is collected and received by the photodetectors 40 and 42, the spherical aberration caused by the difference in thickness between the optical disks 52 and 54 is greatly reduced, and each optical information is received by the other photodetectors so that compatible reproduction can be performed without interference. have.

In the dual focus optical pickup apparatus using numerical aperture adjustment, the spherical aberration caused by the difference in the thickness of the optical disk and the recording surface of the signal information varies the incident light intensity distribution of the light beam incident on the objective lens. Since the NA) is actively converted, the spherical aberration is significantly reduced, so that digital audio and digital video discs can be interchangeably played.

Claims (2)

A phase hologram lattice 22 disposed below the optical disc 50 and selectively diffracted according to the polarization of the laser light, and installed above the phase hologram lattice 22 and opposed to the upper and lower sides to supply power. It is provided between the transparent electrode 26 and the transparent electrode 26 to form a hollow ring portion 25b formed at a predetermined radius inside, and formed outside the hollow ring portion 25b to transmit the laser light when voltage is applied. Liquid crystal hologram composed of a light control liquid crystal plate 24 made of a hollow liquid crystal plate 25 made of a liquid crystal layer 25a, and a wavelength plate 28 for converting a phase of laser light on the light control liquid crystal plate 24. A wave plate 20; A laser diode 10 emitting a laser beam of a predetermined wavelength under the liquid crystal hologram wave plate 20; A plurality of photodetectors 40, 42 for receiving reflected light from both sides of the laser diode 10; An objective lens 30 for condensing laser light between the liquid crystal hologram wave plate 20 and the optical disc 50; Dual focus optical pickup apparatus using a liquid crystal element, characterized in that comprises the component. 2. The dual focus optical pickup apparatus of claim 1, wherein the liquid crystal hologram wave plate (20) adjusts the outermost incident angles (θ1, θ2) of the laser light focused on the optical disc (50).