JPH10116434A - Optical pickup device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent fluctuations of wave front aberration amount at the time of a tracking operation from being generated by moving a polarizing filter controlling an effective numerical aperture together with an objective lens at the time of the tracking operation. SOLUTION: Since a polarizing filter 5 and an objective lens 6 are both fixed to an objective lens holder, the polarizing filter 5 is moved together with the objective lens 6 at the time of the tracking operation and a focusing operation. Since polarization directions of these light beams are parallel with the transmission polarization direction of the polarizing filter 5. the light beams are made incident on the objective lens 6 almost without being interrupted by the filter 5 and the incident light beams on the lens 6 are made to be a converged light from parallel beams to form a spot on the recording surface of a recording medium 14. Although, the thickness of a substrate is different normally in a high density recording medium and in a low density recording medium and the amount of spherical aberration is also different, the compensation for aberration is made sufficiently in this device.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup device, and more particularly, to a plurality of light beams having different polarization directions.
The present invention relates to an optical pickup device sharing the same objective lens.

[0002]

2. Description of the Related Art In recent years, as the application range of optical recording media has been widened, various types of light having different recording densities, substrate (protective layer) thicknesses, substrate (protective layer) materials, recording / reproducing methods, disk sizes and the like have been changed. A recording medium is provided.

In an optical pickup device for handling optical recording media having different recording densities, the diameter of a light beam spot (condensed spot) (hereinafter referred to as a spot diameter) according to the recording density of the optical recording medium. .) Must be changed.

Here, the recording density is defined by the recording mark length and the track pitch. In the case of a low-density recording medium having the shortest recording mark length of about 0.8 μm, the spot diameter is about 1.4 μm. In the case of a high-density recording medium having a minimum recording mark length of about 0.4 μm, the spot diameter needs to be about 0.9 μm.

The spot diameter D is given by the following equation.

D = k · λ / NA Here, k is a proportional constant determined by the intensity distribution and aberration of the light beam, and λ is the wavelength of the light beam. Incidentally, the proportional constant is usually set to k ≒ 0.82.

Further, NA is the numerical aperture of the lens and is given by the following equation.

NA = n × sin θ In this equation, n is the refractive index, and θ is the angle spanned by the exit pupil diameter as shown in FIG. 7 (an explanatory diagram for explaining the numerical aperture of the lens 35). .

As can be seen from these equations, in order to reduce the spot diameter D, it is necessary to shorten the wavelength λ of the light beam or increase the numerical aperture (NA) of the lens, that is, θ. .

However, as the numerical aperture (NA) of the lens increases, the amount of coma caused by the inclination of the recording medium increases significantly. This coma aberration is proportional to the cube of the numerical aperture (NA) and the thickness of the substrate (protective layer) as shown in Equation 1.

[0011]

(Equation 1)

Therefore, when the numerical aperture (NA) of the objective lens increases with the increase in the density of the recording medium, the thickness of the substrate (protective layer) tends to decrease.

When the thickness of the substrate (protective layer) is changed for the reasons described above, the spherical aberration generated in the substrate (protective layer) also changes. This spherical aberration is proportional to the fourth power of the numerical aperture (NA) and the thickness of the substrate (protective layer) as shown in Expression 2, and is usually caused by the optical system (eg, objective lens) of the optical pickup device. Corrections have been made.

[0014]

(Equation 2)

Therefore, when recording media having different thicknesses of substrates (protective layers) are recorded and reproduced by the same optical system, it is not possible to optimally correct spherical aberration for all recording media. As a means for solving this problem, there is known a method of reducing the influence of spherical aberration by changing the effective numerical aperture by narrowing the light beam incident on the objective lens.

For example, in an optical pickup device for recording / reproducing a high-density recording medium (DVD) having a thin substrate (protective layer) and a low-density recording medium (CD) having a thick substrate (protective layer), the optical system must be a DVD. Side, and the effective numerical aperture when recording / reproducing a CD is (NA
=) 0.6 to (NA =) 0.45 to reduce the effect of spherical aberration.

Next, an optical pickup device (JP-A-8-102079) in which the effective numerical aperture is changed using a liquid crystal filter will be described with reference to FIG.

In this optical pickup device,
The light beam emitted from the semiconductor laser 31 is
After being separated into a main beam (for a focus error signal and a recording / reproducing signal) and a sub-beam (for a tracking error signal) in 2, it is incident on a collimator lens 33. The light beam that has entered the collimator lens 33 is converted from divergent light into parallel light, and then enters the beam splitter 34. The light beam that has entered the beam splitter 34 is reflected in the direction of the objective lens 35 and enters the objective lens 35 via the liquid crystal filter 51. The light beam incident on the objective lens 35 is changed from parallel light to convergent light, and forms a spot on the recording surface of the optical recording medium 36.

This light beam is reflected by the recording surface, and is modulated at that time in accordance with information recorded on the recording surface. The light beam reflected by the recording surface is converted into parallel light again by the objective lens 35 and enters the beam splitter 34. The light beam that has entered the beam splitter 34 travels straight through the beam splitter 34 and enters the condenser lens 37. Condensing lens 37
Is converted from parallel light into convergent light, and enters the anamorphic lens 38. The light beam that has entered the anamorphic lens 38 generates astigmatism for detecting a focus error signal, and enters the light receiving element 39. Light receiving element 3
The light beam incident on 9 is converted into an electric signal such as a tracking error signal, a focus error signal, and a recording / reproducing signal.

In order to obtain an accurate electric signal in the optical pickup device, it is necessary to form an appropriate spot on the recording surface, and for that purpose, it is necessary to suppress the influence of spherical aberration. The apparatus uses a liquid crystal filter 51 to reduce the influence of spherical aberration.

FIGS. 5 and 6 show a case where recording and reproduction are performed on a high-density recording medium having a thin substrate (protective layer) and a low-density recording medium having a thick substrate (protective layer) by using the liquid crystal filter 51.

FIG. 5 (a) is a plan view of a liquid crystal filter,
(B) Explanatory diagram showing a light beam transmitted through a liquid crystal filter) shows a case where recording and reproduction are performed on a high-density recording medium.
After passing through the inside uniformly, the light is converged by the objective lens 52. Further, when the light beam passes through the objective lens 52, the light beam is provided with a spherical aberration in the opposite direction to cancel the spherical aberration generated on the substrate (protective layer) 53a of the high-density recording medium (hereinafter, referred to as a “light beam”).
Aberration correction). The convergent light beam thus corrected for aberration passes through the substrate (protective layer) 53a and forms a small spot 55a on the recording surface 54a.

FIG. 6 (a) is a plan view of a liquid crystal filter,
(B) Explanatory diagram showing a light beam passing through a liquid crystal filter) shows a case of recording / reproducing a low-density recording medium. The light enters the lens 52 and is
Is converged light. Here, when the light beam is transmitted through the objective lens 52, the light beam is subjected to the same aberration correction as in the case of the high-density recording medium. The convergent light beam thus corrected for aberration passes through the substrate (protective layer) 53b and forms a spot 55b on the recording surface 54b. However, since θ1> θ2, an effective numerical aperture is obtained. And the spot diameter becomes larger than in the case of reproducing a high-density recording medium.

A low-density recording medium substrate (protective layer) 53
Since b is thicker than the substrate (protective layer) 53a of the high-density recording medium, spherical aberration remains with the same aberration correction as in the case of the high-density recording medium, but θ2 is small in the case of the low-density recording medium because θ2 is small. That is, since the numerical aperture is small, the influence is small.

In other words, the amount of spherical aberration generated is large at the periphery of the light beam (light beam) and extremely small at the center of the light beam (light beam). Even if sufficient, spherical aberration hardly matters.

As described above, in the optical pickup device using the liquid crystal filter 51, by controlling the diameter (effective numerical aperture) of the light beam incident on the objective lens, the low-density recording medium and the high-density recording medium are controlled. In this case, the spot diameter was changed, and the influence of spherical aberration was reduced.

[0027]

By the way, in the optical pickup device, at the time of recording / reproducing, the objective lens has a focus direction (a direction perpendicular to the recording surface of the recording medium) and a tracking direction (parallel to the recording surface of the recording medium and perpendicular to the track). Direction), the optical pickup device using the liquid crystal filter described in Japanese Patent Application Laid-Open No. H8-102079 causes a displacement between the liquid crystal filter and the central axis of the objective lens during the tracking operation. , The amount of wavefront aberration varies. Further, if a liquid crystal filter is provided on a member that moves together with the objective lens during the tracking operation, it becomes difficult to perform a good tracking operation.

Accordingly, the present invention provides an optical element that controls the effective numerical aperture in a simple manner, so that the optical element moves together with the objective lens without hindering the tracking operation and the focusing operation, and the wavefront aberration during the tracking operation is reduced. It is an object of the present invention to provide an optical pickup device in which the amount of fluctuation is reduced.

[0029]

According to a first aspect of the present invention, there is provided an optical pickup device having a light source for generating a light beam, and an optical means for guiding the light beam emitted from the light source to an optical recording medium. In an optical pickup device, an optical element whose transmittance distribution changes depending on the polarization direction of a light beam moves during a tracking operation together with an objective lens.

The optical pickup device according to claim 2 is
2. The optical pickup device according to claim 1, wherein the optical element whose transmittance distribution changes according to the polarization direction of the light beam is an optical element whose effective numerical aperture changes according to the polarization direction of the light beam. It is assumed that.

The optical pickup device according to claim 3 is
The optical pickup device according to claim 1 or 2,
The optical element whose transmittance distribution changes depending on the polarization direction of the light beam is a polarization filter.

The optical pickup device according to claim 4 is
4. The optical pickup device according to claim 1, wherein the optical element whose transmittance distribution changes according to the polarization direction of the light beam is fixed to an edge of the objective lens. is there.

The optical pickup device according to claim 5 is
4. The optical pickup device according to claim 1, wherein the optical element whose transmittance distribution changes according to the polarization direction of the light beam is fixed to an objective lens holder. .

The optical pickup device according to claim 6 is
The optical pickup device according to any one of claims 1 to 5, wherein the optical pickup device has a plurality of light sources.

[0035]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An optical pickup device according to the present invention will be described below with reference to the drawings.

First, the optical pickup device shown in FIG. 1 will be described separately for the case of recording / reproducing a low density recording medium and the case of recording / reproducing a high density recording medium.

In the case of a low-density recording medium, a light beam having an oscillation wavelength of 780 nm output from the LD / PD unit 1 enters the collimator lens 2, is converted from divergent light into parallel light, and is transmitted to the polarizing beam splitter 3. Incident as waves.
The light beam that has entered the polarization beam splitter 3 as an S-wave is reflected in the direction of the rising mirror 4, reflected by the rising mirror 4, and then enters the annular polarizing filter 5 whose central portion has been removed. Since the polarization direction of the light beam is orthogonal to the transmission polarization direction of the polarization filter 5 (the polarization direction of the linearly polarized light transmitted through the polarization filter), only the light flux of the removed part (center part) of the polarization filter 5 is transmitted. The light beam having a reduced beam diameter enters the objective lens 6.
The light beam incident on the objective lens 6 is changed from parallel light to convergent light, and forms a spot on the recording surface of the recording medium 14.

Since the polarizing filter 5 and the objective lens 6 are both fixed to the objective lens holder, the polarizing filter 5 moves together with the objective lens during a tracking operation or a focusing operation.

Thereafter, the light beam reflected by the recording surface of the recording medium 14 passes through the reverse path, that is, the objective lens 6,
The light enters the LD / PD unit 1 via the polarizing filter 5, the rising mirror 4, the polarizing beam splitter 3, and the collimator lens 2.

The LD / PD unit 1 is an optical unit in which a light source (laser diode) and a light receiving element (photodetector) are optically adjusted and integrated.
The focus error signal is detected by the Foucault method, and the track error signal is detected by the push-pull method.

In the case of a high-density recording medium, a light beam having an oscillation wavelength of 650 nm output from the semiconductor laser 7 is incident on the diffraction grating 8, and includes a main beam (for a focus error signal and a recording / reproducing signal) and a sub beam. After being separated into (for a tracking error signal), the light enters the collimator lens 9. The light beam that has entered the collimator lens 9 is converted from divergent light into parallel light, and enters the half mirror 10. The light beam that has entered the half mirror 10 is reflected in the direction of the polarization beam splitter 3 and enters the polarization beam splitter 3 as a P-wave. The light beam that has entered the polarization beam splitter 3 as a P-wave is transmitted in the direction of the rising mirror 4, reflected by the rising mirror 4, and then enters the annular polarizing filter 5 from which the center has been removed. The polarization direction of this light beam is
Since the direction is parallel to the transmission polarization direction of the polarization filter 5 (the polarization direction of linearly polarized light transmitted through the polarization filter), the light beam is incident on the objective lens 6 without being substantially blocked by the polarization filter 5. The light beam incident on the objective lens 6 is changed from parallel light to convergent light, and forms a spot on the recording surface of the recording medium 14.

Thereafter, the light beam reflected on the recording surface of the recording medium 14 travels in the reverse path, that is, the objective lens 6,
The light enters the half mirror 10 via the polarizing filter 5, the rising mirror 4, and the polarizing beam splitter 3. The light beam that has entered the half mirror 10 passes through the half mirror 10 and enters the condenser lens 11. The light beam that has entered the condenser lens 11 is converted from parallel light into convergent light, and enters the anamorphic lens 12. The light beam incident on the anamorphic lens 12 is given astigmatism for detecting a focus error signal, and is condensed on the light receiving element 13.

In the case of a high-density recording medium, a focus error signal is detected by an astigmatism method, and a track error signal is detected by a three-beam method.

In the above optical pickup device, the light beam having a wavelength of 650 nm and the light beam having a wavelength of 780 nm incident on the annular polarizing filter 5 from which the central portion has been removed have the polarization directions orthogonal to each other. The polarization direction of the light beam is the transmission polarization direction of the polarization filter 5 and the wavelength 78.
The polarization direction of the 0 nm light beam is set to a direction orthogonal to the transmission polarization direction of the polarization filter 5. With this setting, a light beam having a wavelength of 780 nm and a wavelength of 6
The effective numerical aperture of the objective lens 8 (substantial numerical aperture with respect to the light beam incident on the objective lens 6) changes with the light beam of 50 nm, and the numerical aperture is large when the light beam has a wavelength of 650 nm. , 780 nm.

Therefore, when reproducing a high-density recording medium, a small spot is formed on the recording surface, and when reproducing a low-density recording medium, a relatively large spot is formed on the recording surface.

In the above optical pickup device, the effective numerical aperture is controlled by the polarizing filter 5. FIG.
Shows this polarization filter and its transmission characteristics. As shown in (a) perspective view and (b) cross-sectional view (AA 'cross section shown in (a)) of FIG.
The center of the film has been removed.

The transmittance distribution along the section AA ′ of this polarizing filter is as shown in FIG. 2C for a light beam having a wavelength of 650 nm, and as shown in FIG. 2D for a light beam having a wavelength of 780 nm. Here, the polarization direction of the light beam having a wavelength of 650 nm is the transmission polarization direction of the polarization filter 5,
The polarization direction of the light beam having a wavelength of 780 nm is determined by the polarization filter 5.
Because it is set in the direction orthogonal to the transmission polarization direction of
Only in the case of a light beam having a wavelength of 780 nm, the outer periphery of the light beam is blocked by the polarizing filter.

The light beam having a wavelength of 650 nm and the light beam having a wavelength of 780 nm may have the same wavelength as long as the light beams have polarization directions orthogonal to each other. Also, the light beam output from the same light source may be switched between linearly polarized light parallel to the transmitted polarization direction and linearly polarized light orthogonal to the transmission polarization direction by using a half-wave plate, a liquid crystal element, or the like.

Further, the substrate (protective layer) usually differs in thickness between the high-density recording medium and the low-density recording medium (normally, the low-density recording medium has a thicker substrate (protective layer)). Although the amount of spherical aberration generated in the protective layer differs, the above-described optical pickup device is set so that sufficient aberration correction for a high-density recording medium is performed by an objective lens or other optical elements. ing.

Therefore, in the case of a low-density recording medium, although the aberration remains, only the light beam at the central portion of the objective lens where the generated spherical aberration is small contributes to the image formation. It can be used without any problem.

In this optical pickup device, the thickness of the substrate (protective layer) of the high-density recording medium is 0.6 mm, and the thickness of the substrate (protective layer) of the low-density recording medium is 1.2 mm.
When reproducing the high-density recording medium, the numerical aperture of the objective lens is 0.6,
The substantial numerical aperture of the objective lens during reproduction of the low-density recording medium was set to 0.45.

In the present invention, a polarizing filter for controlling the effective numerical aperture may be directly bonded to the edge 6a of the objective lens 6, as shown in FIG. In this way, the polarizing filter for controlling the effective numerical aperture can be easily fixed to the objective lens 8.

[0053]

As described above, according to the optical pickup device of the present invention, the polarization filter for controlling the effective numerical aperture moves together with the objective lens during the tracking operation. Fluctuation in the amount of wavefront aberration hardly occurs.

Further, a polarizing filter for controlling the effective numerical aperture hardly burdens the tracking operation and the focusing operation.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a configuration of an optical pickup device of the present invention.

FIGS. 2A and 2B are a perspective view and a cross-sectional view (AA ′ cross-sectional view) showing a polarizing filter for controlling an effective numerical aperture.
It is. (C) and (d) are transmittance distributions along the AA ′ section.

FIG. 3 is a cross-sectional view showing a case where a polarizing filter is attached to an edge of an objective lens.

FIG. 4 is an explanatory diagram showing a configuration of a conventional optical pickup device (when a liquid crystal filter is used).

FIG. 5 is an explanatory view showing a plan view of a liquid crystal filter and a light beam transmitted through the liquid crystal filter (when reproducing a high-density recording medium).

FIG. 6 is an explanatory view showing a plan view of a liquid crystal filter and a light beam transmitted through the liquid crystal filter (when reproducing a low-density recording medium).

FIG. 7 is an explanatory diagram for explaining a numerical aperture.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 LD / PD unit 2, 9 Collimator lens 3 Polarization beam splitter 4 Starting mirror 5 Polarization filter 6 Objective lens 6a Edge 7 Semiconductor laser 8 Diffraction grating 10 Half mirror 11 Condensing lens 12 Anamorphic lens 13 Light receiving element 14 Recording medium

Claims (6)

[Claims] 1. An optical pickup device having a light source for generating a light beam and having optical means for guiding the light beam emitted from the light source to an optical recording medium, wherein the light beam is emitted during a tracking operation. An optical element whose transmittance distribution changes according to the polarization direction of the optical pickup moves together with the objective lens. 2. The optical pickup device according to claim 1, wherein the optical element whose transmittance distribution changes according to the polarization direction of the light beam is an optical element whose effective numerical aperture changes according to the polarization direction of the light beam. An optical pickup device, characterized in that: 3. The optical pickup device according to claim 1, wherein the optical element whose transmittance distribution changes according to the polarization direction of the light beam is a polarization filter. 4. The optical pickup device according to claim 1, wherein the optical element whose transmittance distribution changes according to the polarization direction of the light beam is fixed to an edge of the objective lens. An optical pickup device characterized by the following. 5. The optical pickup device according to claim 1, wherein the optical element whose transmittance distribution changes according to the polarization direction of the light beam is fixed to an objective lens holder. Optical pickup device. 6. The optical pickup device according to claim 1, wherein the optical pickup device has a plurality of light sources.