JP3470781B2 - Optical pickup - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup for recording / reproducing information on / from a recording medium such as an optical disk by using a laser beam or the like.

[0002]

2. Description of the Related Art FIG. 11 is a diagram showing an optical system configuration of an optical pickup used for an optical disk capable of recording and reproducing utilizing a magneto-optical effect which has been conventionally used.

In FIG. 11, a semiconductor laser 1 for emitting laser light, a collimating lens 2 for converting parallel light, a first
The beam splitter 4 and the 45 ° mirror 5 are arranged in a straight line. An objective lens 6 is arranged in a 90 ° reflection direction with respect to the 45 ° mirror 5, and an optical disk 7 is arranged at a position of a focused spot by the objective lens 6. Further, the direction of the second beam splitter 9, the condenser lens 10, and the generatrix of the second beam splitter 9 and the generatrix is set at 45 ° with respect to the paper surface at a position perpendicular to the incident direction of the reflected light on the first beam splitter 4. Cylindrical lens 1
1 Further, the photodetectors 12 are sequentially arranged in a straight line,
The reflected light from the optical disk 7 enters the photodetector 12 and is received.

Further, the condenser lens 13 is placed at a position perpendicular to the incident direction of the reflected light on the second beam splitter 9.
A half-wave plate 14 and a polarization beam splitter 15 are arranged in a straight line. This polarization beam splitter 1
Photodetectors 16 and 17 are arranged in the transmitted light and reflected light directions separated by 5, respectively, and the transmitted light and the reflected light are incident on and received by the photodetectors 16 and 17, respectively. As described above, a conventional recordable / reproducible optical pickup is configured.

With the above structure, the light emitted from the semiconductor laser 1 is converted into parallel light by the collimator lens 2. The parallel light passes through the first beam splitter 4, is further reflected by the 45 ° mirror 5 so as to change the direction of 90 °, and enters the objective lens 6. The light incident on the objective lens 6 forms a focused spot on the optical disk 7 utilizing the magneto-optical effect by the objective lens 6.

The reflected light from the disk 7 returns to the semiconductor laser 1 through the same optical path, but a part of it is reflected by the first beam splitter 4 with its direction changed by 90 °.
The reflected light is separated into a transmitted light and a reflected light by the second beam splitter 9, and the transmitted light is transmitted through the second beam splitter 9 and then the condenser lens 10 and the direction of the generatrix thereof with respect to the paper surface. After passing through the cylindrical lens 11 set in the 45 ° direction, the light is focused on the photodetector 12. In this photodetector 12, the servo error signal of the focus error signal (FES) and the tracking error signal (TES) is generated by the reflected light from the optical disk 7.

On the other hand, the light reflected by the second beam splitter 9 is condensed by another condenser lens 13,
The polarization direction of the light is 45 ° by the half-wave plate 14.
After being rotated, the polarized beam splitter 15 separates the transmitted light and the reflected light. Furthermore, the separated lights are converted into electric signals by the photodetectors 16 and 17, respectively, and the information recorded using the magneto-optical effect is converted into electric signals by differentially detecting the respective outputs. Will be played.

The light quantity of the above optical pickup will be calculated.

In the ordinary optical pickup as shown in FIG. 11, the following values are often used as the optical characteristics of the beam splitters 4, 9 and 15.

Beam Splitter 4 Transmittance: Reflectance = 0.6 to 0.7: 0.4 to 0.3 (P-polarized) Beam Splitter 9 Transmittance: Reflectance = 0.5: 0.5 (P-polarized) Polarized Beam Splitter 15 Transmittance: Reflectance = 0.5: 0.5 (P + 45 ° polarization) Beam splitter 4, 9, 1 with such optical characteristics
5, the light output of the semiconductor laser 1 considering the coupling efficiency of the lens is i ₁ and the intensity of the reflected light from the optical disk 7 is i ₂ , the light output from the objective lens 6 = semiconductor Laser output × beam splitter 4 transmittance = i ₁ × (0.6 to 0.7) = (0.6 to 0.7) ・ light amount received by the i ₁ photodetector 12 = optical disc reflected light × beam splitter 4 reflectance × beam splitter 9
Transmittance = i ₂ × (0.3 to 0.4) × 0.5 = (0.15 to 0.2) ・ light quantity received by the i ₂ photodetector 16.17 = optical disk reflected light × beam splitter 4 reflectance × beam splitter 9
Reflectivity × Bemsplitter 15 reflection, transmittance = i ₂ × (0.3 to 0.4) × 0.5 × 0.5 = (0.075 to 0.1) · i ₂ .

On the other hand, in order to increase the recording density of the optical disc, it is necessary to reduce the diameter of the focused spot formed on the optical disc in order to read information by the optical pickup.

It is generally known that the focused spot diameter is proportional to the wavelength of the laser light source and inversely proportional to the numerical aperture (NA) of the objective lens. However, if the numerical aperture is increased, the tilt due to the warp of the optical disk is caused. The permissible value of is small, and it is difficult to make it larger than the present value.

On the other hand, various studies have been made on a semiconductor laser used as a light source of an optical pickup, but there is a limit to the wavelength range currently put into practical use.

Therefore, various studies on optical super-resolution have been conducted in order to realize a minute focused spot exceeding the diffraction limit of the optical system.

For example, in JP-A-63-247920 and JP-A-2-12624, the central portion of the light beam entering the objective lens is completely shielded from light (transmittance T = 0). Then, a method is disclosed in which only a portion of light passing through an annular (doughnut-shaped) region off the optical axis is condensed by an objective lens and a minute condensed spot below a diffraction limit is obtained by optical super-resolution. ing.

By using this optical super-resolution, the intensity of the light at the central portion of the light beam is attenuated between the objective lens and the semiconductor laser, which is the light source of the optical pickup, without increasing the numerical aperture of the objective lens. Only by arranging a means (hereinafter referred to as a light shield) for making it possible, it is possible to obtain a minute focused spot below the diffraction limit.

Further, as a light shield for generating optical super-resolution, Japanese Patent Laid-Open No. 3-254433 discloses a method of using a diffraction grating to attenuate the 0th-order diffracted light (transmitted light) by generating ± 1st-order diffracted light. However, the characteristics of this diffraction grating are described in JP-A-63-247920 and JP-A-2-
The light intensity on the optical axis is set to be 0 (0th-order diffracted light intensity = 0) similarly to the characteristics of the light shield disclosed in Japanese Patent No. 12624, and the optical characteristics are not optimized at all. Not not.

Further, the ± 1st-order diffracted light generated by this diffraction grating is also used only for attenuating the 0th-order diffracted light (transmitted light), and the ± 1st-order diffracted light is effectively used. It was something I couldn't say.

[0019]

By using the optical super-resolution in the above-mentioned conventional optical pickup, it is possible to focus the focused spot on the optical disc to be smaller than the diffraction limit and to increase the density of the optical disc. Can be planned.

However, a disadvantage of this optical super-resolution is that the side-lobe intensity becomes larger than the main-lobe center intensity (focus spot central intensity) by performing the optical super-resolution. To be

This relationship is shown in FIG. 12, in which the focused spot diameter on the optical disk by the optical pickup shown by the dotted line without using the optical super-resolution (the strength of the main rope 20 'is 1 / the central strength). e ² diameter D ₁ and the side lobe 21 ′ intensity S ₁ for the focused spot by the optical pickup using the optical super-resolution shown by the solid line, the spot diameter (the intensity of the main lobe 20). Is the central strength 1
It can be seen that the diameter D ₂ that becomes / e ² ) becomes smaller and, conversely, the intensity S ₂ of the side lobe 21 becomes larger.

Incidentally, in FIG. 12, the main ropes 20, 2 are
The shape of the focused spot is shown with the central intensity of 0 ′ being constant. In this case, the shape of the light shield is a band, the transmittance T = 0, and the width of the light shield is the effective diameter of the objective lens × 0.5.
It shows the shape of the focused spot calculated by.

As described above, in reproducing information from an optical disk, there is a problem that crosstalk occurs between reproduced signals due to the increase in the strength of the side lobe, and the reproduced signals are deteriorated. To solve this problem, Japanese Patent Laid-Open No. 12624/1990 discloses that a slit is provided in front of a detector for receiving the reflected light from the optical disk so that only the reflected light of the main lobe component is received and the side lobe component Regarding the reflected light, a configuration is disclosed in which the slit does not receive the light.

As described above, in the conventionally proposed optical pickup using the light shield which completely shields light with the transmittance of 0, the focused spot on the disk is reduced by the optical super-resolution. Although it is possible, the side lobe intensity increases significantly and the reproduction signal deteriorates.To prevent this, it is necessary to provide a slit in front of the photodetector, etc., and adjust the position of this slit and the light beam. Therefore, it becomes difficult to prepare the optical pickup, which further increases the cost, and also poses a problem with respect to the reliability after adjustment.

The present invention solves the above-mentioned conventional problems. By optimizing the optical conditions of the light shield, the diameter of the focused spot can be reduced, and the side lobe intensity can be increased. An object of the present invention is to provide an optical pickup capable of suppressing the above.

Further, in the optical pickup using the optical super-resolution, in addition to the problem that the intensity of the side lobe becomes large, the intensity of the light incident on the objective lens is attenuated by the light shield, so that the output of the semiconductor laser is reduced. It has been considered that it is difficult to obtain the light intensity of the focused spot required for recording because the output power of the objective lens becomes smaller and the coupling efficiency deteriorates.

Therefore, many optical pickups using optical super-resolution have been proposed so far as read-only types for reproducing information on an optical disk, and the optical pickups capable of recording and reproducing information have been proposed. The use of resolution has not been proposed.

In the present invention, the attenuating means is composed of a diffraction grating, the optical characteristics of this diffraction grating are optimized, and the disc reflected light is re-incident on the diffraction grating and is generated 1.
Provided is an optical pickup using an optical super-resolution corresponding to an optical disk capable of recording and reproducing by detecting a servo signal from the second-order diffracted light.

[0029]

The optical pickup of the present invention does not make the light intensity of a light-shielded region zero by an attenuating means such as a light-shielding body for generating an optical super-resolution.
The attenuator attenuates the light near the optical axis and allows a part of the light to pass.

The optical characteristic of the attenuating means is that the area of the light whose light intensity is attenuated by the attenuating means has a diameter of A when it has a circular shape, or has a band shape which crosses the light beam. Let the width of the band be a,
Letting T be the transmittance of light that has passed through the light attenuation region of the attenuator, if the shape of the light attenuation region by the attenuator is circular, then (objective lens effective diameter) × 0.5 <A <( Objective lens effective diameter) × 0.75 T> 0.27 Or, when the shape of the light attenuation region by the attenuator is a band, (objective lens effective diameter) × 0.25 <a <(objective lens effective diameter) ) × 0.5 T> 0.54, which achieves the above object.

This optical characteristic may be formed by using a diffraction grating shown below, or a reflecting film satisfying the above conditions may be provided by a dielectric or the like on a part of a parallel plate that transmits light. It may be configured by the above, or may be configured by providing an absorption layer. Further, in another configuration according to the present invention, preferably, in addition to the above configuration, a light beam from a light source is condensed on a recording medium via an objective lens, and a condensing spot obtained on the recording medium forms a recording medium. In an optical pickup utilizing the optical super-resolution effect for recording / reproducing information to / from the optical system, an attenuating means for attenuating the light intensity of the central portion (near the optical axis) in the cross section of the light beam incident on the objective lens is provided. It is formed by a diffraction grating, and the reflected light from the optical disc is configured to generate a servo signal for causing the focused spot to follow the information on the optical disc based on the first-order diffracted light generated when the reflected light is incident on the diffraction grating again. Has been done. In this case, preferably, the amount of light incident on the objective lens becomes approximately 0.8 by the attenuator, and the 0th-order diffraction efficiency of the diffraction grating formed in the attenuator is 0.5 to 0.6.

The operation of the above structure will be described below.

In the optical pickup using the effect of the optical super-resolution according to the present invention, by using the attenuating means having the above-mentioned optical characteristics, the reduction rate of the focused spot diameter obtained by the optical super-resolution is the same. In this case, the transmittance T = 0, which is the optical characteristic of the conventionally proposed attenuator for super-resolution, is used.
The generated side lobe intensity is about 2 /
It becomes possible to suppress to about 3.

By suppressing the side lobe intensity, the side lobe intensity with respect to the main lobe center intensity can be made sufficiently small. Therefore, even if the main rope component and the side lobe component of the optical disk reflected light are simultaneously detected by the photodetector, the side lobe intensity is reduced. The deterioration of the reproduced signal can be ignored.

From this, it is not necessary to provide a slit in front of the photodetector to remove the influence of the side lobe, and the adjustment of the optical pickup becomes easy. Furthermore, in addition to this, the reliability after adjustment is also improved.

Further, the attenuating means is constituted by a diffraction grating, and the reflected light from the optical disk is based on the first-order diffracted light generated by re-incident on the diffraction grating, and the focused spot follows the servo signal for following the information on the optical disk. Is used, the focused spot can be reduced by the optical super-resolution effect, and the coupling efficiency, which is the output power intensity of the objective lens with respect to the output of the semiconductor laser, is It is possible to make it almost the same as an optical pickup that does not.

With this structure, optical super-resolution can be adopted even in an optical pickup capable of recording and reproducing information. As described above, in the optical pickup according to the present invention, the increase in sidelobe intensity is suppressed , and
Since the focused spot below the diffraction limit can be obtained by the optical super-resolution, the density of the optical disc can be increased.

[0038]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below.

(First Embodiment) FIG. 1 is a block diagram of an optical system of an optical pickup according to a first embodiment of the present invention.

In FIG. 1, a semiconductor laser 1, a collimating lens 2 for converting parallel light, an attenuator 3 for super-resolution, a beam splitter 4 and a 45 ° mirror 5 are arranged in a straight line. 90 for this 45 ° mirror 5
The objective lens 6 is arranged in the reflection direction, and the optical disk 7 is arranged at the position of the focused spot by the objective lens 6. Further, the photodetector 8 is arranged at a position perpendicular to the incident direction of the reflected light on the beam splitter 4, and the reflected light from the disk 7 is incident on and received by the photodetector 8.

With the above structure, the light emitted from the semiconductor laser 1 is converted into parallel light by the collimator lens 2, transmitted through the attenuator 3 for super-resolution and the beam splitter 4, and then reflected by the 45 ° mirror 5. The objective lens 6
As a result, the light is focused on the optical disk 7 as a minute spot.

The reflected light from the optical disk 7 is
Although it returns to the semiconductor laser 1 through the same optical path, a part of the reflected light is reflected by the beam splitter 4. In this way, the light reflected by the beam splitter 4 is detected by the photodetector 8
The information received by and recorded on the disk 7 is converted into an electric signal.

In this optical system, although the detection system of the servo signal is not shown, it goes without saying that these detection systems are provided.

FIG. 2 is an enlarged view showing an example of the attenuator 3 of FIG. 1, where a is a front view thereof and b is a side view thereof.
The attenuator in the case where the shape of the light attenuation region is circular is shown.

In FIG. 2, the region 33 of the attenuator 3'is
The parallel luminous flux diameter (diameter C) of the laser light incident on the objective lens 6 is shown, and the area 31 shows the light attenuation area (diameter A) in which the transmitted light quantity is attenuated by the attenuator 3 ′. Its transmittance is set to T.

The attenuation of light by the attenuator 3'is such that when the intensity of light incident on the attenuator 3'is I, the attenuation region 31
The intensity of the light that has entered T.sub.1 is T.I, and the intensity of the light that has entered the donut-shaped region other than the attenuation region 31 in the region 33 is transmitted with the intensity I.

FIG. 3 is an enlarged view showing another example of the attenuator 3 of FIG. 1, where a is a front view thereof and b is a side view thereof, in the case where the shape of the light attenuation region is a band. Attenuator is shown. In FIG. 3, a region 33 of the attenuator 3 ″ indicates a parallel light beam diameter (diameter C) that is incident on the objective lens 6,
Further, the attenuation area 32 indicates an attenuation area (band width a) of the light attenuated by the attenuator 3 ″, and the transmittance thereof is set to T.

With respect to the attenuation of light by the attenuator 3 "as well, when the intensity of the light incident on the attenuator 3" is I, the laser light incident on the band-shaped attenuation region 32 is Its intensity becomes T · I, and this band-shaped attenuation region 32
The laser light incident on the area 33 other than the above area is transmitted with the intensity thereof being I.

First, the diameter A of the circular damping region 31 of FIG.
Also, the optimum value of the width a of the band-shaped attenuation region 32 of FIG. 3 will be considered.

FIGS. 4 and 5 are diagrams showing the super-resolution effect obtained by the optical system of the optical pickup shown in FIG.
Shows the optical characteristics when the shape of the attenuation region is circular (attenuator 3 ′ in FIG. 2), and FIG. 5 shows the optical characteristics when the shape of the attenuation region is band-like (attenuator 3 ″ in FIG. 3). Shows.

FIGS. 4 and 5 show the focused spot diameter when the diameter A and the width a are changed with respect to the parallel light flux diameter (diameter C) of the laser light incident on the objective lens 6. At this time, as the transmittance of the attenuators 3 ', 3 ", T = 0.2 to including the conventionally proposed T = 0.
The respective values when 0.7 is shown.

In this case, the horizontal axis represents the objective lens 6 respectively.
2 and the effective diameter (corresponding to the region 33 (diameter C) of FIGS. 2 and 3) and the diameter A of the damping region 31 of FIG. 2 or the width a of the damping region 32 of FIG. The vertical axis represents the ratio (D ₂ / D ₁ ) of the focused spot diameters.

The ratio of the focused spot diameter on the vertical axis (D ₂ /
D ₁ ) is normalized by the spot diameter D ₁ when the attenuator 3 for super-resolution is not set in the optical system shown in FIG. 1, and the attenuator 3 ′ or 3 ″ for super-resolution is When the spot diameter D ₂ is set, the vertical axis thereof corresponds to D ₂ / D ₁ (ratio of spot diameters) of the focused spot shape shown in FIG.

As can be seen from FIGS. 4 and 5, the transmittance of the attenuator is 0 when the size of the attenuating region is changed.
From the above, it can be seen that the reduction ratio (D ₂ / D ₁ ) of the spot diameter becomes the minimum when the attenuation region has a certain size.

As the optimum value of this attenuation region, when the attenuation region shown in FIG. 4 is circular, the attenuation region (diameter A of FIG. 2) with respect to the effective diameter of the objective lens (corresponding to C of FIG. 2). If the ratio (corresponding to C / A) is set to 0.5 to 0.75, the reduction ratio (D ₁ / D ₂ ) of the main lobe becomes the minimum.

On the other hand, when the attenuation region shown in FIG. 3 is a band, the ratio of the attenuation region (width of a in FIG. 3) to the effective diameter of the objective lens (corresponding to C in FIG. 3) (corresponding to a / C). ) 0.2
If set to 5 to 0.5, the reduction rate (D
₂ / D ₁ ) is the minimum.

Next, the relationship between the transmittance T of the attenuation region 31 of FIG. 2 and the attenuation region 32 of FIG. 3, the focused spot diameter, and the side lobe intensity will be considered.

FIGS. 6 and 7 are diagrams showing the optical characteristics of the focused spot on the optical disk obtained by the optical system of the optical pickup of FIG. 1. In FIG. 6, the shape of the attenuation region is circular (see FIG. 2). FIG. 7 shows the optical characteristics in the case of the attenuator 3 ′), and FIG. 7 shows the optical characteristics in the case where the shape of the attenuation region is a band (the attenuator 3 ″ in FIG. 3).

6 and 7, when the transmittance of the attenuator 3'or 3 "is T, the attenuation rate of light passing through the attenuation regions 31 and 32 shown by (1-T) and T are shown. The transmittance A of the light passing through the attenuation regions 31 and 32 is represented by the horizontal axis, and the diameter A or the width a of the attenuation regions 31 and 32 is shown in FIG.
Based on the results shown in FIG. 5 and FIG. 5, the vertical axis shows the values of the focused spot diameter and the side lobe intensity obtained when the spot diameter on the optical disk is set to be the minimum.

The spot diameter and side lobe intensity are normalized by the spot diameter and side lobe intensity when the super-resolution attenuator 3'or 3 "is not set in the optical system shown in FIG. The vertical axis represents D ₂ / D ₁ (ratio of spot diameters) of the focused spot shape shown in FIG.
And S ₂ / S ₁ (ratio of side strength).

For comparison, the dotted line in FIGS. 6 and 7 is the same spot diameter (D ₂ / D ₂₎ as the minimum spot diameter obtained when the attenuator having each transmittance shown in FIG. 3 is used.
The side lobe intensity when D ₁ ) is realized by using an attenuator (light shield) having a characteristic of light transmittance T = 0 that has been conventionally proposed is shown. As described above, in the optical pickup according to the present embodiment, the intensity of light in the shaded region is not set to 0 by the attenuator that generates optical super-resolution, but the light near the optical axis is attenuated by the attenuator. Dampen,
By adopting a configuration in which a part of the light is passed, as shown in FIGS. 6 and 7, the transmittance T =
Compared to the case of using the attenuator of 0, the increase of the side lobe intensity can be suppressed under the condition that the same spot diameter (D ₂ / D ₁ ) is obtained.

Next, the allowable value for increasing the sidelobe intensity will be considered with reference to FIGS. 6 and 7.

Normally, the crosstalk between reproduced signals is -2.
It is said that good signal reproduction is possible if it is 5 dB or less, and if crosstalk occurs between the reproduced signals due to the side lobes, the main lobe center intensity (focusing spot center intensity) is , The side lobe strength of −25 dB becomes the allowable limit.

This limit is indicated by the value of -25 dB shown in FIGS. 6 and 7, and when the shape of the light attenuation region by the attenuator shown in FIG. 6 is circular, the transmission of the attenuator is shown. The factor T is T> 0.27 (Equation 1), and when the shape of the light attenuation region by the attenuator shown in FIG. 7 is a band, the attenuator transmittance T is T> 0.54 (Equation 2) ).

That is, the circular damping region 31 shown in FIG.
In the case of performing super-resolution in (1), an attenuator that satisfies (Equation 1) and in the case of performing super-resolution in the band-shaped attenuation region 32 shown in FIG. By using it, the sidelobe intensity can be suppressed to about 2/3 and the sidelobe intensity can be reduced as compared with the case of using the conventionally proposed attenuator having the transmittance T = 0. Since it can be set to -25 dB or less with respect to the central intensity (focused spot central intensity), crosstalk between reproduced signals can be suppressed without removing the influence of side lobes by using a slit or the like, which is excellent. It is possible to obtain various reproduced signals.

This performance is obtained when the attenuation area is circular.
0.5 for the effective diameter of the objective lens (corresponding to C in FIG. 2)
A damping region of ~ 0.75 (diameter A in FIG. 2), and
In the case where the attenuation area has a band shape, it is realized by setting the attenuation area (width of a in FIG. 3) of 0.25 to 0.5 with respect to the effective diameter of the objective lens (corresponding to C in FIG. 3). be able to.

As a result, it is not necessary to provide a slit in front of the detector, and it becomes possible to construct an optical pickup which can be easily adjusted, and the reliability after adjustment is also improved.
It is needless to say that the optical disc can be highly densified because the focused spot is reduced by the optical super-resolution.

(Second Embodiment) FIG. 8 is a block diagram of an optical system of a pickup in a second embodiment of the present invention.

In FIG. 8, a semiconductor laser 1 for emitting a laser beam, an attenuator 23 composed of a diffraction grating, a collimating lens 2 for parallel light conversion, a first beam splitter 4 and a 45 ° mirror 5 are aligned in a straight line. It is arranged. An objective lens 6 is arranged in a 90 ° reflection direction with respect to the 45 ° mirror 5, and an optical disk 7 capable of recording / reproducing information using the magneto-optical effect is arranged at a focused spot position by the objective lens 6. ing. A condenser lens 13, a half-wave plate 14 and a polarization beam splitter 15 are arranged in a straight line at a position perpendicular to the incident direction of reflected light on the first beam splitter 4. Photodetectors 16 and 17 are respectively arranged in the transmitted light and reflected light directions separated by the polarization beam splitter 15, and the transmitted light and the reflected light are incident on and received by the photodetectors 16 and 17, respectively. .

A diffraction grating is formed in a part of the attenuator 23, and optical super-resolution is performed based on the attenuation of the 0th-order diffracted light due to the generation of ± 1st-order diffracted light. A photodetector 22 for servo signal detection is arranged at a position where the light diffracted by the attenuator 23 among the reflected light from the optical disc 7 is condensed.

The region of this diffraction grating has the same shape as the attenuation region 31 or 32 shown in FIGS.

With the above structure, the light emitted from the semiconductor laser 1 is converted into parallel light by the collimator lens 2 after passing through the attenuator 23. The parallel light converted by the collimator lens is transmitted through the beam splitter 4 and then reflected by the 45 ° mirror 5 while changing its direction by 90 °, and the reflected light is reflected by the objective lens 6 on the optical disk 7 utilizing the magneto-optical effect. A focused spot is formed on.

The reflected light from the optical disk 7 returns to the semiconductor laser 1 through the same optical path, but a part of the reflected light is reflected by the beam splitter 4. The reflected light from the beam splitter 4 is condensed by the condenser lens 13. The light is rotated by the ½ wavelength plate 14 in the polarization direction of the light by 45 °, and then is converted into the transmitted light by the polarizing beam splitter 15. It is separated into reflected light.

The light separated into the transmitted light and the reflected light is converted into electric signals by the photodetectors 16 and 17, and the respective outputs are differentially detected to be recorded by utilizing the magneto-optical effect. Information is converted into an electric signal and reproduced.

On the other hand, the disc reflected light transmitted through the beam splitter 4 is condensed by the collimator lens 2, but a part of it is diffracted by the attenuator 23 formed of a diffraction grating to detect the servo signal. It is focused on the photodetector 22.

For the detection of the servo signal by the attenuator 23 and the photodetector 22, the attenuator (diffraction grating) 23 is provided with a function of generating astigmatism, and the astigmatism method is used to obtain the focus error signal. Alternatively, the attenuator (diffraction grating) 23 may be provided with a knife edge function to detect the focus error signal by the knife edge method.

Further, regarding the detection of the tracking error signal, by dividing the light receiving surface of the photodetector 22 for detecting the servo signal in the direction parallel to the track direction of the optical disk 7 and performing differential detection, the tracking error by the push-pull method is detected. Signal detection may be performed, or by adding another diffraction grating, three spots may be formed on the optical disc to perform tracking error signal detection by the three-beam method. This three-beam diffraction grating is used as an attenuator 23.
There is no problem even if it is configured on the other side of.

Next, the light quantity of the optical pickup according to the present embodiment will be calculated.

The characteristics of the diffraction grating formed on the attenuator 23 have a 0th-order diffraction index of 0.5 to 0.5 for the reason described later.
The quantity of light incident on the objective lens 6 is set to 0.6 due to the effect of the ± first-order diffracted light generated by the attenuator 23, compared with the case where the attenuator 23 is not set.
It is set to be 8.

The beam splitter 4 shown in FIG.
The optical characteristics of 15 are set as follows.

Beam Splitter 4 Transmittance: Reflectivity = 0.75 to 0.85: 0.25 to 0.15 (P polarized light) Beam Splitter 15 Transmittance: Reflectivity = 0.5: 0.5 (P + 45 ° polarized light) Beam splitter having this optical characteristic When the optical output of the semiconductor laser 1 in consideration of the coupling efficiency of the lenses is i ₁ and the intensity of the reflected light from the optical disk 7 is i ₂ , the light emitted from the objective lens 6 is used. Optical output = semiconductor laser output × light amount loss due to attenuator 21 × beam splitter 4 transmittance = i ₁ × 0.8 × (0.75 to 0.85) = (0.6 to 0.68) ・ i ₁ amount of light received by photodetector 22 = Optical disk reflected light x beam splitter 4 transmittance x amount of light diffracted by attenuator 3'x 1st order light only = i ₂ x (0.75 to 0.85) x 0.2 x 0.5 = (0.075 to 0.085) / i ₂ photo detector 16. Amount of light received by 17 = reflected light of optical disc x beer Staple ° liter 4 reflectance × arsenide Bu Musufu ° splitter 15
Transmission / reflectance = i ₂ × (0.15 to 0.25) × 0.5 = (0.075 to 0.13) · i ₂ .

As described above, in the optical system of FIG. 11 of the conventional example and the optical system of FIG. 8 of the present embodiment, the laser light output emitted from the objective lens 6 and the photodetector 16 can be seen from the above calculation. , 17, the amount of light received is almost equal.

From the above, by adopting the configuration of the optical system shown in FIG. 8, it is possible to obtain almost the same output power of the objective lens as that of the conventional optical system and the light amount for reproducing the information recorded on the optical disc. The same recording power and reproduction signal light amount as in the conventional case can be obtained.

Further, in the optical system according to the present embodiment shown in FIG. 8, the amount of light for detecting the servo signal detected by the photodetector 22 is about half that of the conventional optical system. However, since the servo signal has a sufficient signal quality as compared with the reproduced signal, the reduction in the servo signal light amount does not affect the performance of the optical pickup.

Next, the optimum values of the optical characteristics of the attenuator 23 will be described with reference to FIGS. 9 and 10.

9 and 10 show the loss of light quantity of the semiconductor laser 1 by the attenuator 23 when the diameter A or width a of the attenuating regions 31, 32 shown in FIG. 1 is changed (when the LD power is constant). Of the focused spot intensity on the disk
Is the horizontal axis, and the diameter of the focused spot on the disk is shown by the solid line and the side lobe intensity is shown by the dotted line.

Further, the respective values are shown when the transmittance of the attenuator is set to 0.1 to 0.7.

The focused spot diameter and side lobe intensity are normalized by the spot diameter and side lobe intensity when the super-resolution attenuator 23 is not set in the optical system shown in FIG. The vertical axis represents D ₂ / D ₁ (ratio of spot diameters) of the focused spot shape shown in FIG. 12, and S
Equivalent to ₂ / S ₁ (ratio of sidelobe intensity).

FIG. 9 shows the characteristics when the shape of the attenuation area is circular (the attenuation body of FIG. 2), and FIG. 10 is the characteristics when the shape of the attenuation area is the band (attenuation body of FIG. 3). Is shown. In the above light amount calculation, by setting the total transmitted light amount of the attenuator 23 to 0.8, the same light amount as that of the conventional optical system can be obtained. If the reduction of the center strength of the spot (main strength of the main rope) is limited to 0.8, the following (1) and (2) are obtained.

(1) If the light transmittance T in the attenuation region of the attenuator is 0.5 to 0.6, the reduction of the focused spot diameter is almost the same as the conventionally proposed transmittance T = 0. Is possible.

(2) By setting the transmittance of the attenuator to 0.5 to 0.6, the side rope strength is about 2/3 (attenuation region) as compared with the conventionally proposed case of the transmittance T = 0. Is circular), or about 3/4 of the attenuation region is strip-shaped).

In this way, the attenuator is composed of a diffraction grating,
The diffraction grating has a 0th-order diffraction efficiency of 0.5 to 0.6, and
With the configuration in which the reflected light of the optical disc generates a servo signal based on the first-order diffracted light generated upon incidence on this diffraction grating, it is possible to reduce the focused spot by optical super-resolution, and Optical coupling efficiency similar to that of an optical pickup that does not use super-resolution can be obtained. Therefore, optical super-resolution can be adopted even in an optical pickup capable of recording / reproducing information, and the optical disc can be made higher in density in response to the reduction of the focused spot.

Therefore, in the conventional optical pickup using the optical super-resolution, in addition to the problem that the intensity of the side lobe increases, the light incident on the objective lens is attenuated by the attenuator, so that the output of the semiconductor laser is increased. It has been considered that it is difficult to obtain the light intensity of the focused spot required for recording because the output power of the objective lens becomes smaller and the coupling efficiency deteriorates. Therefore, many optical pickups that use optical super-resolution have been proposed so far as read-only types that reproduce information from optical discs, and this optical super-resolution can be applied to optical pickups that can record and reproduce information. Its use was not suggested. On the contrary,
In the optical pickup of the present embodiment, the attenuator is composed of a diffraction grating, the optical characteristics of the diffraction grating are optimized, and the servo signal is detected from the first-order diffracted light generated when the disc reflected light is incident on the diffraction grating again. By doing so, an optical pickup using optical super-resolution corresponding to an optical disk capable of recording and reproducing is provided.

In each of the above embodiments, in the first embodiment shown in FIG. 1, the attenuator 3 is replaced by the collimator lens 2.
Although an example of arrangement between the beam splitter 4 and the beam splitter 4 is shown,
The attenuator 3 may be arranged at any position between the semiconductor laser 1 and the objective lens 6, and an attenuator satisfying the conditions of the present invention is formed on the surface of another optical component shown in FIG. It can also be realized by doing. Further, in the second embodiment shown in FIG. 8, the semiconductor laser 1 and the photodetector 22 may be housed in the same package, and the attenuator 23 is fixed and integrated on this package. It is also possible.

In the first embodiment, when the attenuation region of the attenuator shown in FIG. 3 has a strip shape, the direction of the strip may be set to either direction. It is better to set the direction orthogonal to the track.
Since the focused spot is reduced in the track direction of the optical disc due to the effect of super-resolution, it is possible to obtain a greater effect in increasing the density of the disc.

Furthermore, FIGS. 4, 5, 6, 7, 9 and 10 are performed under the following conditions.

Wavelength 690 nm Numerical aperture (NA) of objective lens 0.55 (The intensity distribution of light incident on the objective lens is uniform.) Main lobe diameter when no attenuator (light shield) is set
1.04 μmφ (1 / e ² ) Side lobe intensity when no attenuator (light shield) is set 1.86% (when main lobe intensity is 100%) Attenuation (light shield) area of attenuator (light shield) Non-attenuation (shading)
Phase difference of light with area T = 0 When 0 T 0 When T = 0.1 λ / 5.1 When T = 0.2 λ / 5.7 T = 0.3 When λ / 6.3 transmittance T = 0.4 λ / 7.1 transmittance T = 0.5 λ / 8.0 transmittance T = 0.6 λ / 9.2 transmission When the rate T = 0.7, λ / 11.0

[0098]

As described above, according to the present invention, the reduction of the focused spot diameter obtained by the optical super-resolution is the same when the attenuation means having the predetermined optimum optical characteristics is used. Also, compared with the case where the transmittance T = 0, which is the optical characteristic of the attenuation means for super-resolution, which has been conventionally proposed,
The generated side lobe intensity can be suppressed to about 2/3. Therefore, even if the photodetector simultaneously detects the main lobe component and the side lobe component of the optical disc reflected light, the deterioration of the reproduction signal due to the side lobe can be ignored. As a result, it is not necessary to provide a slit in front of the photodetector to eliminate the influence of side lobes, and it is possible to easily adjust the optical pickup and reduce the cost, as well as the reliability after adjustment. As for the above, the performance can be improved.

Further, the attenuating means is composed of a diffraction grating, and the reflected light of the optical disk is based on the first-order diffracted light generated upon incidence on the diffraction grating, and the focused spot follows the servo signal for tracking the information of the optical disk. With the configuration that generates the signal, it becomes possible to generate a signal with the attenuation amount by the attenuating means, which has been a loss when using the optical super-resolution effect for the optical pickup, and to improve the light utilization efficiency. Can be improved.

As described above, by forming the attenuating means by the diffraction grating, the focused spot can be reduced by the effect of optical super-resolution, and the output power intensity of the objective lens with respect to the output of the semiconductor laser can be reduced. Since a certain coupling efficiency can be made almost equal to that of the optical pickup that does not use the super-resolution as shown in the above-mentioned embodiment, the optical pickup which has hitherto been targeted at the reproduction-only pickup for reproducing the information of the optical disc. Super-resolution can be applied to an optical pickup capable of recording and reproducing information.

As described above, in the optical pickup according to the present invention, it is possible to suppress the increase of the side lobe intensity, and it is possible to obtain a minute focused spot below the diffraction limit by the optical super-resolution. It is possible to further increase the density.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an optical system of an optical pickup according to a first embodiment of the present invention.

FIG. 2 is an enlarged view showing an example of the attenuator 3 of FIG.
a is a front view thereof, and b is a side view thereof.

3 is an enlarged view showing another example of the damping body 3 of FIG. 1, in which a is a front view thereof and b is a side view thereof.

4 is a diagram showing a super-resolution effect obtained in an example of the optical system of the optical pickup of FIG. 1, showing optical characteristics when the shape of an attenuation region is circular (attenuator 3 ′ of FIG. 2). It is a figure.

5 is a diagram showing a super-resolution effect obtained in another example of the optical system of the optical pickup of FIG. 1, showing optical characteristics when the shape of the attenuation region is a band (attenuator 3 ″ of FIG. 3). FIG.

6 is a diagram showing the relationship between spot diameter and side lobe intensity with respect to transmittance, which is an optical characteristic of a focused spot on an optical disc obtained by the optical pickup of FIG. It is a figure which shows the case where a shape is circular.

7 is a diagram showing the relationship between the spot diameter and the side lobe intensity with respect to the transmittance, which is the optical characteristic of the focused spot on the optical disc obtained by the optical pickup of FIG. It is a figure which shows the case where a shape is a strip shape.

FIG. 8 is an optical system configuration diagram of an optical pickup according to a second embodiment of the present invention.

9 is a diagram showing the relationship between the spot diameter and the side lobe intensity with respect to the central intensity of the focused spot, which is the optical characteristic of the focused spot on the optical disc obtained by the optical pickup of FIG. It is a figure which shows the case where the shape of the attenuation area is circular.

10 is a diagram showing the relationship between the spot diameter and the side lobe intensity with respect to the central intensity of the focused spot, which is the optical characteristic of the focused spot on the optical disc obtained by the optical pickup of FIG. It is a figure which shows the case where the shape of the attenuation region of FIG.

FIG. 11 is an optical system configuration diagram of a conventional optical pickup used for a recordable / reproducible optical disk utilizing a magneto-optical effect.

FIG. 12 is a diagram showing the shapes of focused spots on an optical disc with and without optical super-resolution.

[Explanation of symbols]

1 Semiconductor laser 3,3 ', 3 ", 23 attenuator 6 Objective lens 7 Optical disc 8,22 photo detector 20, 20 'main rope 21,21 'side lobes 31, 32 Light attenuation area 33 Parallel light flux area incident on the objective lens

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hideo Sato, 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Prefecture Sharp Corporation (56) References JP-A-6-52573 (JP, A) JP-A-7- 98431 (JP, A) JP-A-9-50647 (JP, A) JP-A-7-192302 (JP, A) JP-A-2-294948 (JP, A) JP-A-8-147749 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) G11B ^7/ 12-7/22 G02B 27/42

Claims (3)

(57) [Claims] 1. An optical super-resolution effect is utilized by arranging an attenuating means for attenuating the light intensity of a central portion of a light beam incident on an objective lens between the light source and the objective lens, and the light from the light source is utilized. In an optical pickup that condenses a beam on a recording medium via the objective lens and performs at least reproduction on the recording medium by a converging spot obtained on the recording medium, the attenuating means has a diffraction grating. And the recording medium
So that the reflected light from them enters the diffraction grating of the attenuation means.
Based on the first-order diffracted light of the light incident on the diffraction grating.
Then, the focused spot obtained on the recording medium is
Configuration for generating servo signal for tracking information on medium
An optical pickup, characterized in that the the. 2. The transmittance T of the light passing through the light attenuation region of the attenuator is T ≠ 0, and the light region in which the light intensity is attenuated by the attenuator has a circular shape. If the diameter is A, then (objective lens effective diameter) × 0.5 <A <(objective lens effective diameter) × 0.75, or the region of light in which the light intensity is attenuated by the attenuator is It has a band-like shape and has an optical characteristic given by (objective lens effective diameter) × 0.25 <a <(objective lens effective diameter) × 0.5, where a is the width of the band. The optical pickup according to claim 1, wherein: 3. When the shape of the light attenuation area by the attenuating means is circular, T> 0.27, or when the shape of the light attenuation area by the attenuating means is band-like, T> 0.54. The optical pickup according to claim 2, wherein