JP2000090468A - Optical head device - 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 head device used for an optical disk device and the like, and more particularly to recording and reproduction using a super-resolution technique for generating a small light beam spot with a large depth of focus. The present invention relates to an optical head device.

[0002]

2. Description of the Related Art Methods conventionally known as examples of optical head devices utilizing super-resolution technology according to the prior art include, for example, JP-A-1-315040, "Optical head device and information recording / reproducing method". Japanese Unexamined Patent Application Publication No. Hei 222-2918, "Beam shape conversion device", and Japanese Patent Application Laid-Open No. H5-196896, "Condensing device" are disclosed. FIGS. 11, 12, and 13 show the prior art disclosed above.
This is an example of an optical head device in which portions related to the present invention have been extracted and arranged, and the gist thereof will be described below.

In FIG. 11, a laser beam generated from a semiconductor laser (laser light source) 1 is converted into a parallel beam by a collimating lens 2, passes through a beam splitter 3, and is formed as a beam spot on an optical recording medium 6 by an objective lens 5. It is collected. The light beam condensed on the optical recording medium 6 is a light beam having a small focal depth by shielding the central portion 4d of the light beam by a light shielding plate 47 disposed on the optical axis of the light beam of the laser beam. Spots can be generated. When the optical recording medium 6 is of a rewritable type using a beam spot, information can be written on the optical recording medium 6 by modulating this light beam with a write information signal (not shown).

When reading information written on the optical recording medium 6, the condensed light beam spot having a predetermined constant power is modulated by the optical recording medium 6, and is read from the surface of the optical recording medium 6. The reflected optical signal is again transmitted to the objective lens 5.
To be converted into a light beam, reflected by the beam splitter 3,
Light is condensed on a photodetector 8 via a condensing lens 7. As a result, information written on the optical recording medium 6 can be read.

[0005] The method using the light shielding plate 47 disclosed in Japanese Patent Application Laid-Open No. 1-315040 "Optical head device and information recording / reproducing method" has a problem of light quantity loss accompanying the use of the light shielding plate 47.
As a method for solving this problem, there is a method disclosed in Japanese Patent Application Laid-Open No. 2222918/1990 “beam shape conversion device”. In FIG. 12, the difference from FIG. 11 is that the light shielding plate 47 in FIG. 11 is replaced. In FIG. 12, a conical lens (prism) 48 is arranged on the optical axis of the light beam so as to coincide with the central axis of the conical lens 48. I have.

With this configuration, the light beam transmitted through the collimating lens 2 and the beam splitter 3 is changed in optical path by the conical lens 48, and is converted into a ring-shaped collimated light beam, and the ring-shaped light beam 1a is converted. Super-resolution can be generated by condensing with the objective lens 5. That is, similar to the method using the light shielding plate 47 in FIG. 11,
A region 4d in which no laser light exists can be created in a certain space on the optical axis of the light beam. As a result, as shown in FIG.
Similar to the method using 47, a light beam spot with a minute depth of focus and a large depth of focus is generated, and a strong light beam spot is generated on the optical recording medium 6 as much as the light amount loss of the light beam by the light shielding plate 47 is not caused. be able to.

The optical head device of Japanese Patent Application Laid-Open No. HEI 5-96896 "Condenser" shown in FIG. 13 has a semiconductor laser light source 1 having an elliptical cross-sectional beam shape and generating divergent light, ie, laser light. The laser light from the laser light source 1 is corrected to a substantially circular light beam and an optical recording medium (optical disk)
6, a polarization beam splitter 3A including a beam splitting surface 3a for separating the light beam reflected from the optical disk 6 and the light beam traveling toward the optical disk 6, and the phase of the traveling wavefront of the laser light in an annular shape. And a photodetector 8 that detects the light beam split via the polarizing beam splitter 3A and reproduces information on the optical disk 6.

Further, a collimating lens 2 for collimating divergent laser light between the laser light source 1 and the polarizing beam splitter 3A, and a light transmitting system and a detecting system between the beam splitter 3A and the optical disk 6 A λ / 4 plate 4B for matching the isolation and an objective lens 5 for condensing the light beam on the recording surface of the optical disk 6 are provided. A condensing lens 7A for condensing the light beam separated via the polarizing beam splitter 3A between the polarizing beam splitter 3A and the photodetector 8 on the detection surface of the photodetector 8, On the other hand, a cylindrical lens that generates a control laser beam that enables beam spot control called focusing and tracking so that the light beam that has passed through the objective lens 5 is focused at a desired position on the optical disk 6 with a desired beam spot. 7B.

In such a configuration, the laser light generated from the laser light source 1 is converted into a parallel light beam via the collimating lens 2, and the beam spot is corrected to a substantially circular shape via the polarizing beam splitter 3A. The light enters the light-collecting element 4A. The light beam incident on the light-collecting element 4A is divided into laser light in the central part and the peripheral part of the light beam, and laser light composed of a ring-shaped part in which the phase of the traveling wave front differs from these laser lights by 180 °. The wavefront is split.

[0010] The light beam split into wavefronts is a λ / 4 plate 4B.
The light is converted into circularly polarized light via the objective lens 5 and focused by the objective lens 5 to irradiate the recording surface of the optical disk 6. The light beam irradiating the recording surface is reflected on the recording surface of the optical disk 6, and the reflectance changes locally according to the information recorded on the optical disk 6. The reflected laser beam sequentially passes again through the objective lens 5, the λ / 4 plate 4B, and the condensing element 4A, and is returned to the polarizing beam splitter 3A.
Reflected by the beam splitting surface 3a and condensing lens 7A
The light is condensed on the detection surface of the photodetector 8 via the cylindrical lens 7B, is converted into an electric signal, and is processed by the signal processing circuit 8A.
Thus, information on the recording surface of the optical disk 6 is reproduced. In addition,
In the signal processing circuit 8A, the control signal of the objective lens 5 is simultaneously detected, and the lens coil 5a of the objective lens 5 is energized in accordance with the control signal, and the light beam is emitted to a desired position on the optical disk 6 with a desired beam spot. Can be controlled by a beam spot control called tracking.

[0011]

SUMMARY OF THE INVENTION Prior Art
5040 "Optical head device and information recording / reproducing method", or an optical head device using super-resolution technology disclosed in Japanese Patent Application Laid-Open No. 2222918/1990 However, there is a problem that the entire device becomes large due to the loss of light amount accompanying the use of the lens and the use of a conical lens (prism). Also, with such a super-resolution technique, the diameter of the light beam spot can be made smaller than when the super-resolution effect is not used, but the intensity distribution of the light beam spot is close to the main lobe (main maximum). There is a problem that the intensity of the generated side lobe (sub local maximum) is increased.

Such an increase in the intensity of the side lobe is caused by
When a signal recorded on an optical recording medium is reproduced, the signal becomes a noise component, and the signal-to-noise ratio (S / N ratio) is reduced. Further, the condensing device disclosed in Japanese Patent Application Laid-Open No. H5-196896 uses a super-resolution technique to change the phase of the traveling wavefront of the laser light by 180 ° in an annular region centered on the optical axis of the laser light. It shows that the beam spot diameter is reduced and the light intensity distribution of the side lobe with respect to the main lobe is reduced. However, the relationship between the spot diameter reduction rate of the light beam spot and the reduction of the light intensity distribution of the side lobe relative to the main lobe, which depends on the parameters of the light beam diameter and the inner and outer diameters of the annular zone, are not necessarily clearly disclosed. Not.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to solve the above-mentioned problems and to utilize a super-resolution technique, to reduce the amount of light due to a light-shielding band, and to provide an apparatus using a conical lens (prism). In order to solve the problem that the whole becomes large, a phase changing means for changing the phase of the traveling wavefront of the laser light is provided in the annular area centered on the optical axis of the laser light, and the light beam diameter and the annular area are provided. Clarify the relationship between the spot diameter reduction ratio of the light beam spot and the reduction of the light intensity distribution of the side lobe with respect to the main lobe based on the relationship between the inner diameter and outer diameter, and generate a minute light beam spot with a deep depth of focus. An object of the present invention is to provide an optical head device.

[0014]

According to the present invention, there is provided a laser light source for emitting laser light, an optical element for condensing the laser light on an optical recording medium, and an optical recording medium. An optical head device comprising a photodetector for detecting an optical signal that is modulated and reflected from an optical recording medium surface, wherein the optical head device is disposed on an optical axis between a laser light source and the optical recording medium surface, and includes a light beam cross section of the laser light. Phase changing means for changing the phase of the traveling wavefront of the laser light is provided in an annular area centered on the central axis of the light beam cross section in the plane, and the annular area of the phase changing means is provided with a radius of the inner circumference of the annular area. Assuming that r1, the radius of the outer periphery of the annular zone is r2, and the radius of the light beam cross section of the laser beam is R, the values are selected within the range of 0 <(r1 / R) <1 ≦ (r2 / R).

More preferably, the orbicular zone outer radius r2 is smaller than the radius R of the laser beam, and the ratio ((r2-r1) / R) of the orbicular zone width to the radius of the laser beam cross section is 0.5 or less. It shall be selected within the range. With the above configuration, the light amount loss and the conical lens (prism) due to the use of the above-described light shielding band
The problem that the whole apparatus becomes large can be solved by using. That is, as a more specific means, when the annular zone of the phase changing means is selected in the range of 0 <(r1 / R) <1,
Using the super-resolution technique, the loss of the light amount of the laser light emitted from the laser light source can be reduced, and the entire device can be reduced.

Further, as a more specific means for reducing the side lobe intensity at the time of occurrence of super-resolution, when the annular zone is selected in the range of 0 <(r1 / R) <(r2 / R) ≦ 1, Further, the signal-to-noise ratio (S / N ratio) can be improved by reducing the side lobe intensity at the time of occurrence of super-resolution and reducing the noise component when reproducing the signal recorded on the optical recording medium. In particular, when the orbicular region of the phase changing means is selected so that the ratio ((r2-r1) / R) of the orbicular region width to the radius of the light beam cross section of the laser beam is within 0.5 or less, Since the relationship between the spot diameter reduction rate of the and the reduction of the light intensity distribution of the side lobe with respect to the main lobe can be appropriately selected,
An optical head device that generates a minute light beam spot with a large depth of focus can be provided.

[0017]

FIG. 1 is a block diagram of an optical head device for generating a beam spot on an optical recording medium according to one embodiment of the present invention, and FIG. 2 is an explanatory view for explaining a beam spot relative light intensity distribution according to the present invention. FIG. 3 is an explanatory view for explaining an embodiment of the phase shift plate, FIG. 4 is an explanatory view for explaining another embodiment of the phase shift plate, and FIG. FIG. 6 is an explanatory diagram for explaining the relationship between the beam spot diameter reduction rate as a parameter and the power efficiency concentrated on the main lobe. FIG. 6 shows the beam spot diameter reduction rate using the light beam diameter and the orbicular zone dimension of the phase shift plate as parameters. FIG. 7 is an explanatory diagram for explaining a relationship with a side lobe peak ratio, FIG. 7 is an explanatory diagram for explaining a case where a wavefront is divided into two by a phase shift plate, and FIG. Explain FIG. 9 is an explanatory diagram for explaining a two-divided phase shift plate, FIG. 10 is an explanatory diagram for explaining another example of a two-divided phase shift plate, and Table 1 shows a beam spot diameter reduction rate and (r1 / R ), (r2 / R) are tables describing the relationship with the parameters, and the same members corresponding to FIGS. 11, 12, and 13 are denoted by the same reference numerals.

In FIG. 1, an optical head device includes a laser light source (semiconductor laser) 1 for emitting a laser beam, a laser beam guided to an optical recording medium 6 and reflected from the optical recording medium 6 and reflected from the optical recording medium 6. In the illustrated example, the beam splitter 3 separates the light beam to be transmitted from the laser beam into the optical recording medium 6.
a, 1b, 1c), an optical element (objective lens) 5 for converging the laser beam (1a, 1b, 1c) of the three regions on an optical recording medium 6, and an optical recording device. A photodetector 8 for detecting an optical signal modulated by the medium 6 and reflected from the surface of the optical recording medium 6.

In the illustrated example, a collimating lens 2 for collimating a divergent laser beam between the laser light source 1 and the beam splitter 3 and a beam splitter 3 between the beam splitter 3 and the photodetector 8 are provided. And a condensing lens 7 for condensing the separated light beam on the detection surface of the photodetector 8. The phase changing means 4 is arranged on the optical axis 90 between the laser light source 1 and the surface of the optical recording medium 6, and is centered on the central axis (90) of the light beam cross section in a plane including the light beam cross section of the laser light. The light beam passing through the orbicular zone (4a, 4b, 4c) changes the phase of the wavefront as three regions of laser beam (1a, 1b, 1c) by the orbicular zone (4a, 4b, 4c). I do.

The ring-shaped regions (4a, 4b, 4
c), the radius of the inner periphery of the annular zone is r1, the radius of the outer periphery of the annular zone is r2, and the radius of the light beam cross section of the laser beam is R, the outer peripheral radius of the annular zone r2 is smaller than the radius R of the laser beam, The ratio ((r2-r1) / R) between the width of the annular zone and the radius of the cross section of the laser beam is
Select within 0.5 or less. In such a configuration, the laser light generated from the laser light source 1 is converted into a parallel light beam via the collimator lens 2 and is incident on the phase changing means 4 via the beam splitter 3. The light beam incident on the phase changing means 4 is a laser beam 1a, 1c transmitted through the central portion 4a and the peripheral portion 4c of the phase shift plate 41.
The wavefront is divided into three parts: a laser beam 1b transmitted through the orbicular portion 4b of the phase shift plate, which has a different traveling wavefront from the laser beams 1a and 1c.

This wavefront-divided light beam (1a, 1b, 1c)
Are focused by the objective lens 5 and irradiate the recording surface of the optical recording medium 6. The light beam irradiating the recording surface is reflected by the recording surface of the optical recording medium 6, and a physical quantity such as a reflectance or a polarization locally changes depending on information recorded on the optical recording medium 6. The reflected light beam passes again through the objective lens 5 and the phase shift plate 41, is returned to the beam splitter 3, is reflected by the beam splitting surface 3a, and is detected by the photodetector 8 via the condenser lens 7. The light is condensed on the surface, converted into an electric signal, and the information on the recording surface of the optical recording medium 6 is reproduced by a signal processing circuit (not shown).

In the above configuration, by appropriately selecting the relationship between the reduction ratio of the spot diameter of the light beam spot and the reduction of the light intensity distribution of the side lobe with respect to the main lobe, a fine and deep depth of focus optimal for an optical head device is obtained. A light beam spot can be generated. Next, the relationship between the spot diameter reduction rate and the reduction of the light intensity distribution of the side lobe with respect to the main lobe will be described. First, a description will be given of the case where the wavefront division of the light beam is divided into two (FIGS. 7 to 10) with respect to the reduction of the spot diameter. Next, the case where the wavefront division is divided into three will be described with reference to FIGS.

In FIG. 7A, the optical head device comprises:
A semiconductor laser 1 and a collimating lens 2 are placed on an optical axis 90.
, A beam splitter 3, a phase shift plate 44, an objective lens 5, a condensing lens 7 on an optical axis 91 orthogonal to the optical axis 90 and the beam splitter 3, and a photodetector 8. . The optical head converges and irradiates a laser beam onto the optical recording medium 6 so that information can be recorded. Further, an optical signal modulated by the optical recording medium 6 and reflected from the surface of the optical recording medium 6 is used as a beam splitter. The light is deflected by 3 and guided to a photodetector 8 by a condenser lens 7 so that information can be reproduced.

The phase shift plate 44 constituting the phase changing means 4
As shown in FIG. 7B, as shown in the front view of the phase shift plate 44, the region 4b for changing the phase of the traveling wavefront of the laser beam has a ring shape shown by oblique lines. 4b
The phase of the traveling wavefront of the laser beam transmitted through the laser beam changes by Δθ with respect to the laser beam transmitted through the central portion 4a. The orbicular zone region 4b has a radius defined by equation (1), where r1 is the radius of the inner periphery of the annular zone, r2 is the radius of the outer periphery of the annular zone, and R is the radius of the cross section of the laser beam.

[0025]

0 <(r1 / R) <1 ≦ (r2 / R) (1) Here, the phase change amount of the traveling wavefront of the laser beam transmitted through the annular zone 4b may be any value. Good, but (± 1 /
(2 wavelengths + integer multiple wavelengths), that is, (± π + 2mπ; m is an integer).

Accordingly, the light beam of the laser light transmitted through the phase shift plate 44 shown in FIG. 7 is composed of an annular laser light beam 1b whose phase of the traveling wave front has changed and a cylindrical laser light beam at the center portion without phase change. The beam spot 1a is condensed by the objective lens 5, and the beam spot diameter is reduced on the optical recording medium 6 by the mutual interference between the two light beams 1a and 1b. The inventors have calculated (for example, based on the diffraction theory of Hygens-Fresnel)
Y.Li and E.Wolf, J.Opt.Soc.Am.A.Vol1, No.8,801-808 / A
ug.1984), the intensity distribution of the laser beam spot condensed on the optical recording medium 6 is calculated, and the calculation result shown in the relative light intensity distribution curve of the laser beam spot on the optical recording medium shown in FIG. Have gained. In FIG. 8, the horizontal axis indicates the relative value of the focused spot radius, and the vertical axis indicates the relative value of the light intensity distribution with respect to the main lobe peak value. The intensity distribution of the laser beam spot when condensed on the optical recording medium 6 is shown by a solid line, and the annular zone 4b of the phase shift plate 44 is shown.
The intensity distribution of the laser beam spot when the laser beam whose phase of the traveling wavefront of the laser beam is changed by π using the objective lens 5 is condensed on the optical recording medium 6 is indicated by a dotted line.

From this calculation result, the phase of the traveling wavefront of the laser beam transmitted through the annular zone 4b by the phase shift plate 44 shown by the dotted line is ±± compared to the Airy pattern shown by the solid line without using super-resolution. It can be confirmed that a smaller laser beam spot diameter can be obtained when π is changed. Next, a specific structure example of the phase shift plate 44 will be described. FIG. 9 shows a phase shift plate as one example.
45. The inner radius of the orbicular zone of the orbicular zone 4b is r1, the outer radius of the orbicular zone is r2, the radius of the laser beam cross section is R, and the radius is selected within the range defined by the equation (1). To d,
It is formed of a laser beam transmitting material (glass, plastics, or the like) having a refractive index of n, and a region 4a inside the inner periphery of the annular zone penetrates. The thickness d of the laser beam transmitting material forming the annular zone 4b is designed to a value that satisfies the expression (2), where the generated phase change is Δθ.

[0028]

Δθ = (2π / λa) · d · (1-n) (2) where λa is the wavelength of the laser beam in the air. FIG. 10 shows a phase shift plate 46 as a second structural example. As in the example shown in FIG. 9, the orbicular zone region 4b has a radius of the inner periphery of the orbicular zone r1,
Let r2 be the radius of the outer periphery of the orbicular zone, and R be the radius of the cross section of the light beam of the laser beam, select the range defined by the equation (1), set the thickness of the orbicular zone 4b to d, and transmit the laser beam having the refractive index n. A region (4a) formed of a substance (such as glass or plastics) inside the inner periphery of the orbicular zone has a thickness of d-Δd. Δd, which determines the thickness of the inner region of the inner periphery of the annular zone of the laser light transmitting material that forms the annular zone 4b, is expressed as follows: (3)
It is designed to satisfy each of the equations.

[0029]

Δθ = (2π / λa) · Δd · (1-n) (3) where λa is the wavelength of the laser beam in the air.

[0030]

Embodiment 1 In FIG. 1, an optical head device has an optical axis 90.
The semiconductor laser 1, the collimating lens 2, the beam splitter 3, the phase shift plate 41, the objective lens 5,
And an optical axis 91 orthogonal to the optical axis 90 and the beam splitter 3
It is provided with a condenser lens 7 and a photodetector 8 on the top. This optical head device is configured so that information can be recorded by converging and irradiating a laser beam on the optical recording medium 6. The optical recording medium 6 is modulated by the optical recording medium 6.
The optical signal reflected from the surface is deflected by the beam splitter 3 and guided to the photodetector 8 by the condenser lens 7 to reproduce information.

As shown in the front view of FIG. 1B, the phase shift plate 41 has an annular zone 4b for changing the phase of the traveling wavefront of the laser beam. Changes the phase of the traveling wavefront of the laser light transmitted through Δθ. The orbicular zone region 4b has a radius of the inner periphery of the annular zone as r1, a radius of the outer periphery of the annular zone as r2, and a radius of a light beam cross section of the laser beam as R,
(4) Within the range specified by the formula.

[0032]

0 <(r1 / R) <(r2 / R) <1 (4) Here, the phase change amount of the traveling wavefront of the laser beam transmitted through the annular zone 4b may be any value. Good, but (± 1 /
(2 wavelengths + integer multiple wavelengths), that is, (± π + 2mπ; m is an integer).

Accordingly, the light beam of the laser light transmitted through the phase shift plate 41 shown in FIG. 1 is composed of an annular cross-sectional laser light beam 1b in which the phase of the traveling wave front is changed and a cylindrical light beam having a central portion which is not accompanied by a phase change. Laser beam 1a and annular zone cross section laser beam
1c is condensed by the objective lens 5, and these light fluxes 1a, 1b,
1c is subjected to the mutual interference effect, and the beam spot diameter is reduced on the optical recording medium 6.

With such a configuration, the inventors of the present invention have discussed the intensity distribution of the laser beam spot converged on the optical recording medium, as described above in the case of dividing the wavefront of the light beam into two, as described above. By performing the calculation based on the diffraction theory, the calculation result shown in the relative light intensity distribution curve of the laser beam spot on the optical recording medium shown in FIG. 2 is obtained.

In FIG. 2, the horizontal axis represents the relative value of the focused spot radius, the vertical axis represents the relative value of the light intensity distribution with respect to the main lobe peak value, and the wavefront division of the light beam is performed using the phase shift plate 44. The intensity distribution of the laser beam spot when the laser beam at the time of division is focused on the optical recording medium 6 by the objective lens 5 is indicated by a dotted line, and the light beam is formed by using the orbicular region 4b of the phase shift plate 41 of the present invention. The solid line shows the intensity distribution of the laser beam spot when the laser beam obtained by changing the phase of the traveling wavefront of the laser beam by π when the wavefront is divided into three is converged on the optical recording medium 6 by the objective lens 5. .

From the calculation results, the relative light intensity distribution curve of the laser beam spot when the wavefront of the light beam is divided into two as described with reference to FIG. 8 shows that the beam spot formed by the Airy pattern when the super-resolution is not used. Although a laser beam spot diameter smaller than the diameter was obtained, the side lobe intensity was increased. On the other hand, FIG.
It can be confirmed that when the wavefront division of the light beam shown in FIG. 4 is divided into three, the side lobe intensity can be reduced without increasing the laser beam spot diameter.

[0037]

Embodiment 2 By the way, r1 / R defined by the equation (2) and
FIG. 5 shows the relationship between the reduction ratio of the laser beam spot diameter and the laser power efficiency concentrated on the main lobe when the value of r2 / R is variously changed. Here, the reduction ratio of the laser beam spot diameter is a ratio between the laser beam spot diameter when using the super-resolution according to the present invention and the laser beam spot diameter when not using the super-resolution. The concentrated laser power efficiency η is the ratio of the laser power concentrated on the main lobe to the total laser power incident on the objective lens.

In FIG. 5, the horizontal axis represents the ratio (r1 / R) of the inner radius r1 of the annular zone 4b to the radius R of the laser beam cross section, and the vertical axis represents the reduction rate of the laser beam spot diameter. The solid line shows the reduction rate of the laser beam spot diameter with the ratio (r2 / R) of the ratio (r2 / R) between the outer radius r2 of the annular zone 4b and the radius R of the cross section of the laser beam, and the laser concentrated on the main lobe. A characteristic diagram using the power efficiency η as a parameter is shown by a dotted line.

FIG. 5 shows that the laser power efficiency concentrated on the main lobe is reduced as the reduction ratio of the laser beam spot diameter is increased by super-resolution. For example, the reduction rate of the laser beam spot diameter is 0.75
Laser power efficiency concentrated on the main lobe in the vicinity is 10%
Therefore, the practical setting range of r1 / R and r2 / R is preferably the range shown in FIG.

FIG. 6 shows the relationship between the reduction ratio of the laser beam spot diameter and the side lobe peak ratio when the values of r1 / R and r2 / R are variously changed. That is,
The difference between FIGS. 6 and 5 is that, while FIG. 5 shows the characteristic diagram in which the laser power efficiency η concentrated on the main lobe is a parameter as a dotted line, FIG. 6 shows the characteristic diagram in which the side lobe peak ratio is a parameter as a dotted line. It is illustrated. Here, the side lobe peak ratio is a ratio between the side lobe peak value and the main lobe peak value.

In FIG. 6, for example, the reduction ratio of the laser beam spot diameter is set to 0.86, and to suppress the side lobe peak ratio at this time to about 5%, r1 / R = 0.39, r2 / R
It is understood that R = 0.60 may be set. Table 1 shows design examples of r1 / R and r2 / R from the characteristic diagram shown in FIG. table
From FIG. 1, it is desirable that the values of r1 / R and r2 / R defined by the expression (4) in this embodiment are further set under the condition of the expression (5).

[0042]

0 <(r2 / R) − (r1 / R) ≦ 0.5 (5)

[0043]

[Table 1]

[0044]

Third Embodiment Next, a specific structure example of the phase shift plate 41 will be described. FIG. 3 shows a phase shift plate 42 as one embodiment. Let r1 be the inner radius of the orbicular zone of the orbicular zone region 4b of the phase shift plate 42, r2 be the outer peripheral radius of the orbicular zone, and R be the radius of the light beam cross section of the laser beam. The band region 4b is formed of a laser beam transmitting material (glass, plastics, or the like) having a thickness of d and a refractive index of n, and the thickness of the region inside the inner periphery of the annular zone and the region outside the outer periphery of the annular zone is d. −Δd
Produce to be. The thickness d of the laser beam transmitting material forming the orbicular zone 4b is expressed by the following equation (2), and Δd which determines the thickness of the inner and inner peripheries of the orbital zone is the generated phase. Assuming that the change amount is Δθ, the values are designed to satisfy the respective expressions (6).

[0045]

Δθ = (2π / λa) · Δd · (1-n) (6) where λa is the wavelength of the laser beam in the air.

[0046]

Fourth Embodiment FIG. 4 shows the structure of another embodiment of the present invention in which the phase shift plate 42 shown in FIG. The difference from FIG. 1 is that the phase shift plate 41 (42) of FIG. 1 has been removed, and instead the phase shift hemispherical lens 43 has an optical axis 90 between the objective lens 5 and the optical recording medium 6.
It is the point that is placed above. In FIG. 4, the spherical center of the phase-shift hemispherical lens 43 is the focal point (geometric optical focus) of the traveling wavefront of the laser light converted into a spherical wave by the objective lens 5.
In addition, the optical recording medium 6 is arranged at a point P where the information recording surface of the optical recording medium 6 matches. The phase-shift hemispherical lens 43 has a convex spherical surface on the outside and a concave spherical surface on the inside as shown in FIG. 4A, and the spherical centers of the convex spherical surface and the concave spherical surface coincide with the point P. To be configured. In order to maintain a working distance (working distance) with the optical recording medium 6, the surface of the phase-shift hemispherical lens 43 on the optical recording medium 6 side is cut away from the point P (the objective lens 5 side). Is also good.

In the example shown in FIG. 4, a phase shift hemispherical lens
When expressed as a solid angle θr (half value) centered on the optical axis 90 and the point P on the 43 convex spherical surface, the convergence angle of the laser beam converted into a spherical wave by the objective lens 5 is θR (half value).
If the area defined by the equation (7) is cut out in an annular shape at a depth of Δd, the same operation and effect as the embodiment shown in FIG. 3 can be obtained.

[0048]

0 <θr1 <θr2 <θR (7) By setting the depth Δd to, for example, λa / 2, super-resolution can be efficiently generated.

[0049]

As described above, according to the structure of the present invention, the laser beam is disposed on the optical axis between the laser light source and the surface of the optical recording medium, and the cross section of the light beam cross section is included in the plane including the laser beam cross section. Phase change means for changing the phase of the traveling wavefront of the laser light is provided in an annular area centered on the central axis, and the annular area of the phase change means is selected within a specific range. In addition to reducing the loss of light quantity and reducing the size of the entire device, the light beam spot is determined by the relationship between the light beam diameter using super-resolution technology and the inner and outer diameters of the annular zone. It is possible to clarify the relationship between the spot diameter reduction ratio and the reduction of the light intensity distribution of the side lobes with respect to the main lobe, and to provide an optical head device that generates a minute light beam spot with a large depth of focus.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical head device that generates a beam spot on an optical recording medium according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram illustrating a beam spot relative light intensity distribution according to the present invention.

FIG. 3 is an explanatory diagram illustrating an embodiment of a phase shift plate.

FIG. 4 is an explanatory view illustrating another embodiment of the phase shift plate.

FIG. 5 is an explanatory diagram for explaining a relationship between a beam spot diameter reduction rate using a light beam diameter and an annular zone dimension of a phase shift plate as parameters, and power efficiency concentrated on a main lobe.

FIG. 6 is an explanatory diagram for explaining a relationship between a beam spot diameter reduction rate and a side lobe peak ratio using a light beam diameter and a ring zone dimension of a phase shift plate as parameters.

FIG. 7 is an explanatory diagram for explaining a case where a wavefront of a light beam is divided into two by a phase shift plate.

FIG. 8 is an explanatory diagram for explaining a beam spot relative light intensity distribution in the case of two divisions.

FIG. 9 is an explanatory diagram illustrating a two-phase shift plate.

FIG. 10 is an explanatory diagram illustrating another example of a two-phase shift plate.

FIG. 11 is a configuration diagram of an optical head device using a super-resolution technology according to the related art.

FIG. 12 is a configuration diagram of another optical head device using super-resolution technology according to the related art.

FIG. 13 is a configuration diagram of another optical head device using super-resolution technology according to the related art.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Laser light source 1a, 1b, 1c Laser light 1d Region where laser light does not exist 2 Collimate lens 3 Beam splitter 3A Polarization beam splitter 3a Beam split surface 4 Phase changing means 41,42,44,45,46 Phase shift plate 43 Phase Shift hemispherical lens 47 Light shield plate 48 Conical lens 4a Central part 4b, 4c Ring-shaped area 5 Objective lens 6 Optical recording medium 7,7A Condensing lens 7B Cylindrical lens 8 Photodetector 90,91 Optical axis r1 Radius of inner circumference of ring r2 Radius of outer circumference of annular zone R Radius of cross section of laser beam δθ Phase difference d Thickness of annular zone 4b Δd Thickness of air gap in inner zone 4a of inner circumference of annular zone λa Wavelength of laser beam in air

Claims (3)

[Claims] 1. A laser light source that emits laser light, an optical element that focuses the laser light on an optical recording medium, and a photodetector that detects an optical signal modulated by the optical recording medium and reflected from the optical recording medium surface. An optical head device comprising: a laser light source and an optical recording device that change a phase of a traveling wavefront of the laser light in an annular region centered on a central axis of the light beam cross section in a plane including the light beam cross section of the laser light. Provided on the optical axis between the medium and the medium, the radius of the inner circumference of the annular zone of the annular zone is r
1. An optical head device wherein the annular zone area is defined by the following equation, where r2 is the radius of the outer circumference of the annular zone and R is the radius of the cross section of the light beam of the laser beam. 0 <(r1 / R) <1 ≦ (r2 / R) 2. A laser light source for emitting a laser beam, an optical element for condensing the laser beam on an optical recording medium, and a photodetector for detecting an optical signal modulated by the optical recording medium and reflected from the optical recording medium surface. An optical head device comprising: a laser light source and an optical recording device that change a phase of a traveling wavefront of the laser light in an annular region centered on a central axis of the light beam cross section in a plane including the light beam cross section of the laser light. Provided on the optical axis between the medium and the medium, the radius of the inner circumference of the annular zone of the annular zone is r
1. An optical head device wherein the annular zone area is defined by the following equation, where r2 is the radius of the outer circumference of the annular zone and R is the radius of the cross section of the light beam of the laser beam. 0 <(r1 / R) <(r2 / R) <1 3. A laser light source for emitting laser light, an optical element for condensing the laser light on an optical recording medium, and a photodetector for detecting an optical signal modulated by the optical recording medium and reflected from the optical recording medium surface. An optical head device comprising: a laser light source and an optical recording device that change a phase of a traveling wavefront of the laser light in an annular region centered on a central axis of the light beam cross section in a plane including the light beam cross section of the laser light. Provided on the optical axis between the medium and the medium, the radius of the inner circumference of the annular zone of the annular zone is r
1. An optical head device wherein the annular zone area is defined by the following equation, where r2 is the radius of the outer circumference of the annular zone and R is the radius of the cross section of the light beam of the laser beam. 0 <(r2 / R)-(r1 / R) ≦ 0.5