JPH1186314A - Optical element for spot shaping - Google Patents

Abstract

PROBLEM TO BE SOLVED: To simultaneously embody the degradation of a side lobe and the reduction of a spot diameter while suppressing the degradation in central intensity by forming a phase displacement region specified in an opening diameter and thickness on the opening surface of an optical element consisting of a transparent material and changing the phase of the light past the phase displacement region without changing the phase of the light past the regions exclusive of the phase displacement region. SOLUTION: The optical element 2 for spot shaping is formed of the transparent glass material to nearly a short columnar shape and is arranged between the collimating lens 80 of a light source 1 and a beam splitter 82. The optical element 2 for spot shaping has the concentric belt-like phase displacement region 3. The diameter in the central part of the circular belt of the phase displacement region 3 is >=50% in the opening diameter. The phase displacement region is formed to vary in the thickness from the remaining region in the region on the side inner than the outer periphery of the opening. The phase displacement quantity in the phase displacement region 3 is set at <=1/2 the wavelength, more preferably about 1/5 wavelength. The phase displacement region is formed by partial thin film vapor deposition, etching or lithography.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spot-shaping optical element, and more particularly to a spot-shaping optical element useful for changing the shape of a light spot on a recording medium in an optical head device and improving reading and writing accuracy. Things.

[0002]

2. Description of the Related Art For example, in order to increase the recording density in an optical head device, it is necessary to reduce a condensed spot on a recording medium, and the super-resolution phenomenon is used as an effective means for that. This super-resolution phenomenon is to block the central part of the light beam with an opaque shielding plate and change the spatial distribution of light intensity to obtain a spot smaller than the diffraction limit, but because it blocks the light beam center, There is a disadvantage in that light quantity loss occurs and light from the light source cannot be used effectively.

In order to solve such a drawback, a technique has been proposed in which a phase filter is used to change the phase of the central portion of the light beam by a half wavelength to produce the same effect as that of a shielding plate. Although this method can solve the light amount loss, it cannot solve the following disadvantages associated with the super-resolution phenomenon.

[0004] That is, when the spot diameter is reduced by super-resolution, in addition to the central main spot, an intensity distribution called a side lobe appears remarkably around the main spot. As a result, even if the center spot is reduced, the recording adjacent to the target recording is read at the same time due to the generation of side lobes, causing crosstalk and jitter of the read signal, thereby lowering the signal accuracy.

[0005] In addition, the peak intensity at the center of the spot decreases as the spot diameter decreases due to the super-resolution, which is an important problem at the time of recording and writing. In other words, when recording is performed on the recording medium, the recording medium 5 is localized by the focused laser light to change the recording surface regardless of the method such as the magneto-optical effect, the phase change, and the chemical change. However, high light intensity is required at the center of the spot, but if sufficient light intensity is not obtained, there is a problem in that stable recording is not written or an obstacle to an increase in writing speed.

Further, a semiconductor laser generally used as a light source of an optical head emits laser light diverging from a junction surface between a p-type semiconductor and an n-type semiconductor, and light perpendicular to this surface is emitted in a horizontal direction. The light is emitted while being greatly expanded compared to the light. When this emitted light is converted into a parallel light beam by a collimator lens without performing correction, the intensity in the vertical direction is closer to the uniform intensity in the vertical direction than in the horizontal direction due to the difference in the spread angle of the emitted light. If the intensity distribution of the parallel light beam cross section differs depending on the direction,
The shape of the focused spot also changes depending on the direction. If a light beam having an intensity distribution such that the intensity is significantly reduced when it is separated from the center is condensed, the spot diameter becomes large. If the light intensity distributions differ in directions orthogonal to each other, the spot will be elliptical. For that purpose, it is necessary to correct the intensity distribution in the same way in any direction by correcting the parallel light beam by perfect circle correction.

For this purpose, there is a method in which light emitted from a semiconductor laser is converted into parallel light by a collimating optical system, and then the beam diameter is increased in one direction by a prism optical system. However, this has the disadvantage of being expensive.

The present invention has been made to solve the above-mentioned drawbacks, and provides a spot shaping optical element capable of simultaneously reducing the side lobe and reducing the spot diameter while minimizing the reduction in center intensity. With the goal.

[0009]

According to the present invention, the above object is achieved by disposing a laser beam between a light source 1 and a focusing optical system,
A spot shaping optical element 2 for shaping a converging spot of a converging optical system, which is formed of a transparent material, has a diameter D of 50% or more of an opening diameter D0 on an opening surface of the optical system, and an outer periphery of the opening. The phase shift region 3 is formed by making the thickness different from the remaining region in the inner region, and the phase of light passing through the phase shift region 3 without changing the phase of light passing through the region other than the phase shift region 3. Is achieved by providing the spot shaping optical element 2 that changes

In the present invention, the phase shift region 3 is formed by making the thickness different from the remaining region.
The light beam passing through the phase displacement region 3 having a thickness different from that of the remaining region changes its phase because the optical path length is different from that of the remaining region, and functions as a phase filter as a whole. In the present invention in which the phase filter is formed by changing the thickness, there is an advantage that the manufacturing is simplified as compared with the case of forming a phase shift film, for example. That is, assuming that the refractive index of the material is n, the wavelength of light is λ, the change in thickness is Δd, and the phase displacement is Δφ, there is a relation Δφ = 2π (n−1) · Δd / λ. .. (1) is established.

Now, the refractive index of the material is 1.5 and the wavelength is 68
Assuming a thickness of 5 nm, a thickness change of about 270 nm is required to generate a phase shift of 0.2 wavelength.
Such a change in thickness can be caused by partial thin film deposition, etching,
It can be made relatively easily by lithography or the like.

Further, by making the phase different in the circular band-like region 4 except for the center, it is possible to reduce the peak value of the condensed spot in the condensing optical system and minimize the occurrence of side lobes.

Furthermore, the spot shaping optical element 2 may be formed by superposing two transparent bodies having a difference in refractive index at the contact interface smaller than the difference between the refractive index of each material and air. It is possible, and the phase shift region 3 is formed at the contact interface between these transparent bodies.

Here, to form the phase change region at the contact interface of the transparent body means that the above-mentioned thickness change portion is formed on one of the transparent bodies and the other transparent body is engaged with this thickness change portion. The phase difference is caused by unevenness at the boundary position.

When two transparent bodies having different refractive indices n1 and n2 are superimposed, the relationship given by the above equation (1) is: Δφ = 2π (n1−n2) · Δd / λ. ... (2), and by selecting a material in which n1 and n2 are close to each other, Δd required to generate the same phase difference is larger than Δd in equation (1). can do.

For example, polystyrene (nC = 1.578)
When the glass material BaF3 (nC = 1.57893) is used as a transparent material, the refractive index difference (n1-n2) is 0.00
The thickness change required to generate 0.2 wavelength of the phase difference expands from 270 nm to 140 μm. That is, the amount of phase displacement caused by the displacement of the boundary position at the time of manufacturing the phase filter becomes insensitive, and the processing accuracy can be reduced. In addition, since such an accuracy can be achieved even by using a general molding technique in lens mass production technology, mass production and cost reduction can be easily realized.

In this case, the amount of phase displacement can be set to 、 wavelength, or a value obtained by adding a displacement amount of an integral multiple of the wavelength to １ ／ wavelength. By suppressing the amount of phase displacement to less than 1/2 wavelength, and preferably to about 1/5 wavelength, it is possible to suppress a decrease in center intensity and an increase in side lobes.

It is desirable that the shape of the phase displacement region 3 in a front view is a circular band shape having a width W of about 5% of the opening diameter D0. ., And a different amount of phase displacement can be generated discontinuously in each of the band-shaped regions 4. Here, the term “circular band shape” includes not only a perfect circle but also an ellipse. When a plurality of circular belt-shaped regions 4, 4,... Are formed,
The degree of freedom in spot shape control is improved. When the phase displacement region 3 is constituted by a plurality of circular band regions 4... Having different phase displacement amounts, the phase displacement regions 3 are arranged such that the signs of the phase displacement amounts in adjacent circular band regions 4. It is desirable to design.

The phase displacement region 3 is desirably formed in a concentric perfect circular band having a diameter D of about 60% of the opening diameter D0. With the center at the optical axis, the diameter D is about 60% of the aperture diameter D0,
A narrow circular band (ring shape) whose width W is about 5% of the opening diameter D0
By forming the phase displacement region 3 and generating a phase displacement in the above range of the light beam, unlike the conventional super-resolution filter, not only the spot diameter can be reduced but also the side lobe can be reduced. Here, the diameter D refers to the diameter at the center of the concentric belt.

The phase displacement region 3 can be formed in an elliptical band other than the concentric band as in the second aspect of the present invention. The spot shaping optical element 2 having the elliptical band phase shift region 3 is effective for the intensity distribution of the direction-dependent light beam, and imparts a phase shift to the intensity distribution of the direction-dependent light beam in an elliptical band shape. , The spot shape of the ellipse is corrected to a perfect circle. In this case, the direction in which the emission angle of the light source 1 is maximum and the direction in which the emission angle is minimum are set so as to coincide with the long axis and the short axis.

Furthermore, the perfect circle correction of the spot is carried out in claim 3.
Can be achieved by generating different phase displacements in directions orthogonal to each other on a plane normal to the optical axis of the laser light, in which case the laser light is emitted from the light source 1 in this case. Different phase displacements are generated in different directions at different angles. To generate different phase displacements in different directions, it is only necessary to change the thickness in that direction.

An optical head device using the above-described spot shaping optical element 2 is provided. The optical head device has a light source 1 that generates a laser beam, and is configured to collect light on a recording surface on a recording medium 5 by an optical element and take out light reflected from the recording surface. The spot shaping optical element 2 is arranged between the medium and the medium 5.

In this case, if the hologram diffraction grating 6 is formed on the spot shaping optical element 2, the function of splitting light to the spot shaping optical element and the function of generating a phase shift are described.
Since the two functions can be provided, the number of parts and the assembling process can be prevented from increasing.

Further, when the spot shaping optical element configured to be able to correct the perfect circle as described above for the intensity distribution of the luminous flux having the direction dependence is used, the condensed spot on the recording medium 5 is changed. Recording and reading and writing accuracy are improved by arbitrarily changing the scanning direction.

[0025]

FIG. 1 shows an optical head device using a spot shaping optical element according to the present invention. The optical head device is
The laser beam is focused on the recording surface of the recording medium 5, the light reflected from the recording surface is taken out, and guided to the detection optical system 70. A collimating optical element such as a collimating lens 8 for converting light emitted from the light source 1 into a parallel light beam, and a condensing optical element such as a condensing lens 81 for condensing the parallel light beam on the recording medium 5. The reflected light from the recording medium 5 is converted into a parallel light beam by a condensing optical element such as a condensing lens 81, and then guided to a detection optical system 70 by a beam splitter 82. , And the position shift and the focus shift of the condensed spot are detected.

A spot-shaping optical element formed of a transparent glass material into a substantially low-cylindrical shape is disposed between the collimating lens 80 and the beam splitter 82 on the light source 1 side. In order to change the phase in a concentric band shape, a concentric band-shaped phase displacement region 3 is provided. As shown in FIG. 2, the phase shift region 3 is formed by making the thickness of the glass material different from other regions.

The phase displacement region 3 has a diameter D (diameter at the center of the circular band) of 50% to 70% of the opening diameter D0,
Preferably, at about 60% of the aperture diameter D0, the amount of phase displacement in the phase displacement region 3 is equal to or less than 1/2 wavelength, preferably 1
It is set to be about / 5 wavelength. The thickness change ΔH required to generate a predetermined amount of phase shift is given by the above equation (1), and the phase shift region 3 is formed by partial thin film deposition, etching, and lithography.

FIGS. 3 to 5 show calculation results obtained by setting the width W of the circular band to 5% of the opening diameter D0 in order to confirm the influence of the above-described phase displacement region 3. FIG. First, the graph shown in FIG. 3 shows the change of the side lobe with respect to the diameter D of the circular band (the diameter at the center position of the circular band). Taking the band diameter D, the vertical axis shows the peak intensity of the side lobe when the center intensity is 100%. Further, a solid straight line parallel to the horizontal axis drawn at a value of the side lobe intensity of 1.75 is a case where the spot shaping optical element 2 according to the present invention is not used (hereinafter referred to as “when not used”). Shows the measured values.
Although not plotted on the graph, the side lobe intensity measured using a conventional phase filter that generates a phase displacement at the center of the aperture was 10%.

From the graph shown in the figure, the side lobe increases and decreases as the phase displacement increases, but the tendency of the change is the same regardless of the phase displacement. It can be seen that the side lobes are lower than during use. Conversely, when the circular band is near the center of the aperture, the side lobes increase, and when the amount of phase displacement is large, the side lobes are more than twice that when they are not used, and the effect on the signal reading accuracy becomes significant. In the illustrated graph, the double sidelobe intensity when not used is shown by a broken line parallel to the horizontal axis. From the above results, the diameter D of the circular band affects the side lobe intensity, and the diameter D of the circular band needs to be about 50% or more of the opening diameter D0 in order to suppress the adverse effect of the side lobe.

FIG. 4 shows the change of the spot diameter with respect to the diameter D of the circular band. The phase shift amount is used as a parameter, and the horizontal axis represents the diameter D of the circular band with the aperture diameter D0 being 100%, and the vertical axis represents the spot diameter. , Standardized when not used. According to the graph shown, the diameter D ratio of the circular band = 67 regardless of the phase displacement amount.
It can be confirmed that, when the value is small, the value is reduced when the value is small and the value is enlarged when the value is large. From the above results, the diameter D of the circular band affects the spot diameter. To reduce the spot diameter, the diameter D of the circular band needs to be 70% or less of the opening diameter D0.

When the diameter D of the circular band is fixed to 60% of the opening diameter D0, the change of the center intensity with respect to the amount of phase displacement is shown in FIG. From this graph, the center intensity (vertical axis) decreases as the phase displacement amount (horizontal axis) increases, and the central intensity when not used is 1
If it is set to 00%, in the range of 1.5 (radian) ≒ 0.25 wavelength or less, it is possible to achieve a reduction of at least 80%, that is, 20% or less which does not hinder practical reading and writing. You can see that there is.

A circular band region having a diameter D of 60% of the opening diameter D0 and a width W of 5% of the opening diameter D0 is defined as a phase displacement region 3, and a phase displacement amount of 1/5 wavelength is generated in the phase displacement region 3. FIG. 6 shows an example of a change in spot diameter and a decrease in side lobe when the spot shaping optical element 2 designed to be used is used. In the figure, the solid line shows the case where the spot shaping optical element 2 is used, the broken line shows that it is not used, and the horizontal axis shows the distance from the spot center. From this figure, it can be seen that the side lobe intensity ratio to the center intensity (vertical axis) is reduced by about 19.7%. When the spot diameter and the center intensity of the spot were calculated under the same conditions, the spot diameter was reduced by 1.3% and the center intensity by 19.6% compared to when the spot was not used. It was confirmed that diameter reduction and side lobe reduction were simultaneously achieved.

FIG. 7 shows a second embodiment of the present invention.
In this embodiment, the phase shift regions 3 are adjacent 3
Each of the concentric circular band regions 4 generates a different phase displacement. The width W of each concentric perfect circular band region 4 is 5% or less of the opening diameter D0, and the diameter D is set to 55% to 70% of the opening diameter D0. In addition, the directions of the phase shifts of the three circular zones are such that phase shifts in opposite directions are generated in adjacent circular zones, so that side lobe reduction and spot diameter reduction are simultaneously realized.

Since the phase has periodicity and returns to the original phase when shifted by one wavelength, the phase difference shown in FIG.
Alternatively, by adding a phase difference of an integral multiple of the wavelength, a structure equivalent to this can be obtained as shown in FIG. In this case, since there is no need to generate a positive or negative phase difference, it is not necessary to form a projection with a minute width W on the surface, and processing becomes easy. FIG. 7A shows a cross-sectional shape when the generated phase difference is plotted on the vertical axis.
(B) shows actual design dimensions when the aperture diameter D0 is 3 mm and the refractive index is 1.5.

FIG. 8 shows the result of reducing the focused spot side lobe by using the spot shaping optical element 2 shown in FIG. The width W of each circular zone is 2.2% of the opening diameter D0. In FIG. 8, the vertical axis indicates the ratio of the side lobe intensity to the center intensity, and the spot shape when not used is indicated by a broken line. From this, it can be confirmed that the side lobe is 43% lower than when not in use. At this time, the rate of decrease in the center intensity was suppressed to 15%, and it was confirmed that the spot diameter was smaller than when the spot was not used. This is an example of the effect of the present invention,
If the reduction rate of the side lobe is set to be gentle, it is possible to increase the reduction rate of the spot diameter.

FIG. 9 is a schematic cross-sectional view of the spot shaping optical element 2 in which the amount of phase displacement changes continuously. This is Figure 7
This corresponds to a case in which the number of circular band regions in is increased infinitely and the phase is finely changed, and the degree of freedom of spot control can be increased by adding a complicated phase displacement.

FIG. 10 shows a third embodiment of the present invention. In this embodiment, the spot shaping optical element 2 generates a phase difference by superposing two transparent bodies 90 and 91 having close refractive indexes and changing the position of the boundary. In this embodiment, as described above, the occurrence of the phase displacement amount with respect to the displacement of the boundary position at the time of manufacturing becomes insensitive, so that the processing accuracy can be reduced.

FIG. 11 shows a fourth embodiment of the present invention. In this embodiment, the phase displacement region 3 is formed in an elliptical band shape. As is clear from FIG. 4, the magnitude of the super-resolution effect when the phase is displaced in a circular band shape depends on the diameter D of the circular band, that is, the distance from the center of the beam. Therefore, in the present embodiment in which the phase displacement region 3 is formed in an elliptical band shape, the reduction ratio of the spot diameter increases in the minor axis direction of the elliptical band, and decreases in the major axis direction, or Expanding.

On the other hand, the emitted laser light of a semiconductor laser generally used as the light source 1 of the optical head has direction dependency in the light intensity distribution, and if condensed without correcting this, the spot becomes an ellipse. If the spot diameter deviates from a perfect circle, data reading and writing are adversely affected. According to the present embodiment, the spot shape can be corrected to a perfect circle by changing the reduction ratio of the spot diameter depending on the direction without using a generally used perfect circle correction prism.

Now, assuming that the spot diameter at the time of non-use has a difference of 5% depending on the direction, consider the case where the spot shape is corrected by the spot shaping optical element 2 according to the present embodiment. The amount of phase displacement is fixed at 0.2 wavelength. In the direction in which the spot is small, that is, in the direction in which the width W of the light beam is wide, the diameter D of the elliptical band is increased so that the spot diameter is enlarged.
To 85% of the major axis of the ellipse. In the direction in which the spot is large, that is, in the direction in which the width W of the light beam is narrow, the diameter D of the elliptical band is set to 40% of the opening diameter D0 so as to be the minor axis of the ellipse so as to reduce the spot diameter. According to FIG. 4, the spot diameter is increased by about 3% in the small spot direction, and reduced by about 2% in the large spot direction.
As a result, the spot shape can be corrected to a circle,
The recording and reading accuracy can be improved.

FIG. 12 shows a fifth embodiment of the present invention. This embodiment shows a case where the phase displacement region 3 is formed so as to change the amount of phase displacement depending on the direction. Figure 4
As can be seen from the graph, the magnitude of the super-resolution effect when the phase is displaced in a circular band shape increases with the amount of phase displacement, and becomes maximum at a half wavelength. Therefore, the spot diameter decreases in the direction in which the phase displacement amount is large, and the spot diameter hardly decreases in the small direction. According to the present embodiment, even if the light intensity distribution of the light source 1 of the optical head has direction dependency, it is possible to correct the spot shape to a perfect circle without using a generally used perfect circle correction prism. it can.

Now, assuming that the spot diameter at the time of non-use has a difference of 5% depending on the direction, the case where the spot shape is corrected by the spot shaping optical element 2 according to the present embodiment will be considered. In this case, a phase difference is not generated in the direction where the spot diameter is small, and the diameter D5 is generated in the direction where the spot diameter is large.
The phase shift region 3 is designed so that a phase difference of 0.5 wavelength is generated at 5%. As a result, the spot diameter is reduced by about 5% in the direction in which the spot is larger than in the figure, and the spot shape can be corrected to a circular shape.

FIG. 13 shows a sixth embodiment of the present invention. This embodiment is a particularly effective modification when used as an optical head, and a hologram diffraction grating 6 for dividing a reflected light beam from a recording medium 5 into a plurality of light beams is formed on a transparent body. The hologram diffraction grating 6 has a grating structure formed on a flat substrate at a period several times as long as the wavelength, and the phase displacement region 3 is formed on a surface having no diffraction grating. Therefore, in this embodiment, since the spot shaping optical element 2 also has a function of dividing the light reflected from the recording medium 5 and guiding the light to the detection optical system 70, it is possible to prevent an increase in the number of components and an assembling process. Further, since both the hologram diffraction grating 6 and the phase shift region 3 can be formed by a common method such as etching or lithography, the manufacturing process of the element can be simplified.

[0044]

As is clear from the above description, according to the present invention, it is possible to simultaneously reduce the side lobe and reduce the spot diameter while suppressing the decrease in the center intensity as much as possible. In addition, the spot shape can be rounded and optimized without using another optical element.

Further, according to the optical head device of the present invention, since a small-diameter spot without side lobes can be obtained, it is possible to improve the reading and writing accuracy of the recording medium.

[Brief description of the drawings]

FIG. 1 is a diagram showing a first embodiment of the present invention.

FIG. 2 is an explanatory view showing an optical element for spot shaping;
(A) is a perspective view, (b) is a cross-sectional view cut along a line including the center of (a).

FIG. 3 is a diagram showing a relationship between a diameter ratio of a circular band and a side lobe intensity.

FIG. 4 is a diagram showing a relationship between a diameter ratio of a circular band and a spot diameter.

FIG. 5 is a diagram showing the relationship between the amount of phase displacement and the intensity at the center of a spot.

FIG. 6 is a diagram showing a reduction in side lobes.

FIGS. 7A and 7B are diagrams showing a second embodiment of the present invention, wherein FIG.
Is a sectional view, and (b) is a sectional view showing a phase displacement region equivalent to (a).

FIG. 8 is a diagram showing a reduction in side lobes.

FIG. 9 is a sectional view showing a modification of FIG. 7;

FIG. 10 is a sectional view showing a third embodiment of the present invention.

FIG. 11 is a diagram showing a fourth embodiment of the present invention.

FIG. 12 is a diagram showing a fifth embodiment of the present invention;
(A) is a perspective view, and (b) is a cross-sectional view obtained by cutting (a) along the circumference and developing it.

FIG. 13 is a diagram showing a sixth embodiment of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 light source 2 spot shaping optical element 3 phase displacement area 4 circular band area 5 recording medium 6 hologram diffraction grating D0 aperture diameter W width

Claims (6)

[Claims] 1. A spot shaping optical element arranged between a light source of a laser beam and a converging optical system for shaping a converging spot of the converging optical system, the spot shaping optical element being formed of a transparent material and having a diameter. Light that has passed through a region other than the phase displacement region, where the phase displacement region is formed on the opening surface of the optical system at 50% or more of the opening diameter and in a region inside the outer periphery of the opening with a thickness different from that of the remaining region. Spot shaping optical element that changes the phase of light passing through the phase displacement region without changing the phase of the spot. 2. The method according to claim 1, wherein the phase shift region is an elliptical band.
An optical element for spot shaping according to the above. 3. The spot shaping optical element according to claim 1, wherein the phase displacement regions generate different phase displacements in directions perpendicular to each other on a plane whose normal is the optical axis of the laser beam. 4. An optical head device having a light source for generating a laser beam, condensing light on a recording surface on a recording medium by an optical element, and extracting light reflected from the recording surface, wherein an optical element is provided between the light source and the recording medium. A phase displacement region formed of a transparent material, having a diameter of 50% or more of the opening diameter on the opening surface of the optical system, and having a thickness different from that of the remaining region in a region inside the outer periphery of the opening. A spot shaping optical element is disposed, and the spot on the recording medium is changed by changing the phase of the light passing through the phase shift area without changing the phase of light passing through an area other than the phase shift area by the spot shaping optical element. Optical head device for shaping. 5. A phase displacement region of the spot shaping optical element is an elliptical band, and a major axis and a minor axis of the phase displacement region are orthogonal to a scanning direction of a condensed spot on a recording medium and a direction perpendicular thereto. 5. The optical head device according to claim 4, wherein the optical head device coincides with the direction in which the optical head moves. 6. A phase displacement region of the spot shaping optical element generates different phase displacements in directions orthogonal to each other on a plane normal to the optical axis of the laser beam, and condenses the phase displacements. 5. The optical head device according to claim 4, wherein a scanning direction of the spot on the recording medium is different from a scanning direction orthogonal to the scanning direction.