JP4396294B2 - Optical head device - Google Patents

Description

  The present invention relates to an optical head device that performs recording and / or reproduction of an optical recording medium such as an optical disk.

  In recent years, various types of optical disk devices have been developed and used, and this optical disk device is provided with an optical head device. This optical head device normally collects light emitted from a semiconductor laser as a light source on the recording surface of an optical disk with an objective lens, collects reflected light from the recording surface with an objective lens, and guides it to a detection optical system. It has a configuration.

By the way, glass or plastic is used for the objective lens of the optical head device. In particular, in the case of a plastic lens, birefringence occurs during lens molding. FIGS. 10 and 11 schematically show the distribution in the lens pupil of the retardation and slow axis (azimuth in which the speed of light travel is slow (that is, the refractive index is high)).
In these figures, the direction of the arrow corresponds to the direction of the slow axis β, and the length of the arrow corresponds to the magnitude of the retardation value. In both FIG. 10 and FIG. 11, the retardation is small near the optical axis and large near the optical axis, and the direction of the slow axis β is approximately rotationally symmetric about the optical axis. The direction of the slow axis β may be switched between the fast axis (the direction in which light travels fast (that is, the refractive index is low)) and the slow axis β when the plastic material and molding conditions are changed. And a lens as shown in FIG. 11 is obtained.

  When such a lens has birefringence, astigmatism as shown in FIG. 12 occurs when linearly polarized light is incident. This is because the refractive index for light transmitted through the lens 300 differs within the lens pupil diameter, resulting in a difference in optical path length, and astigmatism occurs. The direction of this astigmatism changes when the polarization direction of incident light is changed.

In order to correct this astigmatism, for example, a method of canceling astigmatism caused by birefringence by generating astigmatism by the lens surface shape has been proposed. Here, the astigmatism generated by the birefringence changes depending on the polarization direction of the incident light. On the other hand, astigmatism generated in the lens surface shape does not change depending on the polarization direction.
JP-A-5-107467

For this reason, when mounting the birefringent objective lens as described above on the optical head device, it is necessary to strictly control the orientation of the objective lens, which is an obstacle to the assembly of the optical head device. It was.
However, astigmatism does not occur when circularly polarized light is used for recording and / or reproducing light in the optical head device. However, when this circularly polarized light is incident, for example, as shown schematically in FIG. 13, an example of the polarization state of the light after passing through the lens 100 having birefringence as shown in FIG. In the spot 400, the center in the pupil plane is the circular polarization state γ, but the periphery changes to an elliptical polarization state δ that is almost linear. The orientation of the major axis and minor axis of elliptically polarized light also changes in the plane. This polarization state changes depending on the retardation of the lens. When there is a polarization state distribution in the pupil plane as described above, there is a problem that the shape of the focused spot on the optical disc surface deteriorates, the jitter of the reproduction signal deteriorates, and the writing characteristics deteriorate.

In recent years, the wavelength of light used in an optical head device tends to be shortened, so that even an objective lens having the same retardation value has a problem of increased birefringence. Furthermore, when a high-density disk is used in the optical disk apparatus, the pit size of the optical disk is reduced with respect to the focused spot, and the spot shape margin is small.
As described above, in the optical head device, there is a demand for improvement in the shape of the focused spot due to birefringence of the objective lens.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an optical head device that can improve the deterioration of a focused spot shape due to birefringence of an objective lens.

The present invention includes a light source, an objective lens that collects outgoing light emitted from the light source on an optical recording medium, and a phase plate that is provided between the light source and the objective lens and changes the polarization state of light. The objective lens is made of a material having birefringence, and the phase plate has a birefringent medium layer made of a birefringent medium, and has a slow phase having a high refractive index in the plane. A retardation value represented by a product of a birefringence amount Δn of the birefringent medium and a thickness of the birefringent medium layer, the axial direction being coincident with a tangential direction or a radial direction of a concentric circle around the optical axis of the light. but the changes with distance from the optical axis, the retardation value of the phase plate provides an optical head device is characterized by having a distribution you cancel the distributions of retardation values of the objective lens.

The phase plate may provide the above optical head device in which the phase difference in the peripheral portion is larger than the phase difference in the vicinity of the optical axis .

The phase plate has a filling medium layer made of a filler, the birefringent medium layer has irregularities that differ in thickness in the radial direction around the optical axis, and the filling medium layer is formed in an uneven depression in the refractive index of the filler is equal to any one of the values of the ordinary refractive index n _o and extraordinary refractive index n _e of the birefringent medium, or, with the ordinary refractive index n _o It provides the above optical head device that having a refractive index between the extraordinary refractive index n _e.

Further, the birefringent medium, provides a liquid crystal above optical head device polymeric Ru crystal der that polymer the.

Further, the phase plate has a transparent substrate which concentric or radial grooves around the optical axis is formed, the optical head device wherein the polymer liquid crystal that has been formed on the groove provide.

Further, the refractive index n _a of the transparent _substrate, the depth d of the groove, the polymer liquid crystal of the ordinary refractive index n _o, when the extraordinary refractive index of the polymer liquid crystal and n _e, | n _o −n _a | · d and | n _e −n _a | · d provide the above-described optical head device, which is not more than one-tenth of the wavelength of light from the light source .

Further, the phase plate, provides the above optical head device formed by laminating at least two of said birefringent medium layer.

Further, the phase plate, one slow axis direction of the birefringent medium layer are the same, the slow axis of the two said birefringent medium layer provides the above optical head device that forms an angle of 45 degrees .

  According to the present invention, it is possible to provide an optical head device that is effective in improving the recording / reproducing characteristics of an optical disk, and that is particularly effective for the demand for improvement in the shape of a focused spot due to birefringence of an objective lens.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
FIG. 1 shows an optical head device 10 according to an embodiment of the present invention. The optical head device 10 collects light emitted from the light source 1 on an optical disc D that is an optical recording medium, records information on the optical disc D, and reproduces information on the optical disc D. A light source 1, a collimator lens 2, a beam splitter 3, a quarter wavelength plate 4, a phase plate 5, an objective lens 6, and a light detection system 7 are provided.

For the light source 1, for example, a semiconductor laser is used. The semiconductor laser of this embodiment oscillates coherent light with a wavelength λ (for example, λ = 405 nm). This semiconductor laser oscillates light in a 650 nm band for DVD or a 780 nm band for CD, for example. Or a plurality of lasers having different wavelengths may be used.

  The quarter-wave plate 4 may not be used, but may be a wave plate having a phase difference other than the quarter-wave plate 4 when used. In addition, the quarter wavelength plate 4 and the phase plate 5 may be bonded together, and such a configuration is preferable because the number of parts can be reduced. Furthermore, the retardation values and slow axis directions of homogeneous wave plates such as the phase plate 5 and the quarter wave plate 4 in the present invention can be combined in a vector manner to form a single phase plate. This is preferable because the refractive layer can be reduced. For example, when the retardation value near the center is 0 and the phase plate 5 and the quarter wavelength plate 4 having a large value around the center are synthesized, the retardation and the slow axis direction equivalent to the quarter wavelength plate are also obtained near the center. It is preferable that the phase plate be provided.

  Therefore, according to the optical head device 10 according to the present embodiment, the light emitted from the semiconductor laser passes through the collimator lens 2, the beam splitter 3, the quarter wavelength plate 4, the phase plate 5 and the objective lens 6 in the present invention. The light is transmitted and collected on the optical disk D. The light reflected from the optical disk D passes through the objective lens 6, is reflected by the phase plate 5, the quarter wavelength plate 4, and the beam splitter 3, and is guided to the light detection system 7.

Next, specific examples of the magnitude of the retardation value (the product of the birefringence amount Δn and the thickness d of the birefringent medium) and the in-plane distribution in the slow axis direction of the phase plate 5 in the present invention are shown in FIGS. Is shown schematically. Here, the direction of the arrow corresponds to the direction of the slow axis β, and the length of the arrow corresponds to the size of the retardation.
In the vicinity of the center of the optical axis α of the light emitted from the light source 1, the retardation value is small and large in the periphery. In the example of FIG. 2, the direction of the slow axis β is parallel to the tangential direction of the concentric circle centered on the optical axis α. On the other hand, in the example of FIG. 3, the direction of the slow axis β is perpendicular to the tangential direction of the concentric circle with the optical axis α as the center.

  As described above, when plastic is used as the material of the objective lens 6, birefringence occurs when the lens is molded. FIGS. 10 and 11 described in the “Background Art” column schematically show the distribution of the retardation value and the slow axis β in the lens pupil. Here, the direction of the arrow corresponds to the direction of the slow axis β, and the length of the arrow corresponds to the magnitude of the retardation value.

  In the present invention, the retardation value distribution of the objective lens 6 is canceled by the retardation value distribution of the phase plate 5, thereby improving the light condensing spot shape on the surface of the optical disk D. For example, when the lens 100 as shown in FIG. 10 has birefringence, the phase plate 5 (5A) having the retardation value distribution as shown in FIG. 3 is combined, and the lens 200 as shown in FIG. In some cases, the retardation between the objective lens 6 and the phase plate 5 can be canceled (cancelled) by combining the phase plate 5 (5B) having the retardation value distribution as shown in FIG.

Further, the in-plane distribution of the retardation value of the phase plate 5 in the present invention may be changed smoothly as shown in FIG. 2 or FIG. 3, or the region is divided so that the uniform retardation value and slow axis are uniform in the region. As the direction distribution, the retardation value or the direction of the slow axis may be changed for each region. In each region, only one of the retardation value or the slow axis direction may be uniform, and the other may be changed smoothly. An example of this is shown as a schematic diagram in FIG. In FIG. 4, the retardation value of the phase plate 5 (5C) is increased stepwise in the order of regions A, B, C, and D, and the direction of the slow axis β is concentric.

  Further, FIG. 7 shows another example of the phase plate 5 in a duplex manner. This phase plate 5D is an example in which each of the regions obtained by dividing the concentric circles radially is uniform in both the magnitude and direction of the retardation value, and the retardation value and direction are changed for each region.

  By the way, when the phase difference of the phase plate 5 with respect to the light of the wavelength λ used in the optical head device 10 is represented by φ (φ = 360 · Δn · d / λ), the phase of the region having the orthogonal birefringence slow axis. Becomes -φ and the sign is reversed. It is also equivalent to shift the phase by an integral multiple of 360 degrees. If this is utilized, the direction of the slow axis can be substituted by a birefringence region in which the slow axis direction is orthogonal. For example, the region of 50 degrees is equivalent to a phase difference of 310 degrees in which the slow axes are orthogonal. In a duplex manner, the phase plate 5D in FIG. 7 and the phase plate 5E in FIG. 8 are equivalent.

The change in the polarization state of transmitted light due to the birefringent medium exhibiting the slow axis in any direction can be synthesized by superimposing two birefringent media whose slow axes intersect at 45 degrees. That is, a phase film having a desired birefringence distribution can be realized by superimposing two layers of birefringent media whose slow axes intersect at 45 degrees and have different retardation values in the plane.
For example, by laminating the birefringent composite layers 50A and 50B whose slow axes β intersect at 45 degrees in FIGS. 9A and 9B, the phase plates 5D and 5E equivalent to FIGS. 7 and 8 are obtained. . Thereby, even when a birefringent medium in which the direction of the slow axis is difficult to change in the plane, a phase plate in which the slow axis direction is changed in the plane as shown in FIGS. 2 and 3 can be realized.

In the present invention, the phase plate 5 is a birefringent medium that processes a single crystal having birefringence such as quartz or LiNbO ₃ (lithium niobate), a resin film having birefringence, it may also be made of work by injection molding. Further, it is preferable to use a polymer liquid crystal obtained by polymerizing a liquid crystal because the slow axis direction can be freely set by controlling the alignment direction of the liquid crystal.

(I) Next, as work made the method of the retardation value distribution, a method of attaching a distribution in the thickness of the birefringent medium layer, there is a method of changing the retardation value was equal to the thickness of the birefringent medium layer.
Therefore, the case where a polymer liquid crystal as a birefringent medium layer, a specific example of the retardation value distribution operation made methods.

First, the configuration of the phase plate 5 in the present invention will be described in detail with reference to the schematic cross-sectional view shown in FIG.
The phase plate 5 includes a transparent first substrate 51, a polymer liquid crystal layer 52 that is a birefringent medium layer formed with a thickness distribution, a filling medium layer 54, and a second substrate 53. It is the composition made to do.

  The thickness of the polymer liquid crystal layer 52 can be formed in a desired distribution by photolithography and an etching process, and the thickness can be changed by providing predetermined unevenness on the first substrate 51. Here, the first and second substrates 51 and 53 can be made of glass or plastic.

Recess of the uneven portions of the polymer liquid crystal layer 52 is a birefringent medium layer is either equal to one value of the ordinary refractive index n _o and extraordinary refractive index n _e of the liquid crystal polymer layer 52, or the 2 It is filled with a filling medium layer 54 which is a filler having a refractive index between two refractive indexes. For this reason, a flat surface having no height difference is formed on the outer surface of the polymer liquid crystal layer 52.
Thus, the refractive index n of the filling medium layer 54, by selecting the refractive index values between the ordinary refractive index n _o and extraordinary refractive index n _e of the liquid crystal polymer layer 52, the transmitted light wave front Disturbance can be suppressed, which is preferable. In particular, the refractive index n of the packed medium layer 54 may either match the n _e or n _o, it is more preferable to adjust the average value of n _e and n _o.

Furthermore, In addition, as a method for manufactured create a retardation distribution can also be realized by making the in-plane distribution of the tilt angle of the liquid crystal polymer 52. Here, the tilt angle refers to an angle formed between the liquid crystal molecules and the substrate in the cross-sectional view of the phase plate 5 shown in FIG. For example, the birefringence amount Δn can be increased with respect to the transmitted light by making the tilt angle close to 90 degrees in the central portion of the element having a small retardation, that is, by bringing the liquid crystal molecules close to the substrate 51 perpendicularly. In addition, as a value of retardation, the magnitude | size which compensates the birefringence amount of the objective lens 6 is preferable, and 40 nm to 200 nm is preferable in the part with the largest retardation value of a peripheral part. Furthermore, 40 nm to 100 nm is preferable.

(II) On the other hand, as a method for controlling the slow axis β direction, when the polymer liquid crystal layer 52 is used, a method of rubbing an alignment film that determines the alignment direction of the liquid crystal in a desired direction (for example, concentric circles) A method of controlling the alignment direction using a material that photo-aligns the film can be used.
Further, the surface in contact with the polymer liquid crystal layer 52 of the first substrate 51 in FIG. 5, to create made minute irregularities groove 51A as shown in FIG. 6, easily aligned liquid crystal molecules in the longitudinal direction of the concavo-convex grooves 51A By using the properties, the orientation direction can also be controlled. By work made of such irregularities grooves 51A in many concentric circles can be achieved concentric slow axis distribution as shown in FIG. It is also possible to realize a radial slow axis distribution as shown in FIG. 3 by manufactured create radial plurality of grooves.

When the depth d (see FIG. 6) for controlling the orientation is increased, the light is scattered and diffracted to cause generation of stray light and a decrease in transmittance. In order to suppress the scattering and diffraction, refractive index of the substrate formed with the uneven groove n _a and a polymer liquid crystal of ordinary direction refractive index n. Or the difference between the extraordinary light direction refractive index n _e, the product of the depth d of the groove
| N _o -n _a | · d and | n _e -n _a | · d
Is preferably less than or equal to 1/10 of the wavelength λ of the light from the light source, in particular, less than or equal to 1/20, that is,
| N _o −n _a | · d ≦ (1/20) λ
and
| N _e −n _a | · d ≦ (1/20) λ
It is preferable that

The refractive index difference between the refractive index n _a and the polymer liquid crystal layer in the groove of the substrate, reflection at the interface between the polymer liquid crystal layer and the substrate, scattering, it is preferable small to diffraction becomes small. For example, 0.25 or less is preferable, and 0.2 or less is particularly preferable. That is,
| N _o −n _a | ≦ 0.2 and | n _e −n _a | ≦ 0.2
It is particularly preferred that

Further, the beam splitter 3 used in the optical head device 10 shown in FIG. 1 has a large number of lenses (here, the collimator lens 2 and the objective lens 6) when the polarization dependency of the reflectivity for guiding light to the photodetector 7 is large. Even when the refraction is small, the polarization state of the return light from the optical disk D changes due to the birefringence distribution in the optical disk D. In this case, since the amount of light reaching the photodetector 7 changes, the signal intensity changes and the reproduction characteristics are deteriorated.
Therefore, in such a case, the fluctuation of the return light quantity can be suppressed by using the phase plate 5 of the present invention. Further, in this case, it is more preferable that the retardation value of the phase plate 5 and the direction of the slow axis have no symmetry when rotated 180 degrees around the optical axis.

Next, an embodiment of the optical head device of the present invention will be specifically described with reference to the schematic diagram shown in FIG.
In this embodiment, a semiconductor laser that oscillates at a wavelength (λ = 405 nm) is used as the light source 1. The light emitted from the semiconductor laser passes through the collimator lens 2, the beam splitter 3, the quarter wavelength plate 4, the phase plate 5 in the present invention, and the objective lens 6, and is condensed on the optical disc D. On the other hand, the light reflected by the optical disk D passes through the objective lens 6, is reflected by the phase plate 5, the quarter wavelength plate 4, and the beam splitter 3, and is guided to the light detection system 7. Here, the objective lens 6 to be used is a plastic lens.

  Here, the semiconductor laser may be a semiconductor laser that emits light in a 650 nm band for DVD or a 780 nm band for CD, or a plurality of laser devices having different wavelengths may be used. The quarter wavelength plate 4 may not be used, or may be a wavelength plate having a phase difference other than the quarter wavelength plate 4. Further, the quarter wave plate 4 and the phase plate 5 of the present invention are preferable because they can be integrated to reduce the number of parts.

  The birefringence of the objective lens 6 has a distribution as shown in FIG. 11, almost no birefringence in the vicinity of the center, a retardation value of 80 nm in the periphery, and the slow axis β is distributed radially. . When the phase plate 5 of the present invention is not used, a polarization state distribution occurs in the light after passing through the lens, which adversely affects the shape of the condensed spot on the surface of the optical disc D, increases the spot diameter, Jitter deteriorates.

Next, the phase plate 5 according to the present invention shown in FIG. 5 will be specifically described.
The phase plate 5 has a polymer liquid crystal layer 52 formed of a birefringent medium (having a birefringent medium layer) with a desired thickness distribution on a transparent first glass substrate 51, on which A second substrate 53 is stacked on the other via a filling medium layer 54. Polymer liquid crystal layer 52, so that the desired thickness distribution, made created by photolithography and edge coating process. The refractive index of the filling medium layer 54 is 1.52, the ordinary refractive index _{n o} is 1.52 of the polymer liquid crystal layer 52, the extraordinary refractive index _{n e} is 1.57, the first, second glass The refractive indexes of the substrates 51 and 53 are 1.52.

Here, as shown in FIG. 5, the surface in contact with the polymer liquid crystal layer 52 of the first substrate 51, by manufactured create fine irregularities groove 51A as shown in FIG. 6, the longitudinal of the uneven grooves 51A The liquid crystal molecules are aligned using the property that the liquid crystal is easily aligned in the direction, and polymerized in that state. Note that by work made an uneven grooves 51A in many concentric circles can be achieved concentric slow axis distribution as shown in FIG.

In FIG. 6, a thin film of refractive index 1.47 is formed on the substrate 51, to create made an uneven groove 51A for controlling alignment of the liquid crystal molecules by the thin film off O preparative lithography over and edge coating techniques. This depth d is 80 nm. Refractive index _{n a} of the concave-convex groove 51A of the substrate is 1.47.

Refractive index difference between the refractive index n _a and the polymer liquid crystal layer in the groove of the substrate 51,
| N. −n _a | = | 1.52-1.47 |
= 0.05
and,
_{| N e -n a | = |} 1.57-1.47 |
= 0.1
And small enough.

Also, the product of the refractive index and n _a substrate of grooved portions, the depth d of the difference between the groove between the ordinary direction refractive index n _o and the extraordinary light direction refractive index n _e of the liquid crystal polymer
| N. -N _a | · d and | n _e -n _a | · d
Are sufficiently small, 4 nm (about (1/100) · λ) and 8 nm (about (1/50) · λ), respectively, and are small because reflection, scattering and diffraction at the interface are small.
The distribution of the retardation value in these elements is such that the amount of birefringence is small at the center of the optical axis of light and large at the periphery. In this embodiment, the center has a retardation value of 0 nm and the periphery is 80 nm.

  Therefore, by using the phase plate 5 of this embodiment, it is possible to actually cancel the birefringence of the lens, the polarization state distribution of the light after passing through the lens is corrected, and the condensed spot on the surface of the optical disc D The shape is improved and the jitter of the reproduced signal is also improved.

  In the present invention, at least one of the slow axis direction and the retardation value in the phase plate is not uniformly distributed in the plane of the phase plate. For example, the slow axis direction in the phase plate is a circle centering on the optical axis. Depending on the circumferential direction or the retardation value is different in the radial direction around the optical axis, the retardation of the objective lens and the like can be canceled to improve the recording and / or reproducing characteristics, and the optical recording of an optical disk or the like. It can be used as an optical head device for recording and / or reproducing a medium.

1 is a schematic diagram showing an example of an optical head device of the present invention. The schematic diagram which shows an example of the birefringence distribution of the phase plate in this invention. The schematic diagram which shows an example of the birefringence distribution of the phase plate in this invention. The schematic diagram which shows an example of the birefringence distribution of the phase plate in this invention. The typical sectional view showing an example of the phase board in the present invention. The schematic diagram which shows an example of the board | substrate shape used for the phase plate in this invention. The schematic diagram which shows another example of the phase plate in this invention from which a slow axis direction changes in a surface. The schematic diagram which shows the further another example of the phase plate in this invention from which a slow axis direction changes in a surface. FIG. 2 shows two birefringent layers combined to form a phase plate in which the slow axis direction is changed in the present invention, wherein (A) is an example of one birefringent layer whose slow axis intersects at 45 degrees. FIG. (B) is a schematic diagram showing an example of the other birefringent layer whose slow axis intersects at 45 degrees. The schematic diagram which shows an example of the birefringence distribution of a plastic objective lens. The schematic diagram which shows an example of the birefringence distribution of a plastic objective lens. The schematic diagram which shows an example of the wavefront distribution of the light which generate | occur | produces by the birefringence of a plastic objective lens. The schematic diagram which shows an example of the polarization state of the light after the lens which has a birefringence shown in FIG.

Explanation of symbols

1 Light source (semiconductor laser)
2 collimator lens 3 beam splitter 4 1/4 wavelength plate 5 phase plate 6 objective lens 7 photodetection system 50A, 50B birefringence synthetic layer 51, 53 substrate 51A uneven groove 52 birefringence medium layer (polymer liquid crystal layer)
54 Filling medium layer (filler)
D Optical disc (optical recording medium)
α Optical axis β Slow axis

Claims (8)

A light source;
An objective lens for condensing outgoing light emitted from the light source on an optical recording medium;
In an optical head device comprising a phase plate that is provided between the light source and the objective lens and changes a polarization state of light,
The objective lens is made of a material having birefringence,
The phase plate has a birefringent medium layer made of a birefringent medium, and a slow axis direction having a high refractive index in the plane coincides with a tangential direction or a radial direction of a concentric circle around the optical axis of the light Furthermore, the retardation value represented by the product of the birefringence amount Δn of the birefringent medium and the thickness of the birefringent medium layer varies with the distance from the optical axis,
The retardation value of the phase plate, the optical head device is characterized by having a distribution you cancel the distributions of retardation values of the objective lens. The optical head device according to claim 1, wherein the phase plate has a larger phase difference in a peripheral portion than a phase difference in the vicinity of the optical axis. The phase plate has a filling medium layer made of a filler,
The birefringent medium layer has irregularities that differ in thickness in the radial direction about the optical axis, and the filling medium layer is formed in the depressions of the irregularities,
Refractive index of the filler is the equal to any one of the values of the ordinary refractive index n _o and extraordinary refractive index n _e of the birefringent medium, or, with the ordinary refractive index n _o and extraordinary refractive index n _e The optical head device according to claim 1 , wherein the optical head device has a refractive index between. The birefringent medium is any one of 3 the liquid crystal from claim 1 is a polymer liquid crystal obtained by polymerizing
The optical head device according to Item. The phase plate has a transparent substrate having concentric or radial grooves formed around the optical axis,
The optical head device according to claim 4 , wherein the polymer liquid crystal is formed on the groove. The refractive index n _a of the transparent _substrate, the depth d of the groove, the polymer liquid crystal of the ordinary refractive index n _o, when the extraordinary refractive index of the polymer liquid crystal and n _e,
6. The optical head device according to claim 5 , wherein | n _o −n _a | · d and | n _e −n _a | · d are equal to or less than one-tenth of the wavelength of light from the light source. The phase plate, the optical head apparatus according to any one of claims 1 to 6 obtained by laminating at least two of said birefringent medium layer. 8. The optical head device according to claim 7 , wherein in the phase plate, the slow axis directions of one of the birefringent medium layers are the same, and the slow axes of the two birefringent medium layers form an angle of 45 degrees.

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