JP4797706B2 - Optical head device - Google Patents

Description

  The present invention relates to an optical head device in which a diffraction grating needs to be switched when recording / reproducing is performed on an optical recording medium (hereinafter referred to as an “optical disk”) such as a CD and a DVD.

  Conventional optical disks include a single-layer optical disk having a single information recording layer and a multi-layer optical disk having a plurality of layers. For example, when recording and reproducing information on a two-layer optical disc having two recording layers, as a tracking method, light from a light source is separated into three beams of 0th-order diffracted light and ± 1st-order diffracted light by a diffraction element, and ± A three-beam method or a working push-pull (DPP) method is adopted using the first-order diffracted light. At this time, the light quantity ratio of the separated light—first order: zero order: +1 order is, for example, 1:10 or more: 1, that is, the light quantity of the 0th-order diffracted light may be larger than the light quantity of ± 1st-order diffracted light. It is advantageous in terms of light utilization efficiency.

  In recording and reproduction, the light returning to the photodetector is affected not only by the layer positioned at the focal point of the objective lens but also by the adjacent layer. Since the interlayer spacing determined by the optical disc standard is determined by an interval at which interlayer crosstalk does not affect the information on the optical disc, the optical pickup is configured so that such interlayer crosstalk does not affect the servo signal. There is a need to. The term “recording / playback” used in this specification is a general term for recording, playback, recording, and playback on an optical disc.

  FIG. 7 shows a schematic diagram of an optical path when reproducing a two-layer optical disk in a conventional optical head device. As shown in FIG. 7, when the layer closer to the light incident surface of the two-layer optical disc is the L1 layer and the far layer is the L2 layer, the L2 layer reflects the light L11 received by the photodetector during reproduction of the L1 layer. The focused light L12 is positioned in front of the light L11. On the other hand, the light L21 reflected by the L1 layer is positioned behind the light L22 with respect to the light L22 received by the photodetector during reproduction of the L2 layer.

  Therefore, when the L1 layer is reproduced, the reflected light from the L1 layer is focused on the photodetector by the 0th order diffracted light and the ± 1st order diffracted light diffracted by the diffraction element, but this light is reflected from the L2 layer. Since the emitted light is not focused on the photodetector, it becomes stray light although the beam diameter is large and the light density is low.

  This stray light is sufficiently low on the photodetector with respect to the 0th-order diffracted light (main beam) from the L1 layer, but originally ± 1st-order diffracted light (sub-beam) whose light quantity is 1/10 or less than the main beam. The amount of light is not negligible. Therefore, the conventional optical head device has a problem that the tracking performance deteriorates due to the stray light.

  As a countermeasure, for example, an optical pickup as shown in Patent Document 1 has been proposed. In this method, a hologram element as shown in FIG. 8 is arranged in a light beam, and a part of reflected light from the optical disk is diffracted to remove stray light irradiated to the sub-beam photodetector.

JP 2005-203090 A

  However, in the case shown in Patent Document 1, not only the stray light from the L2 layer but also the light from the L1 layer from which information is originally read is diffracted by the hologram element, and the signal light intensity entering the photodetector is also increased. There was a problem of being lowered.

  The present invention has been made to solve the conventional problems, and an object of the present invention is to provide an optical head device capable of recording / reproducing a multi-layered optical disk without reducing the signal intensity to the photodetector. To do.

An optical head device according to the present invention includes a light source, a transmissive diffraction element, an objective lens that condenses light emitted from the diffraction element on an optical recording medium, and is collected by the objective lens and is collected by the optical recording medium. And an optical head device that detects and reflects reflected light, and records and reproduces information on and from the optical recording medium. The light emitted from the diffraction element includes zero-order diffracted light, + 1st-order diffracted light, and − The first-order diffracted light is separated into three beams, and the polarization state is different between the zero-order diffracted light and the other diffracted light. After the three beams are condensed and reflected on the optical recording medium, the photodetector The beam intensity of each of the three beams is individually detected by a plurality of light receiving areas arranged on the light receiving surface of the photodetector .

  With this configuration, the polarization state of the 0th-order diffracted light that is the main beam and the ± 1st-order diffracted light that is the sub-beam are different, so that the polarization state of the sub-beam of the reflected light from the recording layer to be read from the optical disk and the stray light from the recording layer The difference in polarization state of the main beam having a high intensity reduces the coherence between the two lights. As a result, it is possible to suppress a decrease in tracking performance due to a change in the intensity of the tracking signal caused by a change in the interference condition between the two lights due to a change in the layer interval or wavelength of the multilayer disc.

  In the optical head device of the present invention, the diffraction element is composed of a polarization diffraction grating having different diffraction efficiency depending on the polarization direction of incident light.

  With this configuration, it is possible to make the polarization states of the 0th-order diffracted light and the 1st-order diffracted light different because the diffraction efficiency of the diffractive element varies depending on the polarization state.

  Furthermore, in the optical head device of the invention, the polarization state of the light incident on the polarization diffraction grating has both a polarization direction component having a high diffraction efficiency and a polarization direction component having a low diffraction efficiency. It is the composition which is.

  With this configuration, the polarization component having a high diffraction efficiency of the polarization diffraction grating among the polarization components incident on the polarization diffraction grating can be set as the first-order diffracted light, and the polarization component having a low diffraction efficiency can be set as the 0th-order diffracted light. It is preferable because the polarization state of the first-order diffracted light can be clearly separated.

  Furthermore, the optical head device of the present invention acts on the diffracted light other than the 0th-order diffracted light in the optical path between the diffractive element and the optical recording medium, and the wavefront of the diffracted light other than the 0th-order diffracted light. A polarization phase difference generating element whose shape is changed is arranged.

  With this configuration, the following effects can be obtained. In general, the main beam is incident perpendicular to the objective lens, but the sub beam is incident obliquely. As a result, off-axis astigmatism of the objective lens occurs only in the sub-beam, and the tracking performance is degraded. Therefore, by arranging a polarization phase difference generating element in the optical path between the diffractive element and the optical recording medium, the in-plane distribution of the phase difference is generated only for the diffracted light such as the first-order diffracted light, and the wavefront shape of only the sub beam is changed. Can be made. In other words, it is preferable to generate astigmatism only in the diffracted light because the astigmatism of off-axis light of the objective lens can be corrected and the tracking performance is improved.

  The present invention can provide an optical head device having an effect that a multilayer optical disk can be recorded and reproduced without reducing the signal intensity to the photodetector.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 is a diagram showing a conceptual configuration of an optical head device 100 according to the present embodiment. In FIG. 1, an optical head device 100 includes a light source 1 that emits a light beam having a predetermined wavelength, and three beams having different polarization states between a main beam and two sub beams by diffracting a part of the light beam emitted by the light source 1. The diffractive element 2, the collimator lens 3 that converts the incident light beam into substantially parallel light, and the three beams emitted from the collimator lens 3 are transmitted and reflected back from the information recording surface 6 a of the optical disk 6. A beam splitter 4 that reflects the returned light of the three beams and guides it to the photodetector 8, an objective lens 5 that focuses the three beams on the information recording surface 6a of the optical disc 6, and a return of the three beams A collimator lens 7 that condenses the light on the photodetector 8 and a photodetector 8 that detects the return light of the three beams are provided.

  A part of the light beam emitted from the light source 1 is diffracted by the diffraction element 2 into three beams including a main beam and two sub beams, which are transmitted through the collimator lens 3, the beam splitter 4, and the objective lens 5 in this order. 6 is condensed on the information recording surface 6a. The above three beams condensed on the information recording surface 6a of the optical disc 6 are reflected by the information recording surface 6a, transmitted through the objective lens 5, and reflected by the beam splitter 4, and are detected from the collimator lens 7 by a photodetector. 8 is incident.

  Here, the output signal of the photodetector 8 is used to generate a read signal, a focus error signal, and a tracking error signal of information recorded on the information recording surface 6a of the optical disc 6. The optical head device 100 controls the lens in the optical axis direction based on the focus error signal (focus servo), and controls the lens in a direction substantially perpendicular to the optical axis based on the tracking error signal. 1 is omitted in the configuration diagram shown in FIG.

  The light source 1 is composed of, for example, a semiconductor laser, and emits a divergent light beam having a wavelength in the vicinity of 650 nm and linearly polarized light. In the above description, the light source 1 is configured to emit a light beam having a wavelength near 650 nm. However, the present invention is not necessarily limited to a configuration in which the light source 1 emits a light beam having a wavelength near 650 nm. For example, it may be configured to emit a light beam having a wavelength near 400 nm, a light beam having a wavelength near 780 nm, or the like, or may be configured to emit a light beam having another wavelength. Here, the wavelength near 400 nm, the wavelength near 650 nm, and the wavelength near 780 nm mean wavelengths in the range of 385 nm to 430 nm, 630 nm to 670 nm, and 760 nm to 800 nm, respectively.

  A so-called hybrid two-wavelength laser light source in which the light source 1 emits light beams having two or three wavelengths and two or three semiconductor laser chips are mounted on the same substrate in the same package. The light source 1 may be configured to form a three-wavelength laser light source. The light source 1 is a monolithic two-wavelength laser light source (for example, refer to Japanese Patent Application Laid-Open No. 2004-39898) having two light emitting points that emit light having different wavelengths, or a monolithic having three light emitting points. A three-wavelength laser light source may be used.

  FIG. 2 is a cross-sectional view schematically showing a polarization diffraction grating as a configuration example of the diffraction element 2 provided in the optical head device 100 according to the present embodiment.

  The light incident on the diffractive element 2 is diffracted and separated into three beams of 0th order diffracted light, + 1st order diffracted light, and −1st order diffracted light. The polarization state of the light is, for example, as shown in FIG. In the parallel direction, the ± first-order diffracted lights 24a and 24b are in a polarization state perpendicular to the paper surface.

  The diffractive element 2 shown in FIG. 2 fills a region sandwiched between a pair of transparent substrates 21, a birefringent medium 22 provided in a lattice shape on the transparent substrate 21, and the pair of transparent substrates 21, for example. The isotropic medium 23 is provided.

  The transparent substrate 21 is preferably composed of a transparent substrate having no birefringence, such as glass or plastic. The birefringent medium 22 is preferably a polymer liquid crystal obtained by polymerizing liquid crystal, for example, but may be composed of other single crystals. The isotropic medium 23 is preferably made of a resin having no birefringence, but may be made of glass, for example.

  In this configuration, the refractive index of the birefringent medium 22 is equal to the refractive index of the isotropic medium 23 for light polarized in parallel to the paper surface, and the refractive index in the polarization direction perpendicular to the paper surface is birefringent. The difference between the neutral medium 22 and the isotropic medium 23 is different by Δn. Here, the product Δn · d of the depth (height) d and Δn of the concavo-convex grating (hereinafter referred to as “concavo-convex grating portion”) composed of the birefringent medium 22 and the isotropic medium 23 is the wavelength λ used. On the other hand, it is preferable to set it to be approximately ½.

  In the diffraction element 2 configured as described above, the refractive index of the concavo-convex grating portion is equal to the light of the polarization component parallel to the paper surface, so that the light is transmitted almost without being diffracted and becomes zero-order diffracted light. . On the other hand, most of the light of the polarization component perpendicular to the paper surface is diffracted by the influence of the λ / 2 phase diffraction grating at the concave / convex grating portion. Therefore, the polarization states of the 0th-order diffracted light and the ± 1st-order diffracted light can be changed by entering light having an appropriate polarization state.

As described above, the polarization diffraction element has different diffraction efficiencies in the case of polarized light parallel to the polarization direction perpendicular to the paper surface. When the polarization direction of linearly polarized light incident on the polarization diffraction element is changed, the 0th-order diffraction efficiency (transmittance) and the first-order diffraction efficiency for the light in the polarization direction with the highest diffraction efficiency are η _0H and η _1H , respectively. A concavo-convex grating for light of high polarization efficiency and perpendicular to the paper surface when the zero-order diffraction efficiency (transmittance) and the first-order diffraction efficiency for light in the polarization direction with low diffraction efficiency are η _0L and η _1L , respectively. The first order diffraction efficiency η _1H of the diffraction grating having a phase difference of λ / 2 at the part is approximately 40%, and the 0th order diffraction efficiency η _0H is approximately 0%. On the other hand, for polarized light having a low diffraction efficiency and parallel to the paper surface, the phase difference is approximately 0 at the concave and convex grating portion, and the ± 1st-order diffraction efficiency η _1L of the diffraction grating is approximately 0%, and the 0th-order diffraction efficiency (transmittance) ) Η _0L is approximately 100%. That is, in calculation, η _1H / η _{1L to} ∞ and η _0L / η _{0H to} ∞, and the polarization dependency of the diffraction efficiency of the polarization diffraction element can be increased. This example is one example in which η _1H / η _1L and η _0L / η _0H can be _maximized .

  In addition, the diffraction efficiency of the diffraction element 2 illustrated in FIG. 2 is that the ± 1st-order diffraction efficiency of the polarization component perpendicular to the paper surface is approximately 40%, so that the polarization state of incident light is 80% of the component parallel to the paper surface. If the polarization is such that the vertical component is 20%, almost all the light of the polarization component parallel to the paper surface becomes 0th order diffracted light, and the amount of 0th order diffracted light becomes 80%. On the other hand, since the light of the polarization component perpendicular to the paper surface is diffracted with a first-order diffraction efficiency of 40%, the light amount of ± first-order diffracted light is 0.4 × 0.2 = 0.08 (8%). Therefore, the diffraction element 2 having a diffraction efficiency ratio of 80% / 8% = 10 between the 0th-order diffracted light and the 1st-order diffracted light can be realized.

  As a method for realizing the above-described incident polarized light, for example, obliquely polarized light whose polarization direction is not perpendicular to or parallel to the paper surface (in this example, polarized light tilted by about 27 degrees perpendicular to the paper surface) is incident. Alternatively, it can be realized by entering elliptically polarized light.

  For example, as a method for realizing oblique linearly polarized light, there is a method in which the semiconductor laser is rotated and installed so that the polarization direction of the semiconductor laser as the light source 1 becomes a predetermined angle. At this time, the polarization direction may change due to individual variations in the polarization direction of the semiconductor laser or temperature changes. When the polarization direction of incident light deviates from a predetermined angle, the diffraction efficiency ratio may change. As a countermeasure, it is preferable to arrange, for example, a polarizing beam splitter at a predetermined angle in the optical path between the light source 1 and the diffraction element 2. As the polarization beam splitter, a polarization beam splitter prism having different transmittance depending on the polarization direction, or an absorption polarizer having different transmittance depending on the polarization direction can be used. In the case of using the above-described polarizing beam splitter prism, it is preferable to use a diffractive polarizing beam splitter because the element size is relatively large and the prism outer shape and the transmission polarization direction cannot be freely selected.

  FIG. 3 shows a configuration example of the diffraction element 2 in which a diffractive polarizing beam splitter and a diffraction grating are integrated. By integrating in this way, the number of parts can be reduced as compared with the case where they are configured separately, and the polarization direction transmitted by the diffractive polarizing beam splitter and the polarization direction of light to be incident on the diffraction grating can be determined. It is preferable because the accuracy of the diffraction efficiency ratio can be increased because the alignment can be easily performed with high accuracy.

  The diffractive element 2 shown in FIG. 3 has a birefringence as a diffractive polarizing beam splitter in addition to the transparent substrate 21, the birefringent medium 22 and the isotropic medium 23 similar to those shown in FIG. A lattice made of the medium 27 is laminated. The refractive index of the birefringent medium 27 with respect to the polarization direction of light transmitted by the birefringent medium 27 (in this example, about 27 degrees in the vertical direction with respect to the paper surface) is made substantially equal to the isotropic medium 23 and orthogonal thereto. It is set so that a difference in refractive index occurs in the polarization direction. With this setting, light having a polarization of about 27 degrees can be transmitted, and light having a polarization direction orthogonal thereto can be diffracted so as to reduce the zero-order transmittance. As a result, the light transmitted through the birefringent medium 27 becomes linearly polarized light in the direction of approximately 27 degrees, which is preferable because the polarization direction incident on the diffraction grating can be stabilized. Here, it can be set so that light of an unnecessary polarization component diffracted by the birefringent medium 27 does not reach the objective lens 5.

  Furthermore, as shown in FIG. 4, the diffraction element 2 can be configured by laminating the polarization direction rotating means 28. By laminating the polarization direction rotating means 28, light having a desired polarization direction can be obtained without rotating the output polarization direction of the semiconductor laser constituting the light source 1. As the polarization direction rotating means 28, for example, a half-wave plate or a twisted polymer liquid crystal element can be used.

  Next, the light collection state on the light receiving surface of the photodetector 8 will be described with reference to FIG. FIG. 5 schematically shows an example of the light condensing state on the light receiving surface of the photodetector 8.

  In FIG. 5, the light receiving surface of the photodetector 8 has a plurality of light receiving areas 51, 52, and 53, and light from an optical disk recording layer to be recorded or reproduced in the light receiving areas is condensed spots 55, 56, and The state of forming 57 is shown. Here, the condensing spot 56 corresponds to 0th-order diffracted light (main beam) emitted from the diffraction element 2, and the condensing spots 55 and 57 correspond to ± 1st-order diffracted light (sub-beams). Further, the condensing spot 58 is a condensing spot of reflected light from the recording layer of the optical disc that becomes stray light, and is defocused on the light receiving surface of the photodetector 8 and has a large size as shown in FIG. Has a radius.

  The stray light condensing spot 58 is applied to the light receiving areas 51, 52, and 53 and interferes with the light of the condensing spots 55, 56, and 57 to generate noise. In particular, since the sub beam has a light amount as small as 1/10 or less as compared with the main beam as described above, conventionally, the influence of this interference of stray light is particularly large, leading to a decrease in tracking performance. Further, when the interlayer distance or wavelength of the optical disk fluctuates due to the influence of stray light interference, the interference condition of light from each layer changes, which is a particular problem.

  As described above, by using the diffractive element 2 according to the present embodiment, the polarization states of the 0th-order diffracted light and the 1st-order diffracted light can be made different. The stray light condensing spot 58 is also a superposition of the 0th-order diffracted light and the 1st-order diffracted light emitted from the diffraction element 2, but the 0th-order diffracted light is 10 times or more larger than the 1st-order diffracted light in terms of light quantity. Considering that the polarization state of the stray light is substantially equal to the polarization state of the 0th-order diffracted light, there is substantially no problem.

That is, by using the diffractive element 2 according to the present embodiment, the polarization state of the sub-beam condensing spots 55 and 57 and the polarization state of the stray light condensing spot 58 can be made different. Lights having different polarization states, because the interference decreases, the optical head apparatus 100 according to the present embodiment, it is possible to suppress the reduction of the tracking performance due to interference between the sub-beams and the stray light.

As described above, the polarization diffraction element can suppress a decrease in tracking performance due to different polarization states of the main beam and the sub beam. Therefore, the polarization of the main beam is dominated by the 0th-order transmitted light (corresponding to η _0L ) in the polarization direction where the diffraction efficiency of the polarization diffraction element is low, and the polarization of the sub beam is the primary in the polarization direction where the diffraction efficiency of the polarization diffraction element is high. The folding light (corresponding to η _1H ) is preferably dominant.

  Therefore, by setting the polarization dependency of the diffraction efficiency of the polarization diffraction element to the expressions (6) and (7), the polarization state of the main beam and the sub beam can be linearly polarized light, and the two light beams This is preferable because interference can be suppressed.

η _0L / η _0H ≧ 5 (6)

η _1H / η _1L ≧ 5 (7)

  More preferably, formulas (8) and (9) are preferable because the effect is further increased.

η _0L / η _0H ≧ 10 (8)

η _1H / η _1L ≧ 10 (9)

  In the above description, in order to simplify the explanation, the polarization directions of the 0th-order diffracted light and the 1st-order diffracted light are parallel and perpendicular to the paper surface as shown in FIG. 2, but the present invention is not limited to this. It is not something. For example, the zero-order diffracted light may be clockwise circularly polarized light, and the first-order diffracted light may be counterclockwise circularly polarized light, elliptically polarized light, or another polarization state.

  Next, in order to express the polarization states of the 0th-order diffracted light and the 1st-order diffracted light, description will be made using Stokes parameters. A detailed description of the Stokes parameters is given in, for example, “Applied Optics 2”, Chapter 5-3 “Polarization Notation” issued by Baifukan. Here, the Stokes parameters will be briefly described.

  When the light traveling in the z direction is selected in the (x, y) coordinate system in a plane perpendicular to the z direction, the light Ex and Ey of the x and y components are expressed by the following equations.

E _x = A _x · exp {i (ωt−k _z + δ _x )} (10)

E _y = A _y · exp {i (ωt−k _z + δy)} (11)

Here, ω is an angular frequency, k is a wave vector, δ _x and δ _y are light phases in the x and y directions, respectively, and A _x and A _y are electric field amplitudes in the x and y directions, respectively.

The polarization state can be expressed by the Stokes parameters (S ₀ , S ₁ , S ₂ , S ₃ ), which are four parameters.

_{^{S 0 = <A x 2> +}} <A y 2> (12)

_{^{S 1 = <A x 2> -}} <A y 2> (13)

_{S 2 = 2 <A x ·A y} ·cosδ> (14)

_{S 3 = 2 <A x ·A y} ·sinδ> (15)

Here, δ = δ _y −δ _x , and the symbol “<>” indicates an average value for a sufficiently long time.

S ₀ because parameters representing the light intensity, the reference Stokes parameters normalized by S _0, can represent a polarization state of light. That is, the standardized Stokes parameter is expressed as follows.

_{^{S 0k = {<A x 2>}} + <A y 2>} / {<A x 2> + <A y 2>} = 1 (16)

_{^{S 1k = {<A x 2>}} - <A y 2>} / {<A x 2> + <A y 2>} (17)

_{S 2k = 2 <A x ·A y} ·cosδ> / {<A x 2> + <A y 2>} (18)

_{S 3k = 2 <A x ·A y} ·sinδ> / {<A x 2> + <A y 2>} (19)

Hereinafter, the standardized Stokes parameters normalized as S ₀ = 1 are used.

When the polarization states of the 0th-order diffracted light and the 1st-order diffracted light are (1, S10, S20, S30) and (1, S11, S21, S31), respectively, the relationship shown by the equation ( 20 ) is 2 It is preferable because the coherence of light in one polarization state can be reduced to half or less.

^{_{(S 10 -S 11) 2 +}} (S 20 -S 21) 2 + (S 30 -S 31) 2 ≧ 2 (20)

Furthermore, the relationship shown in Formula ( 21 ) is preferable.

^{_{(S 10 -S 11) 2 +}} (S 20 -S 21) 2 + (S 30 -S 31) 2 ≧ 2.5 (21)

Furthermore, the relationship shown in the formula ( 22 ) is preferable because the coherence can be greatly suppressed.

^{_{(S 10 -S 11) 2 +}} (S 20 -S 21) 2 + (S 30 -S 31) 2 ≧ 3 (22)

  Furthermore, the relationship shown in the equation (23) is preferable because the coherence can be suppressed very small.

^{_{(S 10 -S 11) 2 +}} (S 20 -S 21) 2 + (S 30 -S 31) 2 = 4 (23)

  Most preferably, interference can be completely suppressed by using the relationship shown in the equation (24).

^{_{(S 10 -S 11) 2 +}} (S 20 -S 21) 2 = 4, S 30 = S 31 = 0 (24)

Specifically, for example, (1, S ₁₀ , S ₂₀ , S ₃₀ ) = (1, 1, 0, 0) and (1, S ₁₁ , S ₂₁ , S ₃₁ ) = (1, -1, 0, In the case of a combination of two polarizations in which linearly polarized light is orthogonal, such as (0), or (1, S ₁₀ , S ₂₀ , S ₃₀ ) = (1, 0, 0, 1) and (1, S ₁₁ , S ₂₁ , S ₃₁ ) = (1, 0, 0, −1).

  Next, an example in which a polarization phase difference generating element that changes the wavefront shape only for the polarized light beam of diffracted light is disposed in the optical path between the diffractive element 2 and the optical disc 6 will be described. The diffracted light does not include 0th-order diffracted light that is transmitted light.

  In general, the main beam is incident perpendicular to the objective lens, but the sub beam is incident obliquely. Therefore, off-axis astigmatism of the objective lens occurs only in the sub beam. The sub beam having astigmatism deteriorates the condensing characteristic on the optical disc, and degrades the tracking performance. As a countermeasure, the wavefront shape of only the sub-beam can be changed by acting only on the polarization of the sub-beam and generating an in-plane distribution of the phase difference. That is, it is preferable to generate astigmatism only in the diffracted light because the astigmatism of off-axis light of the objective lens can be corrected and the tracking performance can be improved.

  The wavefront shape to be changed includes spherical aberration, focus component, coma aberration, etc. in addition to astigmatism, and various aberrations can be generated. Will be described.

  FIG. 6A is a diagram schematically illustrating a configuration example of the polarization phase difference generating element according to the present embodiment.

  The polarization phase difference generating element shown in FIG. 6A is isotropic in which a birefringent medium 62 is provided in a lattice shape on a pair of transparent substrates 61 and is filled in a region sandwiched between the pair of transparent substrates 61. The structure is provided with a conductive medium 63. The transparent substrate 61 is preferably a transparent substrate having no birefringence, such as glass or plastic. The birefringent medium 62 is preferably a polymer liquid crystal obtained by polymerizing liquid crystal, but may be other single crystals. The isotropic medium 63 is preferably a resin having no birefringence, but may be glass, for example.

  In this configuration, when the polarization direction diffracted by the diffraction element 2 is perpendicular to the paper surface as shown in FIG. 2, the refractive index of the birefringent medium 62 and the isotropic medium for polarized light parallel to the paper surface The refractive index of 63 is made equal, and the refractive index in the polarization direction perpendicular to the paper surface is made to differ by Δn between the birefringent medium 62 and the isotropic medium 63. With this configuration, only the light in the polarization direction of the sub-beam corresponds to the shape of the birefringent medium 62, the wavefront shape of the transmitted light changes, and the wavefront shape does not change for the light of the main beam.

  For example, an astigmatism can be generated only in the sub-beam by using a cylindrical hologram diffraction grating as the concavo-convex grating composed of the birefringent medium 62 and the isotropic medium 63. For example, a phase step may be provided instead of the hologram diffraction grating.

  It is also possible to change the wavefront of the sub beam, which is the diffracted light of the diffraction grating, by using a hologram that curves the stripe direction of the diffraction grating instead of a linear grating. However, with this method, the wavefront shape of the ± first-order diffracted light emitted from the diffraction element 2 is reversed. For example, in the case of a hologram that acts as a convex lens for + 1st order diffracted light, the -1st order diffracted light acts as a concave lens. On the other hand, the configuration provided with the polarization phase difference generating element shown in FIG. 6A is preferable because the wavefront shape can be changed in the same manner with respect to ± first-order diffracted light.

  Furthermore, as shown in FIG. 6B, in order to prevent the deterioration of the DPP signal that occurs when the objective lens is displaced, the tracking step performance can be prevented from being degraded when the lens is displaced by generating a phase step 64 in the wavefront of the sub beam. Therefore, it is preferable.

  In addition, if the polarization direction that changes the wavefront of the polarization phase difference generating element and the polarization direction of the diffraction grating of the diffraction element 2 are shifted, the wavefront of the 0th-order diffracted light also slightly changes, so the two polarization directions are accurately matched. There is a need.

  The number of parts can be reduced by integrating the polarization phase difference generating element shown in FIGS. 6A and 6B with the diffraction element 2, and the polarization state incident on the polarization phase difference generating element can be reduced. Since it can match | combine accurately, it is preferable.

  As described above, according to the optical head device 100 according to the present embodiment, the outgoing light transmitted through the diffractive element 2 is separated into light beams including the 0th-order diffracted light and the ± 1st-order diffracted light, and the 0th-order diffracted light and other diffractions Since the polarization state is different from that of light, it is possible to improve the tracking performance at the time of recording / reproducing with respect to the multi-layer disc, and to record / reproduce the multi-layer optical disc without reducing the signal intensity to the photodetector 8. Can do.

  Further, the optical head device 100 according to the present embodiment further includes a polarization phase difference generating element that changes the wavefront shape with respect to the polarized light of the diffracted light, so that, for example, recording of optical disks having different track pitches Tracking can be performed with high accuracy during reproduction.

  As described above, the optical head device according to the present invention is useful as an optical head device having an effect that a multilayer optical disk can be recorded / reproduced without lowering the signal intensity to the photodetector.

The figure which shows the notional structure of the optical head apparatus which concerns on one embodiment of this invention. Sectional drawing which shows typically the structural example of the diffraction element of the optical head apparatus which concerns on one embodiment of this invention Sectional drawing which shows typically the structural example of the diffraction element of the optical head apparatus which concerns on one embodiment of this invention Sectional drawing which shows typically the structural example of the diffraction element of the optical head apparatus which concerns on one embodiment of this invention The schematic diagram which shows an example of the condensing spot which the photodetector of the optical head apparatus which concerns on one embodiment of this invention light-receives Sectional drawing which shows typically the structural example of the polarization phase difference generation element of the optical head apparatus which concerns on one embodiment of this invention Schematic diagram of the optical path when playing a double-layer optical disc Schematic diagram of a conventional hologram element that diffracts part of the reflected light from an optical disk

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light source 2 Diffraction element 3, 7 Collimator lens 4 Beam splitter 5 Objective lens 6 Optical disk 6a Information recording surface 8 Photo detector 21 Transparent substrate 22 Birefringent medium 23 Isotropic medium 24a + 1st order diffracted light 24b -1st order diffracted light 25 0 Next diffracted light 27 Birefringent medium 28 Polarization direction rotating means 51, 52, 53 Light receiving area 55 Main beam condensing spot 56, 57 Sub beam condensing spot 58 Condensing spot of reflected light that becomes stray light 61 Transparent substrate 62 Birefringence Medium 63 isotropic medium 64 phase difference 100 optical head device

Claims (4)

A light source, a transmissive diffractive element, an objective lens for condensing the light emitted from the diffractive element on an optical recording medium, and light for detecting the light collected by the objective lens and reflected by the optical recording medium In an optical head device comprising a detector and recording / reproducing information with respect to the optical recording medium,
The emitted light from the diffractive element is separated into three beams of 0th order diffracted light, + 1st order diffracted light and −1st order diffracted light, and the polarization state is different between the 0th order diffracted light and the other diffracted light,
The light detector individually collects the intensity of each of the three beams after being focused on the optical recording medium and reflected by a plurality of light receiving areas arranged on a light receiving surface of the light detector. An optical head device characterized in that the optical head device is detected.   The optical head device according to claim 1, wherein the diffraction element is configured by a polarization diffraction grating having a diffraction efficiency different depending on a polarization direction of incident light.   3. The optical head device according to claim 2, wherein the polarization state of light incident on the polarization diffraction grating has both a component in a polarization direction with a high diffraction efficiency and a component in a polarization direction with a low diffraction efficiency.   A polarization phase difference generating element that acts on diffracted light other than the 0th-order diffracted light and changes a wavefront shape of diffracted light other than the 0th-order diffracted light in an optical path between the diffractive element and the optical recording medium. 4. The optical head device according to claim 1, wherein the optical head device is disposed.

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