JP3667984B2 - Broadband polarization separation element and optical head using the broadband polarization separation element - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention uses a broadband polarization separation element having a polarization separation function for light of two or more different wavelengths, and two or more types of optical recording media (CD system, DVD) having different use wavelengths using the broadband polarization separation element. The present invention relates to an optical head capable of recording / reproducing information with respect to an optical disc).
[0002]
[Prior art]
An optical head for recording / reproducing information with respect to an optical recording medium such as an optical disk is known. However, in an optical head for an optical disk, light from an optical receiving system can be efficiently transmitted without returning an information signal from the optical disk substrate to the light source. As a means for guiding the light to the detector, a polarization beam splitter is used in combination with a quarter wavelength plate, and the light emitted from the light source and the reflected light from the optical disk are polarized and separated. However, the polarization beam splitter has a structure in which two prisms made of a crystal material having a large birefringence are combined, or a dielectric multilayer on the joint surface (reflection surface) of two prisms made of an isotropic optical medium such as glass. Since it is composed of a structure provided with a film, it has the disadvantages of being large and expensive, and it is difficult to reduce the size and cost of the optical head.
[0003]
In view of this, a birefringent diffraction grating type polarizing plate has been proposed as an extremely thin polarization separation element that eliminates the disadvantages of the conventional polarization separation element (Japanese Patent Laid-Open No. 63-314502). This birefringent diffraction grating type polarizing plate is composed of lithium niobate (LiNbO) which is a birefringent optical crystal._Three ) As a substrate, and has a structure in which proton ions are exchanged in a periodic pattern and a dielectric film is loaded on the proton ion exchange region, and the phase difference of ordinary rays in the proton ion exchange region is determined as a dielectric film. By canceling out, the ordinary ray travels straight and has a function of diffracting only the extraordinary ray, and a thin and small polarization separation element can be realized.
[0004]
[Problems to be solved by the invention]
According to the above prior art, a thin and small polarization separation element having a function of separating ordinary light and extraordinary light can be realized, and a function of polarization separation can be provided for one specific wavelength. It cannot function effectively for the purpose of polarization separation with respect to the wavelength.
For this reason, it is not possible to meet the purpose of recording / reproducing information with respect to two or more types of optical disks (CD system, DVD system, etc.) having different operating wavelengths with a single optical disk head. .
[0005]
The present invention has been made in view of the above circumstances, and it is a first object (problem) to provide a broadband polarization separation element having a good extinction ratio (polarization separation degree) with respect to two or more different wavelengths. And
Further, the present invention provides an optical head for a two-wavelength-mounted optical disc that can record and reproduce information on two or more types of optical discs having a simple configuration and different operating wavelengths, using the broadband polarization separation element. Providing is the second object (problem).
[0006]
[Means for Solving the Problems]
  To achieve the above objective,According to the present inventionThe broadband polarization separation element is a grating-type broadband polarization separation element that separates orthogonally polarized light of incident light into zero-order light and diffracted light by a periodic grating in which birefringent regions and isotropic regions are alternately arranged. Yes, and has the function of polarization separation for light of two or more wavelengths that are different from each otherdoing.
[0007]
  In addition to the above configuration, the wideband polarization separation element according to claim 1 has two wavelengths λ for polarization separation.₁ , Λ₂ Againstλ ₁ And λ ₂ Near the middle ofWavelength λ₃ In contrast, the refractive index for the polarization in the grating vector direction in the birefringence region is n_p , And the refractive index for the polarization in the direction perpendicular to this_s And the refractive index of the isotropic region is n₁ , The depth of the birefringent region is h, the wavelength of light is λ₃ , Where m is a positive / negative natural number including 0 (m = 0, ± 1, ± 2,...)
  (N_p-N₁) H = mλ₃
  (N_s-N₁) H = (m + 1/2) λ₃
Is substantially satisfied.
[0008]
  In addition to the above configuration, the broadband polarization separating element according to claim 2 has two wavelengths λ for polarization separation.₁ , Λ₂ Againstλ ₁ And λ ₂ Near the middle ofWavelength λ₃ In contrast, the refractive index for the polarization in the grating vector direction in the birefringence region is n_p , And the refractive index for the polarization in the direction perpendicular to this_s And the refractive index of the isotropic region is n₁ , The depth of the birefringent region is h, the wavelength of light is λ₃ , Where m is a positive / negative natural number including 0 (m = 0, ± 1, ± 2,...)
  (N_p-N₁) H = (m + 1/2) λ₃
  (N_s-N₁) H = mλ₃
Is substantially satisfied.
[0009]
  Claim3The optical head according to claim 1, wherein a plurality of light sources having different wavelengths, an objective lens disposed between the plurality of light sources and an optical recording medium, and the plurality of light sources and the objective lens are disposed.1 or 2And a quarter-wave plate disposed between the broadband polarization separation element and the objective lens, and a photodetector for detecting diffracted light by the broadband polarization separation element, at least the objective lens Condenses light of different wavelengths from a plurality of light sources on different optical recording medium surfaces, and diffracts and separates the reflected light from the optical recording medium surface for each wavelength by the broadband polarization separation element to detect light for each wavelength. It is characterized by being detected independently by a vessel.
[0010]
  Claim4An optical head according to claim3In addition to the above structure, a ¼ wavelength plate is integrated with the broadband polarization separation element.
[0011]
  Claim5An optical head according to claim3 or 4In addition to the above configuration, at least a plurality of light sources and a plurality of photodetectors are mounted in one package, and the broadband polarization separation element or the quarter wave plate integrated with the package in the package Are integrated by adhesion.
[0012]
  Claim6An optical head according to claim3 or 4 or 5In addition to the configuration described above, the emission surfaces of a plurality of light sources having different wavelengths are arranged so as to be shifted from each other in the optical axis direction of the optical head optical system.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The configuration, operation, and operation of a broadband polarization separation element according to the present invention and an optical head using the broadband polarization separation element will be described in detail below based on the illustrated embodiments.
[0014]
  Example 1
  FirstBasic configuration and operation of broadband polarization separation elementExamples will be described. Figure 1According to the inventionIt is a fragmentary sectional view showing an example of composition of a broadband polarization separation element. In FIG. 1, a polarization separating element 1 is loaded with a birefringent film 3 having a periodic concavo-convex lattice structure on a transparent substrate 2 such as glass or plastic, and is covered with an isotropic overcoat layer 4. It becomes the composition. The birefringent film 3 is a film exhibiting birefringence having different refractive indexes for light oscillating in the direction of the paper in FIG. Next, the operation of this polarization separation element 1 is shown in FIGS.
[0015]
FIG. 2 is a diagram showing an example of the operation of the polarization separation element shown in FIG. 1, and the incident light to the polarization separation element 1 has two wavelengths λ.₁ , Λ₂ It is assumed that the polarized light has vibration components in two directions, that is, the direction of the paper and the direction of polarization perpendicular thereto. In FIG. 2, after passing through the polarization beam splitter 1, the vibration component light in the paper direction travels straight as zero-order light for both wavelengths, and the vibration component light in the vertical direction diffracts as ± first-order light. Where the wavelength λ₁ <Λ₂ Then, for both the + 1st order light and the −1st order light, λ₁ From λ₂ The diffraction angle of light increases. As a result, the two wavelengths λ₁ , Λ₂ The direction of the light changes, and operates as a polarization separation element for two wavelengths.
[0016]
Next, FIG. 3 is a diagram showing another embodiment of the operation of the polarization separation element shown in FIG. 1. In contrast to FIG. 2, light having a vibration component in the polarization direction perpendicular to the paper surface is λ.₁ , Λ₂ The two wavelengths of light travel straight as 0th order light, and light having a vibration component in the direction of the paper surface is diffracted as ± 1st order light.₁ , Λ₂ The light is polarized and separated, resulting in a polarization separation element having a high extinction ratio (polarization separation degree).
[0017]
Note that the polarization separation element having the configuration shown in FIG.₁ , Λ₂ In addition to the high extinction ratio, λ₁ <Λ <λ₂ It operates as a broadband polarization separation element having a high extinction ratio with respect to the wavelength λ satisfying the above condition.
[0018]
As the birefringent film 3 used in the polarization separation element 1 having the configuration shown in FIG.₂O_FiveBirefringence obtained by depositing polydiacetylene on a transparent substrate 2 obliquely deposited on the transparent substrate 2 or by aligning polydiacetylene on the alignment film formed on the transparent substrate 2 and then irradiating it with ultraviolet light. A film or a film obtained by forming a polyimide film on the transparent substrate 2 and stretching it in one direction while heating the polyimide film can be used. Then, these birefringent films are etched to form an uneven periodic grating.
[0019]
The isotropic overcoat layer 4 is loaded using the same material as that of the birefringent film so as to be isotropic. That is, Ta₂O_FiveThen, it is deposited from the vertical direction on the substrate, not diagonally. If polydiacetylene is used, it is loaded without an alignment film. In the case of polyimide, it is not heated and only heated to form an imide film. Moreover, it is good also considering the resin which adjusted the refractive index as an overcoat layer.
[0020]
Incidentally, the birefringent film 3 constituting the polarization separating element is not limited to a film-like one, but as in the embodiment shown in FIG._Three Even if a birefringent crystal 3 ′ such as calcite or quartz is used and this birefringent crystal 3 ′ is etched to form a periodic concavo-convex lattice, an isotropic overcoat layer 4 is loaded thereon. good.
Alternatively, as in the embodiment shown in FIG. 6, after forming a periodic concavo-convex grating by the birefringent film 3 on the substrate 2, another glass substrate 8 is placed on the isotropic resin adhesive (isotropic bonding). The adhesive 7 may be bonded, and the adhesive 7 may serve as an overcoat layer.
[0021]
Further, in the above embodiment, the birefringent medium is described as a periodic concavo-convex shape with a lattice having a structure in which an isotropic material is overcoated thereon. However, the present invention is not limited to this and is shown in the prior art. Such as LiNbO_Three The present invention can also be applied to a lattice in which a birefringent crystal such as is used as a substrate and subjected to ion exchange treatment in a periodic pattern to form a periodic lattice structure.
[0022]
  (Example 2)
  Then claims1Examples will be described. In the present embodiment, the operation of the polarization separation element is analyzed in detail, and the optimum condition for the operation of the polarization separation element shown in FIG. 2 is obtained. The optimum condition here is 2 wavelengths λ₁ , Λ₂ It is a condition with a good extinction ratio for both.
  FIG. 4 is a cross-sectional view showing a main part of the polarization separation element 1 shown in FIGS. In FIG. 4, the periodic grating has an uneven structure in which the birefringent films 3 are regularly arranged on the transparent substrate 2 with the period d, and the depth of the uneven shape (birefringent region) is h.
[0023]
Here, when considering the optimum condition, the third wavelength λ_Three think of. This third wavelength λ_Three Is λ₁ <Λ_Three <Λ₂ Is a wavelength that satisfies₁ And λ₂ A wavelength in the vicinity of the middle is preferable.
This wavelength λ_Three The refractive index of the periodic grating of the birefringent film 3 in FIG._p , N is the refractive index for polarized light (for example, s-polarized light) perpendicular to the paper surface._s And the refractive index of the isotropic overcoat layer 4 is n₁ For example, in FIG. 4, the wavelength λ between the optical paths A and B in FIG._Three The optical path length difference Δ with respect to
Paper direction: Δ_p= (N_p-N₁) H (1)
Perpendicular to paper: Δ_s= (N_s-N₁) H (2)
It becomes. Hereinafter, the paper surface direction is referred to as a lattice vector direction (shown in FIG. 1).
[0024]
As shown in FIG. 2, in order for the vibration component in the lattice vector direction to go straight as zero-order light, and for the vibration component perpendicular to the lattice vector direction to be efficiently diffracted as ± first-order light, the following two expressions are substantially satisfied. It is necessary. That is, the wavelength of light is λ_Three , M is a positive / negative natural number including 0 (m = 0, ± 1, ± 2,...)
(N_p-N₁) H = mλ_Three                ... (3)
(N_s-N₁) H = (m + 1/2) λ_Three    ... (4)
It is.
[0025]
λ₁ And λ₂ When the two wavelengths of light are polarized and separated into 0th order light and 1st order light, it is necessary to satisfy the expressions (3) and (4) in order to increase the degree of polarization separation (extinction ratio).
However, in practice, the refractive index n of the birefringent film 3 is such that the conditions in the vicinity are satisfied even if the expressions (3) and (4) are not strictly satisfied._p , N_s The refractive index n of the overcoat layer 4₁ , The irregularity depth h and the order m of the periodic grating are set.
In addition, the lattice depth satisfying equations (3) and (4) is h.₁ Then, from (4)-(3),
(N_s-N_p) H₁= Λ_Three/ 2 ... (5)
∴h₁= Λ_Three/ {2 (n_s-N_p)} (6)
It becomes.
[0026]
Note that the depth of the ion exchange region is h as described above for a lattice formed by ion exchange of a birefringent crystal as in the prior art.
Needless to say, even if the expressions (3) and (4) are not strictly satisfied, they may be satisfied substantially in the vicinity.
[0027]
Next, specific examples of the present invention will be described. In this embodiment, the birefringent film 3 is Ta₂O_FiveAs an overcoat layer, Ta₂O_FiveThis is the case of using the vertical vapor deposition film.
Λ₁= 635 nm, λ₂A polarization separation element having a good extinction ratio with respect to two wavelengths of light of 780 nm and a third wavelength λ_Three As λ₁ And λ₂ Choose an almost intermediate wavelength of λ_Three= 710 nm.
This wavelength λ_Three In the oblique deposition Ta which is the birefringent film 3₂O_FiveRefractive index n in the film lattice vector direction (paper surface direction)_p Is n_p= 1.944, refractive index n perpendicular to the plane of the paper (lattice vector direction)_s Is n_s= 2.023. Ta₂O_FiveΛ of overcoat layer 4_Three Refractive index n₁ Is n₁= 1.944.
[0028]
FIG. 7 shows the relationship between the diffraction efficiency of such 0th-order light and 1st-order light (one side) and the grating depth h (the figure shows the case where m = 0 in the equations (3) and (4)).
FIG. 7 shows the values of the diffraction efficiency of zero-order light with respect to p-polarization in the grating vector direction and the diffraction efficiency of s-polarization first-order light orthogonal to this when the widths of the concave and convex portions of the grating are equal. Plotted for 635 nm and 780 nm.
Optimal grating depth h derived from Eqs. (3) and (4), which is a condition for increasing the extinction ratio for both wavelengths.₁ Is h in this embodiment.₁= 4.49 μm. As can be seen from FIG. 7, the p-polarized zero-order light has a diffraction efficiency of 100% for both wavelengths, and the s-polarized first-order light has a wavelength of 635 nm of 39% and 780 nm of 40%. Also, the efficiency is high. Since the highest diffraction efficiency of the primary light of the rectangular grating as in the present invention is 40.5%, both the two wavelengths have high efficiencies almost close to the highest diffraction efficiency.
[0029]
As described above, according to the present embodiment, by forming the grating so as to satisfy the conditions of the expressions (3) and (4), the p-polarized light is zero-order light and s-polarized light at both wavelengths of 635 nm and 780 nm. A wave is polarized and separated into ± first-order light, and a broadband polarization separation element having an extremely high extinction ratio (polarization separation degree) is realized.
[0030]
  (Example 3)
  Next claim2Examples will be described. In this embodiment, an optimum condition is obtained when the polarization separation element 1 operates as shown in FIG.
  Also in this embodiment, the two wavelengths λ of interest₁ , Λ₂ For λ₁ <Λ₃ <Λ₂ The third wavelength λ that satisfies₃ In contrast, the refractive index of the birefringent film 3 with respect to the polarization (for example, p-polarized light) in the lattice vector direction (paper surface direction) is expressed as n._p , And the refractive index for polarized light in the direction perpendicular to this (for example, s-polarized light) is n_s And the refractive index of the isotropic overcoat layer 4 is n₁ , H is the uneven depth of the periodic grating, and λ is the wavelength of light.₃ , Where m is a positive / negative natural number including 0 (m = 0, ± 1, ± 2,...), The following equations (7) and (8):
  (N_p-N₁) H = (m + 1/2) λ₃    ... (7)
  (N_s-N₁) H = mλ₃                ... (8)
Is substantially satisfied by the operation shown in FIG. In addition, in order to actually satisfy the expressions (7) and (8), n_p, N_s, N₁, H, m are set.
  For details, the diffraction conditions are reversed for the p-polarized wave and the s-polarized wave.1).
[0031]
  Example 4
  Next claim3Examples will be described. The present embodiment is an embodiment in which the broadband polarization separation element of the present invention described in Embodiments 1 to 3 is applied to an optical head for an optical disk, and FIG. 8 shows an example of an optical head using the broadband polarization separation element. It is a schematic block diagram.
  As shown in FIG. 8, this optical head has a configuration in which two light sources having different wavelengths are used and an optical system is used in common, and two or more optical recording media having different operating wavelengths (optical discs such as CD and DVD). ) Is an optical head capable of recording / reproducing information. This is a CD-R (CD-Recordable) which is a write-once CD that is premised on recording / playback at a wavelength λ = 780 nm in the current optical disk system, and a DVD-R of a write-once DVD that performs recording / playback at a wavelength λ = 635 nm. This is the case when two wavelengths are used interchangeably with one optical head.
[0032]
In FIG. 8, the two light sources are composed of semiconductor lasers 11 and 11 '.₁= 635 nm oscillation wavelength, the other semiconductor laser 11 ′ is λ₂= Oscillation wavelength of 780 nm. In the optical path from the light source 11, 11 ′ of the optical head to the optical disk 14 (or 14 ′), the broadband polarization separation element 19 according to any one of Embodiments 1 to 3 (Claims 1 to 3) and ¼. A wave plate 20, a collimating lens 16, and an objective lens 15 are disposed. The grating vector direction of the periodic grating in the polarization separating element 19 coincides with the paper surface direction of FIG. When the oscillation direction of the emitted light of the semiconductor lasers 11 and 11 ′ is the paper surface direction, it is desirable that the polarization separation element substantially satisfies the above-described equations (3) and (4), and the vibration direction is perpendicular to the paper surface. In some cases, it is desirable to satisfy the expressions (7) and (8).
[0033]
Under these conditions, light emitted from the semiconductor laser 11 (or 11 ') passes through the polarization separating element 19 as zero-order light with almost no loss. The light beam that has passed through the polarization separation element 19 becomes circularly polarized light by the quarter-wave plate, becomes parallel light by the collimating lens 16, and is condensed on the recording surface on the optical disk by the objective lens 15. Here, the beam from the semiconductor laser 11 is focused on a thin substrate optical disk 14 such as a DVD, and the beam from the semiconductor laser 11 'is focused on a thick substrate optical disk 14' such as a CD.
[0034]
The light condensed on the recording surface of the optical disk 14 (or 14 ′) is reflected by the recording surface, and the reflected light returns to the quarter-wave plate 20 through the objective lens 15 and the collimator lens 16, and the quarter wavelength. After passing through the plate 20, the circularly polarized light is converted into linearly polarized light having a vibration direction orthogonal to the vibration direction when emitting the semiconductor laser. Then, the vibration plane orthogonal to the time of emission is almost diffracted as ± first-order light by the polarization separation element 19.
[0035]
In FIG. 8, the short wavelength light from the semiconductor laser 11 indicates the optical path with a solid line, and the long wavelength light from the semiconductor laser 11 ′ indicates the optical path with a dotted line. Each light has a very different optical path. Λ of short wavelength₁= Λ longer wavelength for 635 nm light₂= Because the diffraction angle for light of 780 nm is large, λ₂ On the outside, λ₁ Is diffracted inside. So short wavelength λ inside₁ Optical detectors 12 and 13 for the long wavelength λ₂ The light detectors 12 'and 13' are arranged for detecting each wavelength light. Detection by the optical detectors 12 and 13 (or 12 ′ and 13 ′) is performed by using focus error signals and tracking error signals for focus servo and tracking servo, in addition to information signals recorded on the optical disk 14 (or 14 ′). Is detected.
[0036]
  The polarization separation element 19 used in the optical head of FIG.1(Example 2) or claims2As in Example 3, λ₁ <Λ₃ <Λ₂ The third wavelength λ that satisfies₃ If it is set so that the expressions (3), (4) or (7), (8) are substantially satisfied, λ₁ , Λ₂ Therefore, the two-wavelength optical head can be provided which is most suitable for recording / reproducing in the DVD system and the CD system.
  Therefore, according to the fourth aspect of the present invention, recording and reproduction can be performed with a single optical head for a plurality of optical disks 14 and 14 'having different corresponding wavelengths, which can contribute to downsizing and cost reduction of the optical disk drive.
[0037]
  (Example 5)
  Next claim4Examples will be described. In this embodiment, in the optical head having the configuration shown in FIG. 8, a quarter wavelength plate 20 is integrated with the broadband polarization separation element 19 to form a polarization separation element 17 with a quarter wavelength plate. Specifically, a quarter wavelength plate is adhered and integrated on the isotropic overcoat layer 4 of the polarization separation element having the configuration shown in FIG. 1 or FIG. 5, or the polarization having the configuration shown in FIG. A quarter wavelength plate is bonded and integrated on the glass substrate 8 of the separation element to obtain a polarization separation element 17 with a quarter wavelength plate. As another example, an anisotropic film is formed on the isotropic overcoat layer 4 of the polarization separation element shown in FIG. 1 or FIG. 5 or on the glass substrate 8 of the polarization separation element having the configuration shown in FIG. It is also possible to load and form the used 1/4 wavelength film.
[0038]
When the polarization separation element according to the present invention is used in an optical head, a combination with a quarter-wave plate is essential as shown in FIG. In this case, the polarization separation element and the quarter wavelength plate may be separately formed and arranged. However, the quarter wavelength plate 20 is integrated with the polarization separation element 19 as in the above-described embodiment. By using the polarizing plate-equipped polarization separation element 17, the number of parts of the optical head can be reduced, and the optical head can be made compact. In addition, when a quarter wavelength film is loaded and integrated on the polarization separation element, it is not necessary to use an expensive optical crystal as a quarter wavelength plate, which can contribute to cost reduction. .
[0039]
  (Example 6)
  Next claim5Examples will be described. FIG. 9 claims5FIG. 9 is a schematic configuration diagram illustrating an example of the optical head described, in which the basic configuration and operation are the same as those of the two-wavelength compatible optical head in FIG.₁ Semiconductor laser 11 and corresponding photodetectors 12, 13 and wavelength λ₂ The semiconductor laser 11 ′ and the corresponding photodetectors 12 ′ and 13 ′ are integrally mounted on one package 8, and the quarter wavelength plate 20 and the broadband polarization separating element 19 are integrated on the upper surface of the package 8. The polarized light separating element 17 with a quarter wave plate is bonded and integrated.
[0040]
With the configuration of this embodiment, the optical head using two wavelengths has a simple configuration, the number of assembling adjustment points is reduced, the process is simplified, and a small and low-cost two-wavelength compatible optical head can be realized.
Further, since the main parts such as the semiconductor lasers 11 and 11 ′, the photodetectors 12, 13, 12 ′ and 13 ′, and the polarization separation element 17 with a quarter wavelength plate are integrated in one package 8, The stability of the optical system increases with changes in temperature.
[0041]
  (Example 7)
  Next claim6Examples will be described. FIG. 10 claims6FIG. 10 is a schematic configuration diagram of an essential part showing an example of the described optical head, and is an enlarged view of a package portion of the optical head shown in FIG. 9. Example 4 (claims)3To Example 6 (claims)52), the two semiconductor lasers 11 and 11 ′ are arranged apart from each other in the direction perpendicular to the optical axis of the optical head optical system. However, as shown in FIG. In the direction, they are shifted from each other by a distance ΔZ.
  That is, as shown in FIG. 8 and FIG. 9, there are two semiconductor lasers as shown in FIG. 10 because there are deviations in the condensing position in the optical axis direction in the two types of optical disks 14 and 14 ′ having different substrate thicknesses. 11 and 11 'are shifted from each other by a distance ΔZ in the optical axis direction, so that the condensing position can be adjusted in accordance with the two types of optical disks 14 and 14', without burdening the focus servo system of the objective lens. The same optical system can be used to provide good light condensing performance for the two types of optical disks 14 and 14 'having different substrate thicknesses.
[0042]
In the optical heads having the configurations shown in the fourth to seventh embodiments, a plurality of separated photodetectors 12, 13, 12 ′ and 13 ′ are used as photodetectors. However, the present invention is not limited to this. Instead, the photodetector may be one photodetector in which a plurality of detection regions are formed on one Si substrate.
[0043]
【The invention's effect】
  As explained above,BookIn the invention, in a grating-type broadband polarization separation element that separates orthogonally polarized light of incident light into zero-order light and diffracted light by a periodic grating in which birefringent regions and isotropic regions are alternately arranged, at least mutually Has a function to separate polarized light for two or more different wavelengthsRuThus, light having two or more different wavelengths can be polarized and separated into zero-order light and diffracted light, and a broadband polarization separation element can be realized.
[0044]
  According to the first aspect of the present invention, in the broadband polarization separation element, the two wavelengths λ for polarization separation are provided.₁ , Λ₂ Againstλ ₁ And λ ₂ Near the middle ofWavelength λ₃ In contrast, the refractive index for the polarization in the grating vector direction in the birefringence region is n_p , And the refractive index for the polarization in the direction perpendicular to this_s And the refractive index of the isotropic region is n₁ , The depth of the birefringent region is h, the wavelength of light is λ₃ , Where m is a positive / negative natural number including 0 (m = 0, ± 1, ± 2,...)
  (N_p-N₁) H = mλ₃
  (N_s-N₁) H = (m + 1/2) λ₃
Therefore, the wavelength λ₁ To λ₂ Wavelength λ between₃ The maximum extinction ratio (polarization separation degree) can be given to the light. Further, according to the above conditions, the zero-order light is separated into the polarization in the grating vector direction, and the first-order diffracted light is separated into the polarization in the direction perpendicular thereto, so that the extinction ratio is the highest.
[0045]
  Further, in the invention according to claim 2, in the broadband polarization separation element, two wavelengths λ for polarization separation are provided.₁ , Λ₂ Againstλ ₁ And λ ₂ Near the middle ofWavelength λ₃ In contrast, the refractive index for the polarization in the grating vector direction in the birefringence region is n_p , And the refractive index for the polarization in the direction perpendicular to this_s And the refractive index of the isotropic region is n₁ , The depth of the birefringent region is h, the wavelength of light is λ₃ , Where m is a positive / negative natural number including 0 (m = 0, ± 1, ± 2,...)
  (N_p-N₁) H = (m + 1/2) λ₃
  (N_s-N₁) H = mλ₃
Therefore, the wavelength λ₁ To λ₂ Wavelength λ between₃ The maximum extinction ratio (polarization separation degree) can be given to the light. Also, according to the above conditions, contrary to claim 1, the maximum extinction ratio is given when the zero-order light is polarized in the direction perpendicular to the grating vector and the first-order diffracted light is polarized in the grating vector direction. .
[0046]
  Claim3The optical head according to the invention described above is provided with a plurality of light sources having different wavelengths, an objective lens disposed between the plurality of light sources and the optical recording medium, and disposed between the plurality of light sources and the objective lens.1 or 2And a quarter-wave plate disposed between the broadband polarization separation element and the objective lens, and a photodetector for detecting diffracted light by the broadband polarization separation element, at least the objective lens Condenses light of different wavelengths from a plurality of light sources on different optical recording medium surfaces, and diffracts and separates the reflected light from the optical recording medium surface for each wavelength by the broadband polarization separation element to detect light for each wavelength. Since it can be detected independently by a detector, recording and reproduction can be performed with a single optical head for a plurality of optical recording media with different supported wavelengths, and the number of components is greatly reduced, and two wavelengths with a simple configuration can be obtained. A corresponding optical head can be realized, which can contribute to downsizing and cost reduction of an optical disk drive or the like.
[0047]
  Claim4In the invention described in claim3In the optical head described in (4), the quarter-wave plate is integrated with the broadband polarization separation element, so that the number of parts of the optical head can be reduced and the optical head can be made compact. it can.
[0048]
  Claim5In the invention described in claim3 or 4In the optical head described in (1), at least a plurality of light sources and a plurality of photodetectors are mounted in one package, and a broadband polarization separation element or a quarter-wave plate integrated with the package is integrated into the package. However, it is characterized by being integrated by bonding, so the optical head using two wavelengths has a simple configuration, the number of assembly adjustment points is reduced, the process is simplified, and it is compact and low cost for two wavelengths. An optical head can be realized.
[0049]
  Claim6In the invention described in claim3 or 4 or5. The optical head according to 5, wherein the emission surfaces of a plurality of light sources having different wavelengths are shifted from each other in the optical axis direction of the optical head optical system. The focusing position can be adjusted according to the thickness, and it has good focusing performance for two types of optical recording media with different substrate thicknesses using the same optical system without burdening the focus servo system of the objective lens. Can be made.
[Brief description of the drawings]
FIG. 1 is a partial cross-sectional view showing a configuration example of a broadband polarization separation element according to the present invention.
FIG. 2 is a diagram showing an example of the operation of the broadband polarization separation element shown in FIG.
FIG. 3 is a diagram showing another example of the operation of the broadband polarization separation element shown in FIG. 1;
4 is a cross-sectional view of a main part of a main part of the broadband polarization separation element shown in FIG. 1 partially enlarged. FIG.
FIG. 5 is a partial cross-sectional view showing another configuration example of the broadband polarization separation element according to the present invention.
FIG. 6 is a partial cross-sectional view showing another configuration example of the broadband polarization separation element according to the present invention.
FIG. 7 is a graph showing the relationship between the diffracted light rates of zeroth-order light and first-order light and the grating depth by the broadband polarization separation element of the present invention.
FIG. 8 is a schematic configuration diagram showing an example of an optical head using a broadband polarization separation element according to the present invention.
FIG. 9 is a schematic configuration diagram showing another example of an optical head using a broadband polarization separation element according to the present invention.
FIG. 10 is a schematic configuration diagram of a main part showing an enlarged package portion of the optical head shown in FIG. 9;
[Explanation of symbols]
1,1 ': Broadband polarization separation element
2: Transparent substrate
3: Birefringent film
3 ': Birefringent crystal
4: Isotropic overcoat layer
7: Isotropic adhesive layer
8: Glass substrate
11: Wavelength λ₁ Semiconductor laser
11 ′: wavelength λ₂ Semiconductor laser
12, 13: wavelength λ₁ For light detector
12 ', 13': wavelength λ₂ For light detector
14: Wavelength λ₁ Supported optical disks (DVD, etc.)
14 ': wavelength λ₂ Compatible optical disc (CD, etc.)
15: Objective lens
16: Collimating lens
17: Polarization separating element with quarter wave plate
18: Package
19: Broadband polarization separation element
20: 1/4 wavelength plate

Claims (6)

In a grating-type broadband polarization separation element that separates orthogonally polarized light of incident light into zero-order light and diffracted light by a periodic grating in which birefringent regions and isotropic regions are alternately arranged,
At least function of the polarization separation to two different wavelengths or light, two wavelengths lambda ₁ to polarization _separation, lambda respect lambda _{2 1} And λ ₂ For the wavelength λ ₃ near the middle of the birefringence region, the refractive index for the polarization in the grating vector direction in the birefringence region is n _p , the refractive index for the polarization in the direction perpendicular thereto is n _s, and the refractive index of the isotropic region is When n ₁ , the depth of the birefringent region is h, the wavelength of light is λ ₃ , and m is a positive or negative natural number including 0 (m = 0, ± 1, ± 2,...)
(N _p −n ₁ ) h = mλ ₃
(N _s −n ₁ ) h = (m + 1/2) λ ₃
A broadband polarization separation element substantially satisfying the above. In a grating-type broadband polarization separation element that separates orthogonally polarized light of incident light into zero-order light and diffracted light by a periodic grating in which birefringent regions and isotropic regions are alternately arranged,
At least function of the polarization separation to two different wavelengths or light, two wavelengths lambda ₁ to polarization _separation, lambda respect lambda _{2 1} And λ ₂ For the wavelength λ ₃ near the middle of the birefringence region, the refractive index for the polarization in the grating vector direction in the birefringence region is n _p , the refractive index for the polarization in the direction perpendicular thereto is n _s, and the refractive index of the isotropic region is When n ₁ , the depth of the birefringent region is h, the wavelength of light is λ ₃ , and m is a positive or negative natural number including 0 (m = 0, ± 1, ± 2,...)
(N _p −n ₁ ) h = (m + 1/2) λ ₃
(N _s −n ₁ ) h = mλ ₃
A broadband polarization separation element substantially satisfying the above.   The wide-band polarization separation according to claim 1 or 2, wherein a plurality of light sources having different wavelengths, an objective lens disposed between the plurality of light sources and an optical recording medium, and the plurality of light sources and the objective lens are disposed. Element, a quarter-wave plate disposed between the broadband polarization separation element and the objective lens, and a photodetector for detecting diffracted light by the broadband polarization separation element, and at least from the plurality of light sources by the objective lens Lights with different wavelengths are condensed on different optical recording medium surfaces, and the reflected light from the optical recording medium surfaces is diffracted and separated for each wavelength by the broadband polarization separation element, and is independently detected by a photodetector for each wavelength. An optical head using a broadband polarization separation element.   4. An optical head using a broadband polarization separation element according to claim 3, wherein a quarter wavelength plate is integrated with the broadband polarization separation element.   5. The optical head according to claim 3, wherein at least a plurality of light sources and a plurality of photodetectors are mounted in one package, and the broadband polarization separation element or the quarter wavelength plate is integrated in the package. An optical head using a broadband polarization separation element, wherein the broadband polarization separation element is integrated by bonding.   6. The optical polarization head according to claim 3, wherein the emission surfaces of a plurality of light sources having different wavelengths are arranged so as to be shifted from each other in the optical axis direction of the optical head optical system. Used optical head.

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