JPH11110787A - Diffraction element and optical head device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To read information even on an optical recording medium having a birefringent property. SOLUTION: A diffraction element 23 has an optical material forming a diffraction grating 21 with a rugged cross section and another optical material 22 charged between the projecting parts of the diffraction grating 21 at least. In this case, between two kinds of these optical materials 21 and 22, at least one optical material 21 shows the birefringent property and the other optical material 22 has a refraction factor different from both the normal light refraction factor and the abnormal light refraction factor of one optical material 21 showing the birefringent property.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a diffraction element and an optical head device for writing optical information on an optical recording medium such as a CD and a DVD, and for reading optical information.

[0002]

2. Description of the Related Art In order to write optical information on an optical recording medium such as an optical disk such as a CD or DVD and a magneto-optical disk or read optical information from an optical recording medium, a light flux controlling element such as a diffraction element is used. The optical head device provided is used.

FIG. 8 is a side view schematically showing the above-mentioned optical head device. In the optical head device, light emitted from a light source 1 such as a semiconductor laser is diffracted by a hologram beam splitter or the like which is a kind of light flux controlling element. The light passes through the element 2 and is focused by an objective lens 3 on an optical disc 4 such as a CD or DVD, which is an optical recording medium.

[0004] The reflected light from the optical disk 4 passes through the objective lens 3 again, is diffracted by the diffraction element 2 such as a hologram beam splitter, and reaches the light receiving element 5 constituting a photodetector. The light receiving element 5 converts the received reflected light into an electric signal, the electric signal is amplified by an amplifier 6, and the gain is multiplied by an automatic gain correction circuit 7, so that the signal level is adjusted to a certain range.

As a light flux controlling element such as the diffractive element 2, a non-polarizing hologram beam splitter has been used. FIG. 9 is a side sectional view of a non-polarization hologram beam splitter. The non-polarization hologram beam splitter includes, for example, an isotropic diffraction grating 9 made of an isotropic optical material on an upper surface of a glass substrate 8. Is formed. In the drawing, reference numeral 10 denotes a low-reflection coat applied to both surfaces of the hologram beam splitter.

In the case of a non-polarizing hologram beam splitter, the zero-order transmission efficiency on the outward path is 50% and the first-order diffraction efficiency on the return path is 20%, so that the theoretical reciprocating efficiency is 50% × 20.
% = 10%. However, in practice, the round trip efficiency is 10
% Hologram beam splitter is difficult to produce, and it is only necessary to obtain a reciprocating efficiency of about 6 to 7%. Therefore, a non-polarizing hologram beam splitter has a problem that the reciprocating efficiency is low. Therefore, the light reciprocating efficiency is set to 1
It has been proposed to use a hologram beam splitter whose diffraction efficiency changes depending on the polarization direction of the light in order to raise it to more than 0%.

FIG. 10 is a side sectional view showing a polarization type hologram beam splitter. The polarization type hologram beam splitter is formed on a glass substrate 12 by ordinary light refraction of a birefringent optical material 13 such as a liquid crystal described later. isotropic diffraction grating 14 is formed by an optical material isotropic with the rate n _O or the extraordinary refractive index n _e is equal to the refractive index n _S, coating baking an alignment film 15 on the isotropic diffraction grating 14 After the alignment process is performed, the opposing substrate 17 having the alignment film 16 on which the alignment process has been performed is opposed to each other, and is thermocompression-bonded with a sealing material 18 interposed therebetween, and the birefringent optical material 13 such as liquid crystal is placed inside. It is filled and sealed in a liquid state.

When a hologram beam splitter of a polarization system is used, a 波長 wavelength plate 19 as shown in FIG. The polarization direction of the light when passing through
Rotate degrees.

In a polarization type hologram beam splitter, for example, the ordinary refractive index n _{O of the} birefringent optical material 13 is used.
When the refractive index n _S of the isotropic diffraction grating 14 is equal to (n _O = n _S ), the polarization direction in the outward path of the light emitted from the light source 1 is changed to the direction of the ordinary light refractive index n _O of the birefringent optical material 13. If they are matched, the hologram beam splitter does not function, so that the 0th-order transmission efficiency can be increased. On the return path, the polarization direction of the reflected light is changed by the quarter-wave plate 19 to the extraordinary refractive index of the birefringent optical material 13. since equal to n _e direction, the hologram beam splitter functions, it is possible to increase the first-order diffraction efficiency, as a result, non-polarization-based high reciprocating efficiency than 10% the theoretical reciprocating efficiency of the hologram beam splitter Is obtained.

[0010]

However, the optical head device using the above-mentioned polarizing hologram beam splitter has the following problems. In other words, the optical disc 4 as an optical recording medium is generally a resin molded product, and is required to be molded as homogeneously as possible. In some cases, the flow may be distorted due to, for example, a bias in the flow, and may have partial birefringence at one or more locations.

Then, since the optical disk 4 is rotated at a high speed, the reflected light from the optical disk 4 having one or more partial birefringences is not constant in the polarization direction but fluctuates from time to time. In this case, reflected light whose direction of polarization on the return path is shifted by more than 90 degrees with respect to the direction of polarization of the light, and reflected light whose degree of polarization is completely unknown is returned.

In the worst case, the reflected light from the optical disk 4 may return in the same polarization direction as the outward path.
In such a case, the hologram beam splitter of the polarization system can hardly diffract the reflected light, and the information on the optical disc 4 cannot be read by the light receiving element 5.

SUMMARY OF THE INVENTION In view of the above circumstances, it is an object of the present invention to provide a diffraction element and an optical head device that can read information from an optical recording medium having birefringence.

[0014]

According to the present invention, there is provided a diffraction element comprising an optical material forming a diffraction grating having an uneven cross section and another optical material filled between at least convex portions of the diffraction grating. At least one of the types of optical materials exhibits birefringence, and the other optical material has at least a refractive index different from either the ordinary refractive index or the extraordinary refractive index of one of the birefringent optical materials. A diffractive element having one is provided.

Further, the present invention provides the above-described diffractive element in which one of the two types of optical materials is an isotropic optical material. Further, the refractive index of the isotropic optical material is smaller than the smaller one of the ordinary refractive index and the extraordinary refractive index of the birefringent optical material, or the above-described diffraction element larger than the larger one. provide. Further, the first-order diffraction efficiency in the polarization direction in which the diffraction efficiency is the lowest is 10% or more of the first-order diffraction efficiency in the polarization direction in which the diffraction efficiency is the highest, and the zero-order transmission efficiency in the polarization direction in which the diffraction efficiency is the lowest. The present invention provides the above-described diffraction element using two kinds of optical materials having a refractive index difference of which the product of the first-order diffraction efficiency and the polarization direction at which the diffraction efficiency is the highest is 10% or more.

Further, a light source, a light flux controlling element for passing the light emitted from the light source and changing the traveling direction of the reflected light reflected back from the optical recording medium, and the traveling direction changed by the light flux controlling element. An optical head device provided with a photodetector for detecting information of reflected light, wherein the above-described diffractive element is used as a light flux controlling element.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a side sectional view of a diffraction element of a polarization system according to the first embodiment of the present invention. FIG. 2 is a side cross section of another diffraction element of a polarization system according to the first embodiment of the present invention. FIG. 3 is a side cross-sectional view of a diffractive element of another polarization system according to the first embodiment of the present invention. FIG. 4 is a diagram showing the efficiency and the wavelength according to the first embodiment of the present invention. 6 is a graph showing a relationship with a converted phase difference Δn × d / λ.

The basic configuration of the optical head device is the same as that of FIG. 8, and therefore, FIG. 8 will be referred to as necessary. According to the present invention, at least one of two types of optical materials, that is, an optical material that forms a diffraction grating having an uneven cross section and another optical material that is filled between at least convex portions of the diffraction grating, exhibits birefringence. The feature is that the other optical material has at least one refractive index different from the ordinary light refractive index and the extraordinary light refractive index of the one optical material exhibiting birefringence.

In this embodiment, one of the two types of optical materials is an isotropic optical material. Further, the refractive index of the isotropic optical material may be an intermediate value between the ordinary refractive index and the extraordinary refractive index of the birefringent optical material, but is preferably the ordinary refractive index of the birefringent optical material. Or smaller than the smaller of the extraordinary refractive index,
Or, make it larger than the larger one.

The first-order diffraction efficiency for the polarization direction in which the diffraction efficiency is the lowest is 10% or more of the first-order diffraction efficiency for the polarization direction in which the diffraction efficiency is the highest, and the 0th-order diffraction efficiency for the polarization direction in which the diffraction efficiency is the lowest. The product of the first-order diffraction efficiency with respect to the polarization direction at which the transmission efficiency and the diffraction efficiency are highest is 10
% Of two types of optical materials having a refractive index difference of not less than%.

A light source, a light flux controlling element for passing the light emitted from the light source and changing the traveling direction of the reflected light reflected back from the optical recording medium, and the reflected light the traveling direction of which is changed by the light flux controlling element In the optical head device provided with a photodetector for detecting the above information, the above-described diffraction element is used as a light flux control element.

As a specific structure, for example, FIG.
As shown in FIG. 2, a birefringent diffraction grating 21 having a rectangular cross section is formed on a substrate 20 such as glass by photolithography or etching.
A diffractive element 23 is formed by filling an isotropic filler 22 at least between the convex portions of the optical disc device, and the diffractive element 23 is incorporated between the light source of the optical head device and the optical disk.

As shown in FIG. 1, the birefringent optical material between the projections may be completely removed by utilizing the difference in the etching rate between the substrate 20 and the birefringent optical material. As shown in FIG. 2, in order to adjust the grating depth d of the birefringent diffraction grating 21, a structure may be employed in which a required amount of birefringent optical material is left between the convex portions.

In this case, the birefringent optical material constituting the birefringent diffraction grating 21 is, for example, a polymer liquid crystal obtained by orienting and polymerizing a liquid crystal material or a uniaxially stretched polymer film. Or a birefringent single crystal. Further, as an isotropic optical material constituting the isotropic filler 22, for example, an acrylic resin or an epoxy resin can be used.

Alternatively, as shown in FIG. 3, a thin film of an isotropic optical material formed on a substrate 20 such as glass by a reactive sputtering method is subjected to photolithography, etching, or the like, so as to form a rectangular isotropic wave. Birefringent filler 2 is formed between convex portions of isotropic diffraction grating 24.
5 to form the diffraction element 23, and the diffraction element 23 may be incorporated between the light source of the optical head device and the optical disk. Note that the isotropic diffraction grating 24 may be formed by directly forming irregularities on the substrate 20 such as glass.

In this case, as the isotropic optical material constituting the isotropic diffraction grating 24, for example, glass or SiO—
A thin film of N (a mixture of SiO ₂ and SiN, the refractive index of which can be adjusted by adjusting the ratio of both) and an acrylic resin or an epoxy resin can be used. In addition, as the birefringent optical material constituting the birefringent filler 25, for example, a liquid crystal or a polymer liquid crystal in which a liquid crystal material is aligned and polymerized to polymerize can be used. When the liquid crystal is used in a liquid state, the liquid crystal is sealed using the counter substrate and the sealant 26 as in the case of FIG.

In each case of FIGS. 1 to 3, 1
The 波長 wavelength plate 27 may be integrally formed.
By doing so, the number of parts can be reduced,
This is advantageous because the size and weight can be reduced. In FIG. 3,
Although the opposite substrate is a quarter-wave plate 27, the quarter-wave plate 27 may be provided separately from the opposite substrate.

[0028] Then, first, ordinary refractive index n _O of the birefringent optical material, abnormal light refractive index n _e, the refractive index of the isotropic material and n _S, with n _S ≠ n _e, and, A combination of a birefringent optical material and an isotropic material such that n _S ≠ n _O is selected. By doing so, the isotropic optical material exhibits a different refractive index from the birefringent optical material for either polarized light in the ordinary refractive index direction or polarized light in the extraordinary refractive index direction of the birefringent optical material. Therefore, the diffraction element 23 has diffraction efficiency regardless of the polarization direction of light.

The diffraction efficiency and transmission efficiency of the diffraction element 23 are standardized by the efficiency and the wavelength, where Δn is the refractive index difference between the two optical materials, d is the grating depth, and λ is the wavelength of light. It is determined based on the graph of FIG. 4 showing the relationship with the phase difference Δn × d / λ. Of these, the wavelength λ of light is predetermined as the wavelength at which the light source oscillates, so that the diffraction efficiency is determined by the refractive index difference Δn and the grating depth d. Then, it is difficult to manufacture the diffraction element. Therefore, it is preferable to mainly determine the refractive index difference Δn.

[0030] Here, in the extraordinary refractive index direction of the birefringent optical material, the refractive index difference between the birefringent optical material and an isotropic optical material _{Δn 1 = | n e -n S} | and then, birefringence When the refractive index difference between the birefringent optical material and the isotropic optical material in the ordinary light refractive index direction of the isotropic optical material is Δn ₂ = | n _O −n _S |, the refractive index n _S of the isotropic material is , an intermediate value between the ordinary refractive index n _O and the extraordinary refractive index n _e of the birefringent optical material (n _e
<N _S <n _O or, for the case of the _{n O <n S <n e} ) , the refractive index difference [Delta] n ₁ and [Delta] n ₂ becomes a no great difference values, the settings such as diffraction efficiency, This is disadvantageous in terms of structure because it is inevitable to increase the lattice depth d.

[0031] Therefore, the refractive index n _S of the isotropic material, the ordinary refractive index of the birefringent optical material n _O and the extraordinary refractive index n _e
Is smaller than the smaller one or larger than the larger one (n _S <n _e , n _O or n _e , n _O <n _S ). Then, the refractive index difference Δn ₁
And the value of Δn ₂ , it is possible to easily set the diffraction efficiency and the like without increasing the grating depth d, thereby obtaining a great advantage in structure.

Here, in the polarization type diffraction element 23,
When the polarization direction with the lowest diffraction efficiency is the A direction and the polarization direction with the highest diffraction efficiency is the B direction, the 0th-order transmission efficiency on the outward path is increased (that is, the 1st-order diffraction efficiency is reduced), and the first-order on the return path is reduced. When the folding efficiency is increased, the reciprocation efficiency is increased. Therefore, it is preferable that the polarization direction of the light on the outward path is set to the A direction and the polarization direction of the light on the return path is set to the B direction. It is preferable that the first-order diffraction efficiency with respect to the polarization direction of the return path (B direction) be as high as possible because the reciprocation efficiency of light can be increased.
According to FIG. 4, the first-order diffraction efficiency with respect to the polarization direction in the B direction can secure approximately 40%.

In order to obtain a reciprocating efficiency of 10% or more, which is the theoretical reciprocating efficiency of the non-polarizing hologram beam splitter, the first-order diffraction efficiency on the return path is almost 40% (actually less than this). Therefore, the zero-order transmission efficiency for the polarized light in the outward path (A direction) is approximately 30% with some allowance.
It is necessary to do above.

On the other hand, if the optical disc has birefringence, in the worst case, the polarization direction on the return path may be the same as the polarization direction in the A direction on the outward path. In this case, the light is diffracted by the first-order diffraction efficiency with respect to the polarization direction in the A direction. However, if the first-order diffraction efficiency with respect to the polarization direction equal to the outward path is too low, the light receiving element cannot effectively detect light.

Since the magnification at which a signal can be corrected by the automatic gain correction circuit connected to the light receiving element is approximately up to about 10 times, the first-order diffraction efficiency for polarized light in the A direction is 10% of the first-order diffraction efficiency for polarized light in the B direction. By doing the above,
Light can be effectively detected by the light receiving element. In order to simplify the configuration of the automatic gain correction circuit, the first-order diffraction efficiency with respect to the polarization in the A direction is set to be 1 to the polarization with respect to the polarization in the B direction.
It is preferable to set it to 25% or more of the next diffraction efficiency.

Here, when the 0th-order transmission efficiency is increased, the first-order diffraction efficiency is reduced, and conversely, when the 0th-order transmission efficiency is reduced, the first-order diffraction efficiency is increased. By setting the zero-order transmission efficiency and the first-order diffraction efficiency in the polarization direction of the A direction corresponding to the above to be within a range that satisfies both of the above conditions, information can be read even from an optical disc having birefringence. it can.

Since the optical disk does not always have a portion having birefringence, the diffraction element 23 and the optical disk must be positioned so that the polarization direction of the return path is perpendicular to the polarization direction of the outward path. It is preferable to arrange a 波長 wavelength plate between them.

The above further includes a birefringent optical material smaller than the ordinary refractive index n _O is the extraordinary refractive index n _e, the refractive index n _S
The ratio of but when using a small isotropic material than the ordinary refractive index n _O of the birefringent optical material, i.e., in the case of _{n S <n O <n e} , and the width dimension of the concave portion and the convex portion There 1: the case of using a cross-section rectangular wave diffraction grating, more specifically, the refractive index n _O, n _e, the method of determining n _S will be described.

In the case of using a diffraction grating having a rectangular cross section having a width ratio of the concave portion and the convex portion of 1: 1, the relationship between the efficiency and the phase difference Δn × d / λ normalized by the wavelength is shown in FIG. As can be seen from the figure, when Δn × d / λ = 0.5, the ± 1st-order diffraction efficiency is the highest at 40.5%, respectively.

[0040] In this case, becomes lower direction A direction most diffraction efficiency of the ordinary refractive index n _O, since the direction of the extraordinary refractive index n _e is higher B direction most diffraction efficiency, the A direction outward The direction B is set as the polarization direction, and the direction B is set as the return direction.
Then, the refractive index difference B direction _{Δn 1 = | n e -n S} | if that, to the first-order diffraction efficiency 40.5% which is the maximum value, to optimize the value of [Delta] n ₁ × d , Δn ₁ × d / λ are set to 0.5 or around 0.5.

In order to secure the reciprocating efficiency of 10% or more, which is the theoretical reciprocating efficiency of the non-polarizing type diffraction element, it is necessary to make the 0th-order transmission efficiency of the outward path approximately 30% or more (return path). When the first-order diffraction efficiency is 40.5%,
Calculations show that the 0th-order transmission efficiency on the outward path may be approximately 25% or more, but in practice, the first-order diffraction efficiency on the return path is 4%.
A value as high as 0.5% cannot be obtained), and the value of Δn ₂ × d / λ is approximately 0.32 or less, where Δn ₂ = | n _O −n _S |

Further, the direction of the ordinary light refractive index n _O (A direction)
The first-order diffraction efficiency for the polarized light, 1st-order diffraction efficiency with respect to the polarization direction (B direction) of the extraordinary refractive index n _e, as described above (= 40.5 percent) of 10% or more, preferably 25% or more Since it is necessary, the first-order diffraction efficiency is 40.5% ×
1/10 {4% or more, preferably 40.5% × １ ／}
The value of Δn ₂ × d / λ satisfying this condition is about 0.11 or more, and preferably about 0.17 or more. As a result, when the above is combined, Δn ₂ × d / λ
Is about 0.11 or more, preferably about 0.1.
A range of about 0.32 or less can be obtained with a value of 17 or more.

FIG. 5 is a side sectional view of a polarization type diffraction element according to a second embodiment of the present invention, and FIG. 6 is a diagram showing efficiency and efficiency according to the second embodiment of the present invention. 9 is a graph showing a relationship with a phase difference Δn × d / λ normalized by wavelength.

The present embodiment is characterized in that the cross-sectional shape of the diffraction grating in the first embodiment is formed in a sawtooth shape by hot pressing using a mold or the like. FIG. 5 shows a case where the isotropic diffraction grating 24 of FIG. 3 has a sawtooth shape. However, even if the birefringent diffraction grating 21 of FIG. It may be in a sawtooth shape.

When the diffraction grating has a sawtooth cross section, as shown in FIG. 6, when Δn × d / λ = 1.0, the highest diffraction efficiency of about 100% is obtained. Therefore, the first-order diffraction efficiency for polarized light in the B direction is 100%.
Is set so as to obtain a reciprocating efficiency of 10% or more which is the theoretical reciprocating efficiency of the non-polarization type diffraction element.
The zero-order transmission efficiency for polarized light in the direction is approximately 10% or more, and the value of Δn ₂ × d / λ satisfying this condition is approximately 0.
76 or less.

The first-order diffraction efficiency for polarized light in the A direction, which is 10% or more, preferably 25% or more, of the first-order diffraction efficiency (approximately 100%) for polarized light in the B direction, is approximately 10%.
%, Preferably about 25% or more, and the value of Δn ₂ × d / λ satisfying this condition is about 0.25 or more, preferably about 0.4 or more. As a result, when the above is combined, the value of Δn ₂ × d / λ is approximately 0.25 or more,
Preferably, a range of about 0.4 or more and about 0.74 or less is obtained. Other than the above, the configuration is almost the same as that of the above embodiment, and the same operation and effect can be obtained.

FIGS. 7 (1), (2) and (3) show the relationship between the refractive indices of two birefringent optical materials according to the third embodiment of the present invention. This embodiment is characterized in that both of the two types of optical materials are birefringent optical materials. In this case, at least three of the ordinary light refractive index and the extraordinary light refractive index of the two types of birefringent optical materials are different.

For example, as shown in FIG. 7, two types of birefringent optical materials are referred to as an optical material (1) and an optical material (2), respectively. Among them, the refractive index in the A direction is represented by n _a1 , the refractive index in the B direction is represented by n _b1 , and the refractive index in the A direction of the ordinary light refractive index and the extraordinary light refractive index of the optical material (2) is n _If the refractive index in the _a2 and B directions is represented by n _b2 , the optical material (1)
And the direction of the refractive index of the optical material (2), the ordinary refractive index and the extraordinary refractive index can be set to either n _a1 , n _b1 , or n _a2 , n _b2 . As shown in FIG. 7A, between the ordinary refractive index and the extraordinary refractive index of the optical material (1), there is a higher one of the ordinary refractive index and the extraordinary refractive index of the optical material (2). When the optical material (2) has a lower ordinary light refractive index and an extraordinary light refractive index below the lower ordinary light refractive index and the extraordinary light refractive index of (1), the ordinary light refraction of the optical material (1) is used. The lower one of the refractive index and the extraordinary light refractive index is set to the direction A, and the higher refractive index is set to the direction B, and the ordinary refractive index and the extraordinary refractive index of the optical material (2) are set to the higher refractive index. Is the direction A, and the one with the lower refractive index is the direction B, and the refractive index difference Δn ₂ (= | n
_a1 −n _a2 |) and the refractive index difference Δn _{1 in the} B direction (= | n _b1
−n _b2 |), or as shown in FIG. 7 (2), the lower one of the ordinary refractive index and the extraordinary refractive index of the optical material (1) is set to the A direction, and the refractive index of the optical material (1) is changed to the A direction. The higher direction is set to the B direction, the lower one of the ordinary light refractive index and the extraordinary light refractive index of the optical material (2) is set to the A direction, and the higher refractive index is set to the B direction, and refraction in the A direction is performed. Rate difference Δn ₂ (= | n
_a1 −n _a2 |) and the refractive index difference Δn _{1 in the} B direction (= | n _b1
−n _b2 |) to select a refractive index difference or a phase difference Δn × d / λ normalized by wavelength,
The first-order diffraction efficiency and the zero-order transmission efficiency are set.

As shown in FIG. 7C, the ordinary refractive index and the extraordinary refractive index of the optical material (2) are lower than the lower of the ordinary refractive index and the extraordinary refractive index of the optical material (1). When there is a higher refractive index, the lower one of the ordinary refractive index and the extraordinary refractive index of the optical material (1) is set to the A direction, the higher refractive index is set to the B direction, and Of the ordinary light refractive index and the extraordinary light refractive index in 2), the higher refractive index is set to the A direction, and the lower refractive index is set to the B direction, and the refractive index difference Δn _{2 in the} A direction (= | n _a1 −n _a2). |) And the refractive index difference Δ in the B direction
Although n ₁ (= | n _b1 −n _b2 |) is obtained or not shown, the lower one of the ordinary light refractive index and the extraordinary light refractive index of the optical material (1) is set in the A direction, and the refractive index is changed. The higher one is B
Direction, the higher refractive index of the ordinary light refractive index and the extraordinary light refractive index of the optical material (2) is set to the B direction, and the lower refractive index is set to the A direction, and the refractive index difference Δn _{2 in the} A direction. (=
| N _a1 −n _a2 |) and the refractive index difference Δn _{1 in the} B direction (= |
n _b1 −n _b2 |) to select the refractive index difference or the phase difference Δn × d / λ normalized by the wavelength, and set the first-order diffraction efficiency and zero-order transmission efficiency. .

Similarly, when both the ordinary refractive index and the extraordinary refractive index of the optical material (2) are between the ordinary refractive index and the extraordinary refractive index of the optical material (1), Refractive index difference or phase difference Δn × d / λ normalized by wavelength
And the first-order diffraction efficiency and zero-order transmission efficiency can be set.

By using both types of optical materials as birefringent optical materials as described above, the selection range of the refractive index difference is expanded, so that the first-order diffraction efficiency and the zero-order transmission efficiency can be freely set. The degree increases, which is preferable. In addition to the above,
It has almost the same configuration as the above-described embodiment, and the same operation and effect can be obtained.

[0052]

【Example】

Example 1 A birefringent diffraction grating 21 having a rectangular wave cross section was filled with an isotropic filler 22 to produce a diffraction element 23 substantially similar to that shown in FIG. In this case, the ordinary refractive index n _O = 1.52, the polymer liquid crystal of the extraordinary refractive index n _e = 1.64 is used as the birefringent diffraction grating 21, refractive index n _S as isotropic filler 22 = An acrylic isotropic medium of 1.43 was used. Then, a grating depth d was set to about 1.62 μm, and a light source having a wavelength λ of 650 nm was used.

The refractive index difference Δn _{1 in the} B direction having the highest refractive index of the diffraction element 23 is 1.64-1.43 = 0.21.
Where the depth d of the grating is 1.62 μm and the wavelength λ is 650.
nm, the value of Δn ₁ × d / λ is approximately 0.
5, and the first-order diffraction efficiency for the polarized light in the B direction is
A value of approximately 38%, the theoretical limit, was obtained.

Further, the refractive index difference Δn _{2 in the} A direction having the lowest refractive index of the diffraction element 23 is 1.52-1.43 =
0.09, the grating depth d is 1.62 μm, and the wavelength λ
Is 650 nm, the value of Δn ₂ × d / λ is approximately 0.22, and the zero-order transmission efficiency for polarized light in the A direction is approximately 58%, and the first-order diffraction efficiency is approximately 15%. Was done.

Therefore, in the case of an optical disk having almost no birefringence, the outgoing light polarized in the A direction is transmitted through the diffraction element 23 with a zero-order transmission efficiency of 58%, and is converted into circularly polarized light by a quarter-wave plate. Is reflected by the optical disk and
After being changed to B-direction polarized light by a 偏光 wavelength plate, the diffraction element 2
In No. 3, the light was diffracted with a first-order diffraction efficiency of 38% and reached the light receiving element. The reciprocating efficiency at this time was 58% × 38% ≒ 22%. However, this value is a converted value on the assumption that there is no aperture restriction due to the pupil diameter of the objective lens, and no reflection loss of the disk.

Further, assuming that the optical disk has a large birefringence, the reciprocation efficiency when the polarization direction of the return path is the same as the A direction as the forward path is examined, and is 58% × 15% = 8.
It was 7%.

Embodiment 2 FIG. 3 shows that an isotropic diffraction grating 24 having a rectangular cross section is filled with a birefringent filler 25.
A diffraction element 23 substantially similar to that described above was produced. In this case, the ordinary refractive index n _O = 1.5 as the birefringent filler 25.
2, using the liquid crystal of the extraordinary refractive index n _e = 1.77, SiO-N in the refractive index n _S = 1.58 as isotropic diffraction grating 24
Was used. Then, the grating depth d was set to about 1.72 μm, and a light source having a wavelength λ of 650 nm was used.

The refractive index difference Δn ₁ of the diffraction element 23 in the direction B having the highest refractive index is 1.77−1.58 = 0.19.
Where the depth d of the grating is 1.72 μm and the wavelength λ is 650.
nm, the value of Δn ₁ × d / λ is approximately 0.
5, and the first-order diffraction efficiency for the polarized light in the B direction is
A value of approximately 38%, the theoretical limit, was obtained.

The refractive index difference Δn _{2 in the} direction A having the lowest refractive index of the diffraction element 23 is 1.52 to 1.58 = −
0.06, the grating depth d is 1.72 μm, and the wavelength λ
Is 650 nm, the value of Δn ₂ × d / λ is approximately 0.16, and the zero-order transmission efficiency for the polarized light in the A direction is approximately 74%, and the first-order diffraction efficiency is approximately 9%. Was done.

Therefore, in the case of an optical disk having almost no birefringence, the outgoing light polarized in the A direction is transmitted through the diffractive element 23 with a zero-order transmission efficiency of 74%, and is converted into circularly polarized light by a quarter-wave plate. Is reflected by the optical disk and
After being changed to B-direction polarized light by a 偏光 wavelength plate, the diffraction element 2
In No. 3, the light was diffracted with a first-order diffraction efficiency of 38% and reached the light receiving element. The reciprocating efficiency at this time was 74% × 38% ≒ 28%. However, this value is a converted value on the assumption that there is no aperture restriction due to the pupil diameter of the objective lens, and no reflection loss of the disk.

Assuming that the optical disk has a large birefringence, the reciprocation efficiency when the polarization direction of the return path is the same as the A direction as the forward path is examined, and it is found that 74% × 9% ≒ 6.6.
%.

Example 3 By filling a birefringent filler 25 into an isotropic diffraction grating 24 having a sawtooth cross section, a diffraction element 23 substantially similar to that shown in FIG. 5 was manufactured. In this case, the ordinary refractive index n _O =
1.52, using the liquid crystal of the extraordinary refractive index n _e = 1.77,
An isotropic diffraction grating 24 was formed directly on a glass substrate 20 having a refractive index n _S = 1.59. Then, a grating depth d was set to about 3.6 μm, and a light source having a wavelength λ of 650 nm was used.

The refractive index difference Δn _{1 in the} B direction having the highest refractive index of the diffraction element 23 is 1.77−1.59 = 0.18.
Where the depth d of the grating is 3.6 μm and the wavelength λ is 650 n
m, the value of Δn ₁ × d / λ is approximately 1.0
Thus, the first-order diffraction efficiency with respect to the polarized light in the B direction was about 95%, which is the theoretical limit.

The refractive index difference Δn _{2 in the} direction A having the lowest refractive index of the diffraction element 23 is 1.52 to 1.59 = −
0.07, the depth d of the grating is 3.6 μm, and the wavelength λ is 650 nm. Therefore, the value of Δn ₂ × d / λ is approximately 0.39, and the zero-order transmission efficiency for the polarized light in the A direction is 0. Was about 58%, and the first-order diffraction efficiency was about 22%.

Therefore, in the case of an optical disk having almost no birefringence, the outgoing light polarized in the A direction is transmitted through the diffraction element 23 with a zero-order transmission efficiency of 58%, and is converted into circularly polarized light by a quarter-wave plate. Is reflected by the optical disk and
After being changed to B-direction polarized light by a 偏光 wavelength plate, the diffraction element 2
3 and was diffracted with a first-order diffraction efficiency of 95%, and reached the light receiving element. The reciprocating efficiency at this time was 58% × 95% ≒ 55%. However, this value is a converted value on the assumption that there is no aperture restriction due to the pupil diameter of the objective lens, and no reflection loss of the disk.

Assuming that the optical disk has a large birefringence, the reciprocation efficiency when the polarization direction of the return path is the same as the A direction as the forward path is examined, and is 58% × 22% ≒ 13
%.

It should be noted that the present invention is not limited to the above-described embodiment, and various changes can be made without departing from the scope of the present invention.

[0068]

As described above, according to the diffraction element and the optical head device of the present invention, an excellent effect that information can be read from an optical recording medium having birefringence can be obtained.

[Brief description of the drawings]

FIG. 1 is a side sectional view of a polarizing diffraction element according to a first embodiment of the present invention.

FIG. 2 is a side cross-sectional view of another polarization type diffraction element according to the first embodiment of the present invention.

FIG. 3 is a side cross-sectional view of another polarization type diffraction element according to the first embodiment of the present invention.

FIG. 4 is a diagram showing the efficiency and efficiency according to the first embodiment of the present invention;
9 is a graph showing a relationship with a phase difference Δn × d / λ normalized by wavelength.

FIG. 5 is a side sectional view of a polarizing diffraction element according to a second embodiment of the present invention.

FIG. 6 shows the efficiency and the second embodiment according to the present invention;
9 is a graph showing a relationship with a phase difference Δn × d / λ normalized by wavelength.

FIGS. 7 (1), (2) and (3) are diagrams showing the relationship between the refractive indices of two birefringent optical materials according to a third embodiment of the present invention.

FIG. 8 is a side view schematically showing a conventional optical head device.

FIG. 9 is a side sectional view of a non-polarization hologram beam splitter.

FIG. 10 is a side sectional view showing a polarization type hologram beam splitter.

[Explanation of symbols]

 1: light source 4: optical disk (optical recording medium) 5: light receiving element (photodetector) 6: amplifier (photodetector) 7: automatic gain correction circuit (photodetector) 20: substrate 21: birefringent diffraction grating ( 22: Isotropic filler (optical material) 23: Diffraction element (light flux controlling element) 24: Isotropic diffraction grating (optical material) 25: Birefringent filler (optical material)

Claims (5)

[Claims] 1. A diffraction element comprising an optical material forming a diffraction grating having an uneven cross section and another optical material filled between at least convex portions of the diffraction grating, wherein at least one of the two types of optical materials is used. One shows birefringence, and the other optical material has at least one refractive index different from both the ordinary light refractive index and the extraordinary light refractive index of the one optical material showing birefringence. Diffraction element. 2. The diffraction element according to claim 1, wherein one of the two types of optical materials is an isotropic optical material. 3. The refractive index of the isotropic optical material is smaller than the smaller one of the ordinary light refractive index and the extraordinary light refractive index of the birefringent optical material, or larger than the larger one. 2. The diffraction element according to 2. 4. A method according to claim 1, wherein said light beam has a diffraction efficiency of 1.
The first-order diffraction efficiency is 10% or more of the first-order diffraction efficiency in the polarization direction in which the diffraction efficiency is the highest, and the zero-order transmission efficiency in the polarization direction in which the diffraction efficiency is the lowest and the first-order diffraction in the polarization direction in which the diffraction efficiency is the highest. 4. The diffractive element according to claim 1, wherein two types of optical materials having a difference in the refractive index such that the product of the efficiency and the efficiency is 10% or more are used. 5. A light source, a light flux controlling element for passing the light emitted from the light source and changing the traveling direction of the reflected light reflected back by the optical recording medium, and the traveling direction changed by the light flux controlling element. 5. An optical head device comprising a photodetector for detecting information of reflected light, wherein the diffraction element according to claim 1, 2, 3 or 4 is used as a light flux controlling element.