JPH1164615A - Diffraction element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a diffraction element light in weight, small in size and high in the availability of light by forming a lattice having projections and recesses in the cross section in a double refractive film, filling the recesses of the lattice with an isotropic material having an almost the same refractive index as the refractive index for normal or abnormal light of the double refractive film. SOLUTION: A double refractive film 24 comprising a uniformly oriented polymer liquid crystal having photosetting property is formed on a transparent substrate 20. Then a lattice 27 having projections and recesses in the cross section is formed in the double refractive film 24 produced by forming film and orienting. As for the means to form the lattice 27, an etching method by photolithography or a pressing method with a die having a lattice form can be used. When the incident linearly polarized light is designed to be perpendicular to the orientation direction of the polymer liquid crystal, the recesses are filled with an isotropic material 28 having almost the same refractive index as the refractive index for normal light of the polymer liquid crystal. When the incident linearly polarized light is designed to be parallel to the orientation direction of the polymer liquid crystal, the recesses are filled with an isotropic material having almost the same refractive index as the refractive index for abnormal light of the polymer liquid crystal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a CD, a CD-RO
The present invention relates to a diffractive element used for an optical head device for writing optical information on an optical disk such as a video disk, a video disk, and a magneto-optical disk, and for reading optical information.

[0002]

2. Description of the Related Art An optical head device is used for writing optical information on an optical recording medium such as an optical disk and a magneto-optical disk and reading optical information from the optical recording medium. The optical head device includes an optical component for guiding (beam splitting) the signal light reflected from the recording surface of the disk-shaped optical recording medium to the light detection unit. Conventionally, as this optical component, one using a diffraction element or a hologram element and one using a prism type beam splitter have been known.

[0003] A conventional diffraction element or hologram element for an optical head device is formed by forming a relief-shaped grating having a rectangular cross section on a glass or plastic substrate by dry etching or injection molding. Diffracts light to provide a beam splitting function.

FIG. 4 shows an example of a diffraction element or a hologram element using a dry etching method. SiO ₂ film formed by using a vacuum process such as an evaporation method or a sputtering method on the upper surface of the glass substrate 2 itself having the low reflection coat 1 on the lower surface side, or on the glass substrate ₂
A lattice-shaped photoresist mask is formed on the upper surface of the inorganic thin film 3 by photolithography, and in this state, dry etching is performed to form an inorganic lattice 4 which is a lattice of the inorganic thin film 3 on the portion of the inorganic thin film 3. Further, the photoresist remaining on the inorganic grating 4 is removed, and then a low-reflection coating 5 is applied to produce a diffraction element, which is used as a polarization-independent isotropic diffraction element.

The light use efficiency of such an isotropic diffraction element is as low as about 10%, and if it is desired to increase the light use efficiency to more than 10%, it is conceivable to use polarized light.

Therefore, an optical head device provided with a hologram (diffraction element) using polarized light and having high light use efficiency has been proposed in Japanese Patent Application Laid-Open No. 9-180236. The proposed polarizing diffractive element is that shown in FIG. First, an inorganic isotropic thin film 8 having a refractive index substantially equal to the ordinary refractive index or the extraordinary refractive index of a birefringent material such as a liquid crystal 7 described later is formed on a glass substrate 6. A lattice-shaped photoresist mask is formed on the isotropic thin film 8 by photolithography, and in this state, an inorganic lattice 9 is formed by performing dry etching, and the photo-residue remaining on the inorganic lattice 9 is formed. After the resist is removed, the alignment film 10 is applied and baked to perform an alignment treatment. Then, the opposite substrate 12 having the alignment film 11 that has been subjected to the same alignment treatment is opposed to each other, and the heat is applied through the sealing material 13. A glass substrate in which a diffractive element portion 14 is formed by compression bonding, filling a birefringent material such as a liquid crystal 7 therein, and sealing, and further, an organic retardation film 15 is attached to the lower surface side of the glass substrate 6. 6
To form a phase difference plate portion 17,
Low reflection coats 18 and 19 are applied to both sides.

[0007]

In the case of the polarizing diffractive element shown in FIG. 5, the ordinary liquid crystal 7 is oriented along the extension direction of the uneven portion in the lattice, that is, along the lattice direction. The ordinary light refractive index of the liquid crystal 7 corresponds to polarized light perpendicular to the liquid crystal, and the liquid crystal 7 corresponds to polarized light parallel to the lattice direction.
Corresponds to the extraordinary light refractive index. Therefore, in the optical head device, there is a restriction that the grating direction is either parallel or perpendicular to the polarization direction of the incident light and is substantially uniform in the diffraction element section 14.

Further, since the glass substrate 6, the counter substrate 12, and the sealing material 13 are required for enclosing the liquid crystal 7, there is a problem in the element size with respect to the effective diameter. Further, in the case of using a low-cost organic retardation film 15 for the retardation plate 17 in a diffraction element having a retardation plate 17 for switching polarization, the transmitted wavefront of the organic retardation film 15 alone is used. There is a problem in reliability with respect to distortion and a reaction with the liquid crystal 7 filled in the diffraction element portion 14. To solve this problem, three glasses are required, which is a problem in making the diffraction element thinner and lighter. .

The present invention solves the above-mentioned problems, can incorporate a retardation plate, is lightweight and small, and has no restriction on the angle between the grating direction and the incident polarization direction, and can be industrially manufactured and high. An object is to provide a diffraction element having light use efficiency.

[0010]

The present invention comprises a transparent substrate,
A birefringent film formed on a transparent substrate, the cross section of the birefringent film is irregular, and the concave portion of the lattice is filled with a refractive index substantially equal to the ordinary light refractive index or the extraordinary light refractive index of the birefringent film. A diffractive element comprising: an isotropic material having a refractive index.

[0011] The present invention also provides the diffraction element, wherein the grating is formed on the birefringent film in a direction that is neither parallel nor perpendicular to the refractive index major axis direction of the refractive index ellipsoid. Further, the present invention provides the diffraction element, wherein the birefringent film is formed of a polymer liquid crystal. Further, the present invention provides the diffraction element, wherein the transparent substrate is a transparent substrate having an optical retardation or a transparent substrate having a retardation film formed thereon. Further, the diffractive element integrally provided with a wavelength-selective aperture diameter restricting portion that exhibits different transmittances for two types of light having different wavelengths, at least in a part of an effective diameter of at least one surface of the diffractive element. I will provide a.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, a photo-curing material which is uniformly oriented on a substrate having an optical phase difference such as a quartz substrate or a transparent substrate such as a glass substrate having a phase difference film formed thereon. A birefringent film made of a polymer liquid crystal having a property is formed. The alignment direction of the polymer liquid crystal constituting the birefringent film may be either horizontal to the transparent substrate and perpendicular / parallel to the incident linearly polarized light, and high transmittance can be obtained in any case.

A lattice having an uneven cross section is formed on a birefringent film made of a polymer liquid crystal that has been formed and oriented. As a means for forming the lattice, an etching method using photolithography, a pressing method using a mold having a lattice shape, or the like can be used.

At this time, since the grating is formed on the birefringent film, the grating direction is not restricted by the orientation direction of the polymer liquid crystal and the direction of the incident linearly polarized light. The direction can be neither parallel nor perpendicular to the refractive index major axis direction of the refractive index ellipsoid in the refractive film. Further, the diffraction element can be divided into a plurality of regions, and a grating having a different direction can be formed for each region.

An isotropic medium is filled in the uneven portions of the formed lattice. The refractive index of the isotropic medium is determined by the relationship between the direction of the incident linearly polarized light and the orientation direction of the polymer liquid crystal forming the birefringent film. For example, when the incident linearly polarized light and the orientation direction of the polymer liquid crystal are perpendicular to each other, the concave portion is filled with an isotropic medium having almost the same refractive index as the ordinary light refractive index of the polymer liquid crystal.
When the incident linearly polarized light and the orientation direction of the polymer liquid crystal are parallel, the concave portion is filled with an isotropic medium having a refractive index substantially equal to the extraordinary light refractive index of the polymer liquid crystal. As the isotropic material to be filled, a photocurable polymer, a thermosetting polymer, or the like can be used, and for example, an acrylic ultraviolet curable adhesive can be used.

In order to suppress the transmitted wavefront distortion of the entire diffraction element at the time of filling the isotropic medium, it is easy to sandwich and cure the cover glass or the like provided with a low reflection coating. However, in order to reduce the thickness and weight of the diffraction element, it is preferable to cure the diffraction element without a cover glass. Further, a phase difference plate can be integrated with the diffraction element, which is desirable for reducing the size and weight of the optical head device and the diffraction element.
Therefore, a quartz transparent substrate can be used as the transparent substrate having a phase difference.

Further, a transparent substrate made of glass to which an organic phase difference film is adhered can be used instead of the transparent substrate made of quartz. Instead of the organic phase difference film, an organic phase difference material such as a polymer liquid crystal having photocurability which is horizontally aligned with the transparent substrate can be used. When a substrate having a phase difference or an organic phase difference material is used as a 波長 wavelength plate (converting linearly polarized light into circularly polarized light), the phase difference between these materials is (n + ／) of the used wavelength. The optical axis of the birefringent material and the direction of the incident linearly polarized light are arranged at an angle of 45 °. Here, n is an integer of 0 or more.

The optical head device includes two types of light sources, for example, a semiconductor laser having a wavelength of 650 nm and a semiconductor laser having a wavelength of 780 nm.
In order to improve the aberration at the time of focusing caused by the difference in the thickness of DVD and CD discs when reading and reading CDs, it is known to reduce the amount of light at the outer periphery when reading CDs. It is known to use a wavelength-selective aperture diameter limiting unit as one of the means for achieving this. In the present invention, for this purpose, a wavelength-selective aperture limiter having a transmittance different by 20% or more depending on the wavelength of the two light sources is used.

For example, the wavelength selectivity of an optical multilayer film such as a dichroic mirror that transmits at least 90% of light having a wavelength of 650 nm and reflects at least 90% of light having a wavelength of 780 nm. The aperture limiter can be installed integrally with the outer periphery of the element that is cut off when reading CD data in the effective area, thereby reducing the number of components and reducing the size and weight of the optical head device. can get.

The diffractive element of the present invention has a higher degree of freedom in the design of a grating pattern than conventional diffractive elements, and can be laminated and integrated with optical components for various uses, so that it is possible to reduce the weight and thickness. .

In the case of a diffractive element having both a retardation plate and a wavelength-selective aperture diameter limiting portion, the S-wave laser light from the semiconductor laser having a wavelength of 650 nm is transmitted in the outward path (in the direction from the light source side toward the optical recording medium side). In (1), first, the light passes through the wavelength-selective aperture diameter limiting portion and is incident on all the effective regions of the birefringent film made of the polymer liquid crystal. Then, the refractive index of the convex portion of the polymer liquid crystal oriented in the direction corresponding to the P-wave is 1.
5 (ordinary light refractive index), and the concave portion has a refractive index of about 1.
5, the laser light is transmitted without being diffracted, and the linearly polarized light is converted into the circularly polarized light by the birefringent substrate or the retardation film having the phase difference.

On the return path (in the direction from the optical recording medium side to the light source side), the laser light is reflected by the optical recording medium, becomes reverse circularly polarized light, enters the diffraction element, and has a birefringence having a phase difference. This time, the light is changed to linearly polarized light orthogonal to the outward path by the conductive substrate or the phase difference film, and becomes a P wave. Then, the refractive index of the convex portion formed by the polymer liquid crystal oriented in the direction corresponding to the P wave is about 1.6 (an extraordinary light refractive index), and the refractive index of the concave portion is approximately 1.5. Function as a diffraction element, and light diffraction occurs.

On the other hand, for a laser beam having a wavelength of 780 nm, the above-described wavelength-selective aperture diameter limiting portion functions, so that only the central portion enters the diffraction element portion of the diffraction element. Then, as in the case of the wavelength of 650 nm, there is almost no difference in the refractive index on the outward path, so that the light is transmitted downward without being diffracted. Here, the retardation plate is adjusted to be a １ ／ wavelength plate with respect to the wavelength of 650 nm, so that it is 78
For a wavelength of 0 nm, it functions as an approximately 1/5 wavelength plate,
As a result, the return light from the optical recording medium does not become a complete P-wave, but most of it is diffracted.

Further, the retardation of the retardation plate of the present invention is 650.
For a wavelength of nm, it becomes a ／ wavelength plate, and 780 nm
By making the plate a 4/4 wavelength plate for the wavelength of, a diffraction element that functions for the wavelength of 650 nm, does not function for the wavelength of 780 nm, and transmits most of the light can be obtained. . In this case, a diffraction element may be separately provided before the light source having a wavelength of 780 nm,
By tilting the 780 nm incident polarization direction by 20 ° to 45 ° with respect to the m incident polarization direction, the light may be diffracted in both the forward and backward directions.

In the diffraction element of the present invention, another grating may be formed on the surface on the light source side, in which case a tracking error can be detected by a three-beam method, which is preferable. The pattern of the concave and convex portions (optically anisotropic diffractive element) of the grating in the diffractive element of the present invention may have a curvature in the plane of the diffractive element so that the beam shape of the return light from the optical recording medium has a desired shape. Also, distribution can be added to the lattice spacing.

Various light sources such as a solid-state laser such as a semiconductor laser and a YAG laser and a gas laser such as a He-Ne laser can be used for the diffraction element of the present invention. It is preferable to use a semiconductor laser in terms of miniaturization, lightening, continuous oscillation, maintenance and inspection. In addition, when using a short-wavelength laser such as a blue laser using a harmonic generator that incorporates a nonlinear optical element in a light source section such as a semiconductor laser,
High-density optical recording and reading are possible.

An optical recording medium using an optical head device equipped with the diffraction element of the present invention can record and / or record information by light.
Or a readable medium. Examples include optical disks such as CDs, CD-ROMs and DVDs, magneto-optical disks, and phase-change optical disks.

[0028]

【Example】

Embodiment 1 Embodiment 1 will be described with reference to FIGS. For light of 650 nm wavelength used in CD, 1 /
3 inches in diameter and 0.4 m thick with 4 wavelength phase difference
A phase difference plate portion 21 is formed by a quartz transparent substrate 20 having a low reflection coating 22 on the optical recording medium side surface (the lower surface in the figure) of the quartz transparent substrate 20 constituting the phase difference plate portion 21. Was given.

Next, a polyimide alignment film 23 is formed on the light source side surface (upper surface in the figure) of the quartz transparent substrate 20, and the polyimide alignment film 23 is formed at 45 ° to the optical axis of the quartz transparent substrate 20. Rubbing is performed in the direction of, and first, an unpolymerized liquid of a photo-curable high-molecular liquid crystal is applied to the polyimide alignment film 2.
3 was dropped. Next, using a facing glass substrate (not shown) in which polyimide was applied to the surface and rubbing was performed at 180 ° with the rubbing direction of the transparent substrate 20 of quartz, and the unpolymerized polymer liquid crystal was horizontally The alignment state is obtained, and polymerization is performed by irradiating an ultraviolet light having a light amount of 600 mJ. Thereafter, the above-mentioned opposite glass substrate (not shown) is released from the mold and removed by a 3.5 μm thick horizontally aligned polymer liquid crystal. A refractive film 24 is formed, and the light amount is 3000 m
After performing additional polymerization by irradiating ultraviolet light of J, annealing (annealing) is performed at 140 ° C. for 30 minutes to obtain a birefringent film 24.
Was completely solidified.

On this birefringent film 24 made of a polymer liquid crystal, an SiO ₂ film 25 was formed as a protective film to a thickness of about 50 nm by sputtering. Next, on the SiO ₂ film 25, the stripe direction of the lattice is at an angle of + 45 ° and −45 ° with respect to the rubbing direction by photolithography.
A photoresist mask having a grid shape with a pitch of 6 μm and two regions was formed.

First, using a lattice-shaped photoresist mask, using a fluorocarbon gas such as CF ₄ gas at a flow rate of 100 SCCM, under a condition of a pressure of 0.2 Torr and an output of 300 W for 5 minutes, and an ion etching, the SiO ₂ film 25 to transfer the grating pattern of the photoresist mask, to produce a SiO ₂ selection mask 26.

Next, the prepared SiO ₂ selection mask 2
6, using O ₂ gas at a flow rate of 100 SCCM,
Ashing (ashing treatment) is performed for 20 minutes under the conditions of a pressure of 0.2 Torr and an output of 300 W to remove the remaining photoresist mask by reactive ion etching, and at the same time, a depth of 3.5 μm and a pitch of 6 μm shown in FIG. Lattice 2
7 was formed on a birefringent film 24 made of a polymer liquid crystal.

[0033] After that, this time, the polymer liquid crystal (ordinary refractive index n _o = 1.5 using the birefringent film 24, the extraordinary refractive index n _e
= 1.6) the ordinary refractive index n _o is equal to the refractive index of the (n = 1.
5) filling the ultraviolet-curable isotropic material 28 having
Curing polymerization was performed by irradiating an ultraviolet light with a light amount of 5000 mJ to form a diffraction element section 29.

Further, a glass substrate 30 having a thickness of 0.3 mm
, 90% or more of light having a wavelength of 650 nm used in DVDs is transmitted by a vacuum deposition method and a lift-off method to an outer peripheral portion excluding a central portion of 2.18 mmφ, and
An optical multilayer film 31 that reflects 90% or more of light having a wavelength of 780 nm used in a CD is formed.
An SiO ₂ coat 32 for correcting the phases of the central portion and the outer peripheral portion is formed at a portion of 8 mmφ to form a wavelength-selective aperture diameter limiting portion 33, and the glass substrate 30 of the wavelength-selective aperture diameter limiting portion 33 is formed. Cover the isotropic material 28 with a light intensity of 50
The isotropic material 28 is cured and polymerized by irradiating it with 00 mJ of ultraviolet light, and the wavelength-selective aperture diameter limiting part 33 is integrated. Was given. Finally, it was cut by dicing to produce a diffraction element 35 having an outer diameter of 4 mm × 4 mm and a thickness of about 0.7 mm.

The characteristics of the diffractive element 35 manufactured as described above were examined. As a result, a DVD from a semiconductor laser (light source) (not shown) was used for the polarization of the birefringent film 24 of the polymer liquid crystal in the direction perpendicular to the rubbing direction. Same wavelength as 650nm
It was confirmed that the transmittance of the S-wave (light in the polarization direction parallel to the paper surface in FIG. 1) was 92%. Reflected light from an optical recording medium (not shown) is transmitted through the phase difference plate 21 twice in a reciprocating manner, thereby forming a P wave (light in a polarization direction perpendicular to the paper).
The diffraction efficiency of the + 1st-order diffracted light is 38%, and the diffraction efficiency of the -1st-order diffracted light is 35%.
It was confirmed that the total was 73%.

Therefore, the reciprocating efficiency is 0.92 × 0.
73 = 67%, and a practically sufficiently high diffraction efficiency was obtained. In addition, for a laser beam of 780 nm, which is the same as that of a CD, only the central portion of 2.18 mmφ has a transmittance of 91% on the outward path and a diffraction efficiency of + 1st-order diffracted light on the return path of 17%.
%, The diffraction efficiency of the -1st-order diffracted light was 19%, which was a total of 36%. Therefore, the reciprocating efficiency is 0.9
1 x 0.36 = 33%. The wavefront aberration of the transmitted light is
It was 0.020λ _rms (root mean square) or less at the center of the light entrance / exit surface of the diffraction element 35 (circular range having a diameter of 2 mm).

Embodiment 2 Embodiment 2 will be described with reference to FIG. A transparent substrate 37 made of glass having a diameter of 3 inches and a thickness of 0.5 mm having a low reflection coat 36 on the surface on the optical recording medium side (the lower surface in the figure) is prepared. A polyimide alignment film 38 is formed on the light source side surface (upper surface in the figure), and the polyimide alignment film 38 is subjected to horizontal alignment treatment by rubbing. A molecular liquid crystal is applied and, based on the refractive index difference (Δn ≒ 0.1) of the polymer liquid crystal after polymerization, ultraviolet light is applied so that the phase difference becomes approximately ／ wavelength times of 650 nm, which is the wavelength used for DVD. The polymer liquid crystal was polymerized and cured to form a retardation film 39 of about 8 μm of the polymer liquid crystal, thereby forming a retardation plate portion 40. Thereafter, a 50 nm thick SiO ₂ film 41 was formed as a protective film on the retardation film 39 made of a polymer liquid crystal by a sputtering method.

Next, a polyimide alignment film 42 is formed again on the SiO ₂ film 41, and rubbing is performed on the polyimide alignment film 42 so as to be horizontally oriented in a direction of 45 ° with respect to the optical axis of the retardation plate 40. Processing was performed. Then, an unpolymerized photo-curable polymer liquid crystal is applied on the polyimide alignment film 42, and the polymer liquid crystal is horizontally aligned using a counter glass substrate (not shown), and then irradiated with 600 mJ of ultraviolet light. Then, the opposite glass substrate is released from the mold, and a birefringent film 43 of 3.5 μm-thick horizontally oriented polymer liquid crystal is formed, and further irradiated by irradiating 3000 mJ of ultraviolet light. After the polymerization, annealing was performed at 140 ° C. for 30 minutes to obtain a birefringent film 4 made of a polymer liquid crystal.
3 was completely solidified.

On the birefringent film 43 made of the polymer liquid crystal, an SiO ₂ film 44 was formed to a thickness of about 50 nm by sputtering. Next, a photoresist mask having a lattice shape with a pitch of 6 μm, in which the direction of the lattice is at 45 ° to the rubbing direction, was formed by photolithography.

First, using a lattice-shaped photoresist mask, a pressure of 0.2 Torr and an output of 300 using a fluoride gas such as CF ₄ gas at a flow rate of 100 SCCM.
Under the conditions of W carried reactive ion etching for 5 minutes, then transfer the pattern of the photoresist mask on the SiO ₂ film 44, to prepare a SiO ₂ selection mask 45.

Next, the formed SiO ₂ selection mask 4
5, using O ₂ gas at a flow rate of 100 SCCM,
Ashing is performed for 20 minutes under the conditions of a pressure of 0.2 Torr and an output of 300 W to remove the remaining photoresist by reactive ion etching and at the same time to a depth of 3.5 μm.
A grid 46 having m and a pitch of 6 μm was formed on the birefringent film 43 made of a polymer liquid crystal.

[0042] After that, this time, the polymer liquid crystal (ordinary refractive index n _o = 1.5 using the birefringent film 43, the extraordinary refractive index n _e
= 1.6) the ordinary refractive index n _o is equal to the refractive index of the (n = 1.
The ultraviolet-curable isotropic material 47 having the above-mentioned 5) was applied and filled, and was cured and polymerized by irradiation with an ultraviolet light having a light amount of 5000 mJ to form the diffraction grating portion 48. And the isotropic material 4
7, a 50 nm SiO ₂ film 49 is formed as a protective film, a low reflection coat 50 is applied on the SiO ₂ film 49, and then cut by dicing to obtain an outer diameter of 4 mm × 4 mm.
A diffraction element 51 having a thickness of about 0.5 mm was manufactured.

The characteristics of the diffractive element 51 manufactured in this manner were examined. As a result, a semiconductor liquid (not shown) was applied to the birefringent film 43 of the polymer liquid crystal constituting the diffraction grating portion 48 in the direction perpendicular to the rubbing direction. D from laser (light source)
It was confirmed that the transmittance of an S wave having a wavelength of 650 nm (light in a polarization direction parallel to the paper surface in FIG. 1) equal to VD was 91%. The reflected light from the optical recording medium (not shown) is transmitted through the phase difference plate twice in a reciprocating manner, and becomes a P wave (light in a polarization direction perpendicular to the paper surface) and is incident on the diffraction grating portion 48.
It was confirmed that the diffraction efficiency of the + 1st-order diffracted light was 37% and the diffraction efficiency of the -1st-order diffracted light was 35%, for a total of 72%.

Therefore, the reciprocating efficiency is 0.91 × 0.
72 = 66%, and a practically sufficiently high diffraction efficiency was obtained. The wavefront aberration of the transmitted light is 0.025 at the center of the light entrance / exit surface of the diffraction element 51 (circular range having a diameter of 2 mm).
λ _rms (root mean square).

[0045]

According to the diffractive element of the present invention, the retardation plate can be built in, it is lightweight and small, and there is no restriction on the angle between the grating direction and the incident polarization direction, and industrial production is possible and high. An excellent effect that a diffraction element having light use efficiency can be provided is exhibited.

[Brief description of the drawings]

FIG. 1 is a side sectional view of a diffraction element according to a first embodiment.

FIG. 2 is a view taken in the direction of arrows II-II in FIG.

FIG. 3 is a side sectional view of a diffraction element according to a second embodiment.

FIG. 4 is a side sectional view of a diffraction element or a hologram element using a dry etching method.

FIG. 5 is a side sectional view showing a polarizing diffraction element.

[Explanation of symbols]

 20, 37: transparent substrate 24, 43: birefringent film 27, 46: grating 28, 47: isotropic material 33: wavelength-selective aperture diameter limiting portion 39: retardation film

Claims (5)

[Claims] 1. A transparent substrate, a lattice formed on the transparent substrate and having a birefringent film having a concave and convex cross section, and a concave portion of the lattice filled in a concave portion having a refractive index of ordinary light refraction of the birefringent film. A isotropic material having a refractive index substantially equal to the refractive index or the extraordinary light refractive index. 2. The diffraction element according to claim 1, wherein the grating is formed on the birefringent film in a direction that is neither parallel nor perpendicular to the refractive index major axis direction of the refractive index ellipsoid. 3. The diffraction element according to claim 1, wherein the birefringent film is formed of a liquid crystal polymer. 4. The diffraction element according to claim 1, wherein the transparent substrate is a transparent substrate having an optical retardation or a transparent substrate having a retardation film formed thereon. 5. A wavelength-selective aperture diameter limiting portion that exhibits different transmittances for two types of light having different wavelengths, at least in a part of an effective diameter of at least one surface of the diffraction element. The diffraction element according to claim 1, 2, 3, or 4.