JP3978821B2 - Diffraction element - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a diffractive element used for an optical head device for writing optical information on optical disks such as CDs, CD-ROMs, video disks, and magneto-optical disks, and reading optical information.
[0002]
[Prior 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) signal light reflected from a recording surface of a disk-shaped optical recording medium to a light detection unit. Conventionally known optical components include those using a diffraction element or a hologram element and those using a prism type beam splitter.
[0003]
A conventional diffraction element or hologram element for an optical head device is formed by forming a relief-like grating having a rectangular cross section on a glass or plastic substrate by a dry etching method or an injection molding method. The beam was diffracted and a beam splitting function was added.
[0004]
Among these diffraction elements or hologram elements, an example using a dry etching method is shown in FIG.
The upper surface of the glass substrate 2 itself provided with the low reflection coating 1 on the lower surface side, or the upper surface of the inorganic thin film 3 such as SiO _{2 formed} on the glass substrate 2 by using a vacuum process such as vapor deposition or sputtering. In addition, a lattice-like photoresist mask is prepared by photolithography, and in this state, dry etching is performed to form an inorganic lattice 4 that is a lattice of the inorganic thin film 3 in the inorganic thin film 3 portion. After removing the photoresist remaining on the substrate, a low reflection coating 5 is applied to create a diffraction element, which is used as a polarization-independent isotropic diffraction element.
[0005]
In such an isotropic diffractive element, the light utilization efficiency is as low as about 10%, and it is conceivable to use polarized light when the light utilization efficiency is to be increased beyond 10%.
[0006]
In view of this, an optical head device provided with a hologram (diffraction element) having high light utilization efficiency using polarized light is proposed in Japanese Patent Laid-Open No. 9-180236. The proposed polarizing diffraction element is that shown in FIG. First, an inorganic isotropic thin film 8 having a refractive index substantially equal to an ordinary light refractive index or an extraordinary light refractive index of a birefringent material such as a liquid crystal 7 described later is formed on a glass substrate 6, and then, A lattice-like photoresist mask is formed on the isotropic thin film 8 by photolithography, and in this state, dry etching is performed to form the inorganic lattice 9, and the photoresidue remaining on the inorganic lattice 9 is further formed. After removing the resist, the alignment film 10 is applied and baked to perform the alignment process, and then the counter substrate 12 having the alignment film 11 subjected to the same alignment process is faced to each other, and the sealing material 13 is interposed therebetween to heat the film. A glass substrate in which a diffractive element portion 14 is formed by sealing and filling a birefringent material such as a liquid crystal 7 inside, and an organic retardation film 15 is attached to the lower surface side of the glass substrate 6 Attached so as to sandwich the 6 form a phase feedboard unit 17 performs low-reflective coating 18, 19 on both sides.
[0007]
[Problems to be solved by the invention]
In the case of the polarizing diffractive element shown in FIG. 5, the normal liquid crystal 7 is aligned along the extending direction of the concavo-convex portion in the grating, that is, along the grating direction. 7 corresponds to the ordinary light refractive index, and the extraordinary light refractive index of the liquid crystal 7 corresponds to the polarized light parallel to the lattice direction. Therefore, in the above optical head device, there is a restriction that the grating direction is either parallel or perpendicular to the polarization direction of incident light and is substantially uniform in the diffraction element section 14.
[0008]
Further, since the glass substrate 6, the counter substrate 12, and the sealing material 13 are necessary to enclose the liquid crystal 7, there has been a problem in the element size with respect to the effective diameter.
Further, in the diffraction element having the retardation plate portion 17 for switching the polarization, when an inexpensive organic retardation film 15 is used for the retardation plate portion 17, the transmitted wavefront of the organic retardation film 15 alone is provided. The distortion and the reliability with respect to the reaction with the liquid crystal 7 filled in the diffractive element portion 14 become problems, and three glasses are necessary to solve this problem, which has been a problem in reducing the thickness and weight of the diffractive element. .
[0009]
The present invention solves the above-mentioned problems, can incorporate a phase difference plate, is lightweight and compact, has no restrictions on the angle between the grating direction and the incident polarization direction, can be industrially produced, and has high light utilization efficiency. An object of the present invention is to provide a diffraction element having
[0010]
[Means for Solving the Problems]
The present invention relates to a method for manufacturing a diffraction element, wherein the diffraction element is filled in a transparent substrate, a grating formed on the transparent substrate with a birefringent film having a concavo-convex cross section, and a concave portion of the grating. And a birefringent element comprising an isotropic material having a refractive index substantially equal to an ordinary refractive index or an extraordinary refractive index of the birefringent film, wherein the birefringent film is made of a polymer liquid crystal. A birefringent film comprising the steps of: forming a birefringent film made of a polymer liquid crystal on the transparent substrate and completely solidifying; and etching the birefringent film by photolithography. There is provided a method for manufacturing a diffraction element, comprising: a step of forming; and a step of filling the isotropic material into a grating made of the birefringent film . In this case, the step of forming a lattice by etching by photolithography includes a step of forming a protective film on the birefringent film and a photoresist mask having a lattice shape on the protective film. Preferably, the method includes a step of producing a selective mask made of the protective film by etching using a mask, and a step of producing a grating made of the birefringent film using the selective mask made of the protective film, The protective film is preferably a SiO ₂ film .
[0012]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, from a polymer liquid crystal having photo-curability uniformly oriented on a substrate having an optical retardation such as a quartz substrate or a transparent substrate such as a glass substrate on which a retardation film is formed. A birefringent film is formed. The alignment direction of the polymer liquid crystal constituting the birefringent film may be either horizontal with respect to the transparent substrate and vertical / parallel to the incident linearly polarized light. In either case, high transmission can be achieved.
[0013]
A lattice having a concavo-convex cross section is formed on a birefringent film made of polymer liquid crystal that is formed and oriented. As a means for forming a lattice, an etching method using photolithography, a press method using a mold having a lattice shape, or the like can be used.
[0014]
At this time, since the grating is formed in the birefringent film, the grating direction is not restricted by the alignment direction of the polymer liquid crystal and the incident linear polarization direction, so any direction can be selected, and the birefringent film by the polymer liquid crystal The refractive index ellipsoid can be in a direction that is neither parallel nor perpendicular to the major axis direction of the refractive index. 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.
[0015]
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 incident linear polarization direction and the alignment direction of the polymer liquid crystal constituting the birefringent film. For example, when the incident linearly polarized light and the orientation direction of the polymer liquid crystal are perpendicular, the concave portion is filled with an isotropic medium in which the ordinary refractive index and refractive index of the polymer liquid crystal are approximately equal, and the incident linearly polarized light And the alignment direction of the polymer liquid crystal are parallel, the recess is filled with an isotropic medium having an approximately equal refractive index and 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. For example, an acrylic ultraviolet curable adhesive can be used.
[0016]
In order to suppress the transmitted wavefront distortion of the entire diffractive element at the time of filling the isotropic medium, a method of sandwiching and curing with a cover glass having a low reflection coating is easy. However, in order to reduce the thickness and weight of the diffractive element, it is desirable to cure it without a cover glass. Further, a retardation 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 a transparent substrate having a phase difference.
[0017]
Moreover, the glass transparent substrate which affixed the organic type phase difference film instead of the quartz transparent substrate can also be used. Instead of the organic retardation film, an organic retardation material such as a polymer liquid crystal having photo-curing property that is horizontally aligned with respect to the transparent substrate can also be used. When using a substrate having retardation or an organic retardation material as a quarter-wave plate (converting linearly polarized light to circularly polarized light), the phase difference of these materials is (n + 1/4) of the wavelength used. The optical axis of the birefringent material and the direction of incident linearly polarized light are arranged at an angle of 45 °. Here, n is an integer of 0 or more.
[0018]
In addition, when the optical head device includes two types of light sources, for example, a semiconductor laser with a wavelength of 650 nm and a semiconductor laser with a wavelength of 780 nm, and the same condenser lens is used for DVD reading and CD reading, respectively, DVD and CD In order to improve the aberration at the time of condensing that occurs due to the difference in the thickness of the disk, there is known a method of reducing the amount of light at the outer periphery during reading of the CD, and one of the means for this is wavelength selectivity. It is known to use an opening diameter limiting portion. In the present invention, for this purpose, a wavelength-selective aperture diameter limiter having a transmittance of 20% or more depending on the wavelengths of the two light sources is used.
[0019]
For example, a wavelength-selective aperture diameter limiting unit made of an optical multilayer film such as a dichroic mirror that transmits 90% or more of light having a wavelength of 650 nm and reflects 90% or more of light having a wavelength of 780 nm. Can be integrated and installed on the outer periphery of the element that is discarded when reading out the CD data in the effective area, thereby obtaining the effects of reducing the number of parts and reducing the size and weight of the optical head device.
[0020]
The diffractive element of the present invention has a higher degree of freedom in designing a grating pattern than conventional diffractive elements, and optical components for various applications can be laminated and integrated, so that the weight and thickness can be reduced.
[0021]
In the case of a diffractive element having both a retardation plate and a wavelength-selective aperture diameter limiter, the S-wave laser beam from a semiconductor laser having a wavelength of 650 nm is in the forward path (the direction from the light source side to the optical recording medium side). First, the light passes through the wavelength-selective aperture diameter limiting portion and enters all 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 aligned in the direction corresponding to the P wave is about 1.5 (ordinary refractive index), and the refractive index of the concave portion is also about 1.5. Linearly polarized light is converted into circularly polarized light by a birefringent substrate or retardation film that transmits without being diffracted and has a phase difference.
[0022]
In 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, and enters the diffractive element. By the retardation film, this time the light is changed to linearly polarized light orthogonal to the forward path and becomes a P wave. Then, the refractive index of the convex portion formed by the polymer liquid crystal aligned in the direction corresponding to the P wave is about 1.6 (abnormal light refractive index), and the refractive index of the concave portion is about 1.5. It functions as a diffraction element, and light diffraction occurs.
[0023]
On the other hand, for the laser beam having a wavelength of 780 nm, since the above-described wavelength-selective aperture diameter limiting portion functions, only the central portion enters the diffraction element portion of the diffraction element. Since there is almost no difference in refractive index in the forward path as in the case where the wavelength is 650 nm, the light is transmitted downward without being diffracted. Here, since the retardation plate is adjusted to be a quarter wavelength plate with respect to the wavelength of 650 nm, the retardation plate functions as an about 1/5 wavelength plate with respect to the wavelength of 780 nm, and as a result, an optical recording medium. The return light from the light does not become a perfect P wave, but most of it is diffracted.
[0024]
In addition, the retardation of the retardation plate of the present invention is a 5/4 wavelength plate for a wavelength of 650 nm, and is a 4/4 wavelength plate for a wavelength of 780 nm. It is also possible to make a diffractive element that functions and does not function for a wavelength of 780 nm and transmits most of it. In this case, a diffractive element may be separately provided in front of the light source having a wavelength of 780 nm. By diffracting the incident polarization direction of 780 nm with respect to the incident polarization direction of 650 nm by 20 to 45 °, both the outward and return paths are diffracted. Also good.
[0025]
The diffraction element of the present invention may be formed with another grating on the surface on the light source side, and in that case, tracking error detection by the three-beam method is preferable.
The pattern of the concavo-convex portion (optically anisotropic diffractive element) of the grating in the diffraction element of the present invention has a curvature in the plane of the diffractive element so that the beam shape of the return light from the optical recording medium becomes a desired shape. It is also possible to give a distribution to the lattice spacing.
[0026]
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 reduction in size and weight, continuous oscillation, maintenance and inspection. In addition, when a harmonic generation device incorporating a nonlinear optical element in a light source unit such as a semiconductor laser is used and a short wavelength laser such as a blue laser is used, high-density optical recording and reading can be performed.
[0027]
An optical recording medium using an optical head device equipped with the diffraction element of the present invention is a medium on which information can be recorded and / or read by light. Examples thereof include optical disks such as CDs, CD-ROMs, and DVDs, magneto-optical disks, and phase change optical disks.
[0028]
【Example】
[Example 1]
A first embodiment will be described with reference to FIGS. A phase difference plate portion 21 is constituted by a crystal transparent substrate 20 having a diameter of 3 inches and a thickness of 0.4 mm and having a phase difference of ¼ wavelength with respect to light having a wavelength of 650 nm used in a CD. A low reflection coating 22 was applied to the optical recording medium side surface (the lower surface in the figure) of the quartz transparent substrate 20 constituting the portion 21.
[0029]
Next, a polyimide alignment film 23 is formed on the light source side surface (upper surface in the figure) of the crystal transparent substrate 20, and the polyimide alignment film 23 is oriented at 45 ° to the optical axis of the crystal transparent substrate 20. First, an unpolymerized liquid of a polymer liquid crystal having photocurability was dropped on the polyimide alignment film 23. Next, an unpolymerized polymer liquid crystal is horizontally applied using a counter glass substrate (not shown) which has been subjected to a release treatment after applying polyimide on the surface and rubbing the crystal transparent substrate 20 in the rubbing direction and 180 °. In the aligned state, polymerization is performed by irradiating with ultraviolet light having a light amount of 600 mJ, and then the above-mentioned counter glass substrate (not shown) is removed from the mold, and then the composite liquid crystal is formed by a horizontally aligned polymer liquid crystal having a thickness of 3.5 μm. The refractive film 24 was formed, and after additional polymerization was performed by irradiating ultraviolet light having a light amount of 3000 mJ, annealing (annealing) was performed at 140 ° C. for 30 minutes to completely solidify the birefringent film 24.
[0030]
On this birefringent film 24 made of polymer liquid crystal, a SiO ₂ film 25 of about 50 nm was formed as a protective film by sputtering. Next, on the SiO ₂ film 25, a photoresist mask having a lattice shape of 6 μm pitch having two regions in which the stripe direction of the lattice forms an angle of + 45 ° and −45 ° with respect to the rubbing direction by photolithography. Formed.
[0031]
First, reactive ion etching is performed for 5 minutes under the conditions of a pressure of 0.2 Torr and an output of 300 W using a fluorocarbon gas such as a CF ₄ gas having a flow rate of 100 SCCM, using a lattice-shaped photoresist mask. Then, the lattice pattern of the photoresist mask was transferred to the SiO ₂ film 25 to produce a SiO ₂ selection mask 26.
[0032]
Next, ashing is performed for 20 minutes under the conditions of a pressure of 0.2 Torr and an output of 300 W using an O ₂ gas having a flow rate of 100 SCCM, using the produced SiO ₂ selection mask 26, and reactive. At the same time as removing the remaining photoresist mask by ion etching, a grating 27 having a depth of 3.5 μm and a pitch of 6 μm shown in FIG.
[0033]
Thereafter, this, ordinary refractive index n _o is equal to the refractive index of the liquid crystal polymer used in the birefringent film 24 (ordinary refractive index n _o = 1.5, extraordinary refractive index n _e = 1.6) (n = 1.5) and is cured and polymerized by irradiation with ultraviolet light with a light amount of 5000 mJ to form a diffraction element portion 29.
[0034]
Furthermore, the outer peripheral portion of the glass substrate 30 with a thickness of 0.3 mm excluding the central 2.18 mmφ portion transmits 90% or more of light having a wavelength of 650 nm, which is used in a DVD, by a vacuum deposition method and a lift-off method. In addition, an optical multilayer film 31 that reflects 90% or more of light having a wavelength of 780 nm used in a CD is formed, and an SiO ₂ coat for correcting the phase of the central portion and the outer peripheral portion in the central 2.18 mmφ portion 32 is formed to constitute a wavelength-selective opening diameter restricting portion 33, the isotropic material 28 is covered with the glass substrate 30 of the wavelength-selective opening diameter restricting portion 33, and ultraviolet light having a light amount of 5000 mJ is irradiated. The isotropic material 28 was cured and polymerized, the wavelength-selective opening diameter restricting portion 33 was integrated, and the low-reflection coating 34 was applied on the wavelength-selective opening diameter restricting portion 33. 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.
[0035]
When the characteristics of the diffractive element 35 thus manufactured were examined, the same wavelength as that of a DVD from a semiconductor laser (light source) (not shown) was applied to polarized light in a direction perpendicular to the rubbing direction of the birefringent film 24 by polymer liquid crystal. It was confirmed that the transmittance of S wave of 650 nm (light having a polarization direction parallel to the paper surface in FIG. 1) was 92%. Reflected light from an optical recording medium (not shown) passes through the retardation plate 21 twice in a reciprocating manner, and becomes a P wave (light having a polarization direction perpendicular to the paper surface) and enters the diffraction grating 29. It was confirmed that the diffraction efficiency of the + 1st order diffracted light was 38% and the diffraction efficiency of the −1st order diffracted light was 35%, which was 73% in total.
[0036]
Therefore, the reciprocal efficiency was 0.92 × 0.73 = 67%, and a sufficiently high diffraction efficiency was obtained in practical use. For the same 780 nm laser light as CD, the forward transmittance is 91%, the + 1st order diffracted light diffraction efficiency is 17%, and the −1st order diffracted light is only for the central 2.18 mmφ portion. Was found to be a total of 36% at 19%. Therefore, the round-trip efficiency was 0.91 × 0.36 = 33%. The wavefront aberration of the transmitted light was 0.020λ _rms (root mean square) or less at the central portion (circular range of 2 mm in diameter) of the light incident / exit surface of the diffraction element 35.
[0037]
[Example 2]
A second embodiment will be described with reference to FIG. A glass transparent substrate 37 having a diameter of 3 inches and a thickness of 0.5 mm is prepared by applying a low-reflection coating 36 to the optical recording medium side surface (lower surface in the figure). A polyimide alignment film 38 is formed on the light source side surface (upper surface in the drawing), and after the horizontal alignment treatment by rubbing is performed on the polyimide alignment film 38, an unpolymerized photo-curing high on the polyimide alignment film 38. Molecular liquid crystal is applied, and based on the refractive index difference (Δn≈0.1) of the polymer liquid crystal after polymerization, UV light is used so that the phase difference is about 5/4 times the 650 nm wavelength used for DVD. The polymer liquid crystal was polymerized and cured to form a retardation film 39 made of a polymer liquid crystal having a thickness of about 8 μm. After that, a 50 nm SiO ₂ film 41 was formed as a protective film on the retardation film 39 made of polymer liquid crystal by sputtering.
[0038]
Next, a polyimide alignment film 42 is formed again on the SiO ₂ film 41, and a rubbing process is performed so that the polyimide alignment film 42 is horizontally aligned in a direction of 45 ° with respect to the optical axis of the retardation plate 40. did. Then, an unpolymerized photocurable 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 ultraviolet light having a light amount of 600 mJ. Then, the opposing glass substrate is released and removed to form a birefringent film 43 made of a horizontally oriented polymer liquid crystal having a thickness of 3.5 μm, and further irradiated with ultraviolet light having a light intensity of 3000 mJ. After the polymerization, annealing was performed at 140 ° C. for 30 minutes to completely solidify the birefringent film 43 made of polymer liquid crystal.
[0039]
An SiO ₂ film 44 having a thickness of about 50 nm was formed on the birefringent film 43 made of the polymer liquid crystal by sputtering. Next, a photoresist mask having a lattice shape with a pitch of 6 μm in which the direction of the lattice is 45 ° with respect to the rubbing direction is formed by photolithography.
[0040]
First, reactive ion etching is performed for 5 minutes under the conditions of a pressure of 0.2 Torr and an output of 300 W using a lattice-like photoresist mask and a fluoride gas such as CF ₄ gas having a flow rate of 100 SCCM. Then, a photoresist mask pattern was transferred to the SiO ₂ film 44 to produce a SiO ₂ selection mask 45.
[0041]
Next, ashing was performed for 20 minutes under the conditions of a pressure of 0.2 Torr and an output of 300 W using O ₂ gas having a flow rate of 100 SCCM using the produced SiO ₂ selection mask 45, and remained by reactive ion etching. Simultaneously with the removal of the photoresist, a grating 46 having a depth of 3.5 μm and a pitch of 6 μm was formed on the birefringent film 43 of polymer liquid crystal.
[0042]
Thereafter, this, ordinary refractive index n _o is equal to the refractive index of the polymer liquid crystal used in the birefringent film 43 (ordinary refractive index n _o = 1.5, extraordinary refractive index n _e = 1.6) (n = 1.5) is applied and filled, and is cured and polymerized by irradiation with ultraviolet light with a light amount of 5000 mJ to form a diffraction grating portion 48. Then, a SiO ₂ film 49 having a thickness of 50 nm is formed as a protective film on the isotropic material 47, and the low reflection coating 50 is applied on the SiO ₂ film 49, and then cut by dicing to have an outer diameter of 4 mm × 4 mm. A diffraction element 51 having a thickness of about 0.5 mm was produced.
[0043]
The characteristics of the diffractive element 51 thus manufactured were examined. As a result, a semiconductor laser (light source) (not shown) was used for polarized light in a direction perpendicular to the rubbing direction of the birefringent film 43 by the polymer liquid crystal constituting the diffraction grating portion 48. The transmittance of the S wave having the same wavelength as that of the DVD from 650 nm (light having a polarization direction parallel to the paper surface in FIG. 1) was 91%. Reflected light from an optical recording medium (not shown) passes through the retardation plate twice in a reciprocating manner, and enters a diffraction grating portion 48 as a P wave (light having a polarization direction perpendicular to the paper surface). It was confirmed that the diffraction efficiency of the folded light was 37% and the diffraction efficiency of the −1st order diffracted light was 35%, which was 72% in total.
[0044]
Therefore, the round-trip efficiency was 0.91 × 0.72 = 66%, and a sufficiently high diffraction efficiency was obtained in practical use. The wavefront aberration of the transmitted light was 0.025λ _rms (root mean square) or less at the center of the light incident / exit surface of the diffractive element 51 (circular range with a diameter of 2 mm).
[0045]
【The invention's effect】
According to the diffractive element of the present invention, a retardation plate can be built in, light and small, and there is no restriction on the angle between the grating direction and the incident polarization direction, industrial production is possible, and high light utilization efficiency is achieved. There is an excellent effect that a diffractive element can be provided.
[Brief description of the drawings]
1 is a side cross-sectional view of a diffraction element according to Embodiment 1. FIG.
FIG. 2 is a view taken along arrow II-II in FIG.
3 is a side cross-sectional view of the diffraction element of Example 2. FIG.
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 restricting part 39: retardation film

Claims (3)

A method of manufacturing a diffraction element,
The diffractive element includes a transparent substrate, a grating formed on the transparent substrate with a birefringent film having a concavo-convex cross section, and a refractive index of the birefringent film filled in a concave portion of the birefringent film. A diffractive element comprising an isotropic material having a refractive index substantially equal to the refractive index or extraordinary light refractive index,
The birefringent film is a birefringent film made of a polymer liquid crystal,
The manufacturing method includes:
Forming a completely solidified birefringent film made of a polymer liquid crystal on the transparent substrate;
Forming a grating by photolithography etching the birefringent film;
And a step of filling the isotropic material with a grating made of the birefringent film. Forming a lattice by etching by photolithography,
Forming a protective film on the birefringent film;
Forming a lattice-shaped photoresist mask on the protective film, and etching using the photoresist mask to produce a selective mask made of the protective film;
Producing a grating made of the birefringent film using a selection mask made of the protective film;
A method for manufacturing a diffraction element according to claim 1. The method for manufacturing a diffraction element according to claim _2, wherein the protective film is a SiO ₂ film.

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