Wire grid polarizer and optical isolator
The wire grid polarizer with aluminum and β-FeSi₂/amorphous FeSi₂ layers effectively addresses the issue of infrared reflectivity, enhancing optical isolator performance by blocking return light in infrared communication systems.
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2025-09-16
- Publication Date
- 2026-04-16
AI Technical Summary
Existing wire grid polarizers used as incident polarizers in polarization-dependent optical isolators fail to effectively block infrared reflectivity, leading to issues with return light reflection.
A wire grid polarizer design incorporating a transparent substrate with grid-like protrusions featuring an aluminum layer and additional β-FeSi₂ and amorphous FeSi₂ layers, structured to reduce infrared reflectivity by managing the interference of polarized light waves.
The proposed design significantly reduces infrared reflectivity, enabling effective blocking of return light and improving the performance of optical isolators in infrared light communication systems.
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Figure JP2025032482_16042026_PF_FP_ABST
Abstract
Description
Wire grid polarizers and optical isolators
[0001] This invention relates to a wire grid polarizer and an optical isolator.
[0002] In optical communications using a semiconductor laser that generates infrared light with a wavelength of 1.31 μm or 1.55 μm as the light source and a silica fiber as the transmission path, optical isolators are used to achieve a low error rate. Optical isolators transmit light in the forward direction and block light in the reverse direction (return light), but polarization-dependent optical isolators include an incident polarizer, a Faraday rotator, and an exit polarizer.
[0003] Polarizing glass is known as a polarizer for polarization-dependent optical isolators (see, for example, Patent Document 1). Polarizing glass consists of metal particles with shape anisotropy dispersed in a glass material. Examples of metals that make up the metal particles include copper, silver, gold, and platinum. Polarizing glass is made of inorganic materials and has high heat resistance, making it suitable as a polarizer for polarization-dependent optical isolators.
[0004] On the other hand, a wire grid polarizer that operates in the infrared region is known as a polarizer composed of inorganic materials (see, for example, Patent Document 2). Here, the wire grid polarizer has a first surface on which a plurality of parallel metal wires are formed at periodic intervals less than or equal to the wavelength of the incident light, and a second surface having an anti-reflective structure having a period less than or equal to the wavelength of the incident light. Examples of metals that make up the metal wires include gold, silver, aluminum, nickel, chromium, tungsten, tungsten silicide, and copper.
[0005] Japanese Patent Publication No. 2008-299329 Japanese Patent Publication No. 2013-24982
[0006] However, when the wire grid polarizer described in Patent Document 2 is used as the incident polarizer for a polarization-dependent optical isolator, it is not possible to block the reflected light caused by the reflection of infrared light from the first surface. For this reason, it is desirable to reduce the infrared reflectivity of the wire grid polarizer.
[0007] The present invention aims to provide a wire grid polarizer capable of reducing infrared reflectivity.
[0008] (1) A transparent substrate and a grid-like protrusion arranged on one surface of the transparent substrate at a pitch of 1 / 2 or less of the wavelength of the incident infrared light and extending in a predetermined direction, wherein the grid-like protrusion has an aluminum layer or an aluminum alloy layer, and on one side of the aluminum layer or aluminum alloy layer, β-FeSi ２ Layers and amorphous FeSi ２ A wire grid polarizer with additional layers.
[0009] (2) Starting from the side of the transparent substrate, the β-FeSi ２ Layer and the amorphous FeSi ２ A wire grid polarizer according to (1), having layers.
[0010] (3) The β-FeSi ２ Layer and the amorphous FeSi ２ A wire grid polarizer as described in (1) or (2), wherein the sum of the layer thicknesses is 30 nm or more and 100 nm or less.
[0011] (4) The wire grid polarizer according to any one of (1) to (3), further comprising an inorganic oxide layer formed on the lattice-like protrusions.
[0012] (5) A wire grid polarizer according to any one of (1) to (4), wherein the wavelength of the infrared radiation is 0.8 μm or more and 2.0 μm or less, and the reflectance of the infrared radiation is 10% or less.
[0013] (6) An optical isolator comprising a wire grid polarizer as described in any one of (1) to (5) and a Faraday rotator.
[0014] (7) The optical isolator according to (6), wherein the wire grid polarizer is formed on the Faraday rotor.
[0015] According to the present invention, a wire grid polarizer capable of reducing infrared reflectivity can be provided.
[0016] A cross-sectional view showing a wire grid polarizer according to an embodiment of the present invention. A top view showing the wire grid polarizer of FIG. 1. β-FeSi ２ is the XRD spectrum of. A cross-sectional view showing a wire grid polarizer according to another embodiment of the present invention. A schematic diagram showing an optical isolator according to an embodiment of the present invention. The infrared reflection spectrum of the test piece of Comparative Example 1. The infrared reflection spectrum of the test piece of Example 1. A graph showing the optical admittance locus of the laminate of Example 1. A graph showing the optical admittance locus of the laminate of Example 2. A graph showing the optical admittance locus of the laminate of Example 3. A cross-sectional SEM image of the wire grid polarizer of Example 4. A graph showing the polarization characteristics of the wire grid polarizer of Example 4.
[0017] Hereinafter, embodiments for carrying out the present invention will be described.
[0018] [Wire grid polarizer] FIG. 1 shows a wire grid polarizer according to an embodiment of the present invention.
[0019] The wire grid polarizer 10 includes a transparent substrate 11 and grid-shaped convex portions 12 arranged on one surface of the transparent substrate 11 at a pitch of 1 / 2 or less of the wavelength of the incident infrared rays and extending in the depth direction in the figure. The wire grid polarizer 10 transmits a polarized light wave (TM wave) whose electric field direction is perpendicular and reflects or absorbs a polarized light wave (TE wave) whose electric field direction is parallel with respect to the direction in which the grid-shaped convex portions 12 extend (see FIG. 2). Here, the grid-shaped convex portion 12 has an aluminum layer 12a, and a β-FeSi ２ layer 12b and an amorphous FeSi ２ layer 12c are further provided below the aluminum layer 12a. Therefore, as shown in FIG. 1, when infrared rays are incident from the side where the grid-shaped convex portions 12 of the wire grid polarizer 10 are not formed, that is, from the side where the aluminum layer 12a of the grid-shaped convex portions 12 is not formed, the infrared reflectance of the wire grid polarizer 10 is reduced. At this time, the TE wave is β-FeSi ２ layer 12b and amorphous FeSi ２After passing through the layer 12c, it is reflected by the aluminum layer 12a. However, the TE wave reflected by the aluminum layer 12a interferes and attenuates when passing through the amorphous FeSi ２ layer 12c and β-FeSi ２ layer 12b.
[0020] In contrast, for example, on one side of the aluminum layer 12a, a wire grid polarizer provided with lattice-shaped convex portions having no β-FeSi ２ layer 12b and / or amorphous FeSi ２ layer 12c has a higher infrared reflectivity. When such a wire grid polarizer is used as the incident polarizer of an optical isolator for optical communication, it is impossible to block the return light to a light source (for example, a semiconductor laser) due to the reflection of infrared rays.
[0021] Note that the wire grid polarizer 10 may have an aluminum alloy layer instead of the aluminum layer 12a. Examples of elements other than aluminum constituting the aluminum alloy layer include copper and silicon, and two or more kinds may be used in combination.
[0022] Note that β-FeSi ２ layer 12b and amorphous FeSi ２ layer 12c are not particularly limited in the stacking order. From the viewpoint of ease of manufacturing, the lattice-shaped convex portion 12 preferably has β-FeSi ２ layer 12b and amorphous FeSi ２ layer 12c in this order from the side of the transparent substrate 11.
[0023] Also, FIG. 3 shows the XRD spectrum of β-FeSi ２ .
[0024] β-FeSi ２ layer 12b and amorphous FeSi ２ The sum of the thicknesses of layer 12c is preferably 30 nm or more and 100 nm or less, and more preferably 30 nm or more and 80 nm or less. β-FeSi ２ layer 12b and amorphous FeSi ２When the sum of the thicknesses of the layers 12c is between 30 nm and 100 nm, the infrared reflectivity of the wire grid polarizer 10 is reduced.
[0025] β-FeSi ２ Amorphous FeSi for layer 12b ２ The ratio of the thickness of layer 12c is preferably 0.1 to 1.2, and more preferably 0.1 to 0.3. β-FeSi ２ Amorphous FeSi for layer 12b ２ When the ratio of the thickness of layer 12c is between 0.1 and 1.2, the infrared reflectivity of the wire grid polarizer 10 is reduced.
[0026] β-FeSi ２ Layer 12b and amorphous FeSi ２ The method for forming layer 12c is not particularly limited, but examples include vapor deposition and sputtering. Here, if the temperature of the transparent substrate 11 is raised to 350°C or higher, β-FeSi ２ When layer 12b is formed and the temperature of the transparent substrate 11 is lowered to below 350°C, amorphous FeSi ２ Layer 12c is formed.
[0027] The thickness of the aluminum layer 12a is not particularly limited, but for example, it is 30 nm to 300 nm. The method for forming the aluminum layer 12a is not particularly limited, but examples include vapor deposition and sputtering.
[0028] The wire grid polarizer 10 may further include an inorganic oxide layer formed on the lattice-like protrusions 12. This improves the reliability of the wire grid polarizer 10. The inorganic oxide constituting the inorganic oxide layer is not particularly limited, but examples include silica and alumina.
[0029] The thickness of the inorganic oxide layer is not particularly limited, but for example, it is between 1 nm and 100 nm. The method for forming the inorganic oxide layer is not particularly limited, but examples include CVD and ALD.
[0030] The pitch of the grid-like protrusions 12 is not particularly limited as long as it is less than or equal to half the wavelength of the incident infrared light, but from the viewpoint of ease of manufacture and stability of the wire grid polarizer 10, it is preferably, for example, 100 nm to 200 nm. The pitch of the grid-like protrusions 12 can be measured by observation using a scanning electron microscope or a transmission electron microscope. For example, the pitch of any four locations can be measured using a scanning electron microscope or a transmission electron microscope, and the arithmetic mean can be taken as the pitch.
[0031] The material constituting the transparent substrate 11 is not particularly limited as long as it is transparent to incident infrared rays, but glass is one example.
[0032] In this specification and in the claims, “transparent to incident infrared light” does not mean that the transmittance of incident infrared light is 100%, but rather that the transmittance of infrared light is such that it can maintain its function as a wire grid polarizer. The wavelength of the incident infrared light is, for example, 0.8 μm or more and 2.0 μm or less.
[0033] The thickness of the transparent substrate 11 is not particularly limited, but for example, it is 0.3 mm or more and 1 mm or less.
[0034] Figure 4 shows a wire grid polarizer according to another embodiment of the present invention.
[0035] The wire grid polarizer 10A has β-FeSi on the upper side of the aluminum layer 12a instead of the lattice-shaped protrusions 12. ２ Layer 12b and amorphous FeSi ２ The wire grid polarizer 10 is the same as the wire grid polarizer 10, except that it further includes a lattice-shaped protrusion 12A having a layer 12c. Therefore, as shown in Figure 3, when infrared light is incident on the wire grid polarizer 10A from the side where the lattice-shaped protrusion 12A is formed, that is, from the side where the aluminum layer 12a of the lattice-shaped protrusion 12A is not formed, the reflectivity of infrared light of the wire grid polarizer 10 is reduced.
[0036] [Method for manufacturing a wire grid polarizer] The method for manufacturing the wire grid polarizer 10 involves placing β-FeSi on one side of a transparent substrate 11.２ Layer 12b, amorphous FeSi ２ The process includes the steps of stacking layer 12c and aluminum layer 12a to form a laminate, and selectively etching the laminate to form a grid-like protrusion 12.
[0037] When selectively etching a laminate, a mask pattern is formed on the laminate using a resist, for example, by photolithography or nanoimprint lithography, and then the areas of the laminate where the mask pattern is not formed are etched.
[0038] The etching method is not particularly limited, but one example is a dry etching method that uses an etching gas corresponding to the object to be etched.
[0039] The method for manufacturing the wire grid polarizer 10 may further include a step of forming an inorganic oxide layer on the grid-like protrusions 12.
[0040] [Optical Isolator] Figure 5 shows a polarization-dependent optical isolator as an optical isolator according to one embodiment of the present invention. In Figure 5, the polarization-dependent optical isolator is shown in a state where light in the forward direction is transmitted.
[0041] The optical isolator 100 comprises a wire grid polarizer 10 as an incident polarizer, a Faraday rotator 20, and an exit polarizer 30. In this configuration, infrared light generated by the semiconductor laser 200 is incident on the wire grid polarizer 10 from the side where the grid-like protrusions 12 are not formed. As a result, the reflection of infrared light from the wire grid polarizer 10 back to the semiconductor laser 200 is blocked.
[0042] Furthermore, the output polarizer 30 may be a wire grid polarizer 10 or a known polarizer.
[0043] Alternatively, the wire grid polarizer 10 may be formed on the Faraday rotor 20. In this case, since the wire grid polarizer 10 can be manufactured by a wafer process, the wire grid polarizer 10 can be directly formed on the crystal substrate of the Faraday rotor 20. As a result, there is no need to bond the wire grid polarizer 10 to the Faraday rotor 20, which improves reliability and makes it easier to miniaturize.
[0044] Although embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and the above embodiments may be modified as appropriate within the scope of the spirit of the present invention.
[0045] The following describes embodiments of the present invention, but the present invention is not limited to these embodiments. In these embodiments, a test specimen was used instead of a wire grid polarizer to evaluate the infrared reflectivity.
[0046] [Comparative Example 1] A glass substrate Eagle XG (manufactured by Corning) was heated to room temperature, and a 200 nm thick aluminum layer was formed by sputtering to obtain a test specimen in which an aluminum layer was formed on the glass substrate.
[0047] Using the FTIR method, infrared light was incident from the side where the aluminum layer was not formed, and the infrared reflectance spectrum of the test specimen (see Figure 6) was measured. From Figure 6, it can be seen that the reflectance of infrared light, which has a wavelength of 1.55 μm used in optical communications, is 90% or more.
[0048] [Example 1] A glass substrate Eagle XG (manufactured by Corning) was heated to 390°C, and a 25 nm thick β-FeSi ２ After forming the layer by sputtering, the substrate was thoroughly cooled. Next, the temperature of the glass substrate Eagle XG (Corning) was brought to room temperature, and a 30 nm thick amorphous FeSi layer was formed. ２ After forming a layer by sputtering, a 200 nm thick aluminum layer was formed by sputtering to obtain a test specimen in which a laminate was formed on a glass substrate.
[0049] Using the FTIR method, infrared light was incident from the side where the laminate was not formed, and the infrared reflectance spectrum of the test specimen (see Figure 7) was measured. From Figure 7, it can be seen that the reflectance of infrared light, which has a wavelength of 1.55 μm used in optical communications, has been reduced to 3.6%.
[0050] β-FeSi with a thickness of 25 nm ２ Amorphous FeSi layer, 30 nm thick. ２ A laminate in which layers and a 200 nm thick aluminum layer are sequentially formed was analyzed using optical admittance trajectories (see Figure 8). From Figure 8, β-FeSi ２ When infrared light with a wavelength of 1.55 μm is incident from the side of the layer, a 30 nm thick amorphous FeSi layer is exposed beneath a 200 nm thick aluminum layer. ２ The presence of this layer reduces the infrared reflectivity to around 50%, and the β-FeSi has a thickness of 30 nm. ２ The presence of this layer reduces the infrared reflectivity to as low as 1%.
[0051] [Example 2] Amorphous FeSi with a thickness of 11 nm is placed on a glass substrate. ２ Layer, 44 nm thick β-FeSi ２ The laminate formed on a glass substrate when layers and a 200 nm thick aluminum layer are sequentially formed was analyzed using the optical admittance trajectory (see Figure 9). From Figure 9, amorphous FeSi ２ When infrared light with a wavelength of 1.55 μm is incident from the side of the layer, a 44 nm thick β-FeSi layer is exposed beneath a 200 nm thick aluminum layer. ２ The presence of this layer reduces the infrared reflectivity to around 40%, resulting in an amorphous FeSi with a thickness of 11 nm. ２ The presence of this layer reduces the infrared reflectivity to near 0%.
[0052] [Example 3] A 200 nm thick aluminum layer and a 50 nm thick β-FeSi layer are placed on a glass substrate. ２ Amorphous FeSi with layers and a thickness of 5 nm ２The laminate formed on a glass substrate when layers are formed sequentially was analyzed using the optical admittance trajectory (see Figure 10). From Figure 10, amorphous FeSi ２ When infrared light with a wavelength of 1.55 μm is incident from the side of the layer, a 50 nm thick layer of β-FeSi is formed on top of a 200 nm thick aluminum layer. ２ The presence of this layer reduces the infrared reflectivity to around 10%, resulting in a 5nm thick amorphous FeSi ２ The presence of this layer reduces the infrared reflectivity to near 0%.
[0053] [Example 4] (Fabrication of wire grid polarizer) The temperature of the glass substrate Eagle XG (manufactured by Corning) was set to 390°C, and a β-FeSi with a thickness of 30.5 nm was prepared. ２ After forming the layer by sputtering, the substrate was thoroughly cooled. Next, the temperature of the glass substrate Eagle XG (Corning) was brought to room temperature, and an amorphous FeSi layer with a thickness of 36.5 nm was formed. ２ After forming a layer by sputtering, a 91 nm thick aluminum layer was formed by sputtering, and a laminate was created on a glass substrate. Next, a line and space (L / S) pattern with a 140 nm pitch was formed on the laminate by photolithography, and then chlorine gas was used to selectively etch the pattern to form lattice-like protrusions, thereby obtaining a wire grid polarizer.
[0054] Figure 11 shows a cross-sectional SEM image of a wire grid polarizer.
[0055] (Polarization characteristics of wire grid polarizer) Polarized waves (TM waves and TE waves) were incident on the wire grid polarizer from the side where the lattice-like protrusions are not formed, and the transmittance and reflectance were measured (see Figure 12). From Figure 12, it can be seen that the reflectance of the TE wave, which has a wavelength of 1.9 μm, is reduced to around 0%, excluding interfacial reflection at the surface of the glass substrate.
[0056] 10, 10A Wire grid polarizer 11 Transparent substrate 12, 12A Lattice-shaped protrusions 12a Aluminum layer 12b β-FeSi ２ Layer 12c Amorphous FeSi ２Layer 20 Faraday rotor 30 Exit polarizer 100 Optical isolator 200 Semiconductor laser
Claims
1. A transparent substrate and a grid-like protrusion arranged on one surface of the transparent substrate at a pitch of 1 / 2 or less of the wavelength of incident infrared light and extending in a predetermined direction, wherein the grid-like protrusion has an aluminum layer or an aluminum alloy layer, and on one side of the aluminum layer or aluminum alloy layer, β-FeSi ２ Layers and amorphous FeSi ２ A wire grid polarizer with additional layers.
2. Starting from the side of the transparent substrate, the β-FeSi ２ Layer and the amorphous FeSi ２ A wire grid polarizer according to claim 1, having a layer.
3. The β-FeSi ２ Layer and the amorphous FeSi ２ A wire grid polarizer according to claim 1 or 2, wherein the sum of the layer thicknesses is 30 nm or more and 100 nm or less.
4. The wire grid polarizer according to claim 1 or 2, further comprising an inorganic oxide layer formed on the lattice-like protrusions.
5. The wire grid polarizer according to claim 1 or 2, wherein the wavelength of the infrared radiation is 0.8 μm or more and 2.0 μm or less, and the reflectance of the infrared radiation is 10% or less.
6. An optical isolator comprising a wire grid polarizer according to claim 1 or 2 and a Faraday rotator.
7. The optical isolator according to claim 6, wherein the wire grid polarizer is formed on the Faraday rotor.