Optical transmission line, optical isolator, photonic device, and method of producing optical transmission line

Abstract

To provide an optical transmission line for obtaining an optical isolator that is more compact than conventional products without using cooling or application of an external magnetic field, and to provide an optical isolator using the same.SOLUTION: An optical transmission line disclosed herein comprises an optical waveguide, and a circularly dichroic substance consisting of one of enantiomers and being attached to an outer surface of the optical waveguide.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present disclosure relates to optical devices that allow light to pass in one direction, and in particular to miniature optical isolators that use nanomaterials. [Background technology]

[0002] An optical isolator is an optical element that allows light to pass in only one direction and blocks light in the opposite direction. Backward-propagating light, which is generally caused by reflection, is often a problem in photonic devices and significantly impairs the performance of optical communication systems.

[0003] Optical isolators generally available on the market utilize the Faraday effect and require the combination of a magnetic field and multiple optical elements, and are several centimeters or larger in size, meaning there are limits to how much they can be miniaturized. Figure 1 shows the principle of a conventional optical isolator that uses the Faraday effect. This optical isolator has a structure in which "two polarizers A and B with transmission axes tilted by 45 degrees" and a "Faraday rotator with a Faraday rotation angle of 45 degrees" is inserted between them. Here, polarizer A is set at 0°, and polarizer B is set at 45°. The Faraday effect is a phenomenon in which the plane of polarization of light passing through a magnetic material rotates when the light is incident in a direction parallel to the magnetic field applied to the magnetic material. When a magnetic field is applied by a Faraday rotator, the polarization plane of the input light rotates in opposite directions between the forward and reverse directions, as shown in Figure 1. The direction of rotation here refers to the direction of rotation when viewed from the line of sight of the light's traveling direction. Below is an explanation of Figure 1 viewed from the front. In the forward direction, the input light is linearly polarized at an inclination of 0° by polarizer A, rotated by +45° by the Faraday rotator, and allowed to pass through polarizer B tilted at 45°. In the reverse direction, the input light is linearly polarized at an inclination of 45° by polarizer B, and further rotated by +45° by the Faraday rotator. As a result, it crosses at right angles to polarizer A, which is set at 0°, and the transmitted light suffers a large loss.

[0004] As an example of an optical isolator, Non-Patent Document 1 shows a conventional optical isolator sold by THORLABS. THORLABS offers a wide range of free-space optical isolators designed for the visible spectral region (390-700 nm) as standard products. Their size is several centimeters or more. Similarly, optical isolators of similar size are sold by other optical manufacturers. Although there are some small isolators equipped with fibers, such as those described in Non-Patent Document 2 and Non-Patent Document 3, their size is still several centimeters or more.

[0005] At the forefront of research, Non-Patent Document 4, it has been demonstrated that an optical isolator can be realized by using cold atoms and a magnetic field. Citation from Non-Patent Document 4 As shown in Figure 2(a), cold atoms are adsorbed on the surface of an optical nanofiber, and a magnetic field is applied to it, resulting in the ground state |g> and the counterclockwise circular polarization σ + The action of |e +1 On the other hand, clockwise circular polarization caused by light entering from the right side σ - The action of |e -1 >. Citation from Non-Patent Document 4 Figure 2(c) shows the circularly polarized light σ + The transmission ratio τ + and circular polarization σ - The transmission ratio τ - The results are shown below. It has been shown that it is possible to realize an optical isolator by utilizing the difference in light input from the left and right. Thus, research is progressing at a scale that can be applied to nanophotonic devices, but this research is not suitable for reducing the size of optical isolators because it uses cooling and magnetic fields. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] THORLABS, Visible Free Space Isolator (390~700nm), Internet<https: / / www.thorlabs.co.jp / newgrouppage9.cfm?objectgroup_id=2996&pn=IO-5-532-HP#5089> [Non-Patent Document 2] THORLABS, Visible Fiber Isolator with SM Fiber (650~670nm), Internet<https: / / www.thorlabs.co.jp / newgrouppage9.cfm?objectgroup_id=7299> [Non-Patent Document 3] THORLABS, 1064nm Fiber Isolator, SM Fiber, for Nd:YAG Lasers, Internet<https: / / www.thorlabs.co.jp / newgrouppage9.cfm?objectgroup_id=6177> [Non-Patent Document 4] C. Sayrin, C. Junge, R. Mitsch, B. Albrecht, D. O'Shea, P. Schneeweiss, J. Volz, and A. Rauschenbeutel, “Nanophotonic Optical Isolator Controlled by the Internal State of Cold Atoms”, Phys. Rev. X 5, 041036 (2015). Summary of the Invention [Problem to be solved by the invention]

[0007] Conventional optical isolators such as those described above require the combination of a magnetic field and multiple optical elements, which has reached the limit of miniaturization of optical isolators. As a result, there has been a problem in that optical isolators cannot be incorporated into nanophotonic devices.

[0008] The present disclosure has been made to solve such problems, and aims to provide an optical transmission path for obtaining an optical isolator that is smaller than conventional products without using cooling or application of an external magnetic field, and an optical isolator using the same. [Means for solving the problem]

[0009] The aspects for solving the above problems include the following.

[0010] The first aspect is an optical transmission line comprising an optical waveguide and a substance having circular dichroism, which is made of one of the enantiomers and is attached to the outer surface of the optical waveguide.

[0011] The second aspect is an optical isolator comprising two polarizers having the same transmission axis direction, an optical waveguide disposed between the two polarizers, and a substance having circular dichroism, which is made of one of the enantiomers and is attached to the outer surface of the optical waveguide.

[0012] A third embodiment is the optical isolator of the second embodiment, wherein the diameter of the optical waveguide is equal to or less than the wavelength of the propagating light.

[0013] A fourth aspect is an optical isolator according to the second or third aspect, in which the substance having circular dichroism is attached in a state of contact with the outer surface of the optical waveguide at at least one point.

[0014] A fifth aspect is a photonic device comprising an optical isolator according to any one of the second to fourth aspects.

[0015] A sixth aspect is a method for manufacturing an optical transmission path, comprising the steps of: dripping a solution of a substance having circular dichroism and consisting of one of the enantiomers onto a mesh of an object having a mesh of micron-order size; and contacting and adhering a thin film formed by the dripping so as to cross-link the mesh to an outer surface of an optical waveguide. Effect of the Invention

[0016] According to the present disclosure, it is possible to provide an optical isolator that is smaller than conventional products, by using an optical transmission line that includes an optical waveguide and a substance having circular dichroism. [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 is a diagram illustrating the principle of a conventional Faraday optical isolator. [Diagram 2] This is a quoted diagram from Non-Patent Document 4. This is a diagram explaining an example in Non-Patent Document 4 using cold atoms and a magnetic field. [Diagram 3] FIG. 1 is a schematic diagram illustrating the principle of the present disclosure. [Figure 4] 1 is a photograph of a thin film of a CNT single enantiomer, embodiment 1 of the present disclosure. [Diagram 5] 1 is a photograph showing a nano-optical fiber having a (9,7) CNT single enantiomer attached thereto, which is a second embodiment of the present disclosure. [Figure 6] 1 is a schematic diagram of the adjustments required to realize the present disclosure, showing the positional relationship of circularly polarized light generated by the angle of linearly polarized light input into a nano-optical fiber. [Figure 7] This figure shows embodiment 2 of the invention according to the present disclosure. The transmittance of light input from the forward direction is plotted as a square, and the transmittance of light input from the reverse direction is plotted as a circle. What is written on the figure shows the positional relationship between the circularly polarized light corresponding to the linearly polarized light in the nano-optical fiber and the attached CNT. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. In each drawing, the same or similar reference numerals are used to designate the same or corresponding parts, and duplicated explanations will be omitted. In addition, the present disclosure is not limited to the following embodiment.

[0019] In this specification, "optical waveguide" refers to a device that utilizes optical refractive index to guide light with high linearity. In addition, in this specification, "optical waveguide" refers to a nano-optical waveguide that propagates light in a space where at least one of the three dimensions is on the nanometer scale. "Nanomaterial" refers to a material on the nanometer scale. "Single chirality" refers to a material that has the same chirality (chiral index). "Single enantiomer" refers to a material that consists of only one of the structures of the enantiomers.

[0020] The present disclosure discloses a combination of an optical waveguide and a substance having circular dichroism. The interaction between light propagating through the optical waveguide and a substance having circular dichroism has different effects on the light propagating in the direction of light propagation (forward or backward). By utilizing the difference in this interaction, it is possible to propagate light in only one direction. As a first embodiment for explaining the present disclosure, an optical isolator using a nano-optical fiber as an optical waveguide and a carbon nanotube as a material having circular dichroism will be described below.

[0021] [Optical isolator] The optical isolator according to the present disclosure has a structure in which, as shown in FIG. 3, "two polarizers with the same transmission axis" are inserted between "a nano-optical fiber to which a material having circular dichroism (CNT) is attached." As shown in Figure 3, when light propagates through a nano-optical fiber, counterclockwise circular polarization leaks out onto the fiber surface depending on the direction of propagation. In the forward direction, counterclockwise circular polarization leaks out onto the fiber surface depending on the direction of propagation. In the reverse direction, clockwise circular polarization leaks out onto the fiber surface depending on the direction of propagation. Carbon nanotubes (CNTs) as a substance having circular dichroism are considered to be single enantiomers with a single chirality and the direction of rotation of the helical structure is the same.

[0022] In the forward direction shown in Figure 3, the input light is linearly polarized by polarizer A, and the counterclockwise circularly polarized light on the surface of the nano-fiber interacts with the attached CNT. In the reverse direction, the input light is linearly polarized by polarizer B, and the clockwise circularly polarized light on the surface of the nano-fiber interacts with the attached CNT. By using single enantiomer CNTs, when the rotation direction of the CNT helical structure matches the circular polarization of the nano-fiber, a large interaction (light absorption and scattering) occurs, and the transmitted light suffers a large loss. Figure 3 shows that in the reverse input light, the clockwise circularly polarized light on the surface of the nano-fiber matches the rotation direction of the attached CNT helical structure, so the transmitted light suffers a large loss. In this way, the optical isolator with the structure shown in Figure 3 is capable of propagating only the forward input light.

[0023] [Nano-optical fiber] The nano-optical fiber is not particularly limited as long as it leaks circularly polarized light to the fiber surface. For example, the nano-optical fiber may be made of glass, quartz glass, plastic, etc. It is preferable that the nano-optical fiber is a nano-optical fiber that is normally used in optical communication. The diameter of the nano-optical fiber is preferably equal to or smaller than the wavelength of the propagating light, that is, equal to or smaller than the wavelength of the propagating light. For example, the diameter of the nano-optical fiber may be in the range of 1 / 2 the wavelength of the propagating light to the wavelength of the light. In this way, the diameter of the nano-optical fiber may be appropriately selected based on the wavelength of the propagating light. In addition, the wavelength of the propagating light is not particularly limited as long as it is light that can be propagated by the nano-optical fiber. For example, it may be visible light (400 nm to 800 nm) or near-infrared light (800 nm to 2500 nm). It may be a wavelength commonly used in optical communication (for example, 1300 nm, 1550 nm).

[0024] Furthermore, the shape (cross-sectional shape) of the nano-optical fiber is not particularly limited. In Fig. 6, a circle is depicted as the cross section of the nano-optical fiber, but the cross-sectional shape of the nano-optical fiber is not limited to a circle. For example, it may be an ellipse, or a polygon such as a square, a rectangle, or a pentagon. As long as it is capable of propagating light, the cross-sectional shape is not particularly limited.

[0025] [Substances with circular dichroism] An example of a substance having circular dichroism is carbon nanotubes (CNTs). The chirality of the CNTs is not particularly limited as long as it is a single chirality. It can be appropriately selected depending on the amount of light to be absorbed. In addition, in a single chirality, either structure may be used as a single enantiomer as long as it contains only one structure (i.e., the helical structure has the same rotation direction).

[0026] The CNT is attached by contacting a thin film of the CNT to the surface of the nano-optical fiber. The CNT is preferably equal to or smaller than the diameter of the nano-optical fiber so that it is adsorbed on the surface of the nano-optical fiber. The range in which the CNTs are attached to the surface of the nano-optical fiber may be at least several μm to several tens of μm in the longitudinal direction (length direction) of the nano-optical fiber. For example, it may be 1 μm or more, 5 μm or more, 10 μm or more, or 15 μm or more. The longer the nano-optical fiber in the longitudinal direction, the more light can be attenuated, which is preferable. In the short direction (width direction) of the nano-optical fiber, the shorter the CNTs are, as long as they can interact with the nano-optical fiber. For example, the range is within 1 μm, more preferably within 500 nm, and particularly preferably within 100 nm, on both the left and right sides of the position on the surface of the nano-optical fiber where circularly polarized light is generated in the short direction. Moreover, the position of the CNT attached to the surface of the nano-optical fiber is preferably the closest to the circularly polarized light generated by the angle of the linearly polarized light input into the nano-optical fiber, as shown in Fig. 6. In other words, in Fig. 6, it is the position on either side of the arrows indicating the circularly polarized light generated outside each circle.

[0027] Circular dichroism (CD) is a phenomenon in which a material absorbs circularly polarized light and there is a difference in absorbance between left-handed and right-handed circularly polarized light, and is observed when the material has chirality. Chirality is the property that the structure of a material cannot be superimposed on its mirror image. Therefore, the substance having circular dichroism according to the present disclosure is not limited to CNT, but can be any material having chirality. The difference in absorbance for left and right circularly polarized light is expressed as the magnitude of CD, which is called ellipticity. In terms of material conditions, materials with a large difference in absorbance for left and right circularly polarized light are preferable. In particular, for the materials covered by this disclosure, materials with a larger CD value are preferable. In order to allow the materials to be surface-adsorbed onto the nano-optical fiber, materials with a size equal to or smaller than the diameter of the nano-optical fiber are preferable. Specific examples of materials that are known to have CD include organic compounds such as pinene gas, metal complexes such as optically active complexes in which trivalent cobalt ions are coordinated with ethylenediamine, proteins and polypeptides such as concanavalin A, and nucleic acids such as deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). All materials that have CD and meet the material conditions can be targeted.

[0028] As described above, the condition for attachment is that the CNT single enantiomer is distributed and in contact at one point on the surface of the nano-optical fiber or within a specific range in the width direction on the surface. The specific range is, for example, within the length of the circumference of a semicircle when the cross section of the nano-optical fiber is circular (when the specific angle is the central angle of the circle is within 180°). When the cross section of the nano-optical fiber is circular, the specific angle is, for example, within 180°, more preferably within 90°, and particularly preferably within 45°. When the cross section of the nano-optical fiber is a rectangular parallelepiped waveguide, it is within the width of one of the four surfaces of the rectangular parallelepiped. For example, when the cross section of the waveguide is a rectangular parallelepiped, the specific range is within the width of one surface, more preferably within the width of 1 / 2 surface, and particularly preferably within the width of 1 / 4 surface. Alternatively, the CNT single enantiomer may be a plurality of CNTs of a single chirality aligned in one direction, which may be placed on the surface of the nano-optical fiber so as to satisfy the above-mentioned attachment conditions. For example, the CNT single enantiomer may be placed so as to intersect with the nano-optical fiber.

[0029] [Polarizer] As shown in Fig. 3, the optical isolator according to the present disclosure may include two polarizers arranged to sandwich the nano-optical fiber. The two polarizers preferably have the same transmission axis direction. The transmission axis direction is not particularly limited, and may be a direction that generates linearly polarized light as shown in Fig. 6. Note that in the case where the input light is already polarized, or the like, and the light to be propagated has a known linearly polarized direction, no polarizer is required.

[0030] The optical isolator according to the present disclosure can be constructed from nano-optical fibers and CNTs, and therefore can have a maximum size of about 10 mm, and can also be a few millimeters or less.

[0031] Although the embodiment using a nano-optical fiber as an optical waveguide has been described above, the optical waveguide is not limited to a nano-optical fiber. Anything is acceptable as long as it can propagate light and the circularly polarized light of the propagating light leaks out onto the surface of the optical waveguide. For example, it may be a silicon optical waveguide (silicon photonics). The height of the silicon optical waveguide that serves as the optical path is preferably equal to or smaller than the wavelength of the propagating light, that is, equal to or smaller than the wavelength of the propagating light. For example, it may be a fine optical waveguide with a wavelength of 1550 nm and a height of 200 nm. The size of the silicon optical waveguide is not particularly limited, and the wavelength of the propagating light is not particularly limited as long as it is light that can be propagated through a silicon optical waveguide.

[0032] As described above, an optical isolator that is much smaller than conventional optical isolators can be obtained by using a fine optical waveguide such as an optical fiber or silicon photonics, which is an optical waveguide, and a material having circular dichroism. Therefore, it can be used not only in photonic devices (optical devices) such as optical fibers, LEDs, or optical amplifiers, but also in nanophotonic devices (nano optical devices) such as nano optical circuits and optical integrated circuits, instead of conventional optical isolators. Furthermore, the optical transmission line according to the present disclosure may naturally be used in optical devices other than optical isolators. EXAMPLES

[0033] The present disclosure will be described in more detail below with reference to examples. However, the present disclosure is not limited to these examples, and can be implemented in various forms with various modifications and improvements based on the knowledge of those skilled in the art.

[0034] Here, an example of the fabrication of an optical isolator will be described. By dropping a solution of a CNT single enantiomer onto an "object having a mesh structure (or hollow structure)" of about 10 μm x 10 μm, a CNT thin film bridged in the hollow can be obtained. As the object having a mesh structure, a cut grid for a TEM electron microscope was used. Figure 4 shows the fabricated CNT thin film. The chirality of the CNT is (9,7). A 0.1% DOC (Na deoxycholate) aqueous solution was used as the solution of the CNT single enantiomer. The amount of CNT attached can be adjusted appropriately by changing the CNT concentration in the CNT single enantiomer solution.

[0035] In order to bring the CNT thin film into contact with the nano-optical fiber, a manipulator with a minimum spatial resolution of 0.25 nm was used (product name: Micromanipulator MM3A-EM, manufactured by Kleindiek). The manipulator here refers to a device that has a probe and can be remotely operated over a small distance. The minimum spatial resolution required for a manipulator is about 1 μm, and with this level of resolution, it is possible to contact and attach the CNT to the surface of the nano-optical fiber. By holding a "net-structured object" as shown in Figure 4 at the tip of the manipulator and remotely operating it, it is possible to bring the CNT thin film into contact with the surface of the nano-optical fiber and attach the CNT to a specific position. The nano-optical fiber used was "780HP - Single Mode Optical Fiber, 780 - 970 nm, 0125 μm Cladding" (manufactured by THORLABS) that had been heated with a burner and stretched.

[0036] By contacting and releasing the CNT thin film with the nano-optical fiber, it is possible to attach the CNT single enantiomer to the surface of the nano-optical fiber within a specific angle on the surface circumference. After attachment, the solvent was removed by natural drying. Figure 5 shows optical microscope and SEM images of the nano-optical fiber with the (9,7) CNT single enantiomer attached.

[0037] As shown in Fig. 6, by changing the direction of the linearly polarized light input to the nano-optical fiber, it is possible to move the circularly polarized light on the surface of the nano-optical fiber around the circumference of the surface of the nano-optical fiber. When the circularly polarized light coincides with the position where the CNT is attached on the surface of the nano-optical fiber, a large interaction (light absorption and scattering) occurs, resulting in a large loss of the transmitted light. Therefore, when realizing the optical isolator of the present disclosure, the position is adjusted so that the circularly polarized light and the CNT interact with each other. In addition, when linearly polarized light is input to the nano-fiber, circularly polarized light in the opposite direction is generated above and below the surface of the nano-fiber, as shown in Figure 6. Therefore, the CNT single enantiomer to be attached must be placed so as to satisfy the above-mentioned attachment conditions. Figure 7 shows the results of measurements taken while changing the angle of the linearly polarized light input to the nano-optical fiber. As shown in Figure 7, it can be confirmed that the circularly polarized light generated by the light input from the forward direction is hardly affected by the CNT. On the other hand, the circularly polarized light generated by the light input from the reverse direction was affected on the transmittance when it reached the position of the CNT. Therefore, it is possible to create a difference in the transmittance between the light input from the forward and reverse directions, and the optical isolator shown in this disclosure can be realized.

[0038] As explained above, the function of an optical isolator can be expressed simply by placing a CNT enantiomer on an optical waveguide such as a nano-optical fiber. The optical isolator created in this way has the advantage of being simple in structure and small in size.

[0039] In addition, by selecting the chirality of the CNT enantiomer, it is possible to realize an optical isolator for a specific wavelength. Furthermore, by using multiple chiralities of CNT enantiomers, it is possible to realize an optical isolator for a specific wavelength band. It should be noted that the material is not limited to CNTs or nano-optical fibers, and an optical isolator can be similarly produced by attaching a material having circular dichroism to an optical waveguide.

Claims

1. An optical waveguide; and a substance having circular dichroism, the substance being one of the enantiomers and attached to the outer surface of the optical waveguide. Optical transmission path.

2. Two polarizers having the same transmission axis direction; an optical waveguide disposed between the two polarizers; and a substance having circular dichroism, the substance being one of the enantiomers and attached to the outer surface of the optical waveguide. Optical isolator.

3. The diameter of the optical waveguide is equal to or less than the wavelength of the propagating light.

3. The optical isolator of claim 2.

4. The substance having circular dichroism is attached in a state of contact with the outer surface of the optical waveguide at at least one point.

4. An optical isolator according to claim 2 or 3.

5. The optical isolator according to claim 2 or 3 is provided. Photonic devices.

6. A step of dropping a solution of a substance having circular dichroism composed of one of the enantiomers onto a mesh of an object having a mesh of a size on the order of microns; and contacting and adhering the thin film formed by the dropping so as to bridge the network to an outer surface of the optical waveguide. A method for manufacturing an optical transmission line.

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