Device for coupling a waveguide and an optical fiber, a quantum optical circuit using the same, and a method for fabricating the device.

Abstract

Provided is a device with which the reliability of the coupling of a waveguide and an optical fiber is improved. This device comprises: a waveguide chip (10) that has a cladding layer (12) and an optical waveguide (13) formed on the cladding layer (12); and an optical fiber (20) optically connected to the optical waveguide (13), wherein the cladding layer (12) has a hollow space (15) below a coupling region (C) where the optical waveguide (13) is optically connected to the optical fiber (20), and the optical waveguide (13) is in contact with the optical fiber (20) in the coupling region (C) and bends in the stacking direction of the waveguide chip (10) inside the hollow space (15).

Description

Technical Field

[0001] The present disclosure relates to an apparatus for coupling a waveguide and an optical fiber, a quantum optical circuit using the same, and a method for manufacturing the apparatus.

Background Art

[0002] Towards the development and practical application of quantum computing, many studies have been conducted. One type of quantum computer is a diamond spin-based quantum computer that treats electron spins, nuclear spins, and photon spins (circular polarization) in diamond as qubits. Photons are used in quantum communication because of their long information retention time and are expected to be applied to quantum networks. As single photon sources that emit single spins or photons, nitrogen-vacancy (NV) color centers, silicon-vacancy (SiV) color centers, etc. in diamond are known. Photons extracted from the color centers are coupled from a diamond waveguide to a single-mode optical fiber and detected by a detector (see, for example, Non-Patent Document 1).

[0003] As optical coupling from an optical fiber to an optical waveguide, a configuration is known in which an optical fiber with a tapered tip and an optical waveguide with a tapered tip are brought into contact and adiabatically coupled (see, for example, Non-Patent Document 2). For performing calculations with multiple qubits, a configuration that connects multiple optical waveguides and multiple single-mode fibers with high efficiency is desired. In this case, it is desirable to collectively and stably hold multiple single-mode fibers in a fiber array.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

[0005] [Non-Patent Document 1] SCIENCE Vol. 354, Issue 6314, pp. 847-850 (2016) [Non-Patent Document 2] Optica Vol. 2, Issue 2, pp. 70-75 (2015) [Overview of the project] [Problems that the invention aims to solve]

[0006] When coupling an optical waveguide with a tapered tip to an optical fiber with a tapered tip, a connection position accuracy of 100 nm or less is required. However, the manufacturing accuracy of optical fibers and fiber arrays is only about ±1 μm, resulting in variations in optical coupling efficiency for each optical fiber. Furthermore, since the tapered tip of the optical waveguide and the tapered tip of the optical fiber are only in contact in a minute region, coupling misalignment can easily occur due to external vibrations. One aspect of this disclosure is to provide a device with improved coupling reliability and a quantum optical circuit using this device. [Means for solving the problem]

[0007] The apparatus in one embodiment is A waveguide chip having a cladding layer and an optical waveguide formed on the cladding layer, An optical fiber optically connected to the optical waveguide, Equipped with, The cladding layer has a hollow below the coupling region where the optical waveguide is optically connected to the optical fiber. The optical waveguide contacts the optical fiber in the coupling region and flexes within the hollow interior in the stacking direction of the waveguide chip. [Effects of the Invention]

[0008] A device with improved coupling reliability and a quantum optical circuit using it will be realized. [Brief explanation of the drawing]

[0009] [Figure 1] It is a diagram showing technical problems in evanescent coupling using a tapered structure. [Figure 2] It is a schematic diagram of a waveguide - fiber coupling structure which is an example of the device of the embodiment. [Figure 3] It is an enlarged view of the coupling region in FIG. 2. [Figure 4] It is a schematic diagram of a first modification example of the waveguide - fiber coupling structure. [Figure 5] It is a schematic diagram of a second modification example of the waveguide - fiber coupling structure. [[ID=IP19]] [Figure 6] It is a schematic diagram of a third modification example of the waveguide - fiber coupling structure. [Figure 7] [[ID=IP23]]It is a schematic diagram of a fourth modification example of the waveguide - fiber coupling structure. [[ID=IP24]]It is a schematic diagram of a fifth modification example of the waveguide - fiber coupling structure. [Figure 8] [Figure 9A] It is a process diagram for fabricating a waveguide chip. [Figure 9B] It is a process diagram for fabricating a waveguide chip. [Figure 9C] It is a process diagram for fabricating a waveguide chip. [Figure 9D] It is a process diagram for fabricating a waveguide chip. [Figure 10A] It is a process diagram for fabricating a taper at the tip of an optical fiber. [Figure 10B] It is a process diagram for fabricating a taper at the tip of an optical fiber. [Figure 11] It is a diagram of a stress analysis model of an optical waveguide. [Figure 12] It is a diagram showing stress distribution and the maximum stress point. [Figure 13] It is a diagram showing the relationship between the overlapping amount of an optical fiber and a waveguide and the energy propagation amount. [Figure 14] It is a diagram showing energy propagation to a waveguide with different overlapping amounts. [Figure 15] It is a schematic diagram of a quantum optical circuit using the device of the embodiment. [Modes for carrying out the invention]

[0010] Figure 1 illustrates the technical challenges in optical coupling using a tapered structure. Figure 1(A) is a top view, (B) is a cross-sectional view of (A) along the optical axis AX, and (C) is a cross-sectional view of (A) along AA. In the coordinate system of Figure 1, the optical axis AX or the direction of light propagation is the X direction, the height or stacking direction of the optical waveguide 113 is the Z direction, and the direction perpendicular to the X and Z directions is the Y direction. The optical waveguide 113 is formed on the cladding layer 112, and the tip of a single-mode optical fiber 120 is brought into contact with the optical waveguide 113 to optically connect the optical waveguide 113 and the optical fiber 120. The optical fiber 120 is held in the V-groove 132 of the fiber holder 130 to stabilize the connection.

[0011] The optical waveguide 113 has a taper 113t at the end where it is coupled to the optical fiber 120. The optical fiber 120 has a taper 120t at the end where it is coupled to the optical waveguide 113. The cross-sectional areas of the taper 113t of the optical waveguide 113 and the taper 120t of the optical fiber 120 change gradually in the propagation (X) direction, achieving adiabatic coupling. "Adiabatic coupling" is a coupling that maintains a single mode and keeps all power or energy within that mode. In other words, light is coupled between the optical fiber 120 and the optical waveguide 113 while maintaining a specific mode without power or energy loss or the generation of other modes.

[0012] The optical fiber 120 and the optical waveguide 113 are only lightly in contact at the taper 120t and taper 113t. The positional accuracy of the optical waveguide 113 is determined by the processing accuracy of the optical waveguide 113 by the semiconductor process. On the other hand, the manufacturing accuracy of the fiber holder 130 and the optical fiber 120 is far lower than that of the optical waveguide 113, resulting in variations of several microns in the position of the V-groove 132 and the diameter of the optical fiber 120. In adiabatic coupling using a tapered structure, a connection position accuracy of 100 nm or less is required, but the diameter and the position of the V-groove 132 vary from optical fiber 120 to optical fiber holder 130, causing positional misalignments of Δx, Δy, and Δz in the X, Y, and Z directions, respectively. This problem of positional misalignment becomes more pronounced when multiple optical waveguides 113 and multiple optical fibers 120 are connected in parallel. Furthermore, since the taper 120t of the optical fiber 120 is only in contact with the taper 113t of the optical waveguide 113, even slight external vibrations can easily cause the contact to break, impairing the optical coupling.

[0013] In this embodiment, the reliability of the coupling between the waveguide and the optical fiber is enhanced by improving at least one of the alignment accuracy and resistance to external vibrations in the connection between the optical waveguide and the optical fiber. To achieve this, a portion of the cladding layer supporting the optical waveguide is removed, creating a hollow space directly beneath the coupling region where the optical waveguide and optical fiber are coupled. The optical waveguide contacts the core of the optical fiber at its coupling end and flexes in the stacking direction within the hollow space due to the weight of the core. By optimizing the contact distance between the optical waveguide and the optical fiber in the optical axis direction, the optical fiber core follows the flexure of the optical waveguide, maintaining the optical coupling. The flexure of the optical waveguide and optical fiber core within the hollow space increases the alignment margin in the height (Z) direction and the optical axis (X) direction.

[0014] When a fiber guide is provided in the waveguide tip where the optical waveguide is formed, to guide the tip of the optical fiber into the hollow space, the alignment accuracy in the X and Y directions is improved. An inert gas coating may also be applied to the coupling region between the optical waveguide and the optical fiber. In this case, resistance to external vibrations is improved, and the reliability of the coupling is further enhanced.

[0015] The configuration of the device for coupling a waveguide and an optical fiber will be described below with reference to the drawings. In this embodiment, the device for coupling a waveguide and an optical fiber will be referred to as a "waveguide-fiber coupling structure" for convenience. The following description is intended to embody the technical concept of this disclosure and, unless otherwise specified, does not limit this disclosure to the following description. In each drawing, elements having the same function or configuration are denoted by the same reference numeral, and redundant descriptions may be omitted. To facilitate understanding of the invention, the size and positional relationships of each component in each drawing may be exaggerated.

[0016] <Basic configuration> Figure 2 is a schematic diagram of the waveguide-fiber coupled structure 1 of the embodiment. Figure 2(A) is a top view, and (B) is a cross-sectional view along the optical axis AX. Similar to Figure 1, the direction of the optical axis AX is the X direction, the height or stacking direction of the optical waveguide is the Z direction, and the direction perpendicular to the X and Z directions is the Y direction. The waveguide-fiber coupled structure 1 is applied, for example, to a quantum optical circuit to guide a photon emitted from a single-photon source to a photodetector. As the single-photon source, a quantum dot single-photon source, a composite vacancy center in diamond or Si, or an NV color center can be used. In the embodiment, propagation of red wavelength photons generated by photoexcitation is assumed in an NV color center in diamond or silicon carbide (SiC).

[0017] Waveguide-fiber coupling structure 1 comprises a waveguide chip 10 and an optical fiber 20 optically coupled to an optical waveguide 13 formed on the waveguide chip 10. The waveguide chip 10 has a cladding layer 12 formed on a substrate 11 and an optical waveguide 13 formed on the cladding layer 12. The optical fiber 20 is a single-mode fiber with an outer diameter of 125 μm for the cladding 22 and a core diameter of 6-9 μm. The cladding 22 is quartz, and the core 21 is, for example, Ge-doped quartz with a higher refractive index due to the addition of germanium (Ge).

[0018] The optical waveguide 13 and the core 21 of the optical fiber 20 are optically connected in a coupling region C. The optical waveguide 13 may have a taper 13t that narrows toward the coupling region C. The core 21 of the optical fiber 20 may also have a taper 21t that narrows toward the coupling region C. In this case, the taper angle and taper length of the tapers 13t and 21t are controlled so that an adiabatic coupling is achieved, that is, so that a single mode is maintained without generating other modes.

[0019] The substrate 11 is any substrate capable of supporting the cladding layer 12 and the optical waveguide 13. Insulating substrates such as glass, quartz, sapphire, and magnesium oxide (MgO), or semi-insulating substrates such as silicon (Si) and germanium (Ge) without intentional impurity additions may be used. Alternatively, plastic substrates such as polycarbonate, polyethylene naphthalate, and acrylic resin may be used. The cladding layer 12 is formed from a material with a lower refractive index than the optical waveguide 13; for example, a silicon oxide (SiO2) film can be used. The optical waveguide 13 is formed from a material with a higher refractive index than the cladding layer 12 and is transparent to visible light, including the red wavelength band. As the optical waveguide 13, a diamond crystal thin film, a sapphire (Al2O3 single crystal) thin film, a silicon nitride (SiN) thin film, an aluminum nitride (AlN) thin film, etc., can be used.

[0020] A portion of the cladding layer 12 of the waveguide chip 10 is removed, and a hollow 15 is provided directly beneath the coupling region C between the optical waveguide 13 and the core 21 of the optical fiber 20. The optical waveguide 13 contacts the tip of the core 21 of the optical fiber 20 in the coupling region C, and the weight of the core 21 causes it to bend in the -Z direction (the stacking direction of the waveguide chip 10) inside the hollow 15. In Figure 2, the cladding layer 12 forming the hollow 15 is removed down to the surface of the substrate 11 for clarity, but the depth of the hollow 15 only needs to be deep enough to absorb the bending in the -Z direction at the connection between the optical waveguide 13 and the core 21. The bottom surface 15a of the hollow 15 may be shallower than the surface of the substrate 11. By overlapping the optical waveguide 13 and the core 21 of the optical fiber 20 in the coupling region C, and flexing the optical waveguide 13 together with the core 21 within the hollow 15, positional misalignment or coupling misalignment in the Z and X directions can be suppressed.

[0021] A fiber guide 123 may be provided on the waveguide tip 10 to guide the core 21 of the optical fiber 20 into the hollow 15. The fiber guide 123 extends from the end face 121 of the waveguide tip 10 to the hollow 15 and guides the tip of the core 21 of the optical fiber 20 to the coupling region C. When the core 21 has a taper 21t, the fiber guide 123 has a shape that matches the taper 21t of the core 21. The taper 21t is a cone that is long in the direction of the optical axis, and the fiber guide 123 is a V-groove, rectangular groove, U-groove, etc. that receives the side surface of the cone.

[0022] By providing the fiber guide 123, the alignment accuracy in the Y direction between the optical waveguide 13 and the core 21 is improved. The fiber guide 123 is formed by photolithography and etching in the semiconductor process, and its processing accuracy is as high as that of the optical waveguide 13. Even if there is a variation of about ±1 μm in the manufacturing accuracy of the optical fiber 20 or the fabrication accuracy of the fiber holder 30 that holds the optical fiber 20, the fiber guide 123 guides the tip of the core 21 into the hollow 15, allowing the tips of the optical waveguide 13 and the core 21 to make contact with each other with high precision.

[0023] Figure 3 is an enlarged view of the coupling region. In the coupling region C, the optical waveguide 13 and the tip of the core 21 of the optical fiber 20 are in contact over a predetermined distance d. If tapers 13t and 21t are provided at the tips of the optical waveguide 13 and the core 21, at least one of the width and height of the optical waveguide 13 decreases continuously and gradually towards the tip. Photons propagating through the optical waveguide 13 are coupled to the core 21 by the evanescent field seeping out from the optical waveguide 13. The distance d is designed so that energy propagation in the coupling region C is maximized, that is, so that optical loss is minimized, as will be described later.

[0024] As described above, the weight of the core 21 of the optical fiber 20 causes both the optical waveguide 13 and the core 21 to flex within the hollow 15, making it easier to align the optical waveguide 13 and the core 21 in the X and Z directions. When a fiber guide 123 is formed on the waveguide tip 10, misalignment in the Y direction is also suppressed. If the core 21 is provided with a taper 21t, the fiber guide 123 may be processed to a shape corresponding to the inclination of the taper 21t. In this case, by fitting the core 21 to the fiber guide 123, misalignment in the X direction as well as the Y direction can be suppressed.

[0025] <First variation> Figure 4 is a schematic diagram of the first modified example, the waveguide-fiber coupled structure 1A. Figure 4(A) is a top view, and (B) is a cross-sectional view along the optical axis AX. In the waveguide-fiber coupled structure 1A, at least the portion of the optical waveguide 13 and optical fiber 20 included in the coupling region C is covered with an inert gas coating 18. In the example in Figure 4, the entire optical fiber 20 including the coupling region C and the optical waveguide 13 are covered with an inert gas coating 18. The inert gas coating 18 is formed as follows: The entire waveguide-fiber coupled structure 1A is placed in a chamber set to an extremely low temperature of about 4K (-269°C). By introducing, for example, argon (Ar) gas (melting point -189°C) into the chamber using a solenoid valve, Ar is deposited on the surface of the optical fiber 20 including the coupling region C and the optical waveguide 13.

[0026] The inert gas coating 18 ensures a secure physical connection in the coupling region C, significantly improving resistance to external vibrations. However, even without the inert gas coating 18, resistance to external vibrations is improved by the deflection connection in the -Z direction in the coupling region C and the guidance of the core 21 by the fiber guide 123. When the inert gas coating 18 is provided, the waveguide-fiber coupling structure 1A must be used in an environment lower than -189°C. For example, when using a quantum dot single-photon source, the single-photon source and the waveguide-fiber coupling structure 1A may be housed inside the same cooler. In the configuration shown in Figure 4, the reliability of the coupling between the optical waveguide 13 and the optical fiber 20 is further improved.

[0027] <Second variation> Figure 5 is a schematic diagram of a second modified example, waveguide-fiber coupling structure 1B. Figure 5(A) is a top view, and (B) is a cross-sectional view along line II in (A). In waveguide-fiber coupling structure 1B, the end face 121 of the waveguide tip 10B holds a portion of the outer circumference of the optical fiber together with the taper 21t of the core 21. A groove 114 is formed on the end face 121 of the waveguide tip 10B to receive the outer circumference of the cladding 22 of the optical fiber 20, and the outer circumference of the optical fiber 20 is held by the groove 114. A fiber guide 123B communicating with the groove 114 is provided in the cladding layer 12 of the waveguide tip 10B. The optical fiber 20 is fixed in the groove 114, and the taper 21t of the core 21 is fitted into the fiber guide 123B. The leading edge of the cladding 22 abuts against the back of the groove 114 of the waveguide tip 10B, so the position of the optical fiber 20 in the X direction is automatically determined.

[0028] The grooves 114 and fiber guides 123B formed on the waveguide tip 10B are formed using a semiconductor process, resulting in high processing accuracy. The configuration shown in Figure 5 suppresses misalignment of the optical fiber 20 itself relative to the waveguide tip 10B, improving alignment accuracy in the X and Y directions. A fiber holder 30 may be combined with the configuration shown in Figure 5. Even if there are variations in molding or machining accuracy of the fiber holder 30, the grooves 114 and fiber guides 123B of the waveguide tip 10B can compensate for or absorb variations in accuracy of the optical fiber 20 and the fiber holder 30. The configuration shown in Figure 5 further improves the reliability of the coupling between the optical waveguide 13 and the optical fiber 20.

[0029] <Third variation> Figure 6 is a schematic diagram of a third modified example, waveguide-fiber coupled structure 2. In waveguide-fiber coupled structure 2, multiple optical waveguides 13-1 and 13-2 are formed on the cladding layer 12C of the waveguide tip 10C. Optical waveguides 13-1 and 13-2 are coupled to corresponding optical fibers 20-1 and 20-2, respectively. Optical fibers 20-1 and 20-2 may be held in a fiber holder 30C to form an optical fiber array.

[0030] Optical waveguide 13-1 and optical fiber 20-1 are optically coupled in coupling region C-1, and optical waveguide 13-2 and optical fiber 20-2 are optically coupled in coupling region C-2. The optical coupling in coupling regions C-1 and C-2 may be adiabatic coupling. Hollows 15-1 and 15-2 are formed directly below coupling regions C-1 and C-2. In hollow 15-1, optical waveguide 13-1 and optical fiber 20-1 are in contact with each other and bent in the -Z direction, as shown in Figure 3. In hollow 15-2, optical waveguide 13-2 and optical fiber 20-2 are in contact with each other and bent in the -Z direction. As a result, coupling misalignment in the Z and X directions is suppressed in coupling regions C-1 and C-2, respectively.

[0031] Fiber guides 123-1 and 123-2 may be provided on the end face 121 side of the waveguide tip 10C. In this case, the core 21 of optical fiber 20-1 and the tip of the core 21 of optical fiber 20-2 are fixed, and positional displacement in the Y direction is suppressed. If the tip of the core 21 of optical fiber 20 is tapered, the fiber guides 123-1 and 123-2 can be formed in a shape corresponding to the angle of the taper of the core 21, thereby suppressing positional displacement in both the Y and X directions. The guide configuration on the end face 121 side of the waveguide tip 10C may be configured to hold a part of the outer circumference of the optical fiber 20, as shown in Figure 5. In this case, positional displacement of the optical fibers 20-1 and 20-2 themselves relative to the waveguide tip 10C is suppressed, and the optical fibers 20-1 and 20-2 can be stabilized.

[0032] <Fourth variation> Figure 7 is a schematic diagram of the fourth modified example, waveguide-fiber coupling structure 3. Figure 7(A) is a top view, and (B) is a cross-sectional view along the optical axis AX. The waveguide-fiber coupling structure 3 has a hollow 15D directly below the coupling region C between the optical waveguide 13 and the core 21 of the optical fiber 20. The hollow 15D widens towards the end face 121 of the waveguide tip 10D or towards the coupling region C. There are no particular restrictions on the shape or size of the hollow 15D as long as it can be bent in the -Z direction by bringing the tip of the optical waveguide 13 and the tip (or taper 21t) of the core 21 of the optical fiber 20 into contact. By making the hollow 15D wider towards the tip of the optical waveguide 13 and narrower towards the rear end of the hollow, it becomes possible to narrow the spacing between optical fibers when mounting multiple optical fibers, enabling high-density optical input and output.

[0033] The planar shape of the hollow 15D in the XY plane is not limited to a triangle enclosed by straight lines as shown in Figure 7, but may also be a shape that expands toward the coupling region C in an exponential curve. The entire coupling region C flexes in the -Z direction within the hollow 15D, suppressing coupling misalignment in the Z and X directions, thereby improving the reliability of the waveguide-fiber coupling structure 3.

[0034] <Fifth variation> Figure 8 is a schematic diagram of the fifth modified example, waveguide-fiber coupling structure 4. Waveguide-fiber coupling structure 4 is an extension of waveguide-fiber coupling structure 3 in Figure 7 to multiple arrays. Multiple optical waveguides 13-1 and 13-2 are formed on the waveguide tip 10E. Optical waveguides 13-1 and 13-2 are coupled to the corresponding optical fibers 20-1 and 20-2. Optical fibers 20-1 and 20-2 may be held on a fiber holder 30E to form a fiber array.

[0035] Optical waveguide 13-1 and optical fiber 20-1 are optically coupled in coupling region C-1, and optical waveguide 13-2 and optical fiber 20-2 are optically coupled in coupling region C-2. Hollows 15E-1 and 15E-2 are formed directly beneath coupling regions C-1 and C-2. In hollow 15E-1, optical waveguide 13-1 and optical fiber 20-1 are in contact and bent in the -Z direction. In hollow 15E-2, optical waveguide 13-2 and optical fiber 20-2 are in contact and bent in the -Z direction. As a result, misalignments in the Z and X directions are absorbed in coupling regions C-1 and C-2, respectively. The shape of hollows 15E-1 and 15E-2 allows for a narrower spacing between optical fibers 20-1 and 20-2, enabling high-density optical input and output. When fiber guides 123-1 and 123-2 are provided on the waveguide tip 10E, the positions of the cores of optical fiber 20-1 and optical fiber 20-2 are fixed, and positional displacement in the Y direction is suppressed. When the tip of the core of optical fiber 20 has a taper 21t, positional displacement in both the Y and X directions is suppressed by forming fiber guides 123-1 and 123-2 according to the inclination of the core taper.

[0036] <Fabrication of waveguide chips> Figures 9A to 9D are process diagrams for fabricating a waveguide chip. For example, the waveguide chip 10B shown in Figure 5 is fabricated. In Figure 9A, a substrate 11 for the waveguide chip is prepared, and a cladding layer 12 and a core layer 23 are formed on its entire surface. The substrate 11 is any substrate that can support the cladding layer 12 and the core layer 23. As an example, a Si substrate is used. The cladding layer 12 is formed of a material with a lower refractive index than the core layer 23, and may be SiO2, magnesium fluoride (MgF2), calcium fluoride (CaF2), etc. In this example, an SiO2 cladding layer 12 is formed. When forming the SiO2 cladding layer 12, a Si substrate with a thermal oxide film may be used. The core layer 23 is formed on top of the cladding layer 12. The core layer 23 is formed of a material that has a higher refractive index than the cladding layer 12 and is transparent to the wavelength of the photons to be detected. In this example, a material with a higher refractive index than SiO2 and that is transparent to visible light is used, such as diamond, sapphire (Al2O3 single crystal), SiN, or AlN. As an example, an Al2O3 single crystal thin film with a thickness of several hundred nanometers is formed as a core layer 23 on top of an SiO2 cladding layer 12 by vapor phase growth in a high vacuum or reduced pressure vapor phase growth.

[0037] In Figure 9B, the core layer 23 is processed to form the optical waveguide 13. (A) is a cross-sectional view along the optical axis (X-axis), and (B) is a cross-sectional view taken along line II-II of (A). An electron beam resist is applied to cover the entire surface of the core layer 23, and a resist mask in the shape of the optical waveguide 13 is formed by electron beam exposure and development. The optical waveguide 13 is formed by etching the core layer 23 (see Figure 9A) with the resist mask as a protective film. As an example, an optical waveguide 13 with a thickness of 200 nm and a width of 800 nm is formed. If a taper 13t is to be provided at the tip of the optical waveguide 13, at this stage, etching is performed so that at least one of the width and height of the optical waveguide 13 decreases continuously and gradually towards the tip.

[0038] Figure 9C shows the formation of the fiber holding groove 114 and fiber guide 123. (A) is a cross-sectional view along the optical axis (X-axis), and (B) is a cross-sectional view taken along line III-III of (A). For reference, the outline of the optical fiber 20, which will be set in a later assembly process, is shown by a dashed line. An electron beam resist is applied to the entire surface of the substrate on which the optical waveguide 13 is formed. An electron beam exposure and development process is used to form a resist mask in which only the area where the groove 114 is to be formed is exposed, and the groove 114 is formed by etching. Alternatively, the groove 14 may be formed by conventional photolithography and etching instead of electron beam exposure. After that, the resist mask is removed, a new electron beam resist is applied, and the fiber guide 123 is formed in the same process.

[0039] In Figure 9D, a hollow 15 is formed. A mask covering the entire surface except for the area where the hollow 15 is formed is formed by photolithography, and the cladding layer 12 in a predetermined range is etched away with vaporized hydrofluoric acid. Si is easily etched by fluorine atoms, but Al is difficult to etch by fluorine atoms. The etching selectivity ratio of hydrofluoric acid gas to SiO2 and Al2O3 is large, and the SiO2 cladding layer 12 can be removed with almost no erosion of the optical waveguide 13 formed from a single-crystal thin film of Al2O3. Hydrofluoric acid gas is a reactive gas, and isotropic dry etching is possible. As a result, a hollow 15 is formed over a predetermined area directly below the optical waveguide 13. In Figure 9D, the bottom surface 15a of the hollow 15 reaches the surface of the substrate 11, but it is sufficient to ensure a space for the optical waveguide 13 and the core 21 of the optical fiber 20 to bend in the -Z direction, so the bottom surface 15a of the hollow 15 does not need to reach the surface of the substrate 11. The depth of the hollow section 15 can be designed as appropriate.

[0040] <Formation of a tapered tip in an optical fiber> Figures 10A and 10B show the process of forming the taper 21t at the tip of the optical fiber 20. In Figure 10A, (A) is a top view, and (B) is a cross-sectional view AA of (A). The tip surfaces of optical fibers 20-1 and 20-2 are aligned, and optical fibers 20-1 and 20-2 are placed in the V-groove 32 of the fiber holder 30, fixed with resin 34, and covered with cover 135. Cover 135 is for facilitating handling of the tips of optical fibers 20-1 and 20-2 during tip processing and may be removed after the taper is formed.

[0041] In Figure 10B, the tips of optical fibers 20-1 and 20-2 protruding from the fiber holder 30 are immersed in a hydrofluoric acid solution and gradually withdrawn. Due to the difference in the dissolution rates of the optical fiber core and cladding in the hydrofluoric acid solution, the cladding is preferentially removed. Also, the tip of the core becomes thinner the longer it is immersed in the hydrofluoric acid solution. The taper angle of the core tip is controlled by the withdrawal speed of the optical fiber 20, the concentration of hydrofluoric acid in the solution, etc. Through this wet etching, a taper 21t is formed on each of the optical fibers 20-1 and 20-2.

[0042] <Coupling of optical waveguides and optical fibers> Optical fibers 20-1 and 20-2, each with a tapered 21t, are connected to optical waveguides 13-1 and 13-2 (see Figure 6) on a waveguide tip. The array of optical fibers 20-1 and 20-2, fixed to the fiber holder 30, is placed on a piezo stage with a three-axis actuator, guiding the tapered 21t to the fiber guide 123 on the waveguide tip. While observing the microscope image and optical loss by irradiating test light, the tapered 21t tips of optical fibers 20-1 and 20-2 are superimposed on optical waveguides 13-1 and 13-2 floating above the hollow 15 to determine the position where optical loss is minimized. The taper 21t of optical fibers 20-1 and 20-2 overlaps with optical waveguides 13-1 and 13-2, causing the optical waveguides 13-1 and 13-2 to bend downward (-Z direction) inside the hollow 15 while maintaining contact with optical fibers 20-1 and 20-2. This method provides a waveguide-fiber coupling structure with improved coupling reliability.

[0043] Thus, (a) Form an optical waveguide on a cladding layer formed on a substrate, (b) Remove a portion of the cladding layer below the optical waveguide to form a hollow below the optical waveguide, (c) The core of an optical fiber is brought into contact with the tip of the optical waveguide extending into the hollow, and the optical waveguide and the core are bent in the stacking direction of the optical waveguide inside the hollow. This suppresses coupling misalignment between the optical waveguide and the optical fiber, at least in the Z and X directions, thereby improving the reliability of the coupling between the optical waveguide and the optical fiber.

[0044] <Analysis of coupling between optical waveguides and optical fibers> Figure 11 shows a stress analysis model of an optical waveguide 13 protruding from the hollow 15 of a waveguide tip 10. In the model, a 2 μm thick SiO2 cladding layer 12 is provided on a Si substrate 11, and a sapphire optical waveguide 13 with a width of 800 nm and a thickness of 200 nm is provided on the cladding layer 12. The length L of the optical waveguide 13 protruding into the hollow 15 is set to 20 μm, and the stress is investigated when the tip of the optical waveguide 13 is displaced downward by 0.1 μm (ΔZ = 0.1 μm).

[0045] Figure 12 shows the stress distribution and the point of maximum stress. In the model in Figure 11, when the optical waveguide 13, which protrudes 20 μm, is displaced 0.1 μm downward (in the -Z direction), a maximum stress of 150 MPa is applied in the -Z direction. The point of maximum stress MX is the point of contact between the upper end of the cladding layer 12 forming the hollow 15 and the optical waveguide 13.

[0046] The breaking stress of sapphire, the material of the optical waveguide 13, is approximately 170 MPa. When using a sapphire waveguide, the optical waveguide 13 can be deflected within the hollow 15 if the deflection is 0.1 μm / 20 μm or less. For example, if the length of the optical waveguide 13 that protrudes into the hollow 15 due to the adiabatic change in cross-sectional area is 200 μm, it can be deflected by 1 μm in the -Z direction. The optical waveguide 13 itself may change shape in a tapered manner from a stage before reaching the hollow 15. For example, the total length of the tapered optical waveguide 13 may be 1 mm, of which 400 μm may protrude into the hollow 15.

[0047] Figure 13 shows the relationship between the overlap (μm) of the optical fiber and waveguide in the coupling region C and the amount of energy propagation. The energy propagation on the vertical axis is the relative energy propagation when the input is set to 1. Consider a configuration in which light is incident from an optical fiber with a tapered length of 400 μm into an optical waveguide 13 that protrudes into the hollow 15 at a length L of 200 μm from the maximum stress point MX. The overlap (μm) is defined as the length at which the tapered tip of the optical fiber 20 extending from the X direction contacts the optical waveguide 13 within the hollow 15. A negative value for the overlap indicates that the optical waveguide and the core of the optical fiber are not in contact, but the light emitted from the core propagates through the air and couples with the optical waveguide. If an energy propagation of 0.04 or more is considered acceptable, the overlap between the optical waveguide 13 and the optical fiber 20 is 0 μm or more and 90 μm or less, preferably 0 μm or more and 70 μm or less. When an energy propagation amount of 0.06 or higher is achieved, the overlap is 0 μm to 60 μm, preferably 5 μm to 60 μm, and more preferably 5 μm to 50 μm. Within this range of overlap, when the optical waveguide 13 bends in the -Z direction within the hollow 15, the tip of the optical fiber core 21 follows the displacement of the optical waveguide 13 and bends together with the optical waveguide 13 within the hollow 15, maintaining contact between the optical waveguide 13 and the optical fiber 20, and achieving the desired optical coupling.

[0048] Figure 14 shows the energy propagation for overlap amounts (1), (2), and (3) in Figure 13. Overlap amount (1) is 100 μm, overlap amount (2) is 10 μm, and overlap amount (3) is -50 μm. The amount of energy propagated from the optical fiber to the waveguide is greatest at overlap amount (2), and decreases in the order of (1) and (3). As can be seen from Figure 13, light coupling occurs even if the waveguide and optical fiber are not in contact. It is not the case that more contact between the waveguide and optical fiber is better; there is an optimal contact length for energy propagation, i.e., optical coupling. From the simulation results, the preferred contact length (overlap amount) is greater than 0 μm and 60 μm or less, preferably between 5 μm and 60 μm.

[0049] <Quantum optical circuit> Figure 15 is a schematic diagram of a quantum optical circuit 100 using the waveguide-fiber coupling structure 5 of the embodiment. The waveguide-fiber coupling structure 5 may use any of the configurations shown in Figures 2 to 8, or a combination thereof. In this example, four optical waveguides 13-1, 13-2, 13-3, and 13-4 (collectively referred to as "optical waveguide 13" as appropriate) are formed on the waveguide chip 10, and are connected by the waveguide-fiber coupling structure 5 to four single-mode optical fibers 20-1, 20-2, 20-3, and 20-4 (collectively referred to as "optical fiber 20" as appropriate).

[0050] Hollows 15-1, 15-2, 15-3, and 15-4 (collectively referred to as "hollows 15" as appropriate) are formed in the waveguide-fiber coupling structure 5, and the tip of the optical waveguide 13 and the tip of the optical fiber 20 are in contact in each hollow 15. The end of the waveguide tip 10 opposite to the waveguide-fiber coupling structure 5 is connected to the photon source 40. The photon source 40 has independent single photon sources 41-1, 41-2, 41-3, and 41-4 (collectively referred to as "single photon sources 41" as appropriate).

[0051] The single-photon sources 41-1, 41-2, 41-3, and 41-4 are, for example, NV color centers formed at appropriate intervals in diamond. The NV color centers may be formed by irradiating a diamond crystal thin film containing N impurities at a predetermined concentration with heavy ions such as xenon (Xe) to eject carbon (C) elements and form atomic vacancies. The photons generated by the single-photon source 41 travel through the corresponding optical waveguide 13. The waveguide tip 10 may have gates that manipulate one qubit (give it a predetermined rotation) or gates that act on two qubits.

[0052] The state of photons traveling along the optical waveguide 13 and coupled to the corresponding optical fiber 20 in the waveguide-fiber coupling structure 5 is detected by a detector. The ends of optical fibers 20-1, 20-2, 20-3, and 20-4 opposite the waveguide-fiber coupling structure 5 are connected to detectors 45-1, 45-2, 45-3, and 45-4 (collectively referred to as "detector 45" as appropriate). Detector 45 is a detector made by introducing organic chiral molecules into an inorganic material such as a perovskite halide, and directly detects the right-handed and left-handed polarization of photons. The outputs of detectors 45-1, 45-2, 45-3, and 45-4 may be converted into voltage signals and input to a general-purpose information processing device.

[0053] In each of the hollows 15-1 to 15-4 of the waveguide-fiber coupling structure 5, the corresponding optical waveguide and optical fiber tips overlap and maintain contact while bending in the -Z direction, thus suppressing coupling misalignment in the Z and X directions. If a fiber guide is provided on the waveguide tip 10 to guide the tip of the optical fiber core 20, the Y-direction positional misalignment of the optical waveguide 13 and optical fiber 20 is also suppressed. Since the reliability of coupling is improved in each of the coupling regions C-1 to C-4, the reliability of the operation of the quantum optical circuit 100 is improved.

[0054] If the optical waveguide 13 and optical fiber 20 are coated with an inert gas in at least the coupling region C-1 to C-4, the resistance to external vibrations is further improved, and the reliability of the quantum optical circuit 100 is further enhanced.

[0055] Although embodiments have been described above based on specific configuration examples, this disclosure is not limited to the embodiments described above. Two or more of the embodiments and their modifications described above may be combined with each other. The inert gas coating 18 is applicable to all of the waveguide-fiber coupling structures described above, and the fiber guide configuration of Figure 5 is applicable to all of the waveguide-fiber coupling structures described above. The waveguide-fiber coupling configurations and quantum optical circuits of the embodiments are applicable to quantum computing, quantum information communication, quantum optics, cryptographic communication, etc. [Explanation of Symbols]

[0056] 1, 1A, 1B, 2-5 Waveguide-fiber coupling structure 10, 10A~10E, 10B 11 circuit boards 12 Clad Layers 13, 13-1, 13-2, 13-3, 13-4 Optical waveguide 13t taper 15, 15-1, 15-2, 15-3, 15-4 hollow 18 Coating 20, 20-1, 20-2, 20-3, 20-4 optical fibers 21 cores 21t taper 22 Clad 23 Core Layers 30, 30, 30C, 30D, 30E Fiber Retaining Base 40 photon source 41-1, 41-2, 41-3, 41-4 Single-photon sources 45-1, 45-2, 45-3, 45-4 detectors 100 Quantum optical circuit 114 Groove 123, 123B Fiber Guide AX optical axis C, C-1, C-2, C-3, C-4 binding region

Claims

1. A waveguide chip having a substrate, a cladding layer formed on the substrate, and an optical waveguide formed on the cladding layer, An optical fiber optically connected to the optical waveguide, Equipped with, The cladding layer is not provided below the coupling region where the optical waveguide is optically connected to the optical fiber, and a void is formed between the optical waveguide and the substrate. In the coupling region, the optical fiber contacts the optical waveguide from the opposite side of the hollow, causing the optical waveguide in the coupling region to bend in the direction of the substrate within the hollow. Device.

2. The optical waveguide has a first tapered section in which the height decreases toward the coupling region. The optical fiber has a second tapered portion that becomes narrower toward the coupling region, The second tapered portion overlaps the first tapered portion inside the hollow space. The apparatus according to claim 1.

3. The first tapered portion and the second tapered portion are in contact with each other in the optical axis direction for a length greater than 0 μm and less than or equal to 60 μm. The apparatus according to claim 2.

4. The optical waveguide is formed of a material that is transparent to visible light and has a higher refractive index than the cladding layer. The apparatus according to claim 1.

5. The optical waveguide is formed of sapphire, diamond, silicon nitride, or aluminum nitride. The apparatus according to claim 4.

6. The waveguide tip has a fiber guide that guides the second tapered portion of the optical fiber into the hollow space. The apparatus according to claim 2.

7. The waveguide tip communicates with the fiber guide and has a groove that receives the outer circumference of the optical fiber extending from the second tapered portion. The apparatus according to claim 6.

8. At least the coupling region of the optical waveguide and the optical fiber is coated with an inert gas. The apparatus according to claim 1.

9. The width of the hollow increases toward the side into which the optical fiber is introduced. The apparatus according to claim 1.

10. An apparatus comprising a substrate, a cladding layer formed on the substrate, an optical waveguide formed on the cladding layer, and an optical fiber, A single photon source optically connected to the optical waveguide, A detector connected to the optical fiber for detecting the state of photons, Equipped with, The apparatus is configured such that the cladding layer is not provided below the coupling region where the optical waveguide is optically connected to the optical fiber, and a void is formed between the optical waveguide and the substrate. In the coupling region, the optical fiber contacts the optical waveguide from the opposite side of the hollow, causing the optical waveguide in the coupling region to bend in the direction of the substrate within the hollow. Quantum optical circuit.

11. An optical waveguide is formed on a cladding layer formed on a substrate. By removing a portion of the cladding layer below the optical waveguide, the cladding layer is not provided below the coupling region where the optical waveguide is optically connected to the optical fiber, and a void is formed between the optical waveguide and the substrate. In the coupling region, the optical fiber is brought into contact with the optical waveguide extending into the hollow from the opposite side of the hollow, thereby causing the optical waveguide to bend in the direction of the substrate within the hollow. Method for fabricating the device.

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