Wavelength conversion element and wavelength conversion system

Abstract

To provide a wavelength conversion element and a wavelength conversion system capable of widening the band of the wavelength of output light and stably propagating input light and output light.SOLUTION: According to an embodiment of the present invention, there is provided a wavelength conversion element including: a dielectric substrate formed by periodically forming holes in a nonlinear optical crystal substrate; a line-defect optical waveguide formed in the dielectric substrate; and a periodic domain inversion part provided in the optical waveguide. The wavelength conversion element is configured to convert a wavelength of light passing through the optical waveguide.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a wavelength conversion element and a wavelength conversion system. [Background technology]

[0002] Development of wavelength conversion elements, one type of nonlinear optical element, is progressing. Wavelength conversion elements are expected to be applied and developed in a wide range of fields, including next-generation optical communications and quantum fields. For such wavelength conversion elements, improving conversion efficiency and increasing output power are key issues, and various device structures have been developed. As an example of such a wavelength conversion element, a technology has been proposed in which a thin film layer containing a wavelength conversion material disposed on a substrate is provided with a light confinement section that confines input light and a light emission section (photonic crystal) that emits the converted light in a direction different from the propagation direction of the input light (e.g., Patent Document 1). However, with the technology described in Patent Document 1, the wavelength of the converted light that can be emitted is limited, and it is difficult to propagate the converted light to a desired location. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-209522 Summary of the Invention [Problem to be solved by the invention]

[0004] A primary object of the present invention is to provide a wavelength conversion element and a wavelength conversion system that can widen the wavelength band of output light and can stably propagate input light and output light. [Means for solving the problem]

[0005] [1] A wavelength conversion element according to one embodiment of the present invention comprises: a dielectric substrate formed by periodically forming voids in a nonlinear optical crystal substrate; an optical waveguide having a line defect formed in the dielectric substrate; and a periodic polarization inversion portion provided in the optical waveguide; and is configured to convert the wavelength of light passing through the optical waveguide. [2] In the wavelength conversion element described in [1] above, the dielectric substrate may function as either a photonic crystal or an effective dielectric cladding for at least one of the input lights to the optical waveguide, and may function as either a photonic crystal or an effective dielectric cladding for at least one of the output lights from the optical waveguide. [3] In the wavelength conversion element described in [2] above, the optical waveguide may be configured to receive input light and output first output light and second output light having frequencies lower than that of the input light. [4] In the wavelength conversion element described in [3] above, the dielectric substrate may function as a photonic crystal for the input light and as an effective dielectric cladding for the first output light and the second output light. [5] In the wavelength conversion element described in [3] above, the dielectric substrate may function as a photonic crystal for the input light and the first output light, and as an effective dielectric cladding for the second output light. [6] In the wavelength conversion element described in [1] above, the dielectric substrate may function as an effective dielectric cladding for the input light, the first output light, and the second output light. [7] In the wavelength conversion element according to any one of the above [3] to [6], the input light, the first output light, and the second output light may satisfy the following formula (1-1) and formula (2-1):

number

number

[10] In the wavelength conversion element described in [8] above, the dielectric substrate may function as a photonic crystal for the first input light, the second input light and the first output light, and may function as an effective dielectric cladding for the second output light.

[11] In the wavelength conversion element described in [8] above, the dielectric substrate may function as a photonic crystal for the first input light and the second output light, and may function as an effective dielectric cladding for the second input light and the first output light.

[12] In the wavelength conversion element described in [1] above, the dielectric substrate may function as an effective dielectric cladding for the first input light, the second input light, the first output light, and the second output light.

[13] In the wavelength conversion element described in any one of [8] to

[12] above, the first input light, the second input light, the first output light, and the second output light may satisfy the following formulas (1-2A), (1-2B), and (2-2):

number

number

number

[14] In the wavelength conversion element described in [2] above, the optical waveguide may be configured to receive input light and output output light having a frequency higher than that of the input light.

[15] In the wavelength conversion element described in

[14] above, the dielectric substrate may function as an effective dielectric cladding for the input light and as a photonic crystal for the output light.

[16] In the wavelength conversion element described in [1] above, the dielectric substrate may function as an effective dielectric cladding for the input light and the output light.

[17] In the wavelength conversion element according to any one of the above

[14] to

[16] , the input light and the output light may satisfy the following formula (1-3) and formula (2-3).

number

number

[18] In the wavelength conversion element described in [2] above, the optical waveguide may be configured to receive first input light and second input light having a lower frequency than the first input light, and to output output light having a higher frequency than the first input light and the second input light.

[19] In the wavelength conversion element described in

[18] above, the dielectric substrate may function as a photonic crystal for the output light and as an effective dielectric cladding for the first input light and the second input light.

[20] In the wavelength conversion element described in

[18] above, the dielectric substrate may function as a photonic crystal for the first input light and the output light, and as an effective dielectric cladding for the second input light.

[21] In the wavelength conversion element described in [1] above, the dielectric substrate may function as an effective dielectric cladding for the first input light, the second input light, and the output light.

[22] In the wavelength conversion element according to any one of the above

[18] to

[21] , the first input light, the second input light, and the output light may satisfy the following formula (1-4) and formula (2-4):

number

number

[23] In the wavelength conversion element described in any one of [2] to

[22] above, of the input light and the output light, the light propagated in the dielectric substrate in a photonic crystal mode may satisfy the following formula (3), and the light propagated in the dielectric substrate in an effective dielectric cladding mode may satisfy the following formula (4).

number

number

[24] The wavelength conversion element according to any one of [1] to

[23] above may further comprise a support substrate provided below the nonlinear optical crystal substrate, and a low refractive index portion having a refractive index smaller than that of the nonlinear optical crystal substrate and positioned between the nonlinear optical crystal substrate and the support substrate. At least a portion of the low refractive index portion may overlap the optical waveguide in the thickness direction of the nonlinear optical crystal substrate.

[25] The wavelength conversion element according to any one of [1] to

[24] above may further include a diffraction grating provided in the optical waveguide and arranged to be aligned with the periodic polarization inversion section in the waveguiding direction of the optical waveguide. The wavelength conversion element is configured to emit, from the optical waveguide, light whose wavelength has been converted in the optical waveguide.

[26] The wavelength conversion element according to any one of the above [1] to

[25] may comprise a first electrode and a second electrode electrically connected to the nonlinear optical crystal substrate.

[27] A wavelength conversion system according to another aspect of the present invention includes the wavelength conversion element according to any one of [1] to

[26] above; and a control unit capable of controlling the refractive index of the nonlinear optical crystal substrate.

[28] The wavelength conversion system described in

[27] above may include a first electrode and a second electrode electrically connected to the nonlinear optical crystal substrate, the first electrode and the second electrode being spaced apart from each other; and a power supply capable of applying a voltage to the first electrode and the second electrode. The control unit can control the power supply and adjust the refractive index of the nonlinear optical crystal substrate by controlling the voltage applied to the first electrode and the second electrode. [Effects of the Invention]

[0006] According to the embodiments of the present invention, it is possible to realize a wavelength conversion element and a wavelength conversion system that can widen the wavelength band of output light and stably propagate input light and output light. [Brief explanation of the drawings]

[0007] [Figure 1] Fig. 1(a) is a schematic diagram of a wavelength conversion system including a wavelength conversion element according to an embodiment of the present invention, and Fig. 1(b) is a schematic diagram of a periodically poled portion shown in Fig. 1(a). [Figure 2] FIG. 2 is a schematic perspective view of a wavelength conversion element according to another embodiment of the present invention. [Figure 3] FIG. 3 is a schematic perspective view of a wavelength conversion element according to yet another embodiment of the present invention. [Figure 4] Figures 4(a) to 4(e) are schematic cross-sectional views illustrating a manufacturing method of a wavelength conversion element according to an embodiment of the present invention, where Figure 4(a) shows a step of preparing a nonlinear optical crystal substrate; Figure 4(b) shows a step of bonding the nonlinear optical crystal substrate and a support substrate; Figure 4(c) shows a step of polishing the nonlinear optical crystal substrate; Figure 4(d) shows a step of forming voids; and Figure 4(e) shows a step of forming a first electrode and a second electrode. [Figure 5] FIG. 5 is a graph showing the correlation between the propagation constant and the angular frequency of light in the parametric down-conversion of the first embodiment. [Figure 6] FIG. 6 is a diagram illustrating shifts in band curves of the photonic crystal mode and the EMC mode in the parametric down-conversion of the first embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to these embodiments. A. Overall configuration of wavelength conversion element FIG. 1(a) is a schematic diagram of a wavelength conversion system including a wavelength conversion element according to an embodiment of the present invention; FIG. 1(b) is a schematic diagram of the periodic polarization inversion section shown in FIG. 1(a); FIG. 2 is a schematic perspective view of a wavelength conversion element according to another embodiment of the present invention; and FIG. 3 is a schematic perspective view of a wavelength conversion element according to yet another embodiment of the present invention. As shown in FIG. 1( a), a wavelength conversion element 100 includes a dielectric substrate 10 formed by periodically forming holes 12 in a nonlinear optical crystal substrate 11; a line-defect optical waveguide 13 formed in the dielectric substrate 10; and a periodic polarization inversion portion 14 provided in the optical waveguide 13. The wavelength conversion element 100 is configured to convert the wavelength of light passing through the optical waveguide 13. The optical waveguide 13 is typically a line-defect waveguide defined as a portion of the nonlinear optical crystal substrate 11 where no holes 12 are formed. The frequency of the input light input to the optical waveguide 13 is typically 150 THz or more and 858 THz or less. When the frequency is converted to wavelength, the wavelength of the input light is approximately 350 nm or more and approximately 2 μm or less. The frequency of the output light (converted light) output from the optical waveguide 13 is typically 20 THz or more and 857 THz or less. When the frequency is converted into wavelength, the wavelength of the output light (converted light) is approximately 350 nm or more and approximately 15 μm or less. With this configuration, the periodically poled portion is provided in the optical waveguide, and thus the wavelength of light passing through the optical waveguide can be converted by quasi-phase matching (QPM). Furthermore, since the refractive index of the nonlinear optical crystal substrate can be modulated, the wavelength band of convertible light can be expanded by modulating the refractive index of the nonlinear optical crystal substrate. Furthermore, by adjusting the refractive index of the nonlinear optical crystal substrate, each of the input light and the output light can propagate through the optical waveguide in either the photonic crystal mode or the effective dielectric cladding mode (hereinafter referred to as the EMC mode). Therefore, with this wavelength conversion element, the wavelength band of the output light can be broadened, and the input light and the output light can be stably propagated to the desired location without radiating from the optical waveguide.

[0009] In one embodiment, the dielectric substrate 10 functions as either a photonic crystal or an effective dielectric cladding for at least one of the input lights to the optical waveguide 13, and functions as either a photonic crystal or an effective dielectric cladding for at least one of the output lights from the optical waveguide 13. With this configuration, the wavelength bandwidth of the output light can be sufficiently broadened, and the input light and output light can be propagated more stably. Photonic crystals are multidimensional periodic structures composed of media with high and low refractive indexes at a period comparable to the wavelength of light, and have an optical band structure similar to the band structure of electrons. In photonic crystals, a specific optical forbidden band (photonic band gap) is manifested. Photonic crystals with a forbidden band function as objects that neither reflect nor transmit light of a specific wavelength. When a line defect that disrupts the periodicity is introduced into a photonic crystal with a photonic band gap, a waveguide mode is formed within the frequency region of the band gap, making it possible to realize a waveguide that propagates light with low loss. On the other hand, the effective dielectric cladding does not exhibit a certain forbidden band (photonic band gap) for light. In this case, light is not diffracted by the periodic air holes, and the periodic air holes effectively function as a low-refractive-index region. This is equivalent to behaving as a cladding in an optical fiber. Therefore, if a line defect is introduced into the effective dielectric cladding, the line defect portion behaves as a core in an optical fiber, realizing a waveguide that propagates light with small propagation loss over a wide frequency range. In other words, at least one of the input beams propagates through the optical waveguide 13 in either the photonic crystal mode or the effective dielectric cladding mode (hereinafter referred to as the EMC mode), and at least one of the output beams propagates through the optical waveguide 13 in the other of the photonic crystal mode or the EMC mode.

[0010] In one embodiment, the wavelength conversion element 100 further includes a support substrate 20 and a low-refractive-index portion 30. The support substrate 20 is provided below the dielectric substrate 10 and supports the dielectric substrate 10. This improves the strength of the wavelength conversion element and allows the thickness of the dielectric substrate (nonlinear optical crystal substrate) to be reduced. The low-refractive-index portion 30 is located between the dielectric substrate 10 and the support substrate 20. The refractive index of the low-refractive-index portion 30 is smaller than the refractive index of the nonlinear optical crystal substrate 11. At least a portion of the low-refractive-index portion 30 overlaps with the optical waveguide 13 in the thickness direction of the nonlinear optical crystal substrate 11. This prevents light from leaking into the support substrate when the light propagates through the optical waveguide 13. Therefore, even in an embodiment in which the dielectric substrate is mounted (supported) on the support substrate, light can be stably confined and propagated in the optical waveguide, thereby preventing an increase in propagation loss.

[0011] In one embodiment, the dielectric substrate 10 is directly bonded to the support substrate 20. In this specification, "direct bonding" means that two layers or substrates are bonded together without the use of an adhesive. The form of direct bonding can be appropriately set depending on the configuration of the layers or substrates to be bonded together. More specifically, the wavelength conversion element 100 further includes a bonding portion 80 that bonds the dielectric substrate 10 and the support substrate 20 together. This effectively prevents peeling in the wavelength conversion element, and as a result, effectively prevents damage (e.g., cracks) to the dielectric substrate caused by such peeling.

[0012] In one embodiment, the wavelength conversion element 100 further includes a first electrode 40 and a second electrode 50 electrically connected to the nonlinear optical crystal substrate 11, the first electrode 40 and the second electrode 50 being spaced apart from each other. With this configuration, a voltage can be applied to the nonlinear optical crystal substrate via the first electrode and the second electrode, and the refractive index of the nonlinear optical crystal substrate can be smoothly modulated. As a result, the wavelength of light that can be converted can be stably broadened.

[0013] As shown in FIG. 3, the wavelength conversion element 100 may further include a diffraction grating 15. The diffraction grating 15 is aligned with the periodic polarization inversion units 14 in the waveguiding direction of the optical waveguide 13. The diffraction grating 15 is provided in the optical waveguide 13. More specifically, the diffraction grating 15 is provided in at least one portion selected from the top, left side, and right side of the optical waveguide 13. In this case, the wavelength conversion element 100 can emit wavelength-converted output light from the top surface of the optical waveguide 13. The wavelength conversion element 100 can control the diffraction angle (output angle) of the diffraction grating by changing the wavelength of the output light using the electro-optic effect, thereby functioning as an optical deflector. The light beam (laser light) emitted from the optical waveguide 13 is a so-called fan beam that is linear in plan view (linear in a direction perpendicular to the waveguiding direction) and fan-shaped when viewed from the waveguiding direction. The periodic holes forming the optical waveguide 13 may have different hole periods and hole diameters in the regions where the periodic polarization inversion units 14 and the diffraction grating 15 are provided. The wavelength of the output light from the wavelength conversion element 100 can be broadbanded even without applying a voltage depending on the conditions, and in this case, it is possible to obtain output light from the diffraction grating 15 with different output angles depending on the wavelength, and the output light beam (laser light) from the optical waveguide 13 becomes a beam that is two-dimensionally spread in a planar view.

[0014] In this specification, the term "wavelength conversion element" encompasses both a wafer on which at least one wavelength conversion element is formed (wavelength conversion element wafer) and chips obtained by cutting the wavelength conversion element wafer.

[0015] B. Components of wavelength conversion elements Next, each component of the wavelength conversion element will be described with reference to FIGS. B-1. Nonlinear optical crystal substrate (dielectric substrate) As shown in FIG. 1, the nonlinear optical crystal substrate 11 has an upper surface exposed to the outside and a lower surface located within the composite substrate. The nonlinear optical crystal substrate 11 is made of a nonlinear optical material, and is preferably made of a single crystal of the nonlinear optical material. Any appropriate material can be used as the nonlinear optical material as long as it can provide the effects of the embodiments of the present invention. Representative examples of such materials include lithium niobate (LiNbO3:LN), lithium tantalate (LiTaO3:LT), potassium titanyl phosphate (KTiOPO4:KTP), potassium lithium niobate (KTiOPO4:KTP), and the like. x Li (1-x) NbO2:KLM), potassium niobate (KNbO3:KN), potassium tantalate niobate (KNb x Ta (1-x) O3:KTN), solid solution of lithium niobate and lithium tantalate, KTP (KTiOPO4), KTN (KTa (1-x) Nb x 03), and preferably lithium niobate (LN). When using lithium niobate or lithium tantalate, it is possible to use a crystal doped with MgO or a stoichiometric crystal to suppress optical damage. In addition, organic nonlinear optical crystals (electro-optic polymers) such as 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) and OP-GaAs (orientation-patterned gallium arsenide) crystals may also be used. Nonlinear optical crystal substrate 11 may be an X-cut substrate or a Y-cut substrate. Nonlinear optical crystal substrate 11 is preferably a Y-cut substrate, and more preferably a 5° off-axis Y-cut substrate. The thickness of nonlinear optical crystal substrate 11 may be set to any appropriate value. The thickness of nonlinear optical crystal substrate 11 may be, for example, 0.07 μm to 5.0 μm, or 0.1 μm to 1.5 μm. The refractive index n of the nonlinear optical crystal substrate 11 at 200 THz is typically 2.0 or more, preferably 2.1 or more, and typically 4.0 or less, preferably 3.8 or less.

[0016] In the nonlinear optical crystal substrate 11, holes 12 are periodically formed. The holes 12 may be formed in a periodic pattern as described above. The holes 12 are typically arranged to form a regular lattice. Any appropriate lattice shape may be adopted. Typical examples include a triangular lattice and a square lattice. In one embodiment, the holes 12 may be through-holes. Through-holes are easy to form, and as a result, the refractive index can be easily adjusted. Any appropriate shape may be adopted as the shape of the holes (through-holes) in a plan view. Specific examples include an equilateral polygon (e.g., an equilateral triangle, a square, a regular pentagon, a regular hexagon, or a regular octagon), a substantially circular shape, and an elliptical shape, with a substantially circular shape being preferred. The approximately circular shape preferably has a major axis / minor axis ratio of 0.90 to 1.10, more preferably 0.95 to 1.05. The holes 12 may be low-refractive-index pillars (pillar-shaped portions made of a low-refractive-index material). However, through-holes are easier to form, and are made of air, which has the lowest refractive index, so the refractive index difference with the waveguide can be made larger. The hole diameter may be partially different from the other hole diameters, and the hole period may also be partially different from the other hole period. The vacancy period α (period α of the periodic hole arrangement) is, for example, 0.02 μm or more, preferably 0.10 μm or more, and more preferably 0.30 μm or more, and is, for example, 3.5 μm or less, preferably 1.4 μm or less, and more preferably 0.80 μm or less. The vacancy diameter is preferably 0.25α to 0.95α, and more preferably 0.50α to 0.90α relative to the vacancy period α.

[0017] In the illustrated example, the air holes 12 function as low-refractive-index pillars, the portions between the air holes 12 in the nonlinear optical crystal substrate 11 function as high-refractive-index portions, the low-refractive-index portions function as the lower cladding, and the external environment (air) above the dielectric substrate 10 functions as the upper cladding. The portions of the nonlinear optical crystal substrate 11 where the periodic pattern of the air holes 12 is not formed become line defects, and these line defect portions form the optical waveguide 13. In the illustrated example, the optical waveguide 13 is strip-shaped (linear), but by changing the defect pattern where no periodic pattern is formed, a waveguide of a predetermined shape (and therefore a predetermined waveguiding direction) can be formed. For example, the waveguide may extend in a direction at a predetermined angle (diagonal direction) relative to the long or short side direction of the wavelength conversion element, or may be bent at a predetermined point (the waveguiding direction may change at a predetermined point). The length of the optical waveguide 13 is, for example, 30 mm or less, and preferably 0.1 mm to 10 mm. The width of the optical waveguide 13 may be, for example, 1.01α to 3α (2α in the illustrated example) with respect to the hole period α. The number of hole rows in the waveguide direction (hereinafter sometimes referred to as lattice rows) may be 3 to 10 rows (4 rows in the illustrated example) on each side of the waveguide.

[0018] By appropriately combining and adjusting the diameter of the air holes, the air hole period α, the number of lattice rows, the number of air holes in one lattice row, the thickness of the nonlinear optical crystal substrate, the constituent material of the nonlinear optical crystal substrate (effectively, the refractive index), the width of the line defect portion, etc., it is possible to propagate light in the photonic crystal mode or the EMC mode during the wavelength conversion operation described below.

[0019] B-2. Periodic polarization inversion section The periodically poled portions 14 are provided in at least a portion of the optical waveguide 13. The configuration of the periodically poled portions 14 is not particularly limited as long as they can exhibit quasi-phase matching (QPM). The periodically poled portions 14 typically have first poled portions 14a polarized in the c-axis direction of the nonlinear optical crystal substrate 11 and second poled portions 14b (poled domains) polarized in the opposite direction to the first poled portions 14a, arranged alternately in the waveguiding direction of the optical waveguide 13. In the illustrated example, the first poled portions 14a are polarized in a direction intersecting the waveguiding direction of the optical waveguide 13 and the thickness direction of the nonlinear optical crystal substrate 11. The domain widths of the first poled portions 14a and the second poled portions 14b can be adjusted so that the phases of the output light wavelength-converted by the first poled portions 14a and the second poled portions 14b are aligned. The length of the periodic polarization inversion units 14 in the waveguiding direction of the optical waveguide 13 is, for example, 5 to 95, or for example, 20 to 80, where the total length of the optical waveguide 13 is 100. The polarization inversion period Λ in the periodic polarization inversion units 14 is, for example, 1 μm or more, preferably 3 μm or more, and for example, 50 μm or less, preferably 30 μm or less. The polarization inversion ratio (polarization inversion domain width / polarization inversion period) is, for example, 0.1 or more, preferably 0.3 or more, and for example, 0.9 or less, preferably 0.7 or less.

[0020] B-3.Support board The support substrate 20 has an upper surface located within the composite substrate and a lower surface exposed to the outside. Any appropriate configuration can be adopted for the support substrate 20. Specific examples of materials that can be used to form the support substrate 20 include indium phosphide (InP), silicon (Si), glass, sialon (Si3N4-Al2O3), mullite (3Al2O3·2SiO2, 2Al2O3·3SiO2), aluminum nitride (AlN), magnesium oxide (MgO), aluminum oxide (Al2O3), spinel (MgAl2O4), sapphire, quartz, crystal, gallium nitride (GaN), silicon carbide (SiC), silicon nitride (Si3N4), and gallium oxide (Ga2O3). The support substrate 20 is preferably made of at least one material selected from the group consisting of indium phosphide, silicon, aluminum nitride, silicon carbide, and silicon nitride, and more preferably made of silicon or indium phosphide. It is preferable that the linear expansion coefficient of the material constituting support substrate 20 is as close as possible to the linear expansion coefficient of the material constituting nonlinear optical crystal substrate 11. With such a configuration, thermal deformation (typically, warpage) of the composite substrate can be suppressed. Preferably, the linear expansion coefficient of the material constituting support substrate 20 is within a range of 50% to 150% of the linear expansion coefficient of the material constituting nonlinear optical crystal substrate 11.

[0021] B-4.Low refractive index section In the wavelength conversion element 100 shown in the figure, the low refractive index portion 30 is a cavity 31. In one embodiment, the cavity 31 is defined by the lower surface of the nonlinear optical crystal substrate 11, the upper surface of the support substrate 20, and the bonding portion 80. The low refractive index portion preferably has a refractive index of 4 or less, and may be, for example, an SiO layer, a quartz glass plate, or a resin layer. When the low refractive index portion is a cavity, it can more stably prevent electromagnetic waves propagating through the waveguide from leaking out of the waveguide than when the low refractive index portion is an SiO layer or a quartz glass plate. The width of the low-refractive-index portion 30 (cavity 31) is typically greater than the width of the optical waveguide 13. The low-refractive-index portion 30 (cavity 31) preferably extends from the optical waveguide 13 to at least the third lattice row, and more preferably extends so as to overlap the entire area of ​​the hole-forming portion in the thickness direction of the nonlinear optical crystal substrate. Light not only propagates within the optical waveguide, but part of the optical energy may also diffuse to lattice rows near the optical waveguide. Therefore, by providing a cavity directly below such lattice rows, propagation loss can be suppressed. The dimension of the low refractive index portion 30 (cavity 31) in the thickness direction of the nonlinear optical crystal substrate is, for example, 0.05 μm or more, preferably 0.10 μm or more, and for example, 5.0 μm or less, preferably 1.0 μm or less.

[0022] B-5.Joint part The bonding portion 80 may be a single layer, or may be a laminate of two or more layers. Examples of the bonding portion 80 include an SiO2 layer and an amorphous silicon layer. When the bonding portion 80 is an SiO2 layer, the bonding portion 80 can function as the low refractive index portion 30. The thickness of the bonding portion 80 is, for example, 0.05 μm or more and 5.0 μm or less.

[0023] B-6. First electrode and second electrode In one embodiment, the first electrode 40 and the second electrode 50 are disposed on the surface (upper surface) of the nonlinear optical crystal substrate 11 opposite the support substrate 20. The first electrode 40 and the second electrode 50 are disposed at an interval in a direction perpendicular to the waveguiding direction of the optical waveguide 13. The interval between the first electrode 40 and the second electrode 50 in the direction perpendicular to the waveguiding direction is typically 5 μm to 20 μm. In FIG. 1(a), the first electrode 40 and the second electrode 50 are disposed in the thickness direction of the nonlinear optical crystal substrate 11 so as not to overlap with the holes 12 and the optical waveguide 13. In this case, the first electrode 40 and the second electrode 50 are typically metal electrodes. Examples of materials constituting the metal electrodes include titanium (Ti), platinum (Pt), and gold (Au). The metal electrodes may be single-layered or may be laminated with two or more layers. The thickness of the metal electrode is typically 100 nm or more and 3000 nm or less.

[0024] The first electrode and the second electrode can be disposed at any appropriate position as long as they are electrically connected to the nonlinear optical crystal substrate 11. As shown in FIG. 2, the first electrode 41 and the second electrode 51 are disposed so as to sandwich the dielectric substrate 10 in the thickness direction. In this case, the nonlinear optical crystal substrate 11 is typically a Z-cut substrate. The distance between the first electrode 41 and the second electrode 51 in the thickness direction of the nonlinear optical crystal substrate is typically 5.0 μm or less, preferably 1.3 μm or less, and typically 0.10 μm or more. When the distance between the first electrode and the second electrode is equal to or less than the above upper limit, the first electrode and the second electrode can be disposed near the optical waveguide, and an electric field can be efficiently generated in the optical waveguide when a voltage is applied between the first electrode and the second electrode. The first electrode 41 is disposed on the surface (upper surface) of the dielectric substrate 10 opposite to the support substrate 20, and overlaps with the optical waveguide 13 in the thickness direction of the nonlinear optical crystal substrate 11. In the illustrated example, the first electrode 41 overlaps with all the holes 12 in the thickness direction in addition to the optical waveguide 13. The second electrode 51 is disposed on the surface (lower surface) of the dielectric substrate 10 opposite to the first electrode 41. The second electrode 51 is located between the dielectric substrate 10 and the low refractive index section 30. The second electrode 51 may also be located between the dielectric substrate 10 and the bonding section 80. The second electrode 51 overlaps with the optical waveguide 13 in the thickness direction of the nonlinear optical crystal substrate 11. In the illustrated example, the second electrode 51 overlaps with all the holes 12 in the thickness direction of the nonlinear optical crystal substrate 11. When the first electrode and / or the second electrode overlap the holes, an electric field generated by applying a voltage can be stably applied to the periodic hole portion, which can efficiently change the effective refractive index of the optical waveguide. This can reduce the driving power of the wavelength conversion element. In this case, the first electrode 41 and the second electrode 51 are typically transparent electrodes. Transparent electrodes can prevent light propagating through the optical waveguide from being absorbed by the electrodes. The transparent electrode has a transmittance of light with a wavelength of 1.025 μm of, for example, 80% or more, preferably 90% or more, and for example, 100% or less. Examples of materials constituting the transparent electrode include aluminum-doped zinc oxide (AZO), gallium-doped zinc oxide (GZO), silicon oxide, indium tin oxide (ITO), In-Ga-Zn-O oxide semiconductor (IGZO), and tin oxide. The transparent electrode may be a single layer or a laminate of two or more layers. The thickness of the transparent electrode is typically 50 nm to 300 nm. When the first electrode and / or the second electrode is a metal electrode, a clad layer made of the same material as the low refractive index section 30 can be provided between the dielectric substrate 10 and the metal electrode.

[0025] B-7. Diffraction Grating The diffraction grating 15 is typically provided only directly above the optical waveguide 13. The diffraction grating 15 may be formed on the nonlinear optical crystal substrate 11, or may be formed separately from the nonlinear optical crystal substrate 11, or both. The diffraction grating 15 may have any suitable configuration as long as it can emit light from the upper surface of the optical waveguide 13. For example, the diffraction grating may be planar, concave-convex, or holographic. In the planar type, the diffraction grating pattern is formed by, for example, a refractive index difference; in the concave-convex type, the diffraction grating pattern is formed by, for example, grooves or slits. Typical diffraction grating patterns include stripes, grids, dots, and specific shapes (e.g., stars). The direction and pitch of the stripes, the dot arrangement pattern, and the like can be appropriately set depending on the purpose. In one embodiment, the diffraction grating 15 has multiple grating grooves extending in a direction substantially perpendicular to the waveguiding direction of the optical waveguide 13. The principles of the grating coupler are described in detail in, for example, International Publication No. 2018 / 008183, the entire disclosure of which is incorporated herein by reference. The diffraction grating may also be periodic holes near a line defect portion of an optical waveguide, in which case the period is formed to be different from the period of the holes forming the optical waveguide.

[0026] Next, one embodiment of a method for manufacturing a wavelength conversion element will be described with reference to FIG. C. Manufacturing method of wavelength conversion element As shown in FIG. 4(a), first, a nonlinear optical crystal substrate 11 is prepared, and periodically poled portions 14 are formed in predetermined portions of the nonlinear optical crystal substrate 11. Any appropriate method can be used to form the periodically poled portions. In one embodiment, a comb-like electrode pattern is formed on one surface of the nonlinear optical crystal substrate 11. The period of the comb corresponds to the polarization period Λ of the periodically poled portions 14 described above. Next, a voltage is applied to the nonlinear optical crystal substrate 11 via the electrode pattern in the direction of the crystal axis c. This forms the periodically poled portions 14. Thereafter, the electrode pattern is removed by etching.

[0027] Next, as shown in Figure 4(b), a junction 80 is formed by, for example, sputtering on the surface of nonlinear optical crystal substrate 11 on which the periodic polarization inversion section 14 is formed. Thereafter, intermediate layers 1 and 2 (not shown) are formed by, for example, sputtering on the surface of nonlinear optical crystal substrate 11 on which junction 80 is formed and on support substrate 20. These intermediate layers are then directly bonded together to obtain a composite substrate of nonlinear optical crystal substrate 11 / junction 80 / intermediate layer 1 / intermediate layer 2 / support substrate 20. Intermediate layer 1 and intermediate layer 2 may be omitted.

[0028] Direct bonding can be achieved, for example, by the following procedure: -6 A neutralization beam is irradiated onto the bonding surfaces of the components (layers or substrates) to be bonded at a pressure of about 100 Pa. This activates each bonding surface. Next, the activated bonding surfaces are brought into contact with each other in a vacuum atmosphere and bonded at room temperature. The load during this bonding can be, for example, 100 N to 20,000 N. In one embodiment, when performing surface activation using the neutralization beam, an inert gas is introduced into a chamber, and a high voltage is applied from a DC power supply to an electrode placed in the chamber. With this configuration, an electric field generated between the electrode (positive electrode) and the chamber (negative electrode) causes electrons to move, generating a beam of atoms and ions from the inert gas. Of the beams that reach the grid, the ion beam is neutralized by the grid, and a beam of neutral atoms is emitted from the fast atom beam source. The atomic species constituting the beam are preferably inert gas elements (e.g., argon (Ar) and nitrogen (N)). The voltage during activation by beam irradiation is, for example, 0.5 kV to 2.0 kV, and the current is, for example, 50 mA to 200 mA. The direct bonding method is not limited to this, and other methods such as surface activation using FAB (Fast Atom Beam) or an ion gun, atomic diffusion, and plasma bonding can also be used.

[0029] Next, as shown in FIG. 4(c), the nonlinear optical crystal substrate 11 is polished to the thickness within the above-mentioned range of the nonlinear optical crystal substrate. Then, as shown in FIG. 4(d), a plurality of voids 12 are formed in the nonlinear optical crystal substrate 11 to form the dielectric substrate 10. Specifically, a metal mask (e.g., a Mo mask) is formed on the surface of the nonlinear optical crystal substrate 11 opposite the support substrate 20, and a resin pattern having voids in a predetermined arrangement is formed on the metal mask. Next, voids corresponding to the resin pattern are formed in the metal mask by, for example, dry etching (e.g., reactive ion etching) via the resin pattern. Then, voids are formed in the nonlinear optical crystal substrate 11 by dry etching (e.g., reactive ion etching) via the metal pattern having a plurality of voids. Next, the bonding portion 80 is partially removed by reactive ion etching or wet etching (e.g., immersion in an etching solution) to form the cavities 31 (low refractive index portions 30). The metal mask is then removed by wet etching (e.g., etching solution).

[0030] 4(e), a resist mask pattern that exposes the electrode formation portions is formed on the nonlinear optical crystal substrate 11, for example, by photolithography, and the first electrode 40 and the second electrode 50 are formed through the mask pattern, for example, by sputtering. Thereafter, the resist mask pattern is removed. In this way, the wavelength conversion element 100 can be obtained.

[0031] It goes without saying that processes other than those shown in the drawings can be adopted for fabricating the wavelength conversion element. By appropriately combining the overall structure of the composite substrate, the constituent materials of each layer of the composite substrate, masks, etching methods, etc., it is possible to form holes and cavities in an efficient procedure with high precision, and to fabricate the wavelength conversion element.

[0032] D. Wavelength Conversion System The wavelength conversion element 100 described in the above sections A to C can be applied to a wavelength conversion system 1. As shown in FIG. 1(a), the wavelength conversion system 1 includes the wavelength conversion element 100 and a control unit 70 capable of controlling the refractive index of the nonlinear optical crystal substrate 11. More specifically, the wavelength conversion system 1 further includes a power supply 60 capable of applying a voltage to the first electrode 40 and the second electrode 50. The control unit 70 can control the power supply 60 and adjust the refractive index of the nonlinear optical crystal substrate 11 by controlling the voltage applied to the first electrode 40 and the second electrode 50. The control unit 70 includes, for example, a central processing unit (CPU), ROM, and RAM. As will be described in detail later, the control unit 70 can simulate the band curves of the photonic crystal mode and the EMC mode using the plane wave expansion method. Such a control unit 70 can control the power supply 60 to adjust the refractive index of the nonlinear optical crystal substrate 11 so as to obtain output light having a desired frequency in various wavelength conversion operations.

[0033] E. Wavelength conversion operation The wavelength conversion element 100 and wavelength conversion system 1 described above are capable of performing at least one wavelength conversion operation selected from parametric down-conversion (PDC), optical parametric amplification (OPA), second harmonic generation (SHG), and sum frequency generation (SFG).

[0034] E-1. Parametric Down Conversion (PDC) In one embodiment, the wavelength conversion element 100 is capable of parametric down-conversion (PDC). Specifically, the optical waveguide 13 is configured to receive input light and output first and second output lights. Each of the first and second output lights has a lower frequency than the input light. In one embodiment, one of the first and second output lights is designated as the desired output light, and the frequency of the desired output light is set. The control unit adjusts the refractive index of the nonlinear optical crystal substrate so that input light having a specific frequency is converted into the desired output light. With this configuration, first and second output lights having relatively low frequencies can be obtained from input light having a relatively high frequency.

[0035] The frequency of the input light is typically between 150 THz and 858 THz, which is equivalent to an angular frequency of 9.42 × 10 14 rad / s or more 5.386×10 15 The frequency of each of the first output light and the second output light is typically 20 THz or more and 500 THz or less, which is equivalent to an angular frequency of 1.26 × 10 14 rad / s or more 3.142×10 15 rad / s or less.

[0036] The input light, the first output light, and the second output light satisfy the following formulas (1-1) and (2-1). Formula (1-1) relates to the law of conservation of energy, and formula (2-1) relates to the quasi-phase matching condition. This ensures that the first output light and the second output light can be obtained from the input light.

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[0037] In the above formula (1-1) and formula (2-1), ω OUT‐1 =ω OUT‐2 and ω OUT‐1 >ω OUT‐2 may be. The predetermined temperature in the above formula (2-1) is the temperature of the nonlinear optical crystal substrate during wavelength conversion operation, for example, room temperature (23°C). The predetermined temperatures in the formulas (2-2), (2-3), and (2-4) described below are similarly explained. In this specification, the "speed of light" refers to the speed of light in a vacuum.

[0038] In parametric down-conversion (PDC), the dielectric substrate 10 may function as a photonic crystal for the input light and as an effective dielectric cladding for the first output light and the second output light. Alternatively, the dielectric substrate 10 may function as a photonic crystal for the input light and the first output light and as an effective dielectric cladding for the second output light. Alternatively, the dielectric substrate 10 may function as an effective dielectric cladding for the input light, the first output light, and the second output light. These features allow the input light, the first output light, and the second output light to propagate stably.

[0039] More specifically, the light propagated in the dielectric substrate 10 in the photonic crystal mode (input light, or each of the input light and the first output light) satisfies the following formula (3): The light propagated in the dielectric substrate 10 in the EMC mode (input light, each of the first output light and the second output light, or each of the first output light and the second output light, or the second output light) satisfies the following formula (4):

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[0040] E-2.Optical Parametric Amplification (OPA) In one embodiment, the wavelength conversion element 100 is capable of optical parametric amplification (OPA). Specifically, the optical waveguide 13 is configured to receive first input light as pump light and second input light as signal light, and to output first output light obtained by amplifying the second input light and second output light as idler light. The second input light has a lower frequency than the first input light. The second output light has a lower frequency than the first input light. In one embodiment, the frequency of the first output light is set as the desired output light, and the refractive index of the nonlinear optical crystal substrate is adjusted by the control unit so that the second input light is amplified to the desired first output light. With this configuration, first output light with a relatively high intensity can be obtained from second input light with a relatively low intensity.

[0041] The frequency of the first input light is typically 150 THz or more and 858 THz or less, which is converted to an angular frequency of 9.42 × 10 14 rad / s or more 5.385×10 15The frequency of each of the second input light, the first output light, and the second output light is typically 20 THz or more and 150 THz or less, which is equivalent to an angular frequency of 1.26 × 10 14 rad / s or more 3.142×10 15 rad / s or less.

[0042] The first input light, the second input light, the first output light, and the second output light satisfy the following formulas (1-2A), (1-2B), and (2-2). Formulas (1-2A) and (1-2B) relate to the law of conservation of energy, and formula (2-2) relates to the quasi-phase matching condition. This ensures that the first output light can be obtained from the second input light.

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[0043] In an optical parametric amplifier (OPA), the dielectric substrate 10 may function as a photonic crystal for the first input light and as an effective dielectric cladding for the second input light, the first output light, and the second output light. Alternatively, the dielectric substrate 10 may function as a photonic crystal for the first input light, the second input light, and the first output light, and as an effective dielectric cladding for the second output light. Alternatively, the dielectric substrate 10 may function as a photonic crystal for the first input light and the second output light, and as an effective dielectric cladding for the second input light and the first output light. Alternatively, the dielectric substrate 10 may function as an effective dielectric cladding for the first input light, the second input light, the first output light, and the second output light. These features enable stable propagation of the first input light, the second input light, the first output light, and the second output light.

[0044] More specifically, the light propagated in the dielectric substrate 10 in the photonic crystal mode (the first input light, or each of the first input light, second input light, and first output light, or each of the first input light and second output light) satisfies the above formula (3). The light propagated in the dielectric substrate 10 in the EMC mode (the first input light, second input light, first output light, and second output light, or each of the second input light and first output light, or the second output light, or each of the second input light, first output light, and second output light) satisfies the above formula (4).

[0045] E-3. Second Harmonic Generation (SHG) In one embodiment, the wavelength conversion element 100 is capable of second harmonic generation (SHG). Specifically, the optical waveguide 13 is configured to receive input light and output output light having a frequency higher than that of the input light. In one embodiment, the frequency of the desired output light is set, and the refractive index of the nonlinear optical crystal substrate is adjusted by a control unit so that the input light is converted into the desired output light. With this configuration, output light with a relatively high frequency can be obtained from input light with a relatively low frequency.

[0046] The frequency of the input light is typically between 150 THz and 428 THz, which is equivalent to an angular frequency of 9.42 × 10 14 rad / s or more 2.693×10 15 The frequency of the output light is typically between 300 THz and 857 THz, which is equivalent to an angular frequency of 1.885 × 10 15 rad / s or more 5.386×10 15 rad / s or less.

[0047] The input light and output light satisfy the following formulas (1-3) and (2-3). Formula (1-3) relates to the law of conservation of energy, and formula (2-3) relates to the quasi-phase matching condition. This ensures that output light can be obtained from input light.

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[0048] In second harmonic generation (SHG), the dielectric substrate 10 may function as an effective dielectric cladding for the input light and as a photonic crystal for the output light. The dielectric substrate 10 may also function as an effective dielectric cladding for both the input light and the output light. This allows the input light and the output light to propagate stably. More specifically, the light (output light) propagated in the dielectric substrate 10 in the photonic crystal mode satisfies the above formula (3). The light (input light, or each of the input light and output light) propagated in the dielectric substrate 10 in the EMC mode satisfies the above formula (4).

[0049] E-4. Sum Frequency Generation (SFG) In one embodiment, the wavelength conversion element 100 is capable of performing sum frequency generation (SFG). Specifically, the optical waveguide 13 is configured to receive a first input light and a second input light having a frequency lower than that of the first input light, and to output an output light having a frequency higher than that of the first input light and the second input light. In one embodiment, the angular frequency of the desired output light is set, and the refractive index of the nonlinear optical crystal substrate is adjusted by a control unit so that the first input light and the second input light are converted into the desired output light. With this configuration, output light having a relatively high frequency can be obtained from the first input light and the second input light having a relatively low frequency.

[0050] The frequency of each of the first input light and the second input light is typically 150 THz or more and 428 THz or less, which is converted into an angular frequency of 9.42×10 14 rad / s or more 2.693×10 15 The frequency of the output light is typically between 300 THz and 857 THz, which is equivalent to an angular frequency of 1.885 × 10 15 rad / s or more 5.386×10 15 rad / s or less.

[0051] The first input light, the second input light, and the output light satisfy the following formulas (1-4) and (2-4). Formula (1-4) relates to the law of conservation of energy, and formula (2-4) relates to the quasi-phase matching condition. This ensures that output light can be obtained from the first input light and the second input light.

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[0052] In sum frequency generation (SFG), the dielectric substrate 10 may function as a photonic crystal for the output light and as an effective dielectric cladding for the first input light and the second input light. Alternatively, the dielectric substrate 10 may function as a photonic crystal for the first input light and the output light and as an effective dielectric cladding for the second input light. Alternatively, the dielectric substrate 10 may function as an effective dielectric cladding for the first input light, the second input light, and the output light. These features allow the first input light, the second input light, and the output light to propagate stably. More specifically, the light propagated in the dielectric substrate 10 in the photonic crystal mode (output light, or each of the first input light and output light) satisfies the above formula (3). The light propagated in the dielectric substrate 10 in the EMC mode (each of the first input light, second input light, and output light, or each of the first input light and second input light, or the second input light) satisfies the above formula (4). [Example]

[0053] The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples. In particular, the combination of the propagation modes of the input light and the output light in the present invention is not limited to the combination of the propagation modes of the input light and the output light in the wavelength conversion operation of each example. The method for measuring the propagation loss of the wavelength conversion element is as follows. <Production Example 1> The wavelength conversion element shown in FIG. 1 was fabricated. (1) Periodic polarization reversal Specifically, a comb-shaped electrode pattern with a period of 2.0 μm was formed on a substrate made of MgO-doped 5° off-y-plate lithium niobate (LN) single crystal. A voltage was applied along the crystal c-axis to form a periodic polarization inversion section. The polarization inversion period Λ in the polarization inversion periodic section was 2.0 μm. The depth of the polarization inversion section was 5 μm in the optical waveguide section. After polarization inversion, the comb-shaped electrode was etched away. Next, SiO2 was sputtered onto the polarization inversion surface to form a 1 μm-thick cladding layer. Furthermore, amorphous silicon (a-Si) was sputtered to form a 20 nm-thick intermediate layer. Next, a-Si was sputtered onto a 4-inch-diameter silicon substrate used as a support substrate to form a 20 nm-thick intermediate layer.

[0054] (2)Direct bonding Then, the intermediate layer (a-Si layer) on each of the lithium niobate substrate and the silicon substrate was polished by CMP to reduce the arithmetic mean roughness Ra of each intermediate layer to 0.3 nm or less. Next, the surface of each intermediate layer was cleaned and directly bonded to form a composite wafer. Direct bonding was performed for 10 -6In a vacuum of the Pa range, each bonding surface was irradiated with a high-speed Ar neutral atom beam (accelerating voltage 1 kV, Ar flow rate 60 sccm) for 70 seconds. After irradiation, each substrate was left to cool for 10 minutes, and then the beam-irradiated surfaces of the lithium niobate substrate and the silicon substrate were brought into contact and then pressed together at 4.90 kN for 2 minutes to bond the two substrates.

[0055] (3) Thin plate polishing After bonding, the lithium niobate substrate was polished until its thickness was 0.5 μm, and a composite substrate for photonic crystals was obtained. No defects such as peeling were observed at the bonding interface in the fabricated composite substrate.

[0056] (4) Periodic pore formation Next, a Mo film was formed on the lithium niobate substrate as a metal mask. Next, a resin mask with a periodic hole pattern with a period of 390 nm and a hole radius of 121 nm was formed on the metal mask using nanoimprinting. The resin mask included a hole-free area corresponding to the optical waveguide and etched holes corresponding to the etched grooves. The etched holes had a diameter of 100 μm. The etched holes were formed on both sides of the hole-free area on both the input and output sides of the optical waveguide, at a distance of 10 holes from the hole-free area. In other words, a total of four etched holes were formed.

[0057] Next, the portion of the metal mask exposed from the resin mask was removed using a Mo etching solution, forming a periodic hole pattern and four etching holes in the metal mask. After that, the portion of the lithium niobate substrate exposed from the metal mask was removed using fluorine-based reactive ion etching, forming a periodic hole pattern and four etching grooves in the lithium niobate substrate. This resulted in a dielectric substrate with a periodic hole pattern. At this time, the cladding layer was also removed using fluorine-based reactive ion etching in the portion corresponding to the periodic hole pattern of the lithium niobate substrate. The composite substrate was then immersed in an etching solution of BHF (buffered hydrofluoric acid) to etch the cladding layer (silicon oxide), forming a cavity. The remaining metal mask was then removed with the etching solution.

[0058] (5) Electrode formation Next, a resist was applied to the lithium niobate substrate, and an electrode pattern was formed by exposure and development using a mask aligner. Furthermore, Ti, Pt, and Au films were formed by sputtering to thicknesses of 20 nm, 100 nm, and 0.5 μm, respectively. The resist was then peeled off using an organic solvent, and the first and second electrodes were formed by the lift-off method. The resulting composite substrate was then diced into chips to produce wavelength conversion elements. The optical waveguide length in the wavelength conversion element was 10 mm. The input and output end faces of the optical waveguide in the dielectric substrate were polished. A power supply was electrically connected to the first electrode and the second electrode. A control unit was electrically connected to the power supply. The control unit was capable of controlling the power supply and was able to simulate the band curves of the photonic crystal mode and the EMC mode using the plane wave expansion method.

[0059] <Production Example 2> A wavelength conversion element was manufactured in the same manner as in Manufacturing Example 1, except that the polarization inversion period Λ was changed to 1.0 μm, and a control unit was connected.

[0060] Example 1: Parametric Down Conversion (PDC) In the wavelength conversion element of Production Example 1, a DFB laser (light source) was connected to the input side of the optical waveguide. The DFB laser (light source) had an angular frequency of 2.36×10 15 It is possible to emit laser light (input light) of rad / s (wavelength 0.8 μm, 375 THz). Next, the control unit calculates the angular frequency ω of the first output light (converted light) as the desired output light. OUT‐1 (4.71×10 14 The control unit was set to the temperature (23°C), the angular frequency of the input light ω IN‐1 , the angular frequency ω of the first output lightOUT‐1 , and the range of refractive index (n IN-1 ), and the range of refractive index that the nonlinear optical crystal substrate can take for the first output light (n OUT-1 ) was calculated. The control unit also simulated the band curves of the photonic crystal mode and the EMC mode using the plane wave expansion method, as shown in Figure 5. In Figure 5, the vertical axis represents the angular frequency ω of light, and the horizontal axis represents the propagation constant β, which is (angular frequency ω of light × refractive index n of the nonlinear optical crystal substrate) / speed of light c. Next, the control unit calculates the angular frequency ω of the input light. IN‐1 , the angular frequency ω of the first output light OUT‐1 , the range of refractive index that the nonlinear optical crystal substrate can take n IN-1 , n OUT-1 the input light satisfies the above formula (3) and is located on the band curve of the photonic crystal mode, and the first output light and the second output light each satisfy the above formula (4) and are located on the band curve of the EMC mode, based on the above formula (1-1) and the above formula (2-1), and the angular frequency ω of the input light IN‐1 , the angular frequency ω of the first output light OUT‐1 and the angular frequency ω of the second output light OUT‐2 The combination of (ω IN‐1 ,ω OUT‐1 ,ω OUT‐2 ) was calculated. The resulting combination of angular frequencies (ω IN‐1 ,ω OUT‐1 ,ω OUT‐2 ) was used to determine the voltage (+50V) to be applied to the nonlinear optical crystal substrate. Next, the voltage was applied to the nonlinear optical crystal substrate to adjust the refractive index, and then input light was input into the optical waveguide. As a result, the desired angular frequency of 4.71 × 10 14 The first output light has a wavelength of 4 μm and a frequency of 75 THz. 14 It was confirmed by an optical spectrum analyzer that a second output light having a wavelength of 1 μm and a frequency of 300 THz was output.

[0061] Furthermore, the desired angular frequency ω of the first output light set in the control unit OUT‐1 7.85 x 10 14 The wavelength conversion was carried out in the same manner as above, except that the wavelength was changed to rad / s (wavelength 2.4 μm, 125 THz). When the angular frequency of the desired output light is changed, the band curves of the photonic crystal mode and the EMC mode shift (from the dashed line to the solid line) in a simulation using the plane wave expansion method, as shown in Figure 6. It was confirmed that the photonic crystal mode shifts more than the EMC mode. The control unit controls the angular frequency ω of the input light such that the input light, the first output light, and the second output light satisfy the above formula (1-1) and the above formula (2-1), the input light satisfies the above formula (3) and is located on the band curve of the photonic crystal mode after the shift, and the first output light and the second output light each satisfy the above formula (4) and are located on the band curve of the EMC mode after the shift. IN‐1 , the angular frequency ω of the first output light OUT‐1 and the angular frequency ω of the second output light OUT‐2 The combination of (ω IN‐1 ,ω OUT‐1 ,ω OUT‐2 The resulting angular frequency combination (ω IN‐1 ,ω OUT‐1 ,ω OUT‐2 ) and the voltage (-50V) applied to the nonlinear optical crystal substrate was determined. This also allowed the desired angular frequency ω OUT‐1 It was confirmed that the first output light was output.

[0062] <Example 2: Optical Parametric Amplification (OPA)> In the wavelength conversion element of Production Example 1, a high-power semiconductor laser (first light source) and a laser (second light source) with multiple wavelengths (discretely variable wavelength) were connected to the input side of the optical waveguide. The high-power semiconductor laser (first light source) emitted a laser beam with an angular frequency of 2.36 × 10 15 It is possible to emit laser light (first input light) of rad / s (wavelength 0.8 μm, 375 THz), and the laser (second light source) has an angular frequency of 4.71 × 10 14rad / s (wavelength 4.0μm, 75THz)~1.88×10 15 It is possible to emit laser light (second input light) of rad / s (wavelength 1.0 μm, 300 THz). Next, the control unit calculates the angular frequency ω of the first output light (converted light) as the desired output light. OUT‐1 (4.71×10 14 The control unit was set to the temperature (23°C), the angular frequency ω of the first input light, IN‐1 , the angular frequency ω of the second input light IN‐2 , the angular frequency ω of the first output light OUT‐1 , and the range of refractive index (n IN-1 ), the range of refractive index that the nonlinear optical crystal substrate can take for the second input light (n IN-2 ), and the range of refractive index that the nonlinear optical crystal substrate can take for the first output light (n OUT‐1 ) was calculated. Next, the control unit calculates the angular frequency ω of the first input light. IN‐1 , the angular frequency ω of the second input light IN‐2 , the angular frequency ω of the first output light OUT‐1 , and the range of refractive indexes that the nonlinear optical crystal substrate can take, n IN-1 , n IN-2 , n OUT‐1 the first input light satisfies the above formula (3) and is located on the band curve of the photonic crystal mode, and the second input light, the first output light, and the second output light each satisfy the above formula (4) and are located on the band curve of the EMC mode, based on the above formula (1-2A), the above formula (1-2B), and the above formula (2-2), the first input light satisfies the above formula (3) and is located on the band curve of the photonic crystal mode, and the second input light, the first output light, and the second output light each satisfy the above formula (4) and are located on the band curve of the EMC mode, IN‐1 , the angular frequency ω of the second input light IN‐2 , the angular frequency ω of the first output light OUT‐1 , and the angular frequency ω of the second output light OUT‐2 The combination of (ω IN‐1 ,ω IN‐2 ,ω OUT‐1 ,ω OUT‐2 ) was calculated. The resulting combination of angular frequencies (ω IN‐1,ω IN‐2 ,ω OUT‐1 ,ω OUT‐2 ) was used to determine the voltage (+50V) to be applied to the nonlinear optical crystal substrate. Next, the voltage was applied to the nonlinear optical crystal substrate to adjust the refractive index, and then the first input light and the second input light were input to the optical waveguide. As a result, the desired angular frequency of 4.71 × 10 14 It was confirmed by an optical spectrum analyzer that the amplified first output light had a wavelength of 4.0 μm and 75 THz. The desired angular frequency of the first output light set in the control unit is set to 7.85×10 14 Wavelength conversion was performed in the same manner as above, except that the frequency was changed to rad / s (wavelength 2.4 μm, 124 THz). In this case, the voltage applied to the nonlinear optical crystal substrate was -50 V. This also confirmed that the amplified first output light having the desired angular frequency was output from the optical waveguide.

[0063] Example 3: Second Harmonic Generation (SHG) In the wavelength conversion element of Production Example 2, a titanium sapphire laser (light source) was connected to the input side of the optical waveguide. The titanium sapphire laser had an angular frequency of 1.90 × 10 15 rad / s~2.69×10 15 It is possible to emit laser light (input light) of rad / s (wavelength 700nm to 990nm, 302.8THz to 428.3THz). Next, the control unit calculates the angular frequency ω of the output light (converted light). OUT‐1 The desired value (4.71 x 10 15 The control unit was set to the temperature (23°C), the angular frequency of the input light ω IN‐1 , the angular frequency of the output light ω OUT‐1 , and the range of refractive index (n IN-1 ), and the range of refractive index that the nonlinear optical crystal substrate can take for the output light (n OUT‐1 ) was calculated. Next, the control unit calculates the angular frequency ω of the input light. IN‐1 , the angular frequency of the output light ω OUT‐1 , the range of refractive index that the nonlinear optical crystal substrate can take n IN-1 and n OUT‐1 Based on the above, the input light and the output light satisfy the above formulas (1-3) and (2-3), the output light satisfies the above formula (3) and is located on the band curve of the photonic crystal mode, and the input light satisfies the above formula (4) and is located on the band curve of the EMC mode, and the angular frequency ω of the input light is IN‐1 and the angular frequency of the output light ω OUT‐1 The combination of (ω IN‐1 ,ω OUT‐1 ) was calculated. The resulting combination of angular frequencies (ω IN‐1 ,ω OUT‐1 ) was used to determine the voltage to be applied to the nonlinear optical crystal substrate. Next, a voltage (+50 V) was applied to the nonlinear optical crystal substrate to adjust the refractive index, and then input light (angular frequency 2.35 × 10 15 rad / s) was input. As a result, the desired angular frequency of 4.71 × 10 15 It was confirmed by an optical spectrum analyzer that an output light of rad / s (wavelength 400 nm, 749.5 THz) was being output. The desired angular frequency of the output light set in the control unit is set to 4.19×10 15 The wavelength conversion was performed in the same manner as above, except that the input light was changed to a wavelength of 450 nm and a frequency of 666.2 THz. The angular frequency of the input light was 2.09 × 10 15 rad / s. In this case, the voltage applied to the nonlinear optical crystal substrate was -50 V. This also confirmed that output light of the desired angular frequency was being output from the optical waveguide.

[0064] Example 4: Sum Frequency Generation (SFG) In the wavelength conversion element of Production Example 2, two titanium sapphire lasers (a first light source and a second light source) were connected to the input side of the optical waveguide. The titanium sapphire lasers had an angular frequency of 1.90 × 10 15 rad / s~2.69×10 15It is possible to emit laser light (first input light or second input light) of rad / s (wavelength 700 nm to 990 nm, 302.8 THz to 428.3 THz). Next, the control unit calculates the angular frequency ω of the desired output light (converted light). OUT‐1 (4.71×10 15 The control unit set the temperature (23°C), the angular frequency ω of the first input light, IN‐1 (2.51×10 15 rad / s, wavelength 750 nm, 399.7 THz), angular frequency of the second input light ω IN‐2 (2.19×10 15 rad / s, wavelength 858 nm, 349.4 THz), angular frequency of output light ω OUT‐1 (4.71×10 15 rad / s, wavelength 400 nm, 749.5 THz) and the range of refractive index (n IN-1 ), the range of refractive index that the nonlinear optical crystal substrate can take for the second input light (n IN-2 ), and the range of refractive index that the nonlinear optical crystal substrate can take for the output light (n OUT‐1 ) was calculated. Next, the control unit calculates the angular frequency ω of the first input light. IN‐1 , the angular frequency ω of the second input light IN‐2 , the angular frequency of the output light ω OUT‐1 , the range of refractive index that the nonlinear optical crystal substrate can take n IN-1 , n IN-2 and n OUT‐1 the first input light, the second input light, and the output light satisfy the above formulas (1-4) and (2-4), the output light satisfies the above formula (3) and is located on the band curve of the photonic crystal mode, and the first input light and the second input light each satisfy the above formula (4) and are located on the band curve of the EMC mode, IN‐1 , the angular frequency ω of the second input light IN‐2 , the angular frequency of the output light ω OUT‐1 The combination of (ω IN‐1 ,ω IN‐2 ,ω OUT‐1 ) was calculated. The resulting combination of angular frequencies (ω IN‐1 ,ω IN‐2 ,ω OUT‐1 ), the voltage (+50V) to be applied to the nonlinear optical crystal substrate was determined. Next, the voltage was applied to the nonlinear optical crystal substrate to adjust the refractive index, and then the first input light and the second input light were input to the optical waveguide. As a result, the desired angular frequency (4.71 × 10 15 It was confirmed by an optical spectrum analyzer that output light (radius: rad / s, wavelength: 400 nm, 749.5 THz) was being emitted. In addition, the desired angular frequency of the output light input to the control unit is set to 4.19 × 10 15 The wavelength conversion was performed in the same manner as above, except that the angular frequency of the first input light was changed to rad / s (wavelength 450 nm, 666.2 THz). IN‐1 is 2.19 x 10 15 rad / s (wavelength 860 nm, 349 THz), angular frequency of the second input light ω IN‐2 is 1.97 x 10 15 The frequency was set to rad / s (wavelength 944 nm, 318 THz). In this case, the voltage applied to the nonlinear optical crystal substrate was -50 V. This also confirmed that output light of the desired angular frequency was being output from the optical waveguide.

[0065] <Comparative Example 1> The wavelength conversion element shown in FIG. 4 of JP 2008-209522 A was prepared, and the wavelength conversion element was used to convert light having an angular frequency of 2.35 × 10 15 rad / s (wavelength 800 nm, 374.7 THz) is input to a 15 The wavelength was converted to output light of rad / s (wavelength 400 nm, 749.5 THz). The input light was input to the line-defect waveguide in the photonic crystal mode, but the output light had a wavelength outside the wavelength range corresponding to the photonic bandgap, so it was emitted above the wavelength conversion element without propagating through the line-defect waveguide. [Industrial Applicability]

[0066] The wavelength conversion element according to the embodiment of the present invention can be used in a wide range of fields such as next-generation high-speed communications and quantum fields, and can be particularly suitably used as an optical amplifier or optical modulator. [Explanation of symbols]

[0067] 1. Wavelength conversion system 100 Wavelength conversion element 10 Dielectric substrate 11 Nonlinear optical crystal substrate 12 Vacancies 13 Optical waveguide 14 Periodic polarization inversion section 20 Support substrate 30 Low refractive index section 31 Cavity 40 1st electrode 50 2nd electrode 60 power supply 70 Control Unit

Claims

1. a dielectric substrate formed by periodically forming holes in a nonlinear optical crystal substrate; a line-defect optical waveguide formed in the dielectric substrate; a periodic polarization inversion portion provided in the optical waveguide; a first electrode and a second electrode electrically connected to the nonlinear optical crystal substrate; the first electrode and the second electrode are arranged to sandwich the dielectric substrate in a thickness direction, each of the first electrode and the second electrode is a transparent electrode; A wavelength conversion element configured to convert the wavelength of light passing through the optical waveguide.

2. The wavelength conversion element according to claim 1 , wherein each of the first electrode and the second electrode overlaps the optical waveguide in the thickness direction.

3. The wavelength conversion element according to claim 1, wherein the transparent electrode comprises aluminum-doped zinc oxide (AZO), gallium-doped zinc oxide (GZO), silicon oxide, indium tin oxide (ITO), In-Ga-Zn-O oxide semiconductor (IGZO), or tin oxide.

4. a support substrate provided below the nonlinear optical crystal substrate; a low-refractive-index portion having a refractive index smaller than that of the nonlinear optical crystal substrate, the low-refractive-index portion being located between the nonlinear optical crystal substrate and the support substrate; 2. The wavelength conversion element according to claim 1, wherein at least a portion of the low refractive index portion overlaps with the optical waveguide in the thickness direction of the nonlinear optical crystal substrate.

5. a diffraction grating provided in the optical waveguide and arranged to be aligned with the periodic polarization inversion units in a waveguiding direction of the optical waveguide; 2. The wavelength conversion element according to claim 1, configured to emit light wavelength-converted in said optical waveguide from said optical waveguide.

6. The wavelength conversion element according to any one of claims 1 to 5, a control unit capable of controlling the refractive index of the nonlinear optical crystal substrate.

7. a power source capable of applying a voltage to the first electrode and the second electrode; 7. The wavelength conversion system according to claim 6, wherein the control unit is capable of controlling the power supply and is capable of adjusting the refractive index of the nonlinear optical crystal substrate by controlling the voltage applied to the first electrode and the second electrode.

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