Optical waveguide

Abstract

Provided is an optical waveguide in which a loss wavelength band in which transmittance of light is greatly reduced is not generated in a transmission spectrum of incident light. In an optical waveguide according to the present disclosure, a core and a cladding are made of materials having anisotropy oriented in the same orientation, the refractive index of the core for waves polarized along the ordinary optical axis is higher than the refractive index of the core for waves polarized along the extraordinary optical axis, and the refractive index of the cladding for waves polarized along the ordinary optical axis is higher than the refractive index of the cladding for waves polarized along the extraordinary optical axis.

Description

optical waveguide

[0001] The present disclosure relates to optical waveguides.

[0002] Many nonlinear optical devices and electro-optical devices are being developed for wavelength conversion and optical modulation of optical signals in optical communications, as well as for generation and modulation of coherent light across the ultraviolet, visible, infrared, and terahertz ranges in fields such as optical measurement, optical processing, medicine, and bioengineering.

[0003] Lithium niobate (LiNbO ３ Oxide-based compound substrates such as lithium niobate (hereinafter referred to as LN) have very high second-order nonlinear optical constants and electro-optic constants, and are therefore promising materials for the nonlinear optical medium and electro-optic medium used in such devices. Periodically poled lithium niobate (hereinafter referred to as PPLN) is known as an example of an optical device that utilizes the high nonlinearity of LN. PPLN has been put to practical use as a nonlinear optical medium in wavelength conversion elements that utilize second harmonic generation (SHG), difference frequency generation (DFG), and sum frequency generation (SFG).

[0004] Another example of an optical device that utilizes the second-order nonlinear optical effect is the optical parametric amplifier. Optical parametric amplification is an amplifier that utilizes optical parametric amplification by transferring energy from pump light power to signal light when a wavelength conversion element with high wavelength conversion efficiency is used. Such optical parametric amplifiers, especially phase-sensitive amplifiers that have amplification characteristics according to the phase relationship between pump light and signal light, are expected to be a technology that enables low-noise optical amplification.

[0005] Furthermore, among optical parametric amplification techniques, degenerate optical parametric amplification, in which the signal and idler beams are degenerate, can generate quantum-correlated photon pairs. This makes it possible to generate non-classical states such as squeezed light generation and heralded single-photon states, and such light is expected to be an important resource for optical quantum computers and quantum optical sensing technologies.

[0006] In recent years, there has been a demand for broadband wavelength conversion elements in various technical fields that apply such second-order nonlinear optical effects. For example, in optical communications, broadband wavelength conversion elements would enable the expansion of wavelength resources, thereby enabling increased communication capacity. Furthermore, in the field of quantum optics, applications that make the most of wavelength resources have been proposed (see, for example, Non-Patent Document 1).

[0007] As described above, wavelength conversion elements and optical parametric amplifiers that utilize second-order nonlinear optical effects are expected to be useful in a variety of fields, but currently have the problem of limitations regarding their bandwidth.

[0008] FIG. 1 is a perspective view showing a schematic structure of an optical waveguide 100 according to the prior art. The optical waveguide 100 may be, for example, an optical waveguide-type device configured as a wavelength conversion element. As shown in FIG. 1, the optical waveguide 100 includes a support substrate 110 and a core 120 formed on the support substrate 110. The material of the support substrate 110 is, for example, lithium tantalate (LiTaO ３ The material of the core 120 may be LN doped with zinc oxide (ZnO) (hereinafter referred to as ZnO:LN). In the optical waveguide 100 having such a structure, when incident light (signal light, pump light) is incident on the core 120 as shown by the arrow in the figure, the incident light is confined within the core 120 due to the refractive index differences between the core 120 and the support substrate 110, and between the core 120 and air. Then, wavelength conversion or parametric amplification is performed by the core 120, which has a high second-order nonlinear optical constant, and wavelength-converted light is emitted.

[0009] FIG. 2 shows the transmission spectrum of the optical waveguide 100 when incident light polarized parallel to the extraordinary ray axis of ZnO:LN is incident. Here, the core 120 is configured to have a thickness (length in the Y direction in FIG. 1 ) of 8.2 μm and a width (length in the Z direction in FIG. 1 ) of 8.6 μm. As shown in FIG. 2 , the optical waveguide 100 according to the prior art exhibits wavelength bands in the transmission spectrum where light transmittance drops significantly (corresponding to wavelength bands of 1500 to 1520 nm and 1570 to 1600 nm). It is known that, in response to this phenomenon, slight changes in the structural parameters of the core 120 of the optical waveguide 100 also slightly change the wavelength bands where light transmittance drops significantly. This means that the phenomenon of wavelength bands where light transmittance drops significantly (in other words, the phenomenon of loss wavelength bands) is not caused by the material.

[0010] As described above, the optical waveguide 100 having a waveguide structure according to the prior art has a problem in that a loss wavelength band occurs, making it impossible to perform appropriate wavelength conversion or optical amplification for a wide band of incident light. Currently, the optical waveguide 100 is manufactured so that the loss band is outside the wavelength range to be used, but this is equivalent to setting a limit to the wavelength range that the optical waveguide 100 can accommodate, and appropriate wavelength conversion for a wide band of incident light has not yet been achieved.

[0011] Asuka Inoue et al., “Toward a multi-core ultra-fast optical quantum processor: 43-GHz bandwidth real-time amplitude measurement of 5-dB squeezed light using modularized optical parametric amplifier with 5G technology,” Appl. Phys. Lett. 122, 104001 (2023)D. Marcuse, “Mode Conversion Caused by Surface Imperfections of a Dielectric Slab Waveguide,” The Bell System Technical Journal 48, 3187 (1969)

[0012] The present disclosure has been made in consideration of the above-described problems, and its purpose is to provide an optical waveguide in which a loss wavelength band in which the transmittance of light significantly decreases does not occur in the transmission spectrum of incident light.

[0013] In response to the above-described problems, the present disclosure provides an optical waveguide in which the core and cladding are made of materials having the same anisotropy, the refractive index of the core for waves polarized along its ordinary axis is higher than the refractive index of waves polarized along its extraordinary axis, and the refractive index of the cladding for waves polarized along its ordinary axis is higher than the refractive index of waves polarized along its extraordinary axis.

[0014] 1 is a perspective view showing a schematic structure of an optical waveguide 100 according to the prior art;

[0023] FIG. 1 is a diagram showing a transmission spectrum when incident light polarized parallel to the extraordinary axis of ZnO:LN is incident on the optical waveguide 100;

[0024] FIG. 2 is a diagram showing an example of the intensity distribution of output light when incident light having an arbitrary wavelength included in the loss wavelength band in FIG. 2 is incident on the optical waveguide 100 and the output light is observed through a polarizer, where (a) shows the intensity distribution of output light exhibiting the shape of a first-order mode, and (b) shows the intensity distribution of output light exhibiting the shape of a higher-order mode.

[0025] FIG. 1 is a diagram showing a schematic diagram of the relationship between the refractive index of the core 120 and the support substrate 110 with respect to the polarization of the extraordinary axis and the polarization of the ordinary axis.

[0026] FIG. 2 is a diagram showing the calculation results of the effective refractive index for each mode of light propagating through the optical waveguide 100, where (a) shows the two-dimensional model used in the calculation, and (b) shows a plot of the effective refractive index versus the width of the core 120 obtained by the calculation. 1 is a perspective view showing a schematic structure of an optical waveguide 600 according to the present disclosure. It is a diagram showing the calculation results of the effective refractive index for each mode of light propagating through the optical waveguide 600, where (a) shows a two-dimensional model used in the calculation, and (b) shows a diagram plotting the calculated effective refractive index versus the width of the core 120. It is a diagram showing the transmission spectrum when incident light polarized parallel to the extraordinary ray axis of ZnO:LN is incident on the optical waveguide 600.

[0015] Various embodiments of the present disclosure will be described in detail below with reference to the drawings. The same or similar reference numerals indicate the same or similar elements, and redundant description may be omitted. Materials and numerical values ​​are for illustrative purposes only and are not intended to limit the technical scope of the present disclosure. The following description is an example, and some configurations may be omitted or modified, or additional configurations may be added, as long as they do not deviate from the gist of one embodiment of the present disclosure.

[0016] (Principle of Occurrence of Loss Wavelength Band) Before describing the optical waveguide according to the present disclosure, the phenomenon of occurrence of a loss wavelength band in the optical waveguide 100 according to the conventional technology will be described below.

[0017] 3A and 3B are diagrams illustrating the intensity distribution of the output light when incident light having an arbitrary wavelength included in the loss wavelength band in FIG. 2 is input into the optical waveguide 100 and the output light is observed through a polarizer, where (a) shows the intensity distribution of the output light exhibiting the shape of a first-order mode, and (b) shows the intensity distribution of the output light exhibiting the shape of a higher-order mode.

[0018] Hereinafter, the mode of light propagating through such a core will be represented by symbols using the polarization (extraordinary ray axis or ordinary ray axis) and the order in the vertical direction (Y direction in FIG. 3 ) and the order in the horizontal direction (Z direction in FIG. 3 ) in a plane perpendicular to the optical axis direction. For example, in the case of a mode such as that shown in FIG. 3( a) and polarized along the extraordinary ray axis, this light mode will be represented as "e11" using "e" representing the extraordinary ray axis, "1" representing the order in the vertical direction, and "1" representing the order in the horizontal direction. Similarly, in the case of a mode such as that shown in FIG. 3( b) and polarized along the ordinary ray axis, this light mode will be represented as "o23" using "o" representing the ordinary ray axis, "2" representing the order in the vertical direction, and "3" representing the order in the horizontal direction.

[0019] It is known that when the above-mentioned polarizer is arranged so that it is orthogonal to the polarization of the incident light, the intensity distribution of the output light assumes a higher-order mode shape as shown in FIG. 3(b). This means that part of the energy of the polarization is coupled to the mode of the polarization orthogonal to the extraordinary ray axis. In an ideal situation, the polarizations in the core 120 constituting the optical waveguide 100 are orthogonal, so no energy transition occurs. However, if slight structural irregularities (hereinafter referred to as structural irregularities) occur in the optical waveguide 100, energy transition may occur. In other words, it is believed that the higher-order mode shape of the intensity distribution of the output light is due to such structural irregularities.

[0020] It is known that energy transition due to structural imperfection is likely to occur when the difference in propagation constants of the transitioning modes is close to the reciprocal of the spatial frequency of the structural imperfection (see, for example, Non-Patent Document 2). In general, since the spatial frequency of structural imperfection is smaller than the optical frequency, transition between modes with similar effective refractive indices becomes significant.

[0021] Generally, the highest nonlinear efficiency of ZnO:LN is d ３３ When using a ZnO:LN core, it is desirable that the polarization of light be aligned with the extraordinary axis of ZnO:LN. Focusing on the materials constituting the optical waveguide 100, the ZnO:LN core 120 is a negative uniaxial crystal, while the LT support substrate 110 is a positive uniaxial crystal. It is generally known that the relationship between the refractive index of each of the extraordinary axis and the ordinary axis is as shown in FIG. 4. As can be seen from FIG. 4, when comparing the refractive index difference between the ZnO:LN core 120 and the LT support substrate 110, the refractive index difference for the ordinary axis is greater than the refractive index difference for the extraordinary axis. Furthermore, it can be seen that the refractive index of ZnO:LN (core 120) for waves polarized along the ordinary axis is higher than that of ZnO:LN (core 120) for waves polarized along the extraordinary axis, while the refractive index of LT (support substrate 110) for waves polarized along the ordinary axis is lower than that of LT (support substrate 110) for waves polarized along the extraordinary axis. Therefore, the number of modes excited by waves polarized along the ordinary axis is greater than the number of modes excited by waves polarized along the extraordinary axis. Furthermore, the effective refractive index of higher-order modes of waves polarized along the ordinary axis is close to the lowest-order mode (fundamental mode) of waves polarized along the extraordinary axis that are excited.

[0022] 5A and 5B are diagrams showing the calculation results of the effective refractive index for each mode of light propagating through the optical waveguide 100. (a) shows the two-dimensional model used in the calculation, and (b) shows a plot of the calculated effective refractive index versus the width of the core 120. In addition to the effective refractive index for each mode (o11 to o85) of polarization along the ordinary axis, (b) also shows the refractive index for each polarization along the extraordinary axis and ordinary axis of the ZnO:LN (core 120), the refractive index for each polarization along the extraordinary axis and ordinary axis of the LT (support substrate 110), and the refractive index of the lowest-order mode (fundamental mode) of polarization along the extraordinary axis. These refractive indices are denoted in the figure as "no Core," "ne Core," "no Clad," "ne Clad," and "e11," respectively.

[0023] In addition, this calculation used a two-dimensional model simulating an arbitrary cross section of the optical waveguide 100 perpendicular to the optical axis direction, as shown in FIG. 5( a). The surface of the core 120 perpendicular to the optical axis direction was set to be in the positive direction (in other words, the width and thickness were set to be the same value). The wavelength of the light used in the calculation was 1545 nm. Furthermore, the side of the core 120 in the optical axis direction that was not in contact with the support substrate 110 was assumed to be in contact with air. In other words, in the optical waveguide 100, the support substrate 110 and air function as cladding.

[0024] As mentioned above, the highest nonlinear efficiency of ZnO:LN is d ３３ When using a ZnO:LN optical waveguide, it is desirable that the polarization of light be aligned with the extraordinary axis of the ZnO:LN. However, as shown in Figure 4, in the conventional optical waveguide 100, many higher-order modes of polarization along the ordinary axis are generated, and it can be seen that multiple higher-order modes of polarization along the ordinary axis have effective refractive indices close to the refractive index of the lowest-order mode (fundamental mode: e11) of polarization along the desired extraordinary axis. For example, when the core width (= core thickness) is 8 μm, it can be seen that light modes o16, o26, o45, o54, o62, and o63 have effective refractive indices close to the refractive index of the lowest-order mode (fundamental mode: e11) of polarization along the extraordinary axis.

[0025] Thus, in the optical waveguide 100 according to the prior art, a loss wavelength band occurs due to energy transition of higher-order modes that occur due to structural irregularities for light having a desired polarization. The optical waveguide according to the present disclosure takes this principle into consideration and is characterized by having a structure that reduces light in higher-order modes that undergo energy transition for light having a desired polarization.

[0026] (Structure of Optical Waveguide According to the Present Disclosure) Fig. 6 is a perspective view showing a schematic structure of an optical waveguide 600 according to the present disclosure. As shown in Fig. 6, the optical waveguide 600 includes a support substrate 610 and a core 120 formed on the support substrate 610. The material of the support substrate 610 may be LN doped with magnesium oxide (MgO) (hereinafter referred to as MgO:LN), and the material of the core 120 may be ZnO:LN, as in the optical waveguide 100 according to the prior art. Furthermore, the optical waveguide 600 is configured so that the extraordinary ray axis of the core 120 and the extraordinary ray axis of the support substrate 610 are aligned and perpendicular to the optical axis direction.

[0027] The amount of MgO added to the MgO:LN used in the support substrate 610 may be, for example, 7 mol %, but this is intended as an example. The amount of MgO added to the MgO:LN used in the support substrate 610 may be any value depending on the design.

[0028] Unlike LT, MgO:LN is a negative uniaxial crystal like ZnO:LN (more generally, in the optical waveguide 600, the support substrate 610 is made of a material having anisotropy oriented in the same direction as the core 120). Therefore, the refractive index of the support substrate 610 for waves polarized along the ordinary axis is higher than the refractive index of the support substrate 610 for waves polarized along the extraordinary axis, and accordingly, the number of modes of waves polarized along the ordinary axis is reduced compared to the optical waveguide 100 of the prior art. The relationship between the refractive index of the core 120 for each polarization along the ordinary axis and the extraordinary axis is as shown in FIG. 4.

[0029] 7A and 7B are diagrams showing the calculation results of the effective refractive index for each mode of light propagating through the optical waveguide 600, where (a) shows the two-dimensional model used in the calculation, and (b) shows a plot of the calculated effective refractive index versus the width of the core 120. The calculation conditions are the same as those for the optical waveguide 100 shown in FIG. 5, except that the support substrate 610 is MgO:LN. Similar to FIG. 5B, FIG. 7B also plots the effective refractive index for each mode (o11 to o85) of polarization along the ordinary axis, as well as the refractive index for each polarization along the extraordinary and ordinary axes of the ZnO:LN (core 120), the refractive index for each polarization along the extraordinary and ordinary axes of the MgO:LN (support substrate 610), and the refractive index of the lowest-order mode (fundamental mode) of polarization along the extraordinary axis. These refractive indices are denoted in the figure as "no Core", "ne Core", "no Clad", "ne Clad", and "e11", respectively.

[0030] 7, the number of modes of ordinary axis polarization is reduced in the optical waveguide 600, and higher modes of ordinary axis polarization having effective refractive indices close to the refractive index of the lowest mode (fundamental mode: e11) of the desired extraordinary axis polarization are reduced. Therefore, the optical waveguide 600 suppresses the generation of loss wavelength bands compared to the optical waveguide 100 of the prior art.

[0031] 8 shows the transmission spectrum when incident light polarized parallel to the extraordinary ray axis of ZnO:LN is incident on the optical waveguide 600. Here, the core 120 is configured to have a thickness (length in the Y direction in FIG. 1 ) of 8.2 μm and a width (length in the Z direction in FIG. 1 ) of 8.6 μm, similar to FIG. 2 . As shown in FIG. 8 , the optical waveguide 600 does not exhibit the steep loss wavelength band seen in FIG. 2 . The optical waveguide 600 according to the present disclosure enables appropriate and highly efficient optical propagation, wavelength conversion, and optical amplification even for broadband signal light.

[0032] It was also found that when the thickness and width of the core 120 were changed, no steep loss wavelength band appeared.

[0033] In the optical waveguide 600, the core 120 may be a second-order nonlinear optical medium having a periodically poled structure such as PPLN, so that quasi-phase matching is achieved when light in the communication wavelength band serving as the signal light and the frequency-doubled wave serving as the pump light propagate with polarization along the extraordinary ray axis. In such a case, quasi-phase matching is achieved across the light propagation direction (X direction in FIG. 7 ) of the optical waveguide 600, thereby efficiently utilizing the second-order nonlinear optical effect. However, since the optical waveguide 600 is related to transmittance, which is a basic characteristic of optical waveguides, as described above, its application is not limited to devices that utilize the second-order nonlinear optical effect. For example, the optical waveguide 600 can also be applied to optical modulators using the electro-optic effect and passive waveguides.

[0034] A common approach is to reduce the size of the core 120 to reduce the number of modes on the ordinary ray axis and suppress polarization conversion. However, reducing the core 120 is undesirable because it can increase optical waveguide loss, reduce resistance to high-power light, and reduce device fabrication yield. The optical waveguide 600 according to the present disclosure is therefore more effective for optical waveguide devices with a relatively large core 120. As shown in FIG. 5 , in the optical waveguide 100 according to the conventional technology, when the thickness and width of the core 120 are approximately 3 μm or greater, light in higher-order modes, including third-order and higher-order modes, is excited in the transverse direction. On the other hand, in the optical waveguide 600 according to the present disclosure, as shown in FIG. 7 , only higher-order modes up to second-order are excited in the transverse direction. Therefore, the optical waveguide 600 according to the present disclosure is particularly effective when the thickness and width of the core 120 are approximately 3 μm or greater.

[0035] In the above description, the optical waveguide 600 is a ridge-type optical waveguide in which the side surface of the core 120 in the optical axis direction that is not in contact with the support substrate 110 is in contact with air, but this is intended as an example, and the optical waveguide 600 may have another structure. For example, the optical waveguide 600 may be an embedded-type optical waveguide in which all of the side surfaces of the core 120 in the optical axis direction are covered with a dielectric or the like having a smaller refractive index than the core 120.

[0036] As described above, the optical waveguide according to the present disclosure suppresses the occurrence of loss wavelength bands, and therefore enables appropriate and highly efficient light propagation, wavelength conversion, optical amplification, etc., even for broadband signal light. The optical waveguide according to the present disclosure, which has such characteristics, is expected to be put to practical use in fields such as wavelength conversion elements for broadband signal light, optical parametric amplifiers, optical quantum computers, and sensing technologies using quantum light.

Claims

1. An optical waveguide, wherein the core and cladding are made of materials with anisotropy oriented in the same direction, the refractive index of the core for waves polarized along the ordinary axis is configured to be higher than the refractive index of the core for waves polarized along the extraordinary axis, and the refractive index of the cladding for waves polarized along the ordinary axis is configured to be higher than the refractive index of the cladding for waves polarized along the extraordinary axis.

2. The optical waveguide of claim 1, wherein the orientation of the extraordinary ray axis of said core is configured to coincide with the orientation of the extraordinary ray axis of said cladding.

3. The optical waveguide of claim 1, wherein the extraordinary ray axis of said core is configured to be perpendicular to the optical axis direction.

4. The optical waveguide according to claim 1, wherein the core is made of a second-order nonlinear optical medium having a periodically poled structure.

5. The optical waveguide according to claim 1, wherein said core has a thickness and width of 3 μm or more.

6. An optical waveguide according to any one of claims 1 to 5, wherein the core is made of lithium niobate doped with zinc oxide, and the clad is made of lithium niobate doped with magnesium oxide.

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