JP7577241B1 - How Optical Devices Work - Google Patents

Abstract

A method for operating an optical device that can be easily manufactured is provided.
[Solution] A method for operating an optical device that operates by propagating light of frequency f, the optical device comprises a first optical waveguide and a second optical waveguide arranged side by side on a substrate, and has an optical coupling region in which the distance between the first optical waveguide and the second optical waveguide is narrower than in other regions so that light is coupled between the first optical waveguide and the second optical waveguide, and a grating structure is formed around the first optical waveguide and the second optical waveguide in the optical coupling region, in which a first refractive index material and a second refractive index material having a lower refractive index than the first refractive index material are alternately arranged with a period Λ in the direction of light propagation, and where the speed of light in a vacuum _{is c0} and the effective refractive index corresponding to the waveguide mode in the optical coupling region is _neff , light of frequency f whose period Λ satisfies _11c0 /( _20neff ·f)≦Λ≦ _3c0 /( _4neff ·f) is propagated through the optical device to operate it.
[Selected figure] Figure 3

Description

This disclosure relates to a method for operating an optical device.

In optical devices made from various materials such as silicon (Si), indium phosphide (InP), gallium arsenide (GaAs), gallium nitride (GaN), lithium niobate ( _LiNbO3 ), and compound semiconductors based on these materials, optical waveguides are widely used as a basic component of devices. These waveguides have a higher refractive index than the surrounding area, thereby confining light to a localized region and forming that region into a linear shape, allowing the light to propagate to the desired region.

In addition, various waveguide-type devices based on optical waveguides are also widely used as one of the basic components of optical devices to realize various functions, such as directional couplers, which optically couple the optical propagation modes of the two waveguides by bringing them close to each other at a distance equal to or less than the wavelength of the propagating light, and multimode interference waveguides (MMIs), which form a wide waveguide to convert the eigenmode of the light propagating inside into multiple modes and split the propagating light into an arbitrary number of waves or combine it by optical interference between the modes, polarization multiplexers and splitters, which split the input light into polarization waves and output them in different positions, or the reverse function, and grating couplers, which have a diffraction grating structure in the light propagation direction and diffract light of the desired wavelength or polarization from the optical waveguide in the vertical direction to propagate light into space.

However, in general directional couplers or waveguide-type optical devices such as MMI, in order to fully transfer the optical power propagating in one optical waveguide to the other optical waveguide or to equally distribute the input optical power to multiple outputs, a long distance of about several tens of times the wavelength of the propagating light is required, which causes the device size to become large. In addition, optical devices such as MMI, polarization multiplexer/demultiplexer, and grating coupler have a small range of operating wavelengths or polarizations, and in order to accommodate various wavelengths or polarizations, it is necessary to devise a method such as arranging devices corresponding to light with various attributes in parallel, which also causes the overall device size to become large. For example, 3 dB couplers or 3 dB splitters, which are often used in directional couplers and split 50% of the optical power to the other side, have the problem that the tolerance range of the device length at which the desired transfer rate can be obtained is very narrow, about 1 micrometer. The fact that the device length within the tolerance range is small compared to the overall device length indicates that the directional coupler is a device that is vulnerable to manufacturing errors.

As a technology to solve the above problems, a periodic structure with a period equal to or smaller than the propagating light, known as a sub-wavelength grating (SWG) structure, has been introduced into a Si-based optical device in a direction perpendicular to two optical waveguides, which has succeeded in realizing a smaller device size, polarization independence, and wavelength independence (Non-Patent Documents 1 and 2).

C. Ye et al., “Ultra-Compact Broadband 2 x 23 dB Power Splitter Using a Subwavelength-Grating-Assisted Asymmetric Directional Coupler,” JLT, vol. 38, no. 8 (2020) P. Cheben, “Subwavelength integrated photonics,” Nature, vol. 560, pp. 565-572 (2018)

Non-Patent Documents 1 and 2 disclose techniques for realizing a small directional coupler, but the SWG structure requires a periodic structure with a size equal to or smaller than the wavelength. In general, the refractive index of semiconductor optical materials such as Si and InP that make up the waveguide exceeds 3, so a size of 1/3 or less of the wavelength is essentially required. For example, a wavelength of 1.5 μm requires a periodic structure of 0.5 μm or less, and a fine structure of about 0.25 μm is required if the filling rate of the optical waveguide core material is 50%. Forming a structure of such a size with high precision is difficult in the optical semiconductor process, and a high-precision process such as electron beam (EB) lithography is required, which is a factor in high costs.

The present disclosure has been made to solve the above problems, and aims to provide an operating method for an optical device that requires less machining precision than conventional methods and can be easily manufactured.

A method for operating an optical device according to the present disclosure is a method for operating an optical device that propagates light of frequency f, the method comprising the steps of:
The optical device includes a first optical waveguide and a second optical waveguide arranged side by side on a substrate, and has an optical coupling region in which the distance between the first optical waveguide and the second optical waveguide is narrower than in other regions such that the light is coupled between the first optical waveguide and the second optical waveguide;
a grating structure is provided around the first optical waveguide and the second optical waveguide in the optical coupling region, in which a first refractive index material and a second refractive index material having a refractive index lower than that of the first refractive index material are alternately arranged with a period Λ in a propagation direction of the light,
When the speed of light in a vacuum is c ₀ and the effective refractive index corresponding to the waveguide mode in the optical coupling region is n _eff , the period Λ is expressed as follows:
11c ₀ /(20n _eff・f)≦Λ≦3c ₀ /(4n _eff・f)
The light having the frequency f that satisfies the above condition is propagated to the optical device to operate it.

The method of operating an optical device disclosed herein reduces the required processing precision compared to conventional methods, and allows operation using an optical device that can be easily manufactured.

1 is a schematic top view showing a configuration of an optical device according to a first embodiment. 2 is a schematic enlarged perspective view showing a configuration of an optical coupling region of the optical device according to the first embodiment; FIG. 4 is a diagram illustrating the operating points of the optical device according to the first embodiment in comparison with the operating points of an optical device of a comparative example; 5A to 5C are schematic diagrams for explaining the operation of the optical device according to the first embodiment in comparison with the operation of an optical device of the comparative example. 13 is a diagram showing the branching characteristics of optical power of an optical device of a comparative example. FIG. 13 is a diagram showing the branching characteristics of optical power of an optical device of another comparative example. FIG. 4 is a diagram showing the branching characteristics of optical power of the optical device according to the first embodiment; FIG. FIG. 1 is a diagram showing a calculation model of the FDTD method. FIG. 13 is a diagram showing characteristics of an optical device according to a comparative example. 13 is a diagram showing the characteristics of the optical device according to the second embodiment in comparison with a comparative example. 13A and 13B are diagrams illustrating the operation of the optical device according to the second embodiment in comparison with a comparative example. FIG. 11 is a schematic top view showing the configuration of an optical device according to a third embodiment. FIG. 11 is a schematic enlarged perspective view showing a configuration of an optical coupling region of an optical device according to a third embodiment.

Embodiment 1.
1 and 2 show the configuration of a directional coupler as an optical device according to the first embodiment. As shown in the plan view of FIG. 1, this directional coupler has an area inside with a higher refractive index than the surroundings, and two optical waveguides, a first optical waveguide 11 and a second optical waveguide 21, in which light is locally confined in this area and light is allowed to propagate only in a specific direction, are formed on a substrate 1. If this directional coupler is divided into an input area A, a width shifting area B, a coupling area C, a width expansion area D, and an output area E in this order in the direction of light propagation as shown in FIG. 1, the first optical waveguide 11 and the second optical waveguide 21 are configured as follows in each area. In the input area A, the first optical waveguide 11 and the second optical waveguide 21 are disposed at a sufficient interval of several times or more the wavelength of light. In the width shifting area B, the interval between the first optical waveguide 11 and the second optical waveguide 21 is changed from an interval of several times or more the wavelength of the propagating light in the input area A to be narrowed to about the size of the wavelength. In the width narrowing region B, the length of the region is about 10 times the wavelength or more, so that the first optical waveguide 11 and the second optical waveguide 21 are bent without optical loss, and both the first optical waveguide 11 and the second optical waveguide 21 are bent by drawing a curve such as a combination of arcs, a combination of sine waves or cosine waves, or a cycloid curve or a clothoid curve. In the optical coupling region C, the first optical waveguide 11 and the second optical waveguide 21 are arranged parallel and close to each other so that the interval is kept approximately constant at about the wavelength. Furthermore, in the width expansion region D, the interval between the first optical waveguide 11 and the second optical waveguide 21 is expanded to a sufficient interval of about the wavelength to several times the wavelength of light, in contrast to the width narrowing region B. The width expansion region D is about 10 times the wavelength or more, so that both the first optical waveguide 11 and the second optical waveguide 21 are bent by drawing a curve such as a combination of arcs, a combination of sine waves or cosine waves, or a cycloid curve. Furthermore, in the output region E, the first optical waveguide 11 and the second optical waveguide 21 are disposed at a sufficient interval of at least several times the wavelength of light. This optical device constitutes a directional coupler in which the light of the first optical waveguide 11 and the second optical waveguide 21 are coupled in an optical coupling region C, which is the region where the first optical waveguide 11 and the second optical waveguide 21 are closest to each other.

2 is an enlarged perspective view of the optical coupling region C. As shown in FIG. 2, the first optical waveguide 11 and the second optical waveguide 21 in the optical coupling region C have a substrate 1, a lower cladding layer 5 formed on the substrate 1, and core layers 11a and 21a, which are regions for confining light and are made of a material having a higher refractive index than the lower cladding layer 5 on the lower cladding layer 5. Furthermore, they have a grating structure 3 made of the same material as the core layer, in which protrusions (gratings) 31 are periodically and repeatedly arranged so as to be approximately perpendicular to the light propagation direction and to penetrate both the core layers 11a and 21a. Furthermore, the core layers 11a and 21a and the protrusions 31 are surrounded by a surrounding cladding 6 made of a material having a lower refractive index than the material constituting the core layer, such as SiO ₂ or SiN. That is, the grating structure 3 has a configuration in which protrusions 31 made of a first refractive index material, which is the same material as the core layer, and surrounding cladding 6 made of a second refractive index material having a refractive index lower than that of the first refractive index material, are repeatedly arranged at a constant period, and the entire waveguide has a waveguide structure with a grating. The period Λ of the grating of the grating structure 3 is set so as to generally satisfy the following formula (1).

ここで、ｃ₀は真空中を伝搬する光の速度、ｆは光デバイスを伝搬する光の周波数、ｎ_effは導波モードの実効屈折率、をそれぞれ示す。ここで、実効屈折率は、光導波路の構造および素材で決定されるパラメータであり、光導波路が複雑な構造をもつ場合であっても、導波モードの伝搬速度から、その光導波路が一定の屈折率材料で構成されると仮定したときの、屈折率に相当する値で定義される。なお、図中には、方向性結合器に入力する光４１と、方向性結合器によって分岐した出力光４２、４３を矢印で模式的に示している。また、図１と図２の方向の対応がわかるよう、ｘ、ｙ、ｚ軸を図中に記載している。

Here, c ₀ is the speed of light propagating in a vacuum, f is the frequency of light propagating through the optical device, and n _eff is the effective refractive index of the guided mode. Here, the effective refractive index is a parameter determined by the structure and material of the optical waveguide, and is defined as a value equivalent to the refractive index when the optical waveguide is assumed to be made of a material with a constant refractive index, based on the propagation speed of the guided mode, even if the optical waveguide has a complex structure. In addition, in the figure, the light 41 input to the directional coupler and the output lights 42 and 43 branched by the directional coupler are shown diagrammatically with arrows. In addition, the x, y, and z axes are shown in the figure so that the correspondence of the directions in Figures 1 and 2 can be understood.

Figure 3 shows a schematic diagram of the dispersion relationship of light propagating through an optical waveguide with a grating. The horizontal axis shows wave numbers, and the vertical axis shows frequency. Due to the action of the reciprocal lattice vector of the grating, the dispersion characteristic is folded back at the so-called X point (=π/Λ), and the dispersion characteristic is curved near the X point, resulting in a Bragg reflection region where no propagation mode exists. The lower frequency side than the Bragg reflection region is the SWG region. The optical waveguide with a grating shown in the prior art had the SWG region indicated by the dashed arrow in the figure as its operating point. On the other hand, the higher frequency side than the Bragg reflection region is the radiation region, where the propagating light is radiated outside the waveguide, and the power is attenuated. The operating point of the optical device according to the first embodiment is the region indicated by the solid arrow in Figure 3. The period Λ of the grating is set so that the operating frequency f falls between f_min and f_max shown in Figure 3, that is, so as to satisfy formula (1). The optical device according to the first embodiment is characterized by operating at a frequency that is in the radiation region and shows the same wave number as the SWG device shown in the prior art. By setting Λ in this way, it is possible to make the grating period Λ longer than in the prior art for the frequency f of light propagating through the same optical device, even though it is in the radiation region, which has the effect of relaxing the fineness of the pattern that forms the grating.

The effect of the above structure will be described below. Figure 4 shows a schematic diagram of the relationship between light propagating in a directional coupler based on a normal optical waveguide and a directional coupler based on an optical waveguide with a grating. In a structure based on a normal optical waveguide, the wave vector increases as the frequency of the propagating light increases, but in a structure based on an optical waveguide with a grating, the reciprocal lattice vector acts. When operating in a frequency range higher than the Bragg reflection range, as in the technology disclosed herein, the reciprocal lattice vector acts to fold back the wave vector of the propagating light, and the kz component of the wave vector can be set to a length similar to that of a conventional SWG. As a result, in the present disclosure, even with a long-period grating structure, the same effect as that of a conventional SWG structure can be obtained.

5 to 7 show the results of calculations of the relationship between the length of the optical coupling region and the branching ratio of the optical power when light with a wavelength of 1.55 μm is incident on a normal directional coupler without a grating structure, a directional coupler with a grating of an SWG structure, and a directional coupler with a grating structure according to the first embodiment. In each directional coupler, the first optical waveguide 11 and the second optical waveguide 21 in the optical coupling region are Si waveguides with a refractive index of 3.4, with widths of 0.4 μm and 0.5 μm, an interval of 0.2 μm, a height of 0.25 μm, and surrounded by a SiO ₂ cladding with a refractive index of 1.6 (n _eff = 2.38). FIG. 5 shows the calculation results of a normal directional coupler without a grating structure as a comparative example. Fig. 6 shows the calculation results of a directional coupler having an SWG structure with a period of 0.22 µm (Λ = 0.338c ₀ / (n _eff · f)) and a filling rate of 50% as a comparative example. Fig. 7 shows an example of the calculation results of the directional coupler according to the first embodiment, in which a grating structure with a period of 0.39 µm (Λ = 0.600c ₀ / (n _eff · f)) and a filling rate of 50% is provided as the grating structure 3.

The reason why the widths of the first optical waveguide 11 and the second optical waveguide 21 are different is that by making the widths different, it is possible to shorten the distance required for optical branching. This effect becomes noticeable when the width ratio is about 1.2 times. The optimal ratio varies depending on the branching ratio to be designed. However, if the width ratio becomes too large, such as 2 times or more, the difference in the effective refractive index of the respective guided modes of the first optical waveguide and the second optical waveguide becomes too large, and a coupling mode is not formed.

As can be seen from FIG. 5, a normal directional coupler without a grating cannot achieve 50% power splitting no matter how the length is set, but by adding an SWG structure as shown in FIG. 6, it is possible to achieve approximately 50% power splitting at a length of approximately 6 μm to 8 μm. In addition, in the configuration according to the first embodiment of FIG. 7, even with a grating with a period approximately 1.8 times longer than the SWG, it is possible to achieve approximately 50% power splitting at a length of approximately 6 μm to 8 μm, and it can be seen that characteristics equivalent to those of a conventional SWG structure are obtained. A longer directional coupler length that achieves the same power splitting means that the manufacturing precision required for the directional coupler can be relaxed. At the same time, it also means that the optical length at which the desired characteristics are obtained is longer, which has the effect of expanding the operating wavelength band.

The shape, material, and positional relationship of the directional coupler do not need to be limited to those in the first embodiment. For example, the positions of the input side optical waveguide and the output side optical waveguide may be interchanged, the material of the substrate 1 may be other materials such as Si, GaAs, SiO ₂ , SiN, LiNbO ₃ , and the core layer does not need to be Si, but may be any material having a higher refractive index than the cladding layer, for example, SiO ₂ doped with Ge for SiO ₂ cladding. In this case, the refractive index of the device is generally small, which is disadvantageous in that the device size becomes large, but the optical loss of the propagating light is small, which is advantageous in that a low-loss device can be realized.

ここで、ｃ₀は真空中を伝搬する光の速度、ｆは光デバイスを伝搬する光の周波数、ｎ_effは導波モードの実効屈折率、Λはグレーティング３の周期をそれぞれ示す。 Embodiment 2.
The second embodiment is a modification of the first embodiment, and is characterized in that in the directional coupler shown in FIGS. 1 and 2, the period of the grating 3 generally satisfies the following formula (2).

Here, c ₀ is the speed of light propagating in a vacuum, f is the frequency of light propagating through the optical device, n _eff is the effective refractive index of the guided mode, and Λ is the period of the grating 3.

In the first embodiment, it was confirmed that even with a long-period grating, characteristics equivalent to those of a conventional SWG device can be obtained in terms of the branching ratio characteristics of optical power. However, since this operating region is the radiation region in the dispersion relationship of the optical waveguide, there is a problem that radiation loss is unavoidable in principle. In particular, there is a concern that the impact will be significant when the grating period Λ is large, that is, when the operating frequency is located above the dispersion characteristics in Figure 3. However, if Λ is in a range that satisfies formula (2), it is possible to achieve the effects shown in the first embodiment while suppressing the attenuation of propagating light due to the radiation mode, which is a disadvantage when introducing a long-period grating.

The effects of this embodiment are described below. The radiation loss in each structure was calculated using electromagnetic field simulation (Finite Difference Time Domain Method, FDTD method). Figure 8 shows a calculation model of the FDTD method. An optical waveguide with a grating of 0.22 μm in height is installed in the cladding from 10 μm to 100 μm, and a normal optical waveguide of the same height is used from 0 μm to 10 μm. The refractive index n is 3.4 for the protrusion 31 of the grating structure, 1.6 for the surrounding cladding 6, and 3.4 for the optical waveguide. In this model, the fundamental mode of the optical waveguide is excited at the X = 0 μm position and propagated toward the right of the drawing. The relationship between the propagation distance and the power of the propagated light was calculated when the grating period was changed in various ways.

Figure 9 shows the calculation results of the radiation loss in the SWG region and Bragg reflection region in Figure 3. It can be seen that in the SWG region where the period Λ is 0.20 μm to 0.32 μm (Λ is 0.307 _{c 0} /(n _eff · f) to 0.491 _{c 0} /(n _eff · f)), no excess loss dependent on the grating period occurs. In the Bragg reflection region where the period Λ is 0.34 μm (Λ = 0.522 _{c 0} /(n _eff · f)), there is no propagation mode in the optical waveguide with a grating, so it can be seen that the power attenuates while oscillating greatly according to the propagation distance.

Figure 10 shows the calculation results of the radiation loss in the emission region in Figure 3. In the structure of this embodiment 2, where the period Λ is 0.36 μm to 0.38 μm (Λ is 0.553 _{c 0} /(n _eff · f) to 0.583 c ₀ /(n _eff · f)), although there is a small loss because it is an emission region, it is understood that the loss is suppressed to approximately the same level as the loss characteristics of the SWG region in Figure 9. On the other hand, in the region where the period Λ is larger, 0.40 μm to 0.60 μm (Λ is 0.614 c ₀ /(n _eff · f) to 0.922 _{c 0} /(n _eff · f)), it is understood that the radiation loss is large and the optical power of the propagating light is greatly attenuated.

FIG. 11 shows the calculation results of the electromagnetic field distribution of the propagating light when the period Λ is 0.20 μm (Λ=0.307 _{c 0} /(n _eff f)) (FIG. 11D), 0.34 μm (Λ=0.522 _{c 0} /(n _eff f)) (FIG. 11C) _, 0.38 μm (Λ=0.583 _{c 0} /(n _eff f)) (FIG. 11B), and 0.60 μm (Λ=0.922 c 0 /(n eff f)) (FIG. 11A). In FIG. 11, an upward arrow (↑) indicates a positive electric field, and a downward arrow (↓) indicates a negative electric field, with the darker the line, the larger the absolute value. It can be seen that in the structures with large radiation loss shown in FIG. 9 and FIG. 10, the intensity of the propagating light decreases toward the right.

Embodiment 3.
12 and 13 show the configuration of an optical device according to the third embodiment. This optical device is an optical device in which the optical coupling region is a multi-mode interference (MMI) waveguide. As shown in the plan view of FIG. 12, this optical device has a region inside which the refractive index is higher than the surroundings, and two optical waveguides, a first optical waveguide 11 and a second optical waveguide 21, in which light is locally confined in the region and light is allowed to propagate only in a specific direction, are formed on a substrate 1. If this optical device is divided into regions in the light propagation direction, in the order of an input region A, a width adjustment region B, a coupling region C, a width expansion region D, and an output region E as shown in FIG. 12, the first optical waveguide 11 and the second optical waveguide 21 in each region have the following configuration. In the input region A, the first optical waveguide 11 and the second optical waveguide 21 are arranged at a sufficient interval of several times or more the wavelength of light. In the width shifting region B, the interval between the first optical waveguide 11 and the second optical waveguide 21 is changed from an interval of several times or more of the wavelength of the propagating light in the input region to be narrowed to about the wavelength size. In this width shifting region B, the length of the region is about 10 times or more of the wavelength so that the first optical waveguide 11 and the second optical waveguide 21 bend without optical loss, and both the first optical waveguide 11 and the second optical waveguide 21 are configured to bend by drawing a combination of circular arcs, a combination of sine waves or cosine waves, or a curve such as a cycloid curve or a clothoid curve. In the optical coupling region C, a grating structure 30 is formed to configure an MMI section, and light is configured to be incident on this grating structure 30 from the first optical waveguide 11 and the second optical waveguide 21. Also, light from the grating structure 30 in the optical coupling region C is configured to be coupled again to the first optical waveguide 11 and the second optical waveguide 21 in the width expansion region D. In the width expansion region D, the distance between the first optical waveguide 11 and the second optical waveguide 21 is expanded from the wavelength, which is the distance on the optical coupling region C side, to a sufficient distance of several times the wavelength of light, in contrast to the width adjustment region B. The width expansion region D is configured to have a length of about 10 times the wavelength or more, and both the first optical waveguide 11 and the second optical waveguide 21 are bent by drawing a curve such as a combination of circular arcs, a combination of sine waves or cosine waves, or a cycloid curve. Furthermore, in the output region E, the first optical waveguide 11 and the second optical waveguide 21 are disposed at a sufficient distance of several times the wavelength of light or more.

13, the MMI portion of the optical coupling region C includes a substrate 1, a lower cladding layer 5 formed on the substrate 1, a grating structure 30 made of a material having a higher refractive index than the lower cladding layer 5 and having gratings 33 arranged periodically and repeatedly so as to be approximately perpendicular to the direction of light propagation, and a surrounding cladding 6 made of a material such as SiO ₂ or SiN having a lower refractive index than the material constituting the core layer arranged so as to surround the grating 33. The period of the grating 33 is set to approximately satisfy the following formula (3) (the same as formula (1) in the first embodiment). In order to function as an MMI with sufficiently small optical loss, the width of the grating structure 30 in the x direction, i.e., the direction perpendicular to the direction of light propagation and parallel to the substrate 1, needs to be three times the wavelength of light or more.

ここで、ｃ₀は真空中を伝搬する光の速度、ｆは光デバイスを伝搬する光の周波数、ｎ_effは導波モードの実効屈折率、Λはグレーティング３３の周期をそれぞれ示す。なお、図中には、本実施の形態の方向性結合器に入力する光４１と、方向性結合器によって分岐した出力光４２、４３を矢印で模式的に示している。また、図１２と図１３の方向の対応がわかるよう、ｘ、ｙ、ｚ軸を図中に記載している。

Here, _c0 is the speed of light propagating in a vacuum, f is the frequency of light propagating through the optical device, _neff is the effective refractive index of the waveguide mode, and Λ is the period of the grating 33. In the figure, arrows are used to diagrammatically indicate light 41 input to the directional coupler of this embodiment, and output lights 42 and 43 branched by the directional coupler. In order to understand the correspondence of directions between Figures 12 and 13, x, y, and z axes are indicated in the figure.

By adopting the third embodiment, it is possible to realize power splitting operation over a wide wavelength range, similar to the SWG structure MMI disclosed in the prior art, with a long-period grating that requires less manufacturing precision. This has the advantage of reducing manufacturing costs.

Although various exemplary embodiments and examples are described in this disclosure, the various features, aspects, and functions described in one or more embodiments are not limited to application to a particular embodiment, but may be applied to the embodiments alone or in various combinations. Thus, countless variations not illustrated are anticipated within the scope of the technology of this disclosure. For example, this includes cases in which at least one component is modified, added, or omitted, and even cases in which at least one component is extracted and combined with components of another embodiment.

1 substrate, 3, 30 grating structure, 11 first optical waveguide, 21 second optical waveguide, C optical coupling region, 31, 33 grating

Claims (4)

A method of operating an optical device that propagates and operates light of frequency f, comprising the steps of:
The optical device includes a first optical waveguide and a second optical waveguide arranged side by side on a substrate, and has an optical coupling region in which the distance between the first optical waveguide and the second optical waveguide is narrower than in other regions such that the light is coupled between the first optical waveguide and the second optical waveguide;
a grating structure is provided around the first optical waveguide and the second optical waveguide in the optical coupling region, in which a first refractive index material and a second refractive index material having a refractive index lower than that of the first refractive index material are alternately arranged with a period Λ in a propagation direction of the light,
When the speed of light in a vacuum is c ₀ and the effective refractive index corresponding to the waveguide mode in the optical coupling region is n _eff , the period Λ is expressed as follows:
11c ₀ /(20n _eff・f)≦Λ≦3c ₀ /(4n _eff・f)
A method for operating an optical device, comprising: propagating light of the frequency f that satisfies the above formula through the optical device and operating the optical device. The period Λ is
11c ₀ /(20n _eff・f)≦Λ≦3c ₀ /(5n _eff・f)
2. The method of claim 1, further comprising propagating light of the frequency f that satisfies the above formula through the optical device to operate it. The method for operating an optical device according to claim 1 or 2, wherein the width of the first optical waveguide in the optical device is 1.2 to 2 times the width of the second optical waveguide. A method of operating an optical device that propagates and operates light of frequency f, comprising the steps of:
the optical device comprises a first optical waveguide and a second optical waveguide arranged side by side on a substrate, and a grating structure in which a first refractive index material and a second refractive index material having a refractive index lower than that of the first refractive index material are alternately arranged with a period Λ in the propagation direction of the light,
The light is configured to be incident on the grating structure from the first optical waveguide and the second optical waveguide,
a width of the grating structure in a direction perpendicular to the propagation direction of the light and parallel to the substrate is three times or more the wavelength of the light;
If the speed of light in a vacuum is c ₀ and the effective refractive index corresponding to the waveguide mode in the grating structure is n _eff , the period Λ is 11c ₀ /(20n _eff ·f)≦Λ≦3c ₀ /(4n _eff ·f)
A method for operating an optical device, comprising: propagating light of the frequency f that satisfies the above formula through the optical device and operating the optical device.

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