WO2025109722A1 - Waveguide structure and planar optical wave circuit - Google Patents

Abstract

This waveguide structure is provided with: a first cladding layer (102A); a rib-shaped first core (103A) provided on the first cladding layer (102A); a second core (103B) provided on the first cladding layer (102A) so as to cover at least the first core (103A); and a second cladding layer (102B) provided so as to cover the second core (103B).

Description

Waveguide Structures and Planar Lightwave Circuits

This disclosure relates to waveguide structures and planar lightwave circuits.

In recent years, circuit elements that combine the three primary colors of visible light for applications such as eyeglass-type terminals (which may also be called smart glasses) or projectors, such as RGB (red, green and blue) coupler modules using PLC, have been attracting attention (for example, Non-Patent Document 1). PLC is an abbreviation for planar lightwave circuit.

On the other hand, existing silica-based PLCs generally use silicon dioxide (SiO ₂ ) with germania (GeO ₂ ) added to the core that constitutes the waveguide, in other words, Ge-doped SiO _2. It is known that the effective refractive index of a waveguide with this configuration changes when visible light is input (for example, Non-Patent Document 2).

A. Nakao, R. Morimoto, Y. Kato, Y. Kakinoki, K. Ogawa and T. Katsuyama, “Integrated waveguide-type red-green-blue beam combiners for compact projection-type displays”, Optics Communications 330 pp.45-48 (2014) Y. Hibino, T. Kominato and Y. Ohmori, "Optical frequency tuning by laser-irradiation in silica-based March-Zehnder-type multi/demultiplexers," in IEEE Photonics Technology Letters, vol. 3, no. 7, pp. 640-642 (1991) Y. Fujiwara, J. Sakamoto, T. Hashimoto and K. Watanabe, “Zr-doped Silica-based Planar Lightwave Circuits with High Resistance against Blue Light,” Proc. IDW ’21, pp. 242-244 (2021) Y. Fujiwara, J. Sakamoto, S. Katayose, T. Hashimoto and K. Watanabe, ”Silica-based planar lightwave circuits wi th high resistance against blue light for visible-light application,” 2022 Jpn. J. Appl. Phys. 61 SK1021 (2022) J. Sakamoto, T. Goh, S. Katayose, K. Watanabe, M. Itoh and T. Hashimoto, “Compact and low-loss RGB coupler using mode-conversion waveguides,” Optics Communications, Volume 420, pp. 46-51 (2018) Katsunari Okamoto, "Fundamentals of Optical Waveguides", Corona Publishing, ISBN4-339-00602-5, 1992

When the effective refractive index of a waveguide in a PLC changes, the characteristics of an optical circuit (e.g., a directional coupler or a multimode interferometer) formed by the waveguide may fluctuate. Therefore, one non-limiting objective of the present disclosure is to suppress fluctuations in the optical circuit characteristics.

The waveguide structure according to one embodiment of the present disclosure comprises a first clad layer, a rib-shaped first core provided on the first clad layer, a second core provided on the first clad layer so as to cover at least the first core, and a second clad layer provided so as to cover the second core.

In addition, a planar lightwave circuit according to one aspect of the present disclosure includes a plurality of waveguides each of which guides at least one of a plurality of light beams having different wavelengths, and a coupling section in which optical power is at least partially transferred between any two of the plurality of waveguides, and any one or more of the plurality of waveguides and the coupling section have the above-mentioned waveguide structure.

FIG. 2 is a schematic top view of an optical multiplexing circuit, which is an example of a planar lightwave circuit according to an embodiment. 2 is a cross-sectional view showing a schematic cross section of the optical multiplexing circuit shown in FIG. 1 along line AA'. 2 is a cross-sectional view showing a schematic cross section of the optical multiplexing circuit shown in FIG. 1 along the line BB'. FIG. 4 is a diagram illustrating an example of a transmission spectrum of a first multiplexing section in an embodiment. FIG. 13 is a schematic cross-sectional view of a waveguide structure in a multiplexing portion as a comparative sample having no second core. FIG. 13 is a diagram illustrating an example in which a plurality of optical multiplexing circuits are arranged on a wafer. FIG. 1A is a diagram showing an example of the in-plane distribution of λ1 transmittance and λ2 transmittance in a waveguide structure of one embodiment, and FIG. 1B is a diagram showing an example of the in-plane distribution of λ1 transmittance and λ2 transmittance in a waveguide structure of a comparative sample. FIG. 13 is a diagram showing an example of a transmission spectrum for a directional coupler of a comparative sample. FIG. 13 is a diagram showing an example of the normalized distance dependency of optical power propagating through a cross port of a directional coupler. FIG. 13 is a diagram illustrating an example of the normalized distance dependency of the optical power propagating through a through port of a directional coupler. 13A and 13B are diagrams showing examples of calculation results of mode field distribution for a waveguide cross-sectional structure having a second core and a waveguide cross-sectional structure not having a second core, respectively. FIG. 13 is a diagram illustrating an example of the relationship between the difference in core width and the λ1 transmittance of a cross port of a directional coupler. 11 is a diagram illustrating an example of the relationship between the λ1 transmittance of a cross port of a directional coupler and the relative refractive index difference of a second core. FIG. FIG. 1A is a schematic diagram showing the waveguide structure and relative refractive index difference of the present embodiment having a second core, FIG. 1B is a schematic diagram showing the waveguide structure and relative refractive index difference of a comparative sample having no second core, and FIG. 1C is a schematic diagram showing the waveguide structure and relative refractive index difference with a reduced relative refractive index difference. 15 is a diagram showing an example of the relationship between the transmittance per bending angle of 90° and the bending radius of the bending waveguide in the waveguide structure shown in FIG. 14. 13A and 13B are diagrams showing an example of a transmission spectrum before and after blue light is input.

Below, an embodiment will be described with reference to the drawings. Note that the present invention is not limited to this embodiment. In addition, in the description of the drawings, the same or corresponding elements are appropriately given the same reference numerals. The drawings are schematic, and the dimensional relationships and ratios of each element may differ from reality. Even between drawings, there may be parts in which the dimensional relationships and ratios differ. When numerical values are given in the following description, they are merely examples, and other numerical values may be used in addition or as an alternative.

＜Overview＞
PLCs can achieve various functions by creating optical waveguides on a planar substrate using patterning and etching processes such as photolithography, and combining multiple optical circuits (e.g., directional couplers, multimode interferometers, Mach-Zehnder interferometers, etc.).

As described above, in a silica-based PLC using Ge-doped _SiO2 as a waveguide (core), the effective refractive index may change due to the input of visible light. This change in effective refractive index may vary depending on the wavelength, light intensity, or irradiation amount of the input visible light. In addition, the effective refractive index of a silica-based PLC may change over time as visible light is input, which may result in deterioration of optical circuit characteristics over time.

Therefore, for example, the use of _SiO2 without any additives (non-doped _SiO2 ) or _SiO2 doped with zirconia ( _ZrO2 ) (Zr-doped _SiO2 ) for the waveguide (core) has been considered. In a waveguide of this configuration, the change in the effective refractive index of the waveguide due to the input of visible light can be suppressed (for example, Non-Patent Documents 3 and 4).

These silica-based materials have a smaller change in effective refractive index when visible light is input than Ge-doped SiO ₂ , so they are suitable as materials for the waveguide (core) that constitutes a silica-based PLC for optical devices that handle visible light.

However, when non-doped _SiO2 or Zr-doped _SiO2 is used in the waveguide of a silica-based PLC for optical devices that handle visible light, the optical circuit characteristics may vary. For example, the characteristics of the optical circuit (e.g., directional coupler or multimode interferometer) may vary due to variations in core width or fluctuations in the refractive index of the core caused by variations in PLC fabrication, which may result in a deterioration in yield.

Then, in the embodiment described below, we will explain a waveguide structure that can suppress fluctuations in optical circuit characteristics, and a PLC that has this waveguide structure.

<Embodiment>
An optical multiplexing circuit, which is an example of a planar lightwave circuit (PLC) according to an embodiment of the present disclosure, will be described with reference to Figs. 1 to 16. Fig. 1 is a schematic top view of an exemplary optical multiplexing circuit 1 according to an embodiment. As shown in Fig. 1, the optical multiplexing circuit 1 includes, for example, a first to a third three input ends 10a, 20a, and 30a, a first to a third three input waveguides 10, 20, and 30, a first and a second two multiplexing sections 50 and 70, one output waveguide 90, and one output end 90a on a substrate 101.

The input waveguides 10, 20, and 30 may each be a single waveguide that includes straight and curved portions. Therefore, in FIG. 1, the "input waveguide 20" and the "output waveguide 90" may be understood as convenient names for the "input portion" and "output portion" of a single continuous waveguide, respectively.

In FIG. 1, the Z axis represents the light guide direction, which corresponds to the length of the optical receiver 100 when viewed from above, the X axis represents the direction corresponding to the width of the optical multiplexing circuit 1 when viewed from above, and the Y axis represents the direction corresponding to the thickness (or height) of the optical multiplexing circuit 1 when viewed from the side or in a cross-sectional view. The directions represented by the X axis, Y axis, and Z axis are the same in the drawings used in the following explanation.

<Example of input waveguide configuration>
The first to third input ends 10a, 20a, and 30a are illustratively disposed on any end face of the optical multiplexing circuit 1 (for example, the left end face in the Z-axis direction in FIG. 1 ), and are respectively coupled to the first to third input waveguides 10, 20, and 30. In this embodiment, visible light of a first wavelength λ1 is input from the first input end 10a, coupled to the first input waveguide 10, and propagates therethrough.

Similarly, visible light of a second wavelength λ2 different from wavelength λ1 is input from the second input end 20a, coupled to and propagating through the second input waveguide 20. Visible light of a third wavelength λ3 different from wavelengths λ1 and λ2 is input from the third input end 30a, coupled to and propagating through the third input waveguide 30.

<Configuration example of multiplexing section>
The first multiplexing section 50 is coupled to, for example, the first input waveguide 10, the second input waveguide 20, and the second multiplexing section 70. The first multiplexing section 50 multiplexes, for example, visible light of wavelengths λ1 and λ2 input from the first input waveguide 10 and the second input waveguide 20, respectively, and outputs the multiplexed light to the second multiplexing section 70.

In addition, in the multiplexing section 50, the path of light propagating through each of the two waveguides 10 and 20 that form the coupling section is sometimes referred to as a "through port," and the path of light along which optical power is transferred from one of the waveguides 10 and 20 to the other and propagates is sometimes referred to as a "cross port."

The second multiplexing section 70 is coupled to the first multiplexing section 50, the third input waveguide 30, and the output waveguide 90. The second multiplexing section 70 multiplexes the visible light of wavelengths λ1 and λ2 that have been multiplexed in the first multiplexing section 50 and input through the waveguide 20 with the visible light of wavelength λ3 that has been input through the third input waveguide 30, and outputs the multiplexed light to the output waveguide 90.

For example, the second multiplexing section 70 may be a multiplexer (hereinafter, may be referred to as a "mode coupler") that has a multimode waveguide (MW) 72 and involves mode conversion by the MW 72 (for example, Non-Patent Document 5). Note that the concepts of "through port" and "cross port" may also be applied to the second multiplexing section 70, as in the case of the first multiplexing section 50.

<Example of output waveguide configuration>
The output waveguide 90 is coupled to the second output port of the second multiplexing section 70 and to an output end 90a, and outputs the visible light of wavelengths λ1, λ2 and λ3 that has been multiplexed and input in the second multiplexing section 70 to the output end 90a.

By outputting light from three different input terminals 10a, 20a, and 30a from one output terminal 90a, the optical circuit 1 illustrated in FIG. 1 functions as an optical multiplexing circuit 1 that multiplexes light of multiple different wavelengths λ1, λ2, and λ3.

<Relationship between wavelength and input waveguide>
In this embodiment, the light having wavelengths λ1, λ2, and λ3 is input to the first, second, and third input waveguides 10, 20, and 30, respectively, but is not limited to this. Depending on the design of the multiplexer 50 or 70, the combination of the wavelength of the input light and the input waveguide may be changed as appropriate.

<Wave combining method>
In the configuration illustrated in Fig. 1, the first multiplexing section 50 is a directional coupler, and the second multiplexing section 70 is a mode coupler with mode conversion as shown in Non-Patent Document 5. However, the present disclosure is not limited to this, and for example, one or both of the multiplexing sections 50 and 70 may be replaced with a directional coupler, a multimode interferometer, a mode coupler, a Y-branch circuit, or a Mach-Zehnder interferometer. The configuration described in this embodiment can be effective in any multiplexing method.

<Wavelengths and number of wavelengths to be combined>
In this embodiment, the wavelengths λ1, λ2, and λ3 may be wavelengths belonging to different wavelength ranges constituting the three primary colors of light, namely, blue (e.g., 400 to 495 nm), green (e.g., 495 to 570 nm), and red (e.g., 620 to 750 nm).

Therefore, the optical multiplexing circuit 1 of this embodiment may be referred to as an RGB coupler 1. However, the present disclosure may be effective regardless of the input wavelength. In addition, this embodiment illustrates an example of multiplexing light of three wavelengths, λ1, λ2, and λ3, but the number of wavelengths multiplexed in the optical multiplexing circuit 1 (multiplexing section 50 or 70) may be two or more.

<When multiple wavelengths of light are input from the output terminal>
In addition, when light of multiple wavelengths (e.g., λ1, λ3 and λ3) is input from the output terminal 90a of the optical multiplexing circuit 1 of this embodiment, the optical multiplexing circuit 1 also functions as an optical demultiplexing circuit that outputs light of multiple wavelengths from different input terminals 10a, 20a and 30a.

The above-mentioned directional coupler, mode coupler, Y-branch circuit, and Mach-Zehnder interferometer also function as a splitter due to the reciprocity of light. Therefore, the optical multiplexing circuit 1 may also be called an optical multiplexing/splitting circuit 1.

<Example of the cross-section of a waveguide constituting an RGB optical multiplexing circuit>
Fig. 2 is a cross-sectional view showing a schematic diagram of a waveguide structure constituting an exemplary optical multiplexing circuit 1 according to an embodiment, illustrating the cross-sectional view taken along line A-A' shown in Fig. 1. The waveguide structure shown in Fig. 2 may be applied to any one or more of the waveguides 10, 20 (90) and 30.

Exemplarily, the waveguide structure illustrated in FIG. 2 includes a substrate 101, a lower cladding layer 102A provided on the substrate 101, and a first core 103A provided on the lower cladding layer 102A. The waveguide structure further includes a second core 103B provided (e.g., deposited) on the lower cladding layer 102A so as to cover at least the first core 103A, and an upper cladding layer 102B provided so as to cover the second core 103B.

The shape of the second core 103B (e.g., the shape in a cross-sectional view) is not limited to the shape illustrated in FIG. 2, and may be any shape that covers the first core 103A. For example, the second core 103B may be formed as a layer whose thickness in the Y-axis direction is constant along the X-axis direction and that contains the first core 103A.

In addition, in the example of FIG. 2, the entire upper surface of the lower cladding layer 102A is covered by the second core 103B, but the upper surface of the lower cladding layer 102A may be partially exposed. For example, as long as at least the first core 103A is covered, the width of the second core 103B in the X-axis direction may be smaller than the width of the lower cladding layer 102A. These points are also the same in the example of FIG. 3 described later.

The lower cladding layer 102A is an example of a first cladding layer, and the upper cladding layer 102B is an example of a second cladding layer. Depending on the material of the substrate 101, the lower cladding layer 102A may be omitted. For example, this is the case when the substrate 101 made of a quartz-based material doubles as the lower cladding layer 102A.

The first core 103A forms a convex portion (which may also be called a "rib") along the Y-axis direction relative to the lower cladding layer 102A in a cross-sectional view, and may be referred to as a "rib-type core 103A" for convenience. The second core 103B illustratively has a rib 1031 along the Y-axis direction in a cross-sectional view, and the first core 103A is contained within this rib 1031.

The second core 103B may be conveniently referred to as a "cap layer 103B" for the first core 103A. Also, the waveguide structure having the first core 103A and the second core 103B may be conveniently referred to as a "dual core waveguide."

In addition, three or more cores may be provided between the lower cladding layer 102A and the upper cladding layer 102B. For example, a third core covering the second core 103B may be provided on the second core 103B.

<Refractive index difference between cladding layer and core>
In this embodiment, for example, the lower cladding layer 102A and the upper cladding layer 102B have the same or similar refractive index. Also, the first core 103A has a refractive index that is, for example, about 1.0% higher than the lower cladding layer 102A and the upper cladding layer 102B in terms of relative refractive index difference.

The second core 103B has a refractive index that is, for example, about 0.3% higher than the lower cladding layer 102A and the upper cladding layer 102B, calculated as a relative refractive index difference. In the following description, when there is no need to distinguish between the cladding layers 102A and 102B, they may be abbreviated to cladding layer 102.

The relative refractive index difference (Δ) is expressed by the following formula (1): In formula (1), n _core represents the refractive index of the first core 103A or the second core 103B, and n _clad represents the refractive index of the clad layer 102.

<Relationship between refractive index of core and cladding>
The present disclosure is not limited to the refractive indexes of the first core 103A and the second core 103B being about 1.0% and 0.3% higher, respectively, in terms of relative refractive index difference, than the cladding layer 102. The refractive index of the second core 103B may be lower than the refractive index of the first core 103A and higher than the refractive index of the cladding layer 102. When three or more cores are provided as described above, the refractive index of the core provided higher than the first core 103A may be set lower.

<Examples of core and clad materials>
As one non-limiting example, the lower cladding layer 102A is undoped SiO ₂ and the upper cladding layer 102B is SiO ₂ doped with boron and phosphorus (also called "borophosphosilicate glass" (BPSG)).

The first core 103A and the second core 103B are, by way of a non-limiting example, Zr-doped SiO _2. The first core 103A and the second core 103B can have different refractive indices, for example, by changing the concentration of a dopant.

However, the present disclosure is not limited to the additives described above. For example, the additives in one or both of the first core 103A and the second core 103B may be one or more of hafnium (Hf), titanium (Ti), germanium (Ge), aluminum (Al), phosphorus (P), boron (B), carbon (C), and fluorine (F).

The refractive index of the second core 103B may be increased by increasing the Si component in the composition of SiO _2. The refractive index of the second core 103B may be increased by applying SiO ₂ to which hydrogen has been added and which contains hydroxyl groups (OH groups) in the glass network to the second core 103B. The refractive index of the second core 103B may be increased by diffusing the additive of the first core 103A into the second core 103B by heat treatment. Any of the above-mentioned methods may be appropriately combined.

<An example of a method for producing a waveguide cross-sectional structure>
The lower cladding layer 102A, the first core 103A, and the upper cladding layer 102B illustrated in Fig. 2 can be realized by an existing PLC manufacturing method. For example, the lower cladding layer 102A and the first core 103A are formed by an existing method, and then the first core 103A is formed by photolithography and dry etching. The cross-sectional shape of the first core 103A may be, for example, a rectangle such as a square, or may be a mesa shape in which both side walls located in the X-axis direction are inclined with respect to the Y-axis direction.

After the first core 103A is formed, the second core 103B is formed on the lower cladding layer 102A using, for example, flame deposition (FHD), sputtering, or chemical vapor deposition (CVD). As a result, as shown in FIG. 2, the second core 103B is formed, which has a rib 1031 in a cross-sectional view and contains the first core 103A within the rib 1031.

Then, the upper clad layer 102B is formed on the top of the second core 103B using, for example, an existing method, to form the upper clad layer 102B that covers the second core 103B, thereby realizing the cross-sectional structure illustrated in FIG. 2.

<Example of core dimensions>
In this embodiment, the thickness of the first core 103A in the Y-axis direction is 2 μm as a non-limiting example. The width (core width) of the first core 103A in the X-axis direction may be different between the first multiplexing section 50 and the second multiplexing section 70, as described later.

The thickness of the second core 103B is 0.5 μm as a non-limiting example. The present disclosure is not limited to the thickness described above. The thickness of the second core 103B may be equal to or less than the thickness of the first core 103A.

However, even if the thickness of the second core 103B is thicker than that of the first core 103A, it is possible to provide an optical module that can suppress the fluctuation of the optical circuit characteristics due to the variability during fabrication and realize a high yield. Also, compared to the existing PLC using Ge-doped _SiO2 for the core, it is possible to obtain a PLC that can suppress the deterioration of the optical circuit characteristics caused by the fluctuation of the effective refractive index of the waveguide due to the input of visible light.

If the thickness of the second core 103B is greater than the thickness of the first core 103A, a secondary effect may occur in which the chip size tends to become larger for reasons described below. In addition, since the thickness of the second core 103B is made greater than the thickness of the first core 103A, the film formation cost may increase.

<Optical circuit characteristics of the BG coupler that constitutes the RGB coupler>
Fig. 3 is a schematic cross-sectional view of the first multiplexing section 50 in the optical multiplexing circuit 1 shown in Fig. 1, illustrating the B-B' cross section of the optical multiplexing circuit 1 shown in Fig. 1. Fig. 3 corresponds to a cross-sectional view focusing on the coupling section of the multiplexing section 50 which is a directional coupler, in other words, the coupling section of two waveguides which constitute the directional coupler.

As shown in FIG. 3, the multiplexing section 50 has two first cores 103A arranged in the X-axis direction on the lower cladding layer 102A as cores that constitute the waveguides coupled to the two input waveguides 10 and 20, respectively.

Then, a second core 103B is formed so as to cover each of the two cores 103A. For example, two ribs 1031 are formed on the second core 103B corresponding to each of the two first cores 103A, and each of the ribs 1031 contains the first core 103A.

In the cross-sectional view of FIG. 3, there is a "groove" between the two ribs 1031 that is filled with the upper cladding layer 102B. However, this "groove" is a groove that may be formed, for example, in the formation (e.g., deposition) process of the second core 103B, and may not be formed. In addition, the depth of the "groove" in the Y-axis direction when it is formed is also irrelevant. As long as each first core 103A is covered by the second core 103B, a "groove" may be formed deep enough to reach the top surface of the lower cladding layer 102A.

Here, as a non-limiting example, the width in the X-axis direction (core width) of each of the two cores 103A constituting the two waveguides of the directional coupler is 1 μm, the distance (gap width) between the two cores 103A in the X-axis direction is 1 μm, and the coupling length in the Z-axis direction of the directional coupler is 250 μm. The thicknesses of the first core 103A and the second core 103B, and the relative refractive index difference of the first core 103A with respect to the cladding layer 102 may be the values already described.

The waveguide structure having the second core 103B illustrated in FIG. 2 or FIG. 4 can be similarly applied to the multiplexing section 70.

FIG. 4 shows an example of the transmission spectrum of the first multiplexing section 50 in this embodiment. As shown in FIG. 4, the transmission spectrum 401 of the through port has a peak at 495 to 570 nm, which corresponds to wavelength λ2, and the transmission spectrum 402 of the cross port has a peak at 400 to 495 nm, which corresponds to wavelength λ1.

It can therefore be understood that the light input to each of the two waveguides constituting the multiplexing section 50, which is a directional coupler, is multiplexed into one waveguide. In this embodiment, for convenience, the transmittance at the through port at a wavelength of 550 nm is referred to as the "λ2 transmittance," and the transmittance at the cross port at a wavelength of 440 nm is referred to as the "λ1 transmittance."

<Comparison with a comparative sample not having a second core>
In order to explain the effects obtained in this embodiment, the optical circuit characteristics of the multiplexer 50 fabricated using the waveguide structure having the second core 103B as exemplified in Fig. 2 will be compared with the optical circuit characteristics of a comparative sample. The comparative sample is a multiplexer fabricated using a general waveguide structure not having the second core 103B.

<Differences between the Waveguide Structure of the Present Embodiment and Comparative Samples>
Fig. 5 shows a schematic cross-sectional view of a waveguide structure in a multiplexing section as a comparative sample having no second core. Fig. 5 is a cross-sectional view corresponding to Fig. 2, and as shown in Fig. 5, the waveguide structure of the comparative sample has a substrate 101, a lower clad layer 102A, a rib-shaped first core 103A, and an upper clad layer 102B. The cross-sectional structure shown in Fig. 5 may be understood to be equivalent to the cross-sectional structure of a waveguide in a general silica-based PLC.

The width, thickness, and relative refractive index difference of the core 103A in the comparative sample waveguide shown in FIG. 5 may be equivalent to the width, thickness, and relative refractive index difference of the first core 103A illustrated in FIG. 2. The refractive indexes of the upper cladding layer 102B and the lower cladding layer 102A in the comparative sample in FIG. 5 may also be equivalent to the refractive indexes of the upper cladding layer 102B and the lower cladding layer 102A illustrated in FIG. 2. However, in order to adjust the comparison conditions depending on the presence or absence of the second core 103B, the coupling length is set to 250 μm in the example in FIG. 2 and 320 μm in the comparative sample in FIG. 5.

<In-wafer distribution of optical circuit characteristics of BG coupler>
6 is a diagram showing a schematic example of an arrangement of a plurality of optical multiplexing circuits 1 on a wafer 600. FIG. 6 shows an example in which a plurality of optical multiplexing circuits 1 are arranged in a direction perpendicular to the orientation flat (O.F.) of the wafer 600 and on a line segment passing through the center of the wafer 600. In other words, FIG. 6 shows the in-plane position of the multiplexing section 50 constituting the optical multiplexing circuit 1. The diameter of the wafer 600 is 6 inches (about 15 cm) as a non-limiting example. The wafer surface corresponds to the XZ plane shown in FIGS. 1 to 3, for example.

FIGS. 7(a) and 7(b) are diagrams showing an example of the distribution of λ1 transmittance and λ2 transmittance on a wafer, with the horizontal axis representing the wafer position (cm) and the vertical axis representing transmittance (dB). In FIG. 7(a) and FIG. 7(b), the λ1 transmittance and λ2 transmittance are plotted against the position of the multiplexing section 50 when the center of the wafer 600 is set as the reference (e.g., 0 cm). The square plots indicate the λ1 transmittance, and the diamond plots indicate the λ2 transmittance.

As shown in FIG. 7(a), in the directional coupler 50 having a waveguide structure according to this embodiment with the second core 103B, both the λ1 transmittance and the λ2 transmittance are stable within the 6-inch wafer surface. In contrast, as shown in FIG. 7(b), in the directional coupler having a waveguide structure without the second core 103B, which is the comparative sample, it can be seen that the λ1 transmittance varies greatly between the λ1 transmittance and the λ2 transmittance.

If the target value of λ1 transmittance in the first multiplexing section 50 is, for example, -1.0 dB or more, in FIG. 7(a), all multiplexing sections 50 measured within the 6-inch wafer surface meet the target value. In contrast, in the comparison sample in FIG. 7(b), 8 out of 25 multiplexing sections do not achieve the target value (-1.0 dB) of λ1 transmittance.

From these results, it was confirmed that by adopting the second core 103B in the two waveguide structures that make up the multiplexing section 50, it is possible to suppress the variation in the optical circuit characteristics of the multiplexing section 50. Therefore, the yield of the multiplexing section 50, and therefore the yield of the optical multiplexing circuit 1, is greatly improved.

<Reason for decrease in transmittance of comparison sample>
Next, the cause of the decrease in the transmittance of the comparative sample will be considered. Fig. 8 shows an example of the transmission spectrum of the directional coupler of the comparative sample. The transmission spectrum shown in Fig. 8 corresponds to the transmission spectrum of the directional coupler placed at the position of 0 cm, which corresponds to the center of the wafer within the 6-inch wafer shown in Fig. 6.

In Figure 8, reference numeral 801 indicates the transmission spectrum of the through port, and reference numeral 802 indicates the transmission spectrum of the cross port. It can be seen from Figure 8 that, compared with the transmission spectra 401 and 402 of the multiplexing section (directional coupler) 50 of the configuration illustrated in Figure 4, the λ1 transmittance at a wavelength of 440 nm of the cross port is reduced, and the transmittance at a wavelength of 440 nm of the through port is increased. This is thought to be because, in the directional coupler of the comparison sample, the optical coupling rate at wavelength λ1 is reduced due to, for example, manufacturing errors.

<Causes of decrease in optical coupling rate>
One of the reasons for the decrease in the optical coupling rate of the comparative sample directional coupler is the asymmetry of the effective refractive indexes of the two waveguides that make up the directional coupler. For example, the reason for the decrease in the optical coupling rate of the comparative sample directional coupler can be explained as follows from the mode coupling theory.

The following formulas (2) and (3) show an example of the relationship between the coupling length and the light intensity propagating through the through port and the cross port, respectively (for example, Non-Patent Document 6).

Equation (2) represents the normalized optical intensity propagating through the through port, and equation (3) represents the normalized optical intensity propagating through the cross port. These equations (2) and (3) are equations assuming that light is input from the through port, and z represents the coupling length. Since P _Through (0) = 1 at z = 0 (input section), the optical input of the through port can be confirmed.

F and q in formulas (2) and (3) are represented by the following formulas (4) and (5), respectively.

In formulas (4) and (5), χ (chi) represents the mode coupling constant of the directional coupler, and δ represents the difference between the propagation constants (≈effective refractive index) of the two waveguides that constitute the directional coupler, and is represented, for example, by the following formula (6): In formula (6), β _Through represents the propagation constant of the through port waveguide, and β _Cross represents the propagation constant of the cross port waveguide.

Figure 9 shows an example of the normalized distance dependency of the optical power propagating through the cross port of a directional coupler, derived based on equation (3). The optical power is normalized by the input optical power, and has a maximum of 1. The normalized distance is the product of q and z.

As shown in Figure 9, the optical power periodically reaches a maximum every time the normalized distance changes by, for example, π/2. Figure 9 also illustrates the optical coupling ratio for different F values (for example, F = 1.0, F = 0.6, F = 0.2). It can be seen that the maximum value of the optical power propagating through the cross port tends to decrease as the F value decreases.

Figure 10 shows an example of the normalized distance dependency of the optical power propagating through the through port of a directional coupler, calculated in a similar manner based on equation (2). It can be seen from Figure 10 that the minimum optical power propagating through the through port tends to increase as the F value decreases. Therefore, F can be understood to be a coefficient that determines the maximum value of the optical coupling rate.

When F = 1.0, most of the optical power is transferred from the through port to the cross port, but when F < 1, as the F value becomes smaller, the optical coupling rate to the cross port decreases, and some light remains in the through port.

In the directional coupler of the comparative sample described above, the decrease in the λ1 transmittance of the cross port is believed to be due to the F-factor dropping significantly below 1. The F-factor is determined by the difference δ in the effective refractive index of the two waveguides that make up the directional coupler and the mode coupling constant χ, so it is believed that δ increased in the comparative sample due to variations in the processed shape of the first core. For example, it is suspected that variations have occurred in one or both of the core widths and sidewall angles of the two waveguides that make up the directional coupler.

Even in the waveguide structure of this embodiment having the second core 103B, the first core 103A may be processed by the same method as the core 103B in the comparison sample, for example, and similar variations in the processed shape may occur. However, in the waveguide structure of this embodiment, the presence of the second core 103B can suppress the decrease in the F value and improve the yield. The reason for this is explained below.

The mode coupling constant χ is enhanced by the presence of the second core.
In the configuration of this embodiment, the second core 103B can make the mode coupling constant χ larger than when the second core 103B is not present. By making the mode coupling constant χ larger, a decrease in the F value caused by variations in the processed shape of the first core 103A is suppressed, and a decrease in the optical coupling rate is suppressed.

As already mentioned above about equation (4), F is expressed by the mode coupling constant χ and the difference δ between the propagation constants (≒effective refractive index) of the two waveguides that make up the directional coupler. Therefore, it can be seen that the larger the mode coupling constant χ, the closer the F value is to 1 even with the same δ.

Here, the mode coupling constant χ is qualitatively determined by the extent to which the electric field of light propagating through one of the two waveguides that make up the directional coupler reaches or affects the other waveguide (for example, Non-Patent Document 6). For example, the stronger the electric field of light propagating through one waveguide affects the other waveguide, the larger the mode coupling constant χ becomes.

In the waveguide structure of this embodiment, the mode field diameter of the light propagating through the first core 103A is expanded by using the second core 103B, which has a lower refractive index than the first core 103A and a higher refractive index than the cladding layer 102.

Figure 11 shows an example of the calculation results of the mode field distribution for a waveguide cross-sectional structure having a second core 103B and a waveguide cross-sectional structure not having a second core 103B. Figure 11(a) shows an example of the relationship between position (μm) in the X-axis direction and normalized intensity (arbitrary units (a.u.)), and Figure 11(b) shows an example of the relationship between position (μm) in the Y-axis direction and normalized intensity (a.u.).

The width, thickness, and relative refractive index difference of the first core 103A are as described above. From Figures 11(a) and 11(b), it can be seen that by providing the second core 103B, the mode field diameter is increased compared to the comparison sample. In this embodiment, the second core 103B increases the mode field diameter and increases the mode coupling constant, thereby improving the yield of directional couplers.

In order to confirm the effect of this embodiment, several directional couplers were fabricated in which the effective refractive indexes of the two waveguides constituting the directional coupler were purposely made different, and the transmittance was evaluated. For example, a difference in the effective refractive index of the two waveguides was created by increasing the width of one of the two waveguides, core 103A, by Δw. The width, thickness, relative refractive index difference of the first core 103A, the gap width in the directional coupler, and the coupling length may be understood to be equivalent to the values already described.

Figure 12 shows an example of the relationship between the core width difference Δw and the λ1 transmittance of the cross port of a directional coupler. Figure 12 shows transmittance 1201 for a directional coupler of a waveguide structure having a second core 103B, which is the configuration of this embodiment, and transmittance 1202 for a directional coupler of a waveguide structure not having a second core 103B, which is a comparison sample. In Figure 12, the plots shown with squares and circles represent the measured values of λ1 transmittance, and the dashed line represents the calculated values of λ1 transmittance.

In Figure 12, it can be seen that when Δw is 0.1 μm, the transmittance of the comparison sample drops to -4.7 dB, whereas the drop in transmittance of the directional coupler having the waveguide structure of this embodiment is suppressed to -1.4 dB.

These results confirm that the directional coupler having the waveguide structure of this embodiment can suppress the decrease in transmittance caused by the variation in the processed shape of the first core 103A. The relative refractive index difference of the second core 103B with respect to the cladding layer 102 may be freely selected based on, for example, the degree of variation in the processed shape of the first core 103A and the allowable loss for the multiplexing section 50.

<Refractive index of the second core>
In the above-described embodiment, the relative refractive index difference (Δ) of the second core 103B with respect to the cladding layer 102 is set to about 0.3%, but is not limited to this. If the refractive index of the second core 103B is higher than the refractive index of the cladding layer 102, it is possible to suppress a decrease in transmittance due to variations in the processed shape of the first core 103A.

Figure 13 shows an example of the relationship between the λ1 transmittance of the cross port of a directional coupler and the relative refractive index difference of the second core 103B. In the example of Figure 13, in a directional coupler having the cross-sectional structure illustrated in Figure 3, the first core 103A has a core width of 1 μm and a thickness of 2 μm, and the gap width between the two cores 103A constituting the two waveguides of the directional coupler is 1 μm. In addition, the relative refractive index difference of the first core 103A is 1.0%, the difference Δw between the core widths of the cores 103A constituting each of the two waveguides is 0.06 μm, and the thickness of the second core 103B is 0.5 μm.

In Figure 13, the λ1 transmittance of the cross port is plotted when the relative refractive index difference of the second core 103B is changed. In Figure 13, the λ1 transmittance is plotted when the relative refractive index difference of the second core 103B is 0%, 0.1%, 0.3%, and 0.5%, and it can be seen that the λ1 transmittance increases as the relative refractive index difference of the second core 103B increases.

This shows that the larger the refractive index of the second core 103B, the more the effect of the core width difference Δw can be reduced. Even when the relative refractive index difference of the second core 103B is 0.1%, the λ1 transmittance increases by about 0.5 dB compared to when the relative refractive index difference is the minimum of 0%.

Therefore, it can be confirmed that if the relative refractive index difference of the second core 103B is greater than 0%, the effect of increasing the λ1 transmittance can be obtained. If the refractive index of the second core 103B is higher than that of the cladding layer 102, the mode field expands as described above, and the expected effect of this embodiment can be obtained.

Comparison with other methods that can increase the mode coupling constant
In the above-described embodiment, it has been described that the decrease in the optical coupling rate due to the variation in the processed shape can be suppressed by increasing the mode coupling constant by the second core 103B. Below, the difference from other methods (or approaches) that can increase the mode coupling constant will be described.

<Comparison with the method of increasing the mode coupling constant by narrowing the gap width>
Another approach to increasing the mode coupling constant is to reduce the gap width of the directional coupler: as the gap width is reduced, the electric field of light propagating in one waveguide can more easily reach the other waveguide, thereby increasing the mode coupling constant.

However, in reality, if the gap width is reduced beyond a certain point, the upper cladding layer 102B may not enter the gap of the directional coupler, resulting in the formation of a void. If a void is formed in the gap, the characteristics of the directional coupler may vary significantly, and the circuit may not meet the desired performance or specifications (e.g., product specifications).

Thus, although it depends on the thickness of the first core 103A, in the case where the thickness of the core 103A of this embodiment is 2 μm, the gap width is preferably 0.8 μm or more. In the configuration using the second core 103B of this embodiment, the mode coupling constant can be increased without reducing the gap width, which is also effective in terms of preventing the formation of voids in the gap portion.

<Comparison with the method of increasing the mode coupling constant by reducing the refractive index difference between the core and cladding>
Another approach that can increase the mode coupling constant is to reduce the relative refractive index difference of the first core 103 A. When comparing two waveguides that make up a directional coupler with the same core width, thickness, and gap width, the mode coupling constant increases by reducing the relative refractive index difference of the core 103 A.

However, reducing the relative refractive index difference can increase the chip size of the optical circuit. For example, in order to keep production costs down, planar lightwave circuits are designed to have the smallest chip size possible that achieves the desired functionality. One of the factors that determines the chip size is the bending radius of the curved waveguide. The bending radius is generally set to the minimum within the range of allowable radiation loss. With the approach of reducing the relative refractive index difference, the yield of component circuits such as directional couplers improves, but the bending radius and chip size can increase.

In contrast, the configuration using the second core 103B of this embodiment can reduce the impact on the bending radius more than the approach of reducing the relative refractive index difference. Figure 14(a) shows a schematic diagram of the waveguide structure and relative refractive index difference (Δ) of this embodiment having the second core 103B. Figure 14(b) shows a schematic diagram of the waveguide structure and relative refractive index difference (Δ) of a comparison sample not having the second core 103B. Figure 14(c) shows a schematic diagram of the waveguide structure and relative refractive index difference (Δ) with a reduced relative refractive index difference.

The width, thickness, and relative refractive index difference of the core 103A in the waveguide structure illustrated in Figures 14(a) and 14(b) are as described above. In the waveguide structure illustrated in Figure 14(c), the relative refractive index difference of the core 103a with respect to the cladding layer 102 is reduced to 0.7%. This relative refractive index difference is the same value as the relative refractive index difference between the first core 103A and the second core 103B, 0.7%, in the waveguide structure of this embodiment illustrated in Figure 14(a).

Next, Figure 15 shows an example of the relationship between the transmittance per bending angle of 90° and the bending radius of the bent waveguide in the waveguide structure shown in Figures 14(a) to 14(c). In Figure 15, each plot shows the measured bending loss, and the dashed line shows the calculated value. The wavelength of the light input to the bent waveguide is 440 nm.

For example, in FIG. 15, the square plot and dashed line 1501 respectively show the measured values and calculated values for the waveguide structure of this embodiment illustrated in FIG. 14(a). The circle plot and dashed line 1502 respectively show the measured values and calculated values for the waveguide structure of the comparative sample illustrated in FIG. 14(b). The diamond plot and dashed line 1503 respectively show the measured values and calculated values for the waveguide structure with reduced relative refractive index difference illustrated in FIG. 14(c).

If the minimum bending radius is defined as the bending radius at which the transmittance per bending angle of 90° is 0.9 or more, it can be seen from Figure 15 that the minimum bending radius of the comparison sample is approximately 450 μm, and the minimum bending radius of the waveguide structure of this embodiment is approximately 500 μm. On the other hand, the minimum bending radius of the waveguide structure that adopts the approach of reducing the relative refractive index difference to 0.7% is 1150 μm.

As these results show, the waveguide structure of this embodiment can adopt a minimum bending radius that is roughly equivalent to that of the comparison sample, and can reduce the minimum bending radius significantly more than a waveguide structure that adopts an approach that reduces the relative refractive index difference.

In addition, a waveguide structure that adopts an approach to reduce the relative refractive index difference is essentially equivalent to a structure in which the thickness of the second core 103B is made sufficiently thicker than the thickness of the first core 103A in the waveguide structure of this embodiment. Therefore, the bending radius is increased compared to the waveguide structure of this embodiment. This corresponds to the secondary effect of "the chip size tends to become larger" mentioned above.

<Potential to improve yields of other optical circuits>
In the above embodiment, it has been described that the waveguide structure of this embodiment having the second core 103B contributes to improving the yield when the first multiplexing section 50 is a directional coupler. However, this embodiment is not limited to directional couplers, and may also be effective in other optical circuits including the second multiplexing section 70.

For example, even when optical circuits such as a multimode interferometer (MMI), mode coupler, Y-branch circuit, or Mach-Zehnder interferometer are applied to the multiplexer 50 or 70, the effect of stabilizing the optical circuit characteristics can be expected by increasing the mode coupling constant between the two waveguides.

For example, in an MMI, multiple input and output waveguides connected to a multimode waveguide section form a directional coupler, so application of this embodiment is expected to improve yields. The same is true for the Y-branch circuit. A Mach-Zehnder interferometer is generally constructed by combining two or more of an MMI, a directional coupler, and a Y-branch circuit, so application of this embodiment is also effective.

<Suppression of optical circuit characteristic degradation caused by fluctuations in effective refractive index due to visible light input>
In the above-mentioned embodiment, the yield improvement of element circuits such as directional couplers has been described. In the following, it will be described that the waveguide structure according to this embodiment can also be effective in suppressing the deterioration of optical circuit characteristics caused by the fluctuation of the effective refractive index of the waveguide due to the input of visible light. Note that visible light is an example of light in a wavelength range shorter than the wavelengths generally handled in existing PLCs. In order to handle light in the short wavelength range, the conditions or specifications required as a product may be stricter than, for example, existing PLCs.

<Stabilization of optical circuit characteristics when blue light is input>
16(a) and 16(b) show an example of the transmission spectrum before and after a continuous wave (CW) blue light having a wavelength of 450 nm and an optical power of 60 mW is input for 10,000 hours.

Figure 16(a) shows the transmission spectrum of a directional coupler configured with the waveguide structure of this embodiment having the second core 103B, where reference numeral 1601 indicates the transmission spectrum of the through port and reference numeral 1602 indicates the transmission spectrum of the cross port.

Figure 16(b) shows the transmission spectrum of a directional coupler constructed with the waveguide structure of the comparison sample, where reference numeral 1603 indicates the transmission spectrum of the through port and reference numeral 1604 indicates the transmission spectrum of the cross port.

In addition, in Figures 16(a) and 16(b), the solid line indicates the transmission spectrum before blue light is input, and the dashed line indicates the transmission spectrum after blue light is input. Figures 16(a) and 16(b) show that the shape of the transmission spectrum for the directional coupler of the comparison sample changes significantly before and after the input of blue light, whereas the transmission spectrum of the directional coupler configured with the waveguide structure of this embodiment is stable.

<Changes in transmission spectrum>
The change in the shape of the transmission spectrum observed for the comparative sample directional coupler is believed to be caused by a change in the effective refractive index of the waveguide due to the input of blue light. For example, as described in Non-Patent Documents 2 to 4, it is known that the refractive index changes when ultraviolet or visible light is input to or irradiated onto a quartz-based material.

One of the reasons why the waveguide structure of this embodiment, which has the second core 103B, suppresses such changes in the refractive index is that the second core 103B suppresses changes in the refractive index of the upper cladding layer 102B.

It is known that Zr-doped SiO ₂ or non-doped SiO ₂ has a smaller refractive index change upon input of ultraviolet or visible light than Ge-doped SiO ₂ , which is used as a core material for existing quartz-based PLCs (e.g., Non-Patent Documents 3 and 4).

The waveguide structure having the second core 103B of this embodiment has a structure in which a first core 103A made of Zr-doped SiO ₂ is covered with a second core 103B similarly made of Zr-doped SiO ₂ .

In such a waveguide structure, the light leaking from the first core 103A is covered by the second core 103B, so that the intensity of the blue light that can reach the upper cladding layer 102B (e.g., BPSG) is greatly attenuated.

The change in effective refractive index caused by the input of blue light is caused by a change in the refractive index of the core material and a change in the refractive index of the cladding material, so the presence of the second core 103B reduces the intensity of blue light that can reach the upper cladding layer 102B. This is thought to suppress the change in the refractive index of the upper cladding layer 102B, and therefore the change in the optical circuit characteristics.

<Material of the second core>
In the above-described embodiment, an example in which the second core 103B is made of Zr-doped SiO ₂ has been shown. However, this embodiment is not limited to this, and the second core 103B may be made of, for example, non-doped SiO ₂ or SiO ₂ containing the following bond species. By using these SiO ₂ as the material for the second core 103B, it is expected that the change in the effective refractive index when visible light is input can be further suppressed.

In this case, in order to make the refractive index of the second core 103B higher than that of the cladding layer 102, for example, the ratio of the Si component in the composition of SiO ₂ may be increased. In the sputtering method, the ratio of the Si component in the SiO ₂ film formed can be increased by reducing the ratio of oxygen contained in the atmosphere during film formation. The ratio of the Si component may be, for example, Si/O=1/2 or more.

Additionally or alternatively, the refractive index of SiO ₂ can be increased by diffusing carbon (C) or hydroxyl groups (OH groups) into SiO _2. For example, in the plasma CVD method, the concentration of Si-C bonds and Si-OH bonds in the film can be increased by lowering the wafer temperature during film formation.

Additionally or alternatively, for example, a fluorine-based gas may be introduced into the film-forming raw material to introduce Si—F bonds into SiO _2. Since fluorine (F) has the effect of lowering the refractive index, it is possible to adjust the refractive index of the second core 103B by adding fluorine (F) to SiO ₂ together with other additives.

Effects of the embodiment
According to the present embodiment, it is possible to provide an optical circuit that can suppress the fluctuation of the optical circuit characteristics due to the variation during fabrication, and therefore can realize a high yield. Also, compared with the existing PLC using Ge-doped _SiO2 for the core, it is possible to obtain a PLC that can suppress the deterioration of the optical circuit characteristics caused by the fluctuation of the effective refractive index of the waveguide due to the input of visible light.

In the above embodiment, an example was described in which the input light to the optical multiplexing circuit 1 was visible light, but it is also possible to suppress fluctuations in the optical circuit characteristics when the input light is light in a wavelength range different from that of visible light.

<Terminology Notes>
When the term "connect" or "couple" is used in this disclosure, it may be understood to mean any direct or indirect "connection" or "coupling" between two or more elements. For example, it may be understood that the term also includes an indirect "connection" or "coupling" where one or more intermediate elements are between two elements that are "connected" or "coupled" to each other.

Any reference to an element followed by a designation such as "first...", "second..." does not limit the quantity or order of those elements. These designations are merely used as a convenient way to distinguish between two or more elements. For example, a reference to a first and a second element does not imply that only two elements may be employed, nor does it imply that the first element must precede the second element in any physical quantity.

In this disclosure, the terms "circuit" and "section" may be interpreted as interchangeable, or may be interpreted as other terms such as "structure," "means," "device," or "module," as appropriate.

　Although the present disclosure has been described in detail above, it is clear to those skilled in the art that the intent and scope of the present disclosure are not limited to the contents described throughout the present disclosure. The present disclosure can be implemented in modified and altered forms without departing from the intent and scope of the present disclosure as defined by the claims. Therefore, the description of the present disclosure is intended as an illustrative explanation and does not have any limiting meaning on the intent and scope of the present disclosure.

This disclosure is useful for technologies related to optical devices that handle visible light, such as smart glasses or projectors.

REFERENCE SIGNS LIST 1 Optical multiplexing circuit 10, 20, 30 Input waveguide 10a, 20a, 30a Input end 50, 70 Multiplexing section 90 Output waveguide 90a Output end 101 Substrate 102A Lower cladding layer 102B Upper cladding layer 103A First core 103B Second core 600 Wafer

Claims (8)

a first cladding layer;
a rib-type first core provided on the first clad layer;
a second core provided on the first clad layer so as to cover at least the first core;
a second clad layer provided to cover the second core; and
A waveguide structure comprising: The waveguide structure of claim 1, wherein the refractive index of the second core is higher than the refractive index of the first cladding layer or the second cladding layer. The waveguide structure of claim 1, wherein the refractive index of the second core is lower than the refractive index of the first core. The waveguide structure of claim 1, wherein the thickness of the second core is smaller than the thickness of the first core. The waveguide structure of claim 1, wherein one or both of the first core and the second core are silicon dioxide, and the ratio of silicon to oxygen in the composition of the silicon dioxide is greater than 1/2. The waveguide structure of claim 1, wherein one or both of the first core and the second core are silicon dioxide doped with hydrogen and containing hydroxyl groups in a glass network. The waveguide structure of claim 1, wherein the wavelength of light input to the waveguide structure belongs to one of the different wavelength ranges that make up the three primary colors of light. a plurality of waveguides each guiding at least one of a plurality of light beams having different wavelengths;
a coupling section in which optical power is at least partially transferred between any two of the plurality of waveguides;
A planar lightwave circuit, wherein any one or more of the plurality of waveguides and the coupling portion has a waveguide structure according to any one of claims 1 to 7.

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