JP4114791B2 - Laminated optical waveguide - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a laminated optical waveguide that enables high-density integration, eliminates the difference in polarization dependence between the upper and lower waveguide layers (upper and lower cores), and provides a coupling rate when applied to an interlayer optical directional coupler. It has been devised to reduce the polarization dependence of.
[0002]
[Prior art]
Due to the rapid development of optical communication technology, various optical components have been researched and developed. Among them, the waveguide type optical component based on the optical waveguide on the flat substrate occupies the most important position. This is because the waveguide-type optical component has a feature that it can be mass-produced with high reproducibility with accuracy below the optical wavelength by photolithography technology and microfabrication technology.
[0003]
However, the scale of these waveguide type optical components is smaller than that of the communication system, and it is necessary to realize a system by combining a large number of components. For example, in a cross-connect system that realizes 256 input lines and 256 output lines that are being put to practical use in any combination, even when an 8 × 8 matrix switch is used, 1000 or more modules are required. Accordingly, there is an urgent need to increase the scale of individual optical components.
[0004]
In order to increase the scale of the optical circuit, there are a method of increasing the size of the circuit board and integrating, and a method of increasing the degree of integration by reducing the element elements constituting the circuit. In order to increase the chip size, it is necessary to increase the size of the waveguide fabrication apparatus, and the development thereof requires a lot of time, and it is difficult to cope with it in a short time. Further, an increase in chip size leads to an increase in the size of the module on which the chip is mounted, but the substrate that can be mounted on the module has a prescribed size, and the chip size cannot be increased indefinitely.
[0005]
On the other hand, in order to reduce the size of the element element, by increasing the relative refractive index difference between the core and the surrounding cladding in the optical waveguide, the light confinement effect in the core is increased and the bending radius of the waveguide is reduced. It is effective to do. However, as the relative refractive index difference increases, the optical path length error for the same amount of manufacturing error also increases. Even if the relative refractive index difference changes, the geometrical fabrication accuracy is the same. Therefore, in the waveguide type optical component that controls it by the interference of light, the greater the relative refractive index difference, the more the optical interference state is greatly altered. As a result, the performance of the parts is reduced. Therefore, there is a limit to the method of increasing the relative refractive index difference and increasing the degree of integration.
[0006]
As another method for increasing the degree of integration, there is a laminated circuit structure in which a plurality of optical waveguide structures are formed in multiple layers in a direction perpendicular to the substrate, and the circuits are stacked perpendicular to the substrate. When a laminated circuit structure is used, it is essential to realize an interlayer optical directional coupler that transmits and receives light through different waveguides in the upper and lower layers. As an example of an interlayer optical directional coupler, a report by the inventors (Proceedings of the 62nd Japan Society of Applied Physics Academic Lectures (2001. 9. Aichi Institute of Technology), P900, Lecture Number: 12p-Y-6, “ There is a lamination of quartz-based PLC using a polishing method (“Suzuki et al.”) (See FIG. 8).
[0007]
According to this report, the lower clad, intermediate clad, upper clad, lower core and upper core are formed by the flame deposition method (FHD method), and the intermediate clad layer surface is polished immediately after the intermediate clad layer is deposited. It has been shown that an optical directional coupler can be made.
[0008]
In FIG. 8, interlayer light having a lower cladding layer 2, a lower core 3, an intermediate cladding layer 4, an upper core 5, and an upper cladding layer 6 on a silicon substrate 1, and having a certain gap 8 in the optical coupling linear portion 7. A directional coupler is shown.
[0009]
For easy understanding, the top view of the interlayer optical directional coupler of FIG. 8 is shown in FIG. 9, and the cross-sectional views at positions AA-AA ′, BB-BB ′, CC-CC ′ of FIG. a), (b) and (c), respectively.
[0010]
FIG. 11 shows the coupling characteristics of the interlayer optical directional coupler shown in the report. In FIG. 11, the horizontal axis represents the length of the optical coupling straight line portion 7, and the vertical axis represents the coupling rate with respect to both TE and TM polarized waves. Both the upper core 5 and the lower core 3 are configured by waveguides having a relative refractive index difference of 0.75%, the size (vertical dimension and horizontal dimension of the cross section) is 6 μm × 6 μm, and the gap 8 between the upper and lower cores is 3 μm. . In this reported interlayer optical directional coupler, for the TE polarized wave, the optical coupling linear part 7 has a length of 1400 μm and almost 100% coupling is obtained. Has a bond of about 70% at the maximum.
[0011]
[Non-Patent Document 1]
62nd Japan Society of Applied Physics Academic Lecture Proceedings (2001.9, Aichi Institute of Technology), P900, Lecture Number: 12p-Y-6, “Lamination of Silica-Based PLC Using Polishing”
[0012]
[Problems to be solved by the invention]
As described above, the conventional laminated optical waveguide (interlayer optical directional coupler) has a coupling rate of almost 100% with respect to the TE polarized wave, but about 70% at most with respect to the TM polarized wave. Only a binding rate of was obtained.
[0013]
In general, a quartz-based optical waveguide requires high-temperature heat treatment in its film forming process. This process induces compressive stress in the substrate horizontal direction with respect to the waveguide core. However, in the laminated optical waveguide, the distance between the upper core and the lower core from the silicon substrate is different. Different in the core. In general, the propagation constant of an optical waveguide is greatly influenced by a stress component perpendicular to the polarization direction. Therefore, the difference in the stress in the horizontal direction of the substrate causes a mismatch in propagation constant between the upper and lower cores with respect to the TM polarization, and in the interlayer optical directional coupler, the asymmetric directional coupler with respect to the TM polarization. And induces its polarization dependence.
[0014]
The present invention provides a laminated optical waveguide that reduces the difference in polarization dependence between the upper and lower waveguide layers of the laminated optical waveguide that is a device for optical communication, and reduces the polarization dependence in each layer of the upper and lower waveguides. The purpose is to provide.
[0017]
[Means for Solving the Problems]
The outline of the invention disclosed in the present invention will be briefly described as follows.
On a base plate, a lower cladding layer, the lower core, intermediate cladding layer, an upper core, a laminated optical waveguide comprising an upper clad layer are sequentially laminated, a plane upper core and the lower core at the input curved portion is present in each The upper core extends in parallel while being positioned just above the lower core in the optical coupling straight line portion of a certain length, and gradually approaches the distance causing the optical coupling, and again in the output bending portion. An interlayer optical directional coupler having a structure in which the upper core and the lower core are separated from each other while gradually curving to a distance that does not cause optical coupling between the two cores on a plane in which the upper core and the lower core exist; The lower core that constitutes the constant-length optical coupling linear part, wherein the height Hl of the lower core that constitutes the coupling linear part is equal to the height Hu of the upper core that constitutes the constant-length optical coupling linear part Core width l by the a narrower construction than the core width Wu of the upper core constituting the optical coupling linear portion of the fixed length, the propagation constants for the TE polarization in the interlayer optical directional coupler with the upper core lower It is characterized in that it is made equal between the cores, and the propagation constant for TM polarization is made equal between the upper core and the lower core.
[0018]
This eliminates the polarization dependence of the interlayer optical directional coupler because the birefringence of the upper and lower cores can be made zero in the interlayer optical directional coupler fabricated in the laminated optical waveguide. can do. In the laminated optical waveguide, the substrate vertical component of the stress acting on the upper core and the lower core is almost equal. Therefore, by setting Hl = Hu, the propagation constant for TE polarization can be made equal between the upper core and the lower core. it can. Furthermore, by setting Wl <Wu, the substrate horizontal component of stress is set equal between the lower core and the upper core, and the propagation constant for TM polarization can be made equal between the upper core and the lower core. As a result, the propagation constants in the upper core and the lower core can all be set equal, and the polarization dependence can be reduced in the interlayer optical directional coupler.
[0019]
In addition, in the input curved portion, the lower core approaches the optical coupling linear portion while gradually reducing the core width, and in the output curved portion, the lower core gradually moves from the optical coupling linear portion. You may have an interlayer optical directional coupler of the structure which leaves | separates, expanding a core width.
Further, the laminated optical waveguide may be a glass waveguide configured on a silicon substrate.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments, and the repetitive description thereof is omitted.
[0021]
[Example 1]
1 is a perspective view showing a laminated optical waveguide having an interlayer optical directional coupler according to Example 1 of the present invention, FIG. 2 is a top view showing the interlayer optical directional coupler in the laminated optical waveguide, and FIG. (B), (c), (d), and (e) are cross-sectional views taken along the lines AA ′, BB ′, CC ′, DD ′, and EE ′ of FIG.
[0022]
As shown in FIG. 1, this laminated optical waveguide is formed on a silicon substrate 1 by a flame lamination method (FHD method) to form a lower clad layer 2, a lower core 21, an intermediate clad layer 4, an upper core 22, and an upper portion. This is a silica-based optical waveguide configured by sequentially laminating the cladding layer 6. In addition, 7 is an optical coupling straight line portion having a preset length (fixed length), and 8 is a gap.
[0023]
As shown in FIGS. 1 and 2, the interlayer optical directional coupler includes a lower core 21 and an upper core 22.
[0024]
Among these, the lower core 21 includes a lower input waveguide 11, a lower input bending portion 13 connected to the lower input waveguide 11, a lower optical coupling linear portion 15 connected to the lower input bending portion 13, and a lower optical coupling linear portion. 15, a lower output bending portion 17 connected to 15, and a lower output waveguide 19 connected to the lower output bending portion 17.
[0025]
The upper core 22 includes an upper input waveguide 12, an upper input curved portion 14 connected to the upper input waveguide 12, an upper optical coupling linear portion 16 connected to the upper input curved portion 14, and an upper optical coupling linear portion 16. The upper output bending portion 18 is connected to the upper output bending portion 18, and the upper output waveguide 20 is connected to the upper output bending portion 18.
[0026]
As shown in FIG. 3, the interlayer optical directional coupler includes a lower input curved portion 13 and a lower input waveguide 11 and an upper input waveguide 12 that are positioned at a distance that does not cause optical coupling with each other in a cross section AA ′. Then, after gradually approaching at the upper input bending portion 14 (cross section BB ′), the lower optical coupling linear portion 15 existing at a position where optical coupling occurs is optically connected to the upper optical coupling linear portion 16.
[0027]
In the optical coupling linear part 7 including the cross section CC ′, the upper optical coupling linear part 16 is positioned immediately above the lower optical coupling linear part 15 and optical coupling occurs between them. Here, the upper surface of the lower optical coupling linear part 15 and the lower surface of the upper optical coupling linear part 16 extend in parallel with a certain gap G. The lower optical coupling linear portion 15 and the upper optical coupling linear portion 16 extending in parallel over a certain length L are optically coupled again at the lower output curved portion 17 and the upper output curved portion 18 (cross section DD ′), respectively. Are gradually separated to the lower output waveguide 19 and the upper output waveguide 20 that exist at positions where no cross-section occurs (cross-section EE ′).
[0028]
The heights of the lower core 21 and the upper core 22 are both H throughout, and the width of the upper core is constant at W ₁ throughout.
Further, the lower core 21 is bent while gradually reducing its width from W ₁ to W ₂ at the lower input bending portion 13, and is bent while gradually increasing its width from W ₂ to W ₁ at the lower output bending portion 17. .
[0029]
Therefore, the width of the lower core 21 (lower optical coupling linear portion 15) constituting the optical coupling linear portion 7 is narrower than the width of the upper core 22 (upper optical coupling linear portion 16) constituting the optical linear portion 7. Yes.
[0030]
In the first embodiment, G = 3 μm, H = 6 μm, W ₁ = 7.5 μm, W ₂ = 6.5 μm, and a laminated optical waveguide including an interlayer optical directional coupler is a quartz optical waveguide by a flame deposition method. Produced.
[0031]
Further, the horizontal distance (the distance along the width direction) between the lower input waveguide 11 and the upper input waveguide 12 is set to 200 μm in the first embodiment so as not to cause optical coupling with each other.
Similarly, the horizontal distance (distance along the width direction) between the lower output waveguide 19 and the upper output waveguide 20 is set to 200 μm in Example 1 so as not to cause optical coupling with each other.
[0032]
FIG. 4 is a graph showing a change in the coupling rate with respect to the length L of the optical coupling straight line portion 7 of the interlayer optical directional coupler according to the first embodiment. For the TM polarization mode, the asymmetry of the interlayer optical directional coupler is reduced, and the maximum coupling ratio is improved from the conventional value of 70% to 90%. A 50% coupling length of 240 μm is obtained for both TM and TE polarized waves without polarization dependency.
[0033]
[Example 2]
FIG. 5 is a schematic view of an interlayer Mach-Zehnder interferometer in a laminated optical waveguide in Example 2 of the present invention. FIGS. 6 (f), (g), (h), (i), and (j) are FF ′, FIG. It is GG ', HH', II ', and JJ' sectional drawing.
[0034]
Also in Example 2, as in Example 1, the laminated optical waveguide was manufactured by a silica-based optical waveguide by a flame deposition method.
[0035]
The interlayer Mach-Zehnder interferometer shown in FIGS. 5 and 6 includes interlayer optical directional couplers 31 and 32, a lower arm waveguide 33 formed in the lower waveguide layer, and an upper arm waveguide formed in the upper waveguide layer. 34. A thin film heater 35 is disposed directly above the upper arm waveguide 34. The width Wu of the upper arm waveguide 34 is 7.5 μm over the whole, and the width Wl of the lower arm waveguide 33 is 6.5 μm over the whole. Further, the height Hl of the lower arm waveguide 33 and the height Hu of the upper arm waveguide 34 are both 6 μm. The length of the optical coupling straight part of the interlayer optical directional couplers 31 and 32 was 240 μm, and the interlayer gap was 3 μm.
[0036]
After all, when the aspect ratio of the core cross section of the lower arm waveguide 33 is defined as Rl = Hl / Wl and the aspect ratio of the core cross section of the upper arm waveguide is defined as Ru = Hu / Wu, the aspect ratio Ru of the upper core The aspect ratio Rl of the core is set to satisfy Rl> Ru.
[0037]
The light wave incident on the upper input waveguide 36 of the interlayer Mach-Zehnder interferometer is output to the upper output waveguide 37, but the output intensity changes depending on the power applied to the thin film heater 35.
[0038]
FIG. 7 shows the relationship between the power applied to the thin film heater 35 of the interlayer Mach-Zehnder interferometer and the output intensity to the upper output waveguide 37. As shown in FIG. 7, the OFF power was 0.46 W for the TM polarized wave and 0.48 W for the TE polarized wave, and good characteristics with small polarization dependence were obtained.
[0039]
【The invention's effect】
The effects obtained by the representative ones of the inventions disclosed in the present application will be briefly described as follows. That is, according to the present invention, the aspect ratio Ru of the upper core of the laminated optical waveguide and the aspect ratio Rl of the lower core are such that Rl> Ru, so that the polarization dependence of the upper and lower waveguide layers in the laminated optical waveguide is reduced. The difference can be resolved.
[0040]
Further, when the present invention is applied to the interlayer optical directional coupler, the height H1 of the lower core is equal to the height Hu of the upper core in the optical coupling linear portion, and the core width W1 of the lower core is equal to the core of the upper core. Since it is narrower than the width Wu, the polarization dependency of the coupling rate can be reduced, and perfect coupling can be obtained satisfactorily for both TM and TE polarized waves.
[Brief description of the drawings]
FIG. 1 is a perspective view showing a laminated optical waveguide including an interlayer optical directional coupler according to Embodiment 1 of the present invention.
FIG. 2 is a top view showing a laminated optical waveguide including an interlayer optical directional coupler according to Embodiment 1 of the present invention.
3 is a cross-sectional view showing a laminated optical waveguide including an interlayer optical directional coupler according to Example 1 of the present invention, and FIGS. 3 (a) (b) (c) (d) (e) are diagrams. 2 is a cross-sectional view taken along line AA ′, BB ′, CC ′, DD ′, and EE ′.
FIG. 4 is a graph showing a change in coupling ratio with respect to a length L of an optical coupling linear portion of the interlayer optical directional coupler according to the first embodiment of the present invention.
FIG. 5 is a top view showing an interlayer Mach-Zehnder interferometer according to Embodiment 2 of the present invention.
6 is a cross-sectional view showing an interlayer Mach-Zehnder interferometer according to Embodiment 2 of the present invention, and FIGS. 6 (f), (g), (h), (i), and (j) are FF ′ and G in FIG. It is -G ', HH', II ', and JJ' sectional drawing.
FIG. 7 is a graph showing a relationship between output power and applied power of the interlayer Mach-Zehnder interferometer of Example 2 of the present invention.
FIG. 8 is a perspective view showing a laminated optical waveguide including a conventional interlayer optical directional coupler.
FIG. 9 is a top view showing a laminated optical waveguide including a conventional interlayer optical directional coupler.
10 is a cross-sectional view showing a laminated optical waveguide including a conventional interlayer optical directional coupler, and FIGS. 10 (a), 10 (b), and 10 (c) are AA-AA ′, BB-BB ′, CC of FIG. -CC 'sectional drawing.
FIG. 11 is a characteristic diagram showing coupling characteristics of a conventional interlayer optical directional coupler.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Silicon substrate 2 Lower clad layer 3,21 Lower core 4 Intermediate clad layer 5,22 Upper core 6 Upper clad layer 7 Optical coupling linear part 8 Gap 31,32 Interlayer optical directional coupler 33 Lower arm waveguide 34 Upper arm guide Waveguide 35 Thin film heater

Claims (3)

A laminated optical waveguide obtained by sequentially laminating a lower clad layer, a lower core, an intermediate clad layer, an upper core, and an upper clad layer on a substrate,
In the input curved portion, the upper core and the lower core approach each other while gradually curving up to the distance that causes optical coupling on the plane where each exists, and the upper core is directly above the lower core in the optical coupling straight portion of a certain length. It extends in parallel while being positioned, and again in the output curved part, the upper core and the lower core are separated while gradually curving to a distance that does not cause optical coupling between both cores on the plane where each exists. Having an interlayer optical directional coupler;
The height Hl of the lower core that constitutes the constant-length optical coupling straight line portion is equal to the height Hu of the upper core that constitutes the constant-length optical coupling straight line portion,
By performing the fixed length of narrower construction than the core width Wu of the upper core core width Wl of the lower core constituting the optical coupling linear portion of the fixed length constituting the optical coupling straight portion, said <br / > The propagation constant for TE polarization in the interlayer optical directional coupler is made equal between the upper core and the lower core, and the propagation constant for TM polarization is made equal between the upper core and the lower core. A laminated optical waveguide characterized. In the input curved portion, the lower core approaches the optical coupling straight portion while gradually reducing the core width,
2. The laminated optical waveguide according to claim 1 , wherein the output curved portion includes an interlayer optical directional coupler having a structure in which the lower core is separated from the optical coupling straight portion while gradually increasing the core width. . 3. The laminated optical waveguide according to claim 1, wherein the laminated optical waveguide is a glass waveguide configured on a silicon substrate.

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