JP5559381B1 - Optical waveguide - Google Patents

Abstract

In an optical waveguide capable of guiding a plurality of light beams in a guided mode, a problem caused by light in a higher mode can be solved.
A first core 102 formed on a lower clad layer 101 and an upper clad layer 103 formed on the lower clad layer 101 so as to cover the first core 102 are provided. In addition, the bent portion 111 of the first core 102 in which the waveguide direction is changed includes a second core 104 partially disposed in parallel with the first core 102. The second core 104 is in a state of affecting higher-order mode light among a plurality of modes of light propagating along the first core 102.
[Selection] Figure 1A

Description

  The present invention relates to an optical waveguide capable of guiding light of a plurality of waveguide modes.

  In recent years, research and development of silicon optical waveguides have been actively promoted with the aim of miniaturizing optical circuits. The configuration of a conventional silicon fine wire optical waveguide is shown in FIGS. 6A and 6B. This optical waveguide is composed of a lower clad layer 601, a core 602, and an upper clad layer 603. 6A is a plan view, and FIG. 6B is a cross-sectional view perpendicular to the waveguide direction. In FIG. 6A, the upper clad layer 603 is omitted.

  The lower cladding layer 601 and the upper cladding layer 603 are made of, for example, silicon oxide, silicon oxynitride, silicon nitride, epoxy-based polymer, polyimide-based polymer, and the like, and the core 602 is made of silicon. 6A and 6B illustrate a so-called channel type optical waveguide, but a rib type optical waveguide is also used.

  In such an optical waveguide, the core cross section has a dimension that realizes a single mode condition in order to prevent light loss and mode conversion accompanying deflection. The wavelength of light propagating through the optical waveguide is preferably 1200 to 1700 nm, in which silicon is transparent and the light source is easily available. For this wavelength, the core cross-sectional dimension of the channel-type optical waveguide is 450 nm in width, A height of about 200 nm is the upper limit value of the single mode condition (see FIG. 1.2 of Non-Patent Document 1). In addition, in such a micro silicon optical waveguide having a core cross-sectional dimension of several hundreds of nanometers, the influence of light scattering due to the irregularities on the core side wall of the optical waveguide is large, and the propagation loss is 1.2 dB / s even using the latest silicon processing technology. It is about cm (see FIG. 1.11 of Non-Patent Document 1).

  On the other hand, a silicon chip constituting an optical device has a size of about 25 mm square in recent years. Therefore, when a complicated optical circuit is formed in this silicon chip, the length of the optical waveguide is the outer peripheral length of the chip. About 100 mm is required. In this case, assuming a propagation loss of 1.2 dB / cm by the latest processing technology, the propagation loss is 12 dB, and the light intensity is attenuated to about 1/16.

  In order to reduce the propagation loss of such an optical waveguide, it is effective to widen the core width of the optical waveguide to reduce the influence of irregularities on the core side wall (see FIG. 1.12 of Non-Patent Document 1). . Further, according to Non-Patent Document 1, it is shown that when the cross-sectional size of the core is 200 nm, the propagation loss can be almost halved in the optical waveguide with the core width of 500 nm as compared with the single mode optical waveguide with the core width of 440 nm. Yes.

D. Lockwood, L. Pavesi (Eds.), "Silicon Photonic Wire Waveguides: Fundamentals and Applications" in "Silicon Photonics II", Springer, 2011.

  However, an optical waveguide having a wider core width than a single mode optical waveguide having a core width of 500 nm, for example, is a multimode optical waveguide capable of guiding light of a plurality of waveguide modes. During the distance propagation, a small but higher order mode is excited. This higher-order mode propagation light interferes with the fundamental mode, causing a problem that the transmission characteristic of the optical waveguide varies with wavelength. Further, since the propagation constant is different from the fundamental mode, there is a problem that the wavelength filter does not operate normally.

  The present invention has been made in order to solve the above-described problems. In an optical waveguide in which a plurality of waveguide modes of light can be guided, the problem due to the light of higher-order modes can be solved. The purpose is to.

An optical waveguide according to the present invention includes a core which is comprised of silicon formed on the lower cladding layer, and an upper clad layer formed on the lower cladding layer covering the core, the guiding direction is changed and a core-like structure Turkey a bent portion odor Te made of silicon arranged in partially predetermined gap in parallel to the core, the gap, the higher order modes from the input fundamental mode in the core The gap is in the range where the conversion efficiency is the smallest .

In the above optical waveguide, the core-like structures, may be formed at the same height as the core.

  As described above, according to the present invention, since the second core is partially arranged in parallel with the first core in the bent portion, the excitation of higher-order modes is suppressed, and a plurality of waveguide modes In the optical waveguide in which the light can be guided, the excellent effect that the problem due to the light in the higher order mode can be solved is obtained.

FIG. 1A is a plan view showing a configuration of an optical waveguide according to Embodiment 1 of the present invention. FIG. 1B is a cross-sectional view showing the configuration of the optical waveguide according to Embodiment 1 of the present invention. FIG. 2 shows a fundamental mode (a) and a higher-order mode (b) of TE polarized wave propagating through an optical waveguide by a silicon core having a core width of 500 nm and a core height of 200 nm covered with a core made of silicon oxide. It is a distribution map which shows the light intensity distribution in a cross section. FIG. 3 shows the output light depending on the presence or absence of the core structure 104 when light of the fundamental mode is incident on the optical waveguide made of the core 102 made of silicon having a bent portion with a curvature radius of 5 μm that changes the waveguide direction by 90 degrees. It is a characteristic view which shows the state of a fundamental mode and the excited higher order mode. FIG. 4A is a plan view showing the configuration of the optical waveguide according to Embodiment 1 of the present invention. FIG. 4B is a cross-sectional view showing the configuration of the optical waveguide according to Embodiment 1 of the present invention. FIG. 5 shows the output light depending on the presence / absence of the core structure 204 when the light of the fundamental mode is incident on the optical waveguide made of the core 102 made of silicon having a bent portion with a curvature radius of 5 μm that changes the waveguide direction by 90 degrees. It is a characteristic view which shows the state of a fundamental mode and the excited higher order mode. FIG. 6A is a plan view showing a configuration of a silicon fine wire optical waveguide. FIG. 6B is a cross-sectional view showing a configuration of the silicon fine wire optical waveguide.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[Embodiment 1]
First, Embodiment 1 of the present invention will be described with reference to FIGS. 1A and 1B. 1A is a plan view showing the configuration of the optical waveguide according to Embodiment 1 of the present invention, and FIG. 1B is a cross-sectional view showing the configuration of the optical waveguide according to Embodiment 1 of the present invention. FIG. 1B shows a cross section perpendicular to the waveguide direction.

The optical waveguide includes a core 102 formed on the lower clad layer 101 and an upper clad layer 103 that covers the core 102 and is formed on the lower clad layer 101. In addition, the bent portion 111 of the core 102 that the guiding direction is changed, comprises a core structure 104 which is partially disposed in parallel to the core 102. The core structure 104 is in a state of affecting higher-order mode light among a plurality of modes of light propagating along the core 102. Here, the core structure 104 is arranged so as to influence higher-order mode light and to suppress higher-order mode excitation. In the first embodiment, the bent portion 111 changes the waveguide direction by 90 °. In the first embodiment, the core structure 104 is arranged on the center side of the arc of a predetermined curvature in the bent portion 111 when viewed from the core 102.

The lower cladding layer 101 and the upper cladding layer 103 are made of, for example, silicon oxide, silicon oxynitride, silicon nitride, epoxy polymer, polyimide polymer, or the like. The core 102 and the core structure 104 are made of, for example, silicon. For example, a well-known SOI (Silicon on Insulator) substrate may be used, the buried oxide film may be the lower cladding layer 101, and the surface silicon layer may be patterned to form the core 102 and the core structure 104. For example, the core 102 and the core structure 104 can be formed by selectively etching the surface silicon layer by a known dry etching using a mask pattern formed by a known photolithography technique.

The upper cladding layer 103 is formed by depositing silicon oxide after forming the core 102 and the core structure 104 by a deposition technique such as a well-known chemical vapor deposition (CVD) method or sputtering method. What is necessary is just to form. Further, the upper clad layer 103 may be formed by applying a resin such as an epoxy polymer or a polyimide polymer.

Here, the core structure 104 modulates the higher-order mode light so that the higher-order mode excited from the fundamental mode is less likely to be excited while propagating through the bent portion 111. The presence of the core structure 104 does not affect the fundamental mode light. The core structure 104 only needs to be formed on the same surface (lower surface) of the lower cladding layer 101 as the core 102. The upper cladding layer 103 is formed on the lower cladding layer 101 so as to cover the core 102 and the core structure 104.

  The light intensity in the fundamental mode and the light intensity in the higher order mode are distributed as shown in FIG. FIG. 2 shows a fundamental mode (a) and a higher-order mode (a) of TE polarized waves propagating through an optical waveguide through a silicon core having a core width of 500 nm and a core height (thickness) of 200 nm covered with a core made of silicon oxide. It is a distribution map which shows the light intensity distribution in the cross section of b). As shown in FIG. 2B, the higher-order mode has a weaker light confinement in the core than the fundamental mode.

Therefore, the core structure provided on the side of the core 102 with respect to the fundamental mode and the higher order mode of the optical waveguide uniquely determined by the shape and refractive index of the core 102, the lower cladding layer 102, and the upper cladding layer 103. By means of the body 104, the modulation applied to the fundamental mode can be kept small and the higher order modes can be provided with a large modulation. At this time, by appropriately setting the positional relationship between the core 102 and the core structure 103, the higher-order mode is modulated so that the excitation of the higher-order mode by the fundamental mode is as small as possible.

Next, the effect obtained by providing the core structure 104 will be described with reference to FIG. FIG. 3 shows the output light depending on the presence or absence of the core structure 104 when light of the fundamental mode is incident on the optical waveguide made of the core 102 made of silicon having a bent portion with a curvature radius of 5 μm that changes the waveguide direction by 90 degrees. It is a characteristic view which shows the state of a fundamental mode and the excited higher order mode. In FIG. 3, the horizontal axis represents the change in the gap between the core 102 and the core structure 104. Here, the core cross section of the core 102 has a core width of 500 nm and a core height (thickness) of 200 nm. Here, the core structure 104 also has a core cross section with a core width of 500 nm and a core height of 200 nm.

  In the calculation for obtaining the results shown in FIG. 3, the eigenmode is calculated using the film mode matching method, and the S matrix representing the transmission of each eigenmode and the mode conversion between eigenmodes is calculated using the eigenmode expansion method. Then, the transmission efficiency from the input basic mode to the output basic mode and the conversion efficiency from the input basic mode to the higher order mode were obtained.

3A shows a change in the transmission efficiency of the fundamental mode light when the core structure 104 is not provided, and FIG. 3B shows the fundamental mode light when the core structure 104 is provided. The change in transmission efficiency is shown. These correspond to the left vertical axis. On the other hand, FIG. 3C shows a change in conversion efficiency to higher-order mode light when the core structure 104 is not provided, and FIG. 3D shows a higher order when the core structure 104 is provided. It shows the change in conversion efficiency to mode light. These correspond to the right vertical axis.

As shown in FIG. 3, when the core structure 104 is not provided, the conversion efficiency to the output higher-order mode is −33 dB. On the other hand, by setting the gap (gap) between the core 102 and the core structure 104 to about 300 nm, the conversion efficiency to the output higher-order mode can be suppressed to −40 dB or less. This effect is obtained when the core structure 104 has the same core height as the core 102.

As described above, according to the first embodiment, since the core structure is partially arranged in parallel with the core at the bent portion of the core where the waveguide direction is changed, It becomes difficult to be excited. As a result, high-order mode excitation at the bent portion of the core, which could not be suppressed in the past, can be suppressed, and in the optical waveguide in which light of a plurality of waveguide modes can be guided, The problem can be solved. In addition, the core structure can be easily manufactured at the same time as the core, and it is not necessary to use a special material or an additional process, so that the productivity is excellent.

[Embodiment 2]
Next, Embodiment 2 of the present invention will be described with reference to FIGS. 4A and 4B. 4A is a plan view showing the configuration of the optical waveguide according to Embodiment 2 of the present invention, and FIG. 4B is a cross-sectional view showing the configuration of the optical waveguide according to Embodiment 2 of the present invention. FIG. 4B shows a cross section perpendicular to the waveguide direction.

The optical waveguide includes a core 102 formed on the lower clad layer 101 and an upper clad layer 103 that covers the core 102 and is formed on the lower clad layer 101. In addition, the bent portion 111 of the core 102 that the guiding direction is changed, comprises a core structure 204 which is partially disposed in parallel to the core 102. The core structure 204 is in a state of affecting higher-order mode light among a plurality of modes of light propagating along the core 102. Here, the core structure 304 is arranged so as to influence the higher-order mode light and to suppress the higher-order mode excitation. Also in the second embodiment, the bent portion 111 changes the waveguide direction by 90 °. In the second embodiment, the core structure 204 is arranged outside the center of the arc of a predetermined curvature in the bent portion 111 when viewed from the core 102.

The lower cladding layer 101, the core 102, and the upper cladding layer 103 are the same as those in the first embodiment. The material is the same as that of the first embodiment described above, including the core structure 204. For example, the core 102 and the core structure 204 can be formed by using a well-known SOI substrate, using the buried oxide film as the lower cladding layer 101, and patterning the surface silicon layer.

Also in the second embodiment, the core structure 204 modulates the higher-order mode light so that the higher-order mode excited from the fundamental mode is less likely to be excited while propagating through the bent portion 111. The presence of the core structure 204 does not affect the fundamental mode light. The core structure 204 only needs to be formed on the same surface (lower surface) of the lower cladding layer 101 as the core 102. The upper cladding layer 103 is formed on the lower cladding layer 101 so as to cover the core 102 and the core structure 204.

Next, the effect of providing the core structure 204 in Embodiment 2 will be described with reference to FIG. FIG. 5 shows the presence or absence of the core structure 204 in the night output light when the fundamental mode light is incident on the optical waveguide made of the core 102 made of silicon having a bent portion with a curvature radius of 5 μm that changes the waveguide direction by 90 degrees. It is a characteristic view which shows the state of a fundamental mode and the excited higher order mode. In FIG. 5, the horizontal axis represents the change in the gap between the core 102 and the core structure 204. Here, the core cross section of the core 102 has a core width of 500 nm and a core height (thickness) of 200 nm. The core structure 204 also has a core cross section with a core width of 500 nm and a core height of 200 nm.

  In the calculation for obtaining the result shown in FIG. 5, the eigenmode is calculated using the film mode matching method, and the S matrix representing the transmission of each eigenmode and the mode conversion between eigenmodes is calculated using the eigenmode expansion method. Then, the transmission efficiency from the input basic mode to the output basic mode and the conversion efficiency from the input basic mode to the higher order mode were obtained.

5A shows a change in the transmission efficiency of the fundamental mode light when the core structure 204 is not provided, and FIG. 5B shows the fundamental mode light when the core structure 204 is provided. The change in transmission efficiency is shown. These correspond to the left vertical axis. On the other hand, FIG. 5C shows a change in conversion efficiency to higher-order mode light when the core structure 204 is not provided, and FIG. 5D shows a higher order when the core structure 204 is provided. It shows the change in conversion efficiency to mode light. These correspond to the right vertical axis.

As shown in FIG. 5, when the core structure 204 is not provided, the conversion efficiency to the output higher-order mode is −33 dB. On the other hand, by setting the gap (gap) between the core 102 and the core structure 204 to about 510 nm, the conversion efficiency to the output higher-order mode can be suppressed to −50 dB or less. This effect is obtained when the core structure 204 has the same core height as the core 102.

As described above, also in the second embodiment, since the core structure is partially arranged in parallel with the core at the bent portion of the core where the waveguide direction is changed, the higher-order mode is excited. It becomes difficult to be done. As a result, high-order mode excitation at the bent portion of the core, which could not be suppressed in the past, can be suppressed, and in the optical waveguide in which light of a plurality of waveguide modes can be guided, The problem can be solved. In addition, the core structure can be easily manufactured at the same time as the core, and it is not necessary to use a special material or an additional process, so that the productivity is excellent.

The present invention is not limited to the embodiment described above, and many modifications and combinations can be implemented by those having ordinary knowledge in the art within the technical idea of the present invention. It is obvious. For example, the core structure may be arranged on the upper side of the core or the lower side of the core in the stacking direction of the lower clad layer, the core , and the upper clad layer.

DESCRIPTION OF SYMBOLS 101 ... Lower clad layer, 102 ... Core , 103 ... Upper clad layer, 104 ... Core structure .

Claims (2)

A core composed of silicon formed on the lower cladding layer;
And an upper clad layer formed on the lower cladding layer before covering Kiko A,
And a core-like structure made of silicon, which is arranged in a predetermined gap to partially parallel before logger A at the bent portion of the front Kiko A waveguiding direction is changed,
The optical waveguide is characterized in that the gap is a gap in a range in which the conversion efficiency from the input fundamental mode to the higher order mode in the core is minimized. The optical waveguide according to claim 1,
It said core-like structure, the optical waveguide, characterized in that it is formed prior to the same height as the Kiko A.