JP4410149B2 - Polarization rotator and optical circuit - Google Patents

Description

  The present invention relates to an optical communication device, and more particularly to a polarization rotation element that realizes polarization independence of a high refractive index difference optical waveguide, and an optical circuit using the polarization rotation element.

The major challenges of current optical communication devices are miniaturization and high integration. There are two main methods for downsizing a planar optical circuit.
One method is to break the limits of the bending radius of an optical waveguide by partially using a reflective element while being based on a glass optical waveguide. According to such a method, although there is a characteristic deterioration such as an increase in insertion loss due to the reflection type element, there is an advantage that the size can be reduced to about 1/10 of the conventional product and the polarization dependency is small. However, the reduction ratio of the device is about 1/10, and in principle, further miniaturization cannot be expected.

  Another method is to use a high refractive index difference waveguide (see, for example, Non-Patent Document 1). In this method, the device area when compared with the glass waveguide can be reduced to 1/10000 or less. In particular, a waveguide having a silicon core is considered to be able to provide a large amount of inexpensive devices, and many studies are in progress.

K. Yamada et al., “Microphotonics Devices Based on Silicon Wire Waveguiding System”, IEICE TRANS.ELECTRON., Vol.E87-C, No.3, 2004, p.351-358

However, the high refractive index difference waveguide has a big problem that it has dependence on the polarization of incident light. In particular, it has been pointed out that this polarization dependence becomes a major obstacle when used in an optical communication device. In the high refractive index difference waveguide, since the core width and height are 1 μm or less, it is difficult to obtain an accurate square core cross section, and it is also difficult to make the refractive indexes of the upper and lower clads of the core exactly equal. Therefore, it is virtually impossible to make the polarization independent by simply improving the processing accuracy.
In addition, since the optical confinement is strong in the high refractive index difference waveguide, there is a problem that the insertion loss becomes too large if the polarization is made independent using the wave plate like the glass waveguide. There was also a problem that the mounting cost was high.

  The present invention has been made in order to solve the above-described problems. Polarization capable of realizing a polarization-independent device that has a small element size, enables high-density integration, and is inexpensive and easy to mount. An object is to provide a rotating element and an optical circuit.

The polarization rotation element of the present invention includes a first core having a rectangular cross section perpendicular to the light propagation direction, and a second core having a rectangular cross section perpendicular to the light propagation direction, which is disposed so as to cover the first core. And a clad disposed so as to cover the second core, the refractive index of the first core is larger than the refractive index of the second core and the clad, and the second core greater than the refractive index of the core is the refractive index of the cladding, the first Ri core center axis Do different in the light propagation direction and the center axis of the light propagation direction of the second core, the light of the second core The length in the propagation direction is such that the polarization of the incident linearly polarized light rotates 90 degrees while propagating through the first and second cores .
In one configuration example of the polarization rotation element of the present invention, the first core is made of silicon, and the second core is a silicon nitride film, a silicon oxynitride film, or a refractive index of 1.5 or more. The clad is made of a polymer material, and the clad is made of a silicon oxide film, a polyimide resin, or an epoxy resin.

The polarization rotator of the present invention has a first core whose section perpendicular to the light propagation direction is rectangular, and a section perpendicular to the light propagation direction, which is disposed so as to cover the first core. A second core, an input waveguide core integrally formed with the first core, disposed at an input end of the first core, and an output waveguide core of the first core, An output waveguide core integrally formed with the first core; and the input core comprising the second core, the input waveguide core, and a clad disposed so as to cover the output waveguide core. The waveguide core for use includes a flat input portion having a constant cross-sectional area, a connection portion connected to the first core and having the same cross-sectional shape as the first core, and the input portion to the connection portion. A taper part with a gradually decreasing cross-sectional area with a constant height toward the output waveguide, A is a flat output portion having a constant cross-sectional area, a connection portion connected to the first core and having the same cross-sectional shape as the first core, and from the connection portion toward the output portion. A taper portion with a gradually increasing cross-sectional area with a constant height, and refractive indices of the first core, the input waveguide core, and the output waveguide core are refracted by the second core and the cladding. The refractive index of the second core is greater than the refractive index of the cladding, and the central axis of the light propagation direction of the first core is different from the central axis of the light propagation direction of the second core. The length of the second core in the light propagation direction is such that the polarization of the incident linearly polarized light rotates 90 degrees while propagating through the first and second cores. It is a feature .
In one configuration example of the polarization rotation element of the present invention, the first core, the input waveguide core, and the output waveguide core are made of silicon, and the second core is a silicon nitride film. The clad is made of a silicon oxynitride film or a polymer material having a refractive index of 1.5 or more, and the clad is made of a silicon oxide film, a polyimide resin, or an epoxy resin.

  The optical circuit of the present invention includes a polarization rotation element, a polarization separation element that separates incident light according to a polarization plane, and one of the polarization components separated by the polarization separation element. A first waveguide guided to the rotation element; a second waveguide guiding the other of the polarization components separated by the polarization separation element; the polarization component passing through the polarization rotation element; and the second And a multiplexing element for multiplexing the polarization component that has passed through the waveguide.

  According to the present invention, the first core, the second core, and the clad are provided, and the eccentric two in which the central axis of the first core in the light propagation direction does not coincide with the central axis of the second core in the light propagation direction. The polarization state can be changed by adopting a heavy core structure and propagating linearly polarized light through the optical waveguide having the eccentric double core structure for a certain distance. Therefore, in the present invention, by providing a polarization separation element in front of the polarization rotation element, the light in which the TE polarization component and the TM polarization component are mixed is separated according to the polarization plane, and the one polarization is separated. Rotate the component (eg TM) by 90 ° to change to the other polarization component, and combine the polarization component that has passed through the polarization rotator and the polarization component that has not passed through, making it polarization independent as a whole A type waveguide can be realized. As a result, according to the present invention, there is provided a device structure that significantly reduces the polarization dependence, which has been a big problem for a high refractive index difference waveguide having a large refractive index difference between the core and the clad, such as a silicon waveguide. Can be realized. Further, in the present invention, polarization independence can be realized in a high refractive index difference waveguide, so that miniaturization and high density integration can be realized, and there is no need to use a wave plate for polarization independence. Therefore, an inexpensive device that can be easily mounted can be realized.

[First Embodiment]
Hereinafter, embodiments of the present invention will be described with reference to the drawings. The present embodiment includes a first core made of a high refractive index material, a second core made of a medium refractive index material, and a clad made of a low refractive index material. Polarization rotation is realized by using an eccentric double-core optical waveguide in which the central axis of the light propagation direction of the two cores is different.

  FIG. 1 is a block diagram showing a configuration of a polarization-independent waveguide using the polarization rotation element of the present embodiment. Incident light 11 in which a TE (Transverse Electric mode) polarization component and a TM (Transverse Magnetic mode) polarization component are mixed is separated into a TE polarization component 13 and a TM polarization component 14 by a polarization separation element 12. The TE polarization component 13 propagates through the waveguide 15, and the TM polarization component 14 propagates through another waveguide 16. A polarization rotation element 17 is connected to the waveguide 16, and the TM polarization component 14 is rotated 90 ° into the TE polarization component by passing through the waveguide 16. The waveguides 15 and 16 are connected to the multiplexing element 18, and the TE polarization component 13 and the TE polarization component that has passed through the polarization rotation element 17 are combined by the multiplexing element 18.

  Two orthogonal TE polarization components and TM polarization components have little loss in each of the separation, rotation, and multiplexing processes, so that signal attenuation is small, and the signal is sufficient if the waveguide length of the polarization rotation element 17 is sufficiently short. There is no distortion. Therefore, according to the configuration of FIG. 1, polarization independence can be realized while using a waveguide having large polarization dependence as a base.

  FIG. 2 is a cross-sectional view showing the configuration of the polarization rotation element 17. In FIG. 2, the direction perpendicular to the paper surface is the light propagation direction. The polarization rotation element 17 includes a first core 17a, a second core 17b, and a clad 16z. In the example of FIG. 2, silicon having a width of 0.2 μm × height of 0.2 μm (refractive index of 3.5) is assumed as the first core 17a, and silicon acid having a width of 1 μm × height of 1 μm is assumed as the second core 17b. A nitride film (refractive index 1.6) is assumed, and a silicon oxide film (refractive index 1.44) is assumed as the cladding 16z.

  The amount of eccentricity between the central axis of the first core 17a and the central axis of the second core 17b is 0.4 μm in each of two orthogonal directions (vertical direction and horizontal direction in FIG. 2). In FIG. 2, the A-axis direction and the B-axis direction are axes of propagation eigenmodes. Since the propagation constants of vertical polarization and horizontal polarization are different, when the incident light is vertical polarization (TM mode), it receives rotation during propagation and becomes horizontal polarization (TE mode) after propagating for a certain distance. .

  FIGS. 3A and 3B are diagrams showing how light having a wavelength of 1550 nm propagates through the polarization rotation element 17 of FIG. 2 over a length of 30 μm. Polarization at the time of incidence is assumed to be TM mode. FIG. 3 (a) shows only the TM polarization component, and FIG. 3 (b) shows only the TE polarization component. Further, the light propagation states shown in FIGS. 3A and 3B indicate that the light power is stronger in the brighter part.

  As is apparent from FIGS. 3A and 3B, immediately after entering the polarization rotator 17, it is composed only of the TM polarization component. When the TM polarization component gradually changes to the TE polarization component and propagates through the polarization rotator 17 by about 15 μm, the TM polarization component intensity and the TE polarization component intensity become substantially equal, and the propagation is about 30 μm. It completely changes to TE polarization component. Accordingly, it can be seen that the polarization rotation element 17 having a length of 30 μm functions as an element that rotates the TM polarization to the TE polarization by 90 ° for light having a wavelength of 1550 nm.

  FIG. 4 is a perspective view showing a specific example of the polarization rotation element 17 of the present embodiment. The polarization rotation element 17 includes an input waveguide core 16a, a first core 17a, a second core 17b, an output waveguide core 16b, an input waveguide core 16a, and a second core 17b. The clad (not shown) covers the output waveguide core 16b. The input waveguide core 16a, the first core 17a, and the output waveguide core 16b are made of the same material, are integrally formed, and the central axes in the propagation direction coincide with each other. On the other hand, the first core 17a and the second core 17b have an eccentric double core configuration in which the central axes in the propagation direction do not coincide.

  Incident light that is linearly polarized light is incident from the input waveguide core 16a, and the polarization is rotated when propagating through the first and second cores 17a and 17b. It propagates through the waveguide 16b and exits.

As the material of the input waveguide core 16a, the first core 17a, and the output waveguide core 16b, a high refractive index material such as silicon is used. The cross-sectional area of these cores is 0.1 μm ² or less when silicon is used. As the cladding material, a material having a lower refractive index than the first and second cores 17a and 17b is used. Specific examples include silicon oxide, epoxy resin, polyimide, and various other polymers. As the material of the second core 17b, a silicon nitride film or silicon oxynitride film having a refractive index intermediate between the first core 17a and the clad, a polymer material having a refractive index of 1.5 or more, or the like is used. When the material of the first core 17a is silicon and the material of the second core 17b is a silicon oxynitride film having a refractive index of 1.6, the cross-sectional area of the second core 17b is 1.5 μm ² or less.

  As described above, in the polarization rotation element 17 of the present embodiment, the eccentric double core in which the central axis in the light propagation direction of the first core 17a does not coincide with the central axis in the light propagation direction of the second core 17b. The polarization state can be changed by adopting the structure and propagating linearly polarized light through the optical waveguide having the eccentric double core structure for a certain distance. By using this polarization rotation element 17, a polarization-independent waveguide can be realized.

[Second Embodiment]
Next, a second embodiment of the present invention will be described. FIG. 5 is a perspective view showing the configuration of the polarization rotation element 17 of the present embodiment. The polarization plane rotation element 17 of the present embodiment has a flat input waveguide core (input part) 16c having a constant cross-sectional area, and toward the connection part between the waveguide core 16c and the first core 17a. From the connecting portion of the tapered input waveguide core 16d, the first core 17a, the second core 17b, and the first core 17a, whose sectional area gradually decreases with the height being constant, to the output portion. The output waveguide core 16e having a tapered shape whose cross-sectional area gradually increases with a constant height, a flat output waveguide core (output portion) 16f having a constant cross-sectional area, and the input waveguide core 16c. 16d, the second core 17b, and the output waveguide cores 16e and 16f. Here, flat means that W> H, where W is the width and H is the height.

  The input waveguide cores 16c and 16d, the first core 17a, and the output waveguide cores 16e and 16f are made of the same material, are integrally formed, and the central axes in the propagation direction coincide with each other. On the other hand, the first core 17a and the second core 17b have an eccentric double core configuration in which the central axes in the propagation direction do not coincide.

  Incident light that is linearly polarized light enters from the input waveguide core 16c, but the polarization is maintained because the input waveguide core 16c has a flat shape. Then, when the incident light propagates through the first and second cores 17a and 17b through the tapered input waveguide core 16d, the polarization rotates, and after rotating by a desired rotation angle, the tapered output is performed. The light is output to the output waveguide 16f through the waveguide core 16e. Since the output waveguide core 16f has a flat shape, the polarization is maintained.

As a material for the input waveguide cores 16c and 16d, the first core 17a, and the output waveguide cores 16e and 16f, a high refractive index material such as silicon is used. The cross-sectional area of these cores is 0.1 μm ² or less when silicon is used. As the cladding material, a material having a lower refractive index than the first and second cores 17a and 17b is used. Specific examples include silicon oxide, epoxy resin, polyimide, and various other polymers. As the material of the second core 17b, a silicon nitride film or silicon oxynitride film having a refractive index intermediate between the first core 17a and the clad, a polymer material having a refractive index of 1.5 or more, or the like is used. When the material of the first core 17a is silicon and the material of the second core 17b is a silicon oxynitride film having a refractive index of 1.6, the cross-sectional area of the second core 17b is 1.5 μm ² or less.
According to the present embodiment, the same effect as in the first embodiment can be obtained.

[Third Embodiment]
In the second embodiment, as shown in FIG. 5, the second core 17b covers only the first core 17a. However, the present invention is not limited to this, and the second core 17b Not only the core 17a but also a part or the whole of the input waveguide cores 16c and 16d and the output waveguide cores 16e and 16f may be covered.

  The present invention can be applied to an optical communication device.

It is a block diagram which shows the structure of the polarization-independent type | mold waveguide using the polarization rotation element used as the 1st Embodiment of this invention. It is sectional drawing which shows the structure of the polarization rotation element of FIG. It is a figure which shows a mode that light propagates the polarization | polarized-light rotation element of FIG. It is a perspective view which shows the specific example of the polarization rotation element of FIG. It is a perspective view which shows the structure of the polarization rotation element used as the 2nd Embodiment of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... Incident light, 12 ... Polarization separation element, 13 ... TE polarization component, 14 ... TM polarization component, 15, 16 ... Waveguide, 16a, 16c, 16d ... Waveguide core for input, 16b, 16e, 16f ... Output waveguide core, 17... Polarization rotation element, 17 a... First core, 17 b.

Claims (5)

A first core having a rectangular cross section perpendicular to the light propagation direction;
A second core having a rectangular cross section perpendicular to the light propagation direction, disposed so as to cover the first core;
The clad arranged to cover this second core,
The refractive index of the first core is greater than the refractive index of the second core and the cladding;
The refractive index of the second core is greater than the refractive index of the cladding;
The first Ri core center axis Do different in the light propagation direction and the center axis of the light propagation direction of the second core, the length of the light propagation direction of the second core, the light of linear polarization incident polarizations the first polarization rotation device characterized lengths der Rukoto rotating 90 degrees while propagating the second core. The polarization rotation element according to claim 1,
The first core is made of silicon,
The second core is made of a silicon nitride film, a silicon oxynitride film or a polymer material having a refractive index of 1.5 or more,
The polarization rotation element, wherein the clad is made of a silicon oxide film, a polyimide resin, or an epoxy resin. A first core having a rectangular cross section perpendicular to the light propagation direction;
A second core having a rectangular cross section perpendicular to the light propagation direction, disposed so as to cover the first core;
An input waveguide core integrally formed with the first core, disposed at the input end of the first core;
An output waveguide core formed integrally with the first core, disposed at the output end of the first core;
The second core, the input waveguide core and the clad disposed to cover the output waveguide core,
The input waveguide core includes a flat input portion having a constant cross-sectional area, a connection portion connected to the first core and having the same cross-sectional shape as the first core, and the input portion A taper part in which the cross-sectional area gradually decreases while maintaining a constant height toward the connection part,
The output waveguide core includes a flat output portion having a constant cross-sectional area, a connection portion connected to the first core and having the same cross-sectional shape as the first core, and the connection portion from the connection portion. A taper portion where the cross-sectional area gradually increases while maintaining a constant height toward the output portion,
The refractive index of the first core, the input waveguide core and the output waveguide core is larger than the refractive index of the second core and the clad,
The refractive index of the second core is greater than the refractive index of the cladding;
The first Ri core center axis Do different in the light propagation direction and the center axis of the light propagation direction of the second core, the length of the light propagation direction of the second core, the light of linear polarization incident polarizations the first polarization rotation device characterized lengths der Rukoto rotating 90 degrees while propagating the second core. The polarization rotation element according to claim 3,
The first core, the input waveguide core and the output waveguide core are made of silicon,
The second core is made of a silicon nitride film, a silicon oxynitride film or a polymer material having a refractive index of 1.5 or more,
The polarization rotation element, wherein the clad is made of a silicon oxide film, a polyimide resin, or an epoxy resin. The polarization rotation element according to any one of claims 1 to 4,
A polarization separation element that separates incident light according to the polarization plane;
A first waveguide for guiding one of the polarization components separated by the polarization separation element to the polarization rotation element;
A second waveguide for guiding the other of the polarization components separated by the polarization separation element;
An optical circuit comprising: a multiplexing element that combines the polarization component that has passed through the polarization rotation element and the polarization component that has passed through the second waveguide.

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