TWI843248B - Optical mode converter - Google Patents

Abstract

An optical mode converter, including a straight waveguide body and a zigzag recess, is provided. The zigzag recess is formed on a surface of the straight waveguide body in a block shape and extends across the straight waveguide body along a direction perpendicular to a transmission direction. The zigzag recess includes a first recess unit and a second recess unit which are adjacent and opposite to each other. The first recess unit and the second recess unit each includes a rectangular block, and a first outer protrusion and a second outer protrusion which are respectively formed on two opposite sides of the rectangular block. The first outer protrusion has a triangular profile and the second outer protrusion has a fork-shaped profile.

Description

Optical Mode Converter

The present invention relates to an optical mode converter, and in particular to an optical mode converter having a zigzag block-shaped groove.

In modern times, the demand and volume of data transmission (such as images and cloud data) have increased dramatically, and optical fiber networks and database centers (DCNs data center networks) urgently need to improve the speed and efficiency of optical signal transmission.

In terms of materials, silicon has the characteristics of high refractive index, making it a better choice for operating optical signal output. In addition, the silicon photonics process uses SOI (Silicon On Insulator) technology, which is highly compatible with the current mature complementary metal oxide semiconductor (CMOS) technology. Silicon photonics with high transmission rate and transmission capacity will replace the copper wires facing the bottleneck.

In the area of optical signal transmission technology, multiplexing technology (also known as multiplexing technology) has begun to be used to provide more bandwidth. However, for example, Wavelength Division Multiplexing (WDM) relies on the characteristics of low-loss optical fibers and optical signal amplifiers, so it is easy to reach hardware limitations; Polarization Division Multiplexing (PolM) can only use the polarization of TE waves (Transverse Electro waves) and TM waves (Transverse Magnetic waves) to increase the transmission capacity by a factor of 2, which is limited.

Therefore, Mode Division Multiplexing (MDM), which is often used in the field of fiber optics or free space optics, is being tried in the planar waveguide system of silicon photonics. By transmitting optical signals of different modes simultaneously, the transmission efficiency is improved and the high-speed computing requirements are met.

As shown in Figure 1, an optical signal transmission system using mode multiplexing must have a mode converter and a mode (de)multiplexer to connect transmitters Tx1, Tx2 and receivers Rx1, Rx2. The mode converter is particularly important. Its form may include the Y-branch mode converter 2 in Figure 2A, the asymmetric directional coupler 4 (ADC) in Figure 2B, and the multimode interference coupler 6 (MMI multimode interference couplers) in Figure 2C. The Y-branch form can achieve a smaller size, but the sharp angle at the input makes this type of mode converter difficult to manufacture. Both asymmetric directional couplers and multimode interference couplers are difficult to reduce in size.

The zigzag mode converter 8 shown in FIG. 2D can be compressed to a few micrometers ( μm ) in size and achieve a transmission loss of less than 1 dB. However, the multiple sharp corners (e.g., 20-degree sharp corners) generated by its periodic perturbation waveguide structure are difficult to complete during the manufacturing process. Moreover, the more periodic structures there are (e.g., the 6 repeated zigzag structures in the figure), the more sharp corners there are, and the greater the impact on the process yield. In addition, the periodic perturbation waveguide structure needs to be defined by at least seven parameters, namely: period a, length b, length c, waveguide width d, duty cycle, number of zigzags, and depth of zigzag recesses, which increases the difficulty in design.

Therefore, how to improve the process yield while maintaining small size and low loss has become a topic of concern in the industry.

It should be noted that the above technical contents are used to help understand the problems to be solved by the present invention, and they are not necessarily disclosed or known in the art.

One purpose of the present invention is to provide an optical mode converter that has low loss and better process yield while maintaining a small size.

The optical mode converter provided by the present invention may include a straight waveguide body and a meandering groove. The meandering groove is formed in a block shape on a surface of the straight waveguide body and penetrates the straight waveguide body along a direction perpendicular to a transmission direction. The meandering groove includes a first groove unit and a second groove unit adjacent to each other and in opposite directions, and the first groove unit and the second groove unit each include a rectangular block, and a first outer protrusion block and a second outer protrusion block formed on both sides of the rectangular block, respectively, and the first outer protrusion block has a triangular profile, and the second outer protrusion block has a fork-shaped profile.

By implementing the perturbation structure of a mode converter with a zigzag groove, it is beneficial to complete the mode conversion in a smaller space, so that the optical mode converter can have a small size. Furthermore, since the zigzag groove is block-shaped and there is no other structure in the groove, only the structure of the outer contour of the zigzag groove needs to be considered during manufacturing. In this way, fewer parameters are used to determine the zigzag block groove, and the optimization algorithm can be used in a shorter time to make the mode conversion loss less than 1dB, thereby shortening the design time, reducing the risk of process errors and having a better process yield.

In one embodiment, the first groove unit and the second groove unit have the same volume.

In one embodiment, the first outer protrusion and the second outer protrusion of the first groove unit and the second groove unit respectively have the same volume.

In one embodiment, the meandering groove is defined according to the following four parameters: a horizontal distance between an outer vertex of the first outer protrusion of the first groove unit and an outer vertex of the first outer protrusion of the second groove unit, a height of the first outer protrusion of the first groove unit, a width of the straight waveguide body, and a depth of the meandering groove.

In one embodiment, the horizontal distance between the outer vertex of the first outer protrusion of the first groove unit and the outer vertex of the first outer protrusion of the second groove unit is 3.12 μm , the height of the first outer protrusion of the first groove unit is 0.775 μm , the width of the straight waveguide body is 1.2 μm , and the depth of the zigzag groove is 0.089 μm .

In one embodiment, the straight waveguide body is disposed on a buffer layer and is covered by a cladding layer.

In one embodiment, the zigzag groove further includes a third groove unit adjacent to and opposite to the second groove unit, the third groove unit is located on the opposite side of the first groove unit, and the third groove unit includes a rectangular block, and a first outer protrusion block and a second outer protrusion block formed on both sides of the rectangular block, respectively, and the first outer protrusion block has a triangular profile, and the second outer protrusion block has a fork-shaped profile.

In one embodiment, the zigzag groove further includes a fourth groove unit adjacent to and opposite to the third groove unit, the fourth groove unit is located on the opposite side of the second groove unit, and the fourth groove unit includes a rectangular block, and a first outer protrusion block and a second outer protrusion block formed on both sides of the rectangular block, respectively, and the first outer protrusion block has a triangular profile, and the second outer protrusion block has a fork-shaped profile.

2: Traditional Y-bifurcation mode converter

4: Traditional asymmetric directional coupler

6: Traditional multimode interference coupler

8: Traditional modal converter for tortuous structures

10, 10’: Optical mode converter

100: Straight waveguide body

110: Surface of the straight waveguide body

120: long side

130: long side

200, 200’: Zigzag groove

210: First groove unit

212: Rectangular block

214: First outer protrusion

216: Second outer protrusion

220: Second groove unit

222: Rectangular block

224: First outer protrusion

226: Second outer protrusion

230: Third groove unit

300: Buffer layer

400: Coating layer

D1: Transmission direction

D2: vertical transmission direction

L1: horizontal distance

L2: Height

P1: Outer vertex

P2: Outer vertex

TE ₀ , TE ₁ , TE ₂ : transverse magnetic wave mode

Tx1, Tx2: Transmitter

Rx1, Rx2: Receiver

a: Cycle

b: Length

c: Length

d: Waveguide width

d1: depth

w: width

θ: half angle

Figure 1 is a schematic diagram of a conventional optical split mode multiplexing system; Figures 2A to 2D are schematic diagrams of different conventional optical mode converters; Figures 3A to 3B are schematic diagrams of a top view and a side view in the longitudinal direction of an optical mode converter of an embodiment of the present invention; Figures 4A to 4B are schematic diagrams of the separation and combination of two groove units of an optical mode converter of an embodiment of the present invention; Figure 5 is Figure 6 is a diagram of the mode conversion transmittance of the optical mode converter of one embodiment of the present invention; Figure 7 is a schematic diagram of the optical mode converter of one embodiment of the present invention from a top view; Figure 8 is a schematic diagram of the optical mode converter of one embodiment of the present invention from a three-dimensional view; and Figure 9 is a schematic diagram of the width direction of the optical mode converter of one embodiment of the present invention from a side view.

The following will describe specific embodiments of the present invention; however, without departing from the spirit of the present invention, the present invention can be implemented in a variety of different forms of embodiments, and the scope of protection of the present invention should not be interpreted as limited to those described in the specification.

Unless the context clearly indicates otherwise, the singular forms "a", "the" and similar terms used herein also include the plural forms, and the terms "first", "second", etc. are used herein to refer to various components or features, rather than to indicate that these components or features have a necessary order or priority. In addition, the directions (such as front, back, top, bottom, left, right, side, etc.) described are relative positions, which can be defined according to the placement state of the optical mode converter, and do not indicate or imply that the structure or feature needs to be placed in a specific direction, nor can it be understood as a limitation of the present invention.

The dimensions, ratios and sizes in the figures are for convenience only and are not intended to be limiting.

The optical mode converter 10 of the present invention is an improved mode converter compatible with semiconductor processes. In some embodiments, its operating wavelength may include the C band (1530nm~1565nm), and has advantages such as small size and improved process yield. The optical mode converter 10 may include a straight waveguide body 100 and a zigzag groove 200.

FIG. 3A is a schematic top view of an optical mode converter 10 of an embodiment of the present invention. FIG. 3B is a schematic side view of an optical mode converter 10 of an embodiment of the present invention. The optical mode converter can convert the transverse magnetic wave mode TE ₀ into the transverse magnetic wave mode TE ₁ . As shown in FIG. 3A and FIG. 3B , the optical mode converter 10 may include a straight waveguide body 100 and a meandering groove 200. The straight waveguide body 100 may be in a long rectangular shape, have a width w, and have a surface 110 and two long sides 120 and 130 connected to the surface 110 and opposite to each other. In this embodiment, the light wave has a transmission direction D1 in the straight waveguide body 100. The zigzag groove 200 is formed in a block-like manner on the surface 110 of the straight waveguide body 100 and occupies at least a portion of the surface 110. It is recessed downward from the surface 110 by a depth d1 and penetrates the two long sides 120 and 130 of the straight waveguide body 100 along a direction D2 perpendicular to the transmission direction D1 (along a direction parallel to a width of the straight waveguide body 100). When viewed from a top view of the straight waveguide body 100, the zigzag groove 200 presents a zigzag shape.

In detail, please refer to FIG. 4A-4B. If the zigzag groove 200 is cut symmetrically along the transmission direction D1, the zigzag groove 200 may be equivalent to including a first groove unit 210 and a second groove unit 220 adjacent to each other and closely connected. The first groove unit 210 includes a rectangular block 212, a first outer protrusion block 214 formed on the left side of the rectangular block 212, and a second outer protrusion block 216 formed on the right side of the rectangular block 212 (the different patterns are only for the convenience of understanding, and are not used to indicate that it is divided into three independent blocks). The rectangular block 212 may have an outer contour of a square or a rectangle, the first outer protrusion block 214 may have a triangular contour, such as an isosceles triangle contour, and the second outer protrusion block 216 may have a fork-shaped contour or an M-shaped contour, such as two triangles of equal volume connected to each other at a vertex. In some embodiments, the first outer protrusion block 214 may have the same volume as the second outer protrusion block 216, but is not limited thereto. In this embodiment, the first groove unit 210 and the second groove unit 220 have the same volume, and both have the same but horizontally opposite outer contours. That is, the second groove unit 220 also includes a rectangular block 222, a second outer protrusion 226 formed on the left side of the rectangular block 222, and a first outer protrusion 224 formed on the right side of the rectangular block 222, and the first outer protrusion 224 can have the same volume as the second outer protrusion 226. In addition, a vertex where a first outer protrusion 214 of the first groove unit 210 is connected to the rectangular block 212 is connected to a vertex of the second outer protrusion 226 of the second groove unit 220 to form a zigzag groove 200.

Please refer to Figure 3A and Figure 3B again. When the first groove unit 210 and the second groove unit 220 have the same volume, the zigzag groove 200 of the optical mode converter 10 of the present invention can be defined by the following four parameters: a horizontal distance L1 between an outer vertex P1 of the first outer protrusion 214 of the first groove unit 210 and an outer vertex P2 of the first outer protrusion 224 of the second groove unit 220, a height L2 of the first outer protrusion 214 of the first groove unit 210, a width w of the straight waveguide body 100, and a depth d1 of the zigzag groove 200.

The above four parameter values can be optimized by various known optimization algorithms. For example, the above four parameter values can be determined by using a genetic algorithm based on the desired mode conversion loss value optimization process. For example, in one embodiment, when the horizontal distance L1 is 3.12μm, the height L2 is 0.775μm, the width w is 1.2μm, and the depth d1 is 0.089μm, the input loss is 0.01dB, the output loss is 0.29dB, the mode conversion loss is 0.68dB, and the mode conversion penetration rate in the C band reaches 85.5% (as shown in Figure 5). The field simulation diagram can refer to Figure 6. At this time, the half angle θ of the first outer protrusion 214 of the first groove unit 210 is 21.16°, and is the same as the half angle θ of the angle between the two triangular blocks included in the second outer protrusion 226 of the second groove unit 220.

In some embodiments, the zigzag groove 200 may include one or more other groove units in addition to the first groove unit 210 and the second groove unit 220, so as to convert the transverse magnetic wave mode TE ₀ into other types of modes other than the transverse magnetic wave mode TE _1. In one embodiment, such as shown in FIG. 7, the zigzag groove 200' of the optical mode converter 10' of the present invention may include a third groove unit 230 in addition to the first groove unit 210 and the second groove unit 220, so as to convert the transverse magnetic wave mode TE ₀ into the transverse magnetic wave mode TE _2. Similarly, in other embodiments, the zigzag groove 200' may further include a fourth and a fifth groove unit (not shown) ... etc., so as to convert the transverse magnetic wave mode TE ₀ into the transverse magnetic wave mode TE ₃ , TE ₄ ... etc. Among them, these groove units can have the same volume, each groove unit is opposite to its adjacent groove unit (horizontally flipped), but in the same direction as the next groove unit of its adjacent groove unit. Therefore, the third groove unit 230 is in the same direction as the first groove unit 210, the fourth groove unit is in the same direction as the second groove unit 220 but in the opposite direction to the first groove unit 210, and so on.

Although in the above embodiments, each groove unit has the same volume, the present invention is not limited to this. In other words, in some embodiments, the multiple groove units included in the zigzag groove of the present invention may also have different volumes, for example, the height of the first outer protrusion of the first groove unit is different from the height of the first outer protrusion of the second groove unit, but the contour and depth of the zigzag groove can still be determined by the optimization algorithm, thereby obtaining the advantages of good penetration rate and improved process yield.

FIG8 and FIG9 respectively illustrate a three-dimensional schematic diagram and a side view schematic diagram in the width direction of an optical mode converter 10' of an embodiment of the present invention. As shown in FIG8 and FIG9, the straight waveguide body 100 is disposed on a buffer layer 300, the meandering groove 200' has three groove units, and the straight waveguide body 100 is coated by a cladding layer 400. In some embodiments, the cladding layer 400 may be composed of silicon dioxide ( _SiO2 ).

In summary, the optical mode converter of the present invention has a smaller size than asymmetric directional couplers and multimode interference couplers, and can be used in compact devices. Moreover, because it has a block-shaped zigzag groove, it has fewer sharp corners than Y-branch form and periodic structure mode converters, which can greatly shorten the design time, reduce human errors and reduce the risk of process errors. In addition, the zigzag groove of the present invention only requires four parameters to define, which is conducive to accelerating the processing of the optimization algorithm, so it has the effect and characteristics of high transmission efficiency and small structure.

The above embodiments are only used to illustrate the implementation of the present invention and to explain the technical features of the present invention, and are not used to limit the scope of protection of the present invention. Therefore, the changes or equivalent arrangements that can be easily completed by those with ordinary knowledge in the technical field based on the above disclosure should also fall within the scope advocated by the present invention, and the scope of protection of the present invention should be based on the scope of the patent application.

10:Optical mode converter

100: Straight waveguide body

110: Surface of the straight waveguide body

120: long side

130: long side

200: Zigzag groove

D1: Transmission direction

D2: vertical transmission direction

L1: horizontal distance

L2: Height

P1: Outer vertex

P2: Outer vertex

TE ₀ , TE ₁ : transverse magnetic wave mode

w: width

θ: half angle

Claims (7)

An optical mode converter compatible with semiconductor processes includes: a straight waveguide body; the converter has only a single meandering groove, which has no other structure, is formed in a block shape on a surface of the straight waveguide body, and penetrates the straight waveguide body along a direction perpendicular to a transmission direction, wherein the meandering groove includes a first groove unit and a second groove unit adjacent to each other and in opposite directions, and each of the first groove unit and the second groove unit includes a rectangular block, and a first outer protrusion block and a second outer protrusion block formed on both sides of the rectangular block, respectively, the first outer protrusion block has a triangular profile, and the second outer protrusion block has a fork-shaped profile. An optical mode converter as described in claim 1, wherein the first groove unit and the second groove unit have the same volume. An optical mode converter as described in claim 1, wherein the first outer protrusion and the second outer protrusion of the first groove unit and the second groove unit respectively have the same volume. An optical mode converter as described in claim 1, wherein the meandering groove is defined according to the following four parameters: a horizontal distance between an outer vertex of the first outer protrusion of the first groove unit and an outer vertex of the first outer protrusion of the second groove unit, a height of the first outer protrusion of the first groove unit, a width of the straight waveguide body, and a depth of the meandering groove. An optical mode converter as described in claim 1, wherein the straight waveguide body is disposed on a buffer layer and is covered by a cladding layer. An optical mode converter as described in claim 1, wherein the zigzag groove further includes a third groove unit adjacent to and opposite to the second groove unit, the third groove unit is located on the opposite side of the first groove unit, and the third groove unit includes a rectangular block, and a first outer protrusion block and a second outer protrusion block formed on both sides of the rectangular block, respectively, the first outer protrusion block has a triangular profile, and the second outer protrusion block has a fork-shaped profile. An optical mode converter as described in claim 6, wherein the zigzag groove further includes a fourth groove unit adjacent to and opposite to the third groove unit, the fourth groove unit is located on the opposite side of the second groove unit, and the fourth groove unit includes a rectangular block, and a first outer protrusion block and a second outer protrusion block formed on both sides of the rectangular block, respectively, the first outer protrusion block has a triangular profile, and the second outer protrusion block has a fork-shaped profile.

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