Multimode polarization-independent optical cross waveguide based on two-dimensional sub-wavelength grating structure

Abstract

The invention provides a multi-mode polarization independent optical cross waveguide based on a two-dimensional sub-wavelength grating structure. The multi-mode polarization independent optical cross waveguide comprises a substrate, a cladding, a first multi-mode input waveguide arm, a first multi-mode output waveguide arm, a second multi-mode input waveguide arm, a second multi-mode output waveguide arm and a central cross area, the waveguide arms and the central crossing area are integrated on the upper surface of the substrate, and the cladding covers the whole structure; each waveguide arm and the central crossing area have the same width and height, and each waveguide arm and the central crossing area form an orthogonal crossing layout. According to the invention, low-loss and low-crosstalk cross transmission of a fifth-order TE mode and a second-order TM mode in a broadband range can be supported at the same time. The cross waveguide is compact in structure, the preparation process is compatible with a standard CMOS (complementary metal oxide semiconductor) process, and the cross waveguide is suitable for a mode-polarization hybrid multiplexing on-chip optical interconnection system with high-density integration and high capacity.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of integrated optics, in particular to a multi-mode polarization-independent optical cross waveguide based on two-dimensional sub-wavelength grating structure. BACKGROUND

[0002] With the rapid development of optical communication, data center interconnection and on-chip photonic integration technology, photonic integrated circuits play an increasingly important role in high-speed, high-capacity and low-power optical communication systems. To improve the communication capacity of on-chip links, multi-dimensional multiplexing technologies such as wavelength division multiplexing, mode division multiplexing and polarization multiplexing are widely studied. Among them, mode-polarization hybrid multiplexing technology can simultaneously transmit multiple modes and two orthogonal polarization states at a single wavelength, significantly improving system capacity and integration, and becoming a current research hotspot.

[0003] In the mode-polarization hybrid multiplexing system, optical cross waveguide is an indispensable key element. However, the traditional cross structure generally has the problems of limited mode support, high polarization sensitivity, large insertion loss and crosstalk, narrow working bandwidth, complex structure and difficult process, etc. Therefore, it is of great significance to develop a compact, polarization-independent, multi-mode supporting, low insertion loss, low crosstalk and wide working bandwidth cross waveguide structure for promoting the development of high-density photonic integrated circuits. SUMMARY

[0004] The present application is to solve the problems of limited mode support, high polarization sensitivity, large insertion loss and crosstalk, narrow working bandwidth, complex structure and difficult process of traditional optical cross waveguide. The present application provides a multi-mode polarization-independent optical cross waveguide based on two-dimensional sub-wavelength grating structure, which is suitable for low-loss, low-crosstalk on-chip optical interconnection system supporting multiple modes and polarization states.

[0005] Technical scheme: A multi-mode polarization-independent optical cross waveguide based on two-dimensional sub-wavelength grating structure, comprising: a substrate, a cladding, a first multi-mode input waveguide arm, a first multi-mode output waveguide arm, a second multi-mode input waveguide arm, a second multi-mode output waveguide arm and a central cross region.

[0006] The upper surface of the substrate is integrated with each waveguide arm and the central cross region, and the cladding covers the entire structure; each waveguide arm and the central cross region have the same width and height, and each waveguide arm and the central cross region form an orthogonal cross layout.

[0007] Further, each waveguide arm and the central cross region form an orthogonal cross layout, specifically comprising:

[0008] The right port of the first multimode input waveguide arm is connected with the left port of the center cross region, the right port of the center cross region is connected with the left port of the first multimode output waveguide arm, the lower port of the second multimode input waveguide arm is connected with the upper port of the center cross region, and the lower port of the center cross region is connected with the upper port of the second multimode output waveguide arm.

[0009] Further, the first multimode input waveguide arm comprises a first two-dimensional subwavelength grating structure, the first two-dimensional subwavelength grating structure is composed of a cross-shaped silicon column array with constant period and constant duty cycle, and the cross-shaped silicon column is formed by etching square holes at four corners of a square silicon column.

[0010] Further, the first multimode output waveguide arm is obtained by rotating the first multimode input waveguide arm by 180 degrees counterclockwise around a center point, and the second multimode input waveguide arm and the second multimode output waveguide arm are obtained by rotating the first multimode input waveguide arm by 90 degrees and 270 degrees counterclockwise, respectively.

[0011] Further, the center cross region comprises a second two-dimensional subwavelength grating structure, the second two-dimensional subwavelength grating structure is composed of a hollow square silicon column array with constant period and constant duty cycle; the hollow square silicon column is obtained by forming a square groove by shallow etching in the center of a standard square silicon column, and the square groove presents a gradually changing size distribution with a small middle size and a large size at both ends in the transverse and longitudinal directions, thereby constructing a gradient refractive index distribution in the cross region.

[0012] Beneficial effects: The application comprises four multimode waveguide arms and a center cross region, which are integrated on a silicon-based substrate and covered with a cladding. The four waveguide arms are connected to the center cross region in a cross-shaped symmetry, wherein the ends of the waveguide arms are provided with a periodic two-dimensional subwavelength grating structure composed of cross-shaped silicon columns, and the center cross region is composed of a hollow square silicon column array, in which the size of the square groove in the center of each silicon column presents a gradually changing distribution with a small middle size and a large size at both ends in the transverse and longitudinal directions, forming an equivalent gradient refractive index distribution, thereby providing stronger light field constraint in the cross region. This design utilizes the spatial modulation of the two-dimensional subwavelength structure to realize efficient transmission of multi-order modes and different polarization states while maintaining four-fold rotational symmetry. The device can simultaneously support low-loss and low-cross-talk cross transmission of 5-order TE mode and 2-order TM mode in a wide band range. The cross waveguide structure is compact, the preparation process is compatible with the standard CMOS process, and is suitable for high-density integrated, large-capacity mode-polarization hybrid multiplexing on-chip optical interconnection systems.

[0013] (1) Multi-mode and polarization independent transmission: can support 5th order TE mode and 2nd order TM mode simultaneously, breaking the limitation of traditional cross waveguide which is limited to single mode or single polarization, greatly improving the information carrying capacity of single wavelength channel.

[0014] (2) Excellent optical performance: simulation shows that in the wavelength range of 1450~1650 nm, the insertion loss of all supported modes is lower than 0.8 dB, the bypass crosstalk is lower than –30 dB, and the inter-mode crosstalk is lower than –22 dB, with wideband, low loss and low crosstalk.

[0015] (3) Compact structure and process compatible: the overall size of the device is only 5×5 μm², which is suitable for high-density integration; the manufacturing is based on 220 nm SOI standard process, only one shallow etching (for forming gradient holes) and one deep etching (for defining waveguide profile) are needed, the process steps are simple, compatible with existing CMOS process, and conducive to large-scale manufacturing.

[0016] (4) The present application realizes low-loss and low-crosstalk transmission of multi-order modes under TE / TM dual polarization by introducing two-dimensional sub-wavelength structure and its spatial modulation in silicon-based waveguide, and the structure of the present application is simple, occupies small area, and is suitable for polarization and mode mixed multiplexing large-scale on-chip photonic integrated circuit. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 is a top view of the overall structure of a multi-mode polarization independent optical waveguide cross waveguide based on two-dimensional sub-wavelength grating structure proposed by the present application;

[0018] Figure 2 is a local enlarged view of the first multi-mode input waveguide arm;

[0019] Figure 3 is a schematic diagram of the two-dimensional sub-wavelength grating structure of the central cross region;

[0020] Figure 4 is a schematic diagram of the cross section of the multi-mode waveguide arm and the central cross region;

[0021] Figure 5 is a curve of the relationship between the transmission characteristics of the device and the working wavelength, wherein Figure 5 (a)-(e) in (a)-(e) are TE0-TE4 modes; Figure 5 (f)-(g) in (f)-(g) are TM0-TM1 modes;

[0022] Figure 6 is a distribution diagram of the electric field intensity along the propagation direction when each mode passes through the device at 1550 nm wavelength. DETAILED DESCRIPTION

[0023] The technical solutions of the present application will be further described in combination with the drawings and examples.

[0024] As shown in Figure 1 , the embodiment of the application proposes a two-dimensional sub-wavelength grating structure-based multi-mode polarization-independent optical waveguide cross-waveguide based on an SOI platform, which comprises a substrate 6 and a cladding layer 7. The upper surface of the substrate 6 is integrated with each waveguide arm and a central cross region, and the cladding layer covers the entire structure. Each waveguide arm and the central cross region have the same width and height. Specifically, the substrate 6 is provided with a first multi-mode input waveguide arm 1, a first multi-mode output waveguide arm 2, a second multi-mode input waveguide arm 3, a second multi-mode output waveguide arm 4, and a central cross region 5. The right port of the first multi-mode input waveguide arm 1 is connected to the left port of the central cross region 5, the right port of the central cross region 5 is connected to the left port of the first multi-mode output waveguide arm 2, the lower port of the second multi-mode input waveguide arm 3 is connected to the upper port of the central cross region 5, and the lower port of the central cross region 5 is connected to the upper port of the second multi-mode output waveguide arm 4, forming a cross layout. The width and height of each waveguide arm and the central cross region are consistent, and are all multi-mode waveguides supporting TE0-TE4 and TM0-TM1.

[0025] As shown in Figure 1 , the right end of the first multi-mode input waveguide arm 1 is provided with a two-dimensional sub-wavelength grating structure. The first multi-mode output waveguide arm 2 is obtained by counterclockwise rotating the first multi-mode input waveguide arm 1 by 180°, the second multi-mode input waveguide arm 3 is obtained by counterclockwise rotating the first multi-mode input waveguide arm 1 by 90°, and the second multi-mode output waveguide arm 4 is obtained by counterclockwise rotating the first multi-mode input waveguide arm 1 by 270°, so that the entire device has four-fold rotational symmetry. The waveguide arms are connected to the central region in a cross manner to form a "cross" structure.

[0026] As shown in Figure 2 , the first multi-mode input waveguide arm 1 is composed of a strip-shaped multi-mode waveguide 1-1 and a two-dimensional sub-wavelength grating structure 1-2 at the end of the strip-shaped multi-mode waveguide 1-1. The two-dimensional sub-wavelength grating structure 1-2 is composed of periodically arranged cross-shaped silicon pillars 1-2-1, and the period and duty cycle are constant. The cross-shaped silicon pillars 1-2-1 are formed by etching square holes of the same size on the four corners of the standard square silicon pillars.

[0027] As shown in Figure 3 , the central cross region 5 is provided with a two-dimensional sub-wavelength grating structure, which is periodically arranged by hollow square silicon pillars 5-1, and the period and duty cycle are constant. The hollow square silicon pillars 5-1 form square grooves by shallow etching in the center of the square silicon pillars, and the size of the square grooves gradually changes from small in the middle to large at both ends along the horizontal and vertical directions of the central cross region, thereby forming a gradient refractive index distribution in the cross region, and further enhancing the confinement ability of the optical field.

[0028] The optical cross waveguide supports simultaneous transmission of five TE0-TE4 transverse electric modes and two TM0-TM1 transverse magnetic modes, has the characteristics of large bandwidth, low loss, low crosstalk, compact structure and easy preparation.

[0029] Specifically, the transmission process of the optical signal in the optical cross waveguide is as follows: taking the TE mode (TE0, TE1, TE2, TE3, TE4) or the TM mode (TM0, TM1) as an example, when the optical signal is input from the first multimode input waveguide arm 1, the two-dimensional subwavelength grating structure at the end of the second multimode input waveguide arm 3 and the second multimode output waveguide arm 4 can constrain the transmission of the optical signal in the central cross region, effectively suppressing the leakage of the optical field to the adjacent waveguide arms; at the same time, together with the gradient refractive index of the central cross region 5, it provides further mode constraint when the optical wave passes through, forms strong transverse optical field constraint, greatly suppresses the coupling leakage of energy to the vertical direction waveguide, and reduces crosstalk. The optical signal is then coupled to the first multimode output waveguide arm 2 efficiently, completing the cross transmission. Since the optical cross waveguide structure has four-fold rotational symmetry, the reverse or other port incidence is the same.

[0030] As shown in Figure 5 In the wavelength range of 1450-1650 nm, the insertion loss of all 7 modes is less than 0.8 dB, the bypass crosstalk is less than -30 dB, and the intermodal crosstalk is less than -22 dB, showing excellent broadband, low loss and high isolation characteristics.

[0031] Figure 6 The figure is the electric field distribution diagram of each mode passing through the device at 1550 nm wavelength. The results show that the broad square of the 7 modes is effectively limited in the target path, without obvious bypass leakage, further confirming the high efficiency of the cross ability of the embodiment of the application to the multimode polarization signal.

Claims

1. A multi-mode polarization independent optical cross-waveguide based on two-dimensional subwavelength grating structure, characterized in that: The application relates to a substrate, a cladding layer, a first multimode input waveguide arm, a first multimode output waveguide arm, a second multimode input waveguide arm, a second multimode output waveguide arm and a central crossing region. The waveguide arms and the central crossing region are integrated on the upper surface of the substrate, and the cladding layer covers the whole structure; the waveguide arms and the central crossing region have the same width and height, and the waveguide arms and the central crossing region form a right-angle crossing layout. The waveguide arms and the central crossing region form a right-angle crossing layout, and specifically include the following steps:

2. The multi-mode polarization-independent optical cross-waveguide based on two-dimensional subwavelength grating structure according to claim 1, characterized in that: The right port of the first multimode input waveguide arm is connected with the left port of the central crossing region, the right port of the central crossing region is connected with the left port of the first multimode output waveguide arm, the lower port of the second multimode input waveguide arm is connected with the upper port of the central crossing region, and the lower port of the central crossing region is connected with the upper port of the second multimode output waveguide arm. The first multimode input waveguide arm comprises a first two-dimensional subwavelength grating structure, the first two-dimensional subwavelength grating structure is composed of a cross-shaped silicon column array with constant period and constant duty ratio, and the cross-shaped silicon column is formed by etching square holes at four corners of a square silicon column.

3. The multi-mode polarization independent optical cross-waveguide based on two-dimensional subwavelength grating structure according to claim 1, characterized in that: The first multimode output waveguide arm is obtained by rotating the first multimode input waveguide arm 180 degrees counterclockwise around a central point, and the second multimode input waveguide arm and the second multimode output waveguide arm are respectively obtained by rotating the first multimode input waveguide arm 90 degrees and 270 degrees counterclockwise.

4. The multi-mode polarization-independent optical cross-waveguide based on two-dimensional subwavelength grating structure according to claim 1 or 3, characterized in that: The central crossing region comprises a second two-dimensional subwavelength grating structure, the second two-dimensional subwavelength grating structure is composed of a hollow square silicon column array with constant period and constant duty ratio; the hollow square silicon column is obtained by forming a square groove through shallow etching at the center of a standard square silicon column, and the square groove presents a gradually changing size distribution with small middle size and large size at two ends in both the transverse direction and the longitudinal direction, so that a gradient refractive index distribution is constructed in the crossing region.

5. The multi-mode polarization independent optical cross-waveguide based on two-dimensional subwavelength grating structure according to claim 1, characterized in that: ​

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