A dual-band polarization beam splitter on a silicon substrate

Abstract

This invention discloses an on-chip dual-band polarization beamsplitter on a silicon substrate, belonging to the field of silicon-based photonics. It includes a dual-band mode multiplexer, a first Bragg grating, and a second Bragg grating. The dual-band mode multiplexer consists of two tapered waveguides, where the wider waveguide is the bus waveguide and the narrower waveguide is the access waveguide, separated by an S-bend. The Bragg grating is formed by etching holes internally and is used for mode conversion between the 1550nm and 2000nm bands, converting the forward-propagating TE0 mode to the backward-propagating TE1 mode, then demultiplexing the backward-propagating TE1 mode back into the TE0 mode in the access waveguide and separating it. The TM0 mode can be directly transmitted through the dual-band mode multiplexer and the Bragg grating, thus forming an on-chip dual-band polarization beamsplitter. This invention significantly improves the operating bandwidth of the device, meets the size requirements of integrated optoelectronic devices, and will be more flexibly applied in optical interconnect systems.

Description

Technical Field

[0001] This invention belongs to the field of silicon-based photonics, and more specifically, relates to a silicon-based on-chip dual-band polarization beam splitter. Background Technology

[0002] Photonic integration technology based on the silicon-on-insulator (SOI) platform has been widely applied in various fields such as communication and sensing due to its advantages, including low loss, compact structure, and compatibility with CMOS processes. These advantages allow the fabrication of photonic integrated circuits (PICs) using electronic devices. Initially, due to manufacturing limitations, the focus was mainly on large-section SOI waveguides to achieve low-loss optical transmission. For large-size ridge waveguides, the bending radius is generally above 100 μm, making it difficult to achieve ultra-miniature integrated photonic devices. In recent years, with continuous improvements in manufacturing processes, SOI waveguides have been miniaturized, enabling highly compact photonic integrated devices such as grating couplers and multimode interference couplers to be realized on-chip. However, silicon photonic waveguides typically exhibit birefringence, resulting in high polarization dependence. This birefringence introduces strong polarization sensitivity issues in PICs. For fiber optic communication systems, light from the fiber typically has random polarization states, thus the signal-to-noise ratio decreases after passing through a polarization-sensitive PIC. In theory, polarization insensitivity of PICs can be achieved through meticulous design; however, due to the nanoscale precision required for these high-contrast waveguides, it is technically difficult to achieve.

[0003] A common solution to the polarization sensitivity problem of silicon-based nanowaveguide-based photonic integrated devices (PICs) is to use polarization diversity techniques. The basic principle is as follows: the input light is split by a polarization beamsplitter, resulting in two polarized beams (transverse electric mode TE and transverse magnetic mode TM). One polarized beam (e.g., TM) is then converted into an orthogonal beam (TE) using a polarization rotator. The two beams, with the same polarization state, then enter two identical PICs. As a core component, the polarization beamsplitter plays a crucial role in achieving polarization-insensitive PICs. Furthermore, polarization beamsplitters are key devices in many applications, such as coherent optical communication, which has attracted considerable attention in long-distance fiber optic communication, improving spectral efficiency and thus enhancing communication system capacity. However, almost all current polarization beamsplitters cannot operate in two widely separated bands, limiting their future application in multi-band optical communication systems. Therefore, there is an urgent need to design a polarization beamsplitter that operates in widely separated bands to meet the requirements of future multi-band polarization multiplexing systems. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a dual-band polarization beamsplitter on a silicon substrate, aiming to increase the bandwidth of the polarization beamsplitter on the silicon substrate so that a single polarization beamsplitter can be used simultaneously for two discrete bands.

[0005] To achieve the above objectives, this invention provides an on-chip dual-band polarization beamsplitter fabricated on a silicon-based SOI wafer. The beamsplitter includes an on-chip dual-band mode multiplexer, a first Bragg grating, and a second Bragg grating. The dual-band mode multiplexer comprises two tapered waveguides of different widths, wherein the wider waveguide is a bus waveguide and the narrower waveguide is an access waveguide. The access waveguide is separated from the bus waveguide at both ends by S-bend waveguides, and the bus waveguide is directly connected to the first Bragg grating. The dual-band mode multiplexer is used to convert the backward propagation TE1 mode of the first and second bands into the TE0 mode in the access waveguide. The first Bragg grating is used to convert the forward propagation TE0 mode of the first band into the backward propagation TE1 mode, and the second Bragg grating is used to convert the forward propagation TE0 mode of the second band into the backward propagation TE1 mode. Preferably, the first band is a 1550nm band, and the second band is a 2000nm band.

[0006] During operation, the TE0 modes in the 1550nm and 2000nm bands are input from the bus waveguide of the dual-band mode multiplexer at the left end of the device. After passing through the dual-band mode multiplexer, they are converted into the backward-transmitted TE1 mode by the Bragg grating, then converted back into the TE0 mode in the waveguide by the dual-band mode multiplexer, and finally output through the S-bend waveguide. The TM0 modes in the 1550nm and 2000nm bands are input from the bus waveguide of the dual-band mode multiplexer at the left end of the device, pass directly through the dual-band mode multiplexer and the Bragg grating with low loss, and are finally output at the right port of the device.

[0007] As an improvement to the above technology, the dual-band mode multiplexer employs a dual-core tapered waveguide to demultiplex the backpropagating TE1 mode into a TE0 mode. The wider waveguide serves as the bus waveguide, the narrower waveguide as the access waveguide, and the S-waveguide is used to guide the demultiplexed TE0 mode. The tapered waveguide can increase the bandwidth of mode conversion and simultaneously increase the device's process tolerance.

[0008] As a further improvement to the above scheme, the Bragg grating is formed by etching two rows of periodically distributed holes internally. The two rows of holes are staggered by half a period to generate a phase difference of π, enabling the forward-propagating TE0 mode to be converted into the backward-propagating TE1 mode, and increasing the reflection bandwidth. These holes can be rectangular or circular. The size of the holes can be selected as needed, the period of the holes is calculated based on the Bragg reflection condition, and the duty cycle can be arbitrarily selected according to the requirements of the feature size.

[0009] As a further improvement to the above scheme, the Bragg periods of the first and second Bragg gratings are different, corresponding to the first and second wavelength bands respectively, and the larger the wavelength, the larger the period. The size of the period is calculated based on the Bragg reflection condition.

[0010] As a further improvement to the above scheme, the apertures inside the Bragg grating are apodized; that is, the period and width of the apertures along both sides of the middle of the Bragg grating gradually decrease, in order to reduce scattering loss and further increase the reflection bandwidth. Two Bragg gratings are cascaded, converting the forward-propagating TE0 modes of the two discrete bands into the backward-propagating TE1 modes, which are then demultiplexed into TE0 modes by a dual-band mode multiplexer and output from the access waveguide.

[0011] Compared with the prior art, the above-described technical solutions conceived in this invention can achieve the following results.

[0012] Beneficial effects:

[0013] (1) Compared with traditional polarization beam splitters, the present invention can be used for two discrete bands at the same time, and the size of the device is relatively small, which meets the size requirements of integrated optoelectronic devices and can be flexibly applied to on-chip multi-band polarization multiplexing systems.

[0014] (2) The multi-band mode multiplexer and Bragg grating used in this invention can be flexibly designed according to the two required bands, not limited to the 1550nm band and the 2000nm band. It has a certain degree of flexibility and scalability in design, and provides a new idea for the multi-band operation of polarization beam splitters.

[0015] (3) By performing apodization on the holes inside the Bragg grating, this invention achieves the goal of reducing scattering loss and increasing reflection bandwidth, providing a new approach for designing broadband Bragg reflectors.

[0016] (4) The silicon-based dual-band mode multiplexer of the present invention is based on mature semiconductor technology and only requires one etching step, making the manufacturing process simple. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the on-chip dual-band polarization beam splitter structure provided by the present invention.

[0018] Figure 2 This is the transmission status of the present invention in TE0 mode and TM0 mode during operation. (a) is the transmission status in TE0 mode, and (b) is the transmission status in TM0 mode.

[0019] Figure 3The transmission spectra of the TEO and TMEO modes in different ports are simulated according to the theory of this invention. (a) is the transmission spectrum of the TEO mode in the 1550nm band in different ports, (b) is the transmission spectrum of the TMEO mode in the 1550nm band in different ports, (c) is the transmission spectrum of the TEO mode in the 2000nm band in different ports, and (d) is the transmission spectrum of the TMEO mode in the 2000nm band in different ports.

[0020] Figure 4 The following are the optical field propagation conditions of the TEO and TMEO modes simulated by this invention: (a) is the optical field propagation condition of the 1550nm band TEO mode, (b) is the optical field propagation condition of the 1550nm band TMEO mode, (c) is the optical field propagation condition of the 2000nm band TEO mode, and (d) is the optical field propagation condition of the 2000nm band TMEO mode.

[0021] Figure 5 These are the transmission spectra of the TEO and TMEO modes at different ports as measured experimentally in this invention. (a) is the transmission spectrum of the TEO mode at 1550 nm at different ports, (b) is the transmission spectrum of the TMEO mode at 1550 nm at different ports, (c) is the transmission spectrum of the TEO mode at 2000 nm at different ports, and (d) is the transmission spectrum of the TMEO mode at 2000 nm at different ports. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0023] like Figure 1 As shown, the silicon-based dual-band polarization beam splitter provided by the present invention comprises a dual-band mode multiplexer 1, a first Bragg grating 2, and a second Bragg grating 3.

[0024] The dual-band mode multiplexer 1 consists of two tapered waveguides, where the wider waveguide is the bus waveguide and the narrower waveguide is the access waveguide. Its function is to demultiplex the TE1 modes propagating backward from the 1550nm and 2000nm bands into TE0 modes in the access waveguide. The first Bragg grating 2 and the second Bragg grating 3 are formed by etching two rows of holes inside the multimode waveguide. These two rows of holes are staggered by half a cycle to generate a phase difference of π. These holes can be rectangular or circular, and they are apodized to reduce insertion loss and increase the operating bandwidth. The first Bragg grating 2 and the second Bragg grating 3 respectively convert the forward-propagating TE0 modes of the 1550nm and 2000nm bands into backward-propagating TE1 modes.

[0025] like Figure 2 As shown in (a) and (b), when the TE0 mode is input in the 1550nm and 2000nm bands, it is converted into the backward propagating TE1 mode by the first Bragg grating 2 and the second Bragg grating 3, respectively. The backward propagating TE1 mode is then converted into the TE0 mode in the access waveguide by the dual-band mode multiplexer 1, and finally output at the left port of the access waveguide. When the TM0 mode is input in the 1550nm and 2000nm bands, it is directly output at the right port of the device through the dual-band mode multiplexer 1, the first Bragg grating 2, and the second Bragg grating 3.

[0026] The simulation results for this structure are as follows: Figure 3 and Figure 4 As shown in the figure. In the simulation, by selecting different polarizations for different bands, the simulated transmission spectra of the TE0 and TM0 modes in the 1550nm and 2000nm bands can be obtained at different ports. Figure 3 In the diagram, (a) is the transmission spectrum of the TE0 mode in the 1550nm band at different ports, (b) is the transmission spectrum of the TM0 mode in the 1550nm band at different ports, (c) is the transmission spectrum of the TE0 mode in the 2000nm band at different ports, and (d) is the transmission spectrum of the TM0 mode in the 2000nm band at different ports. Figure 4 The display shows the optical field propagation diagrams in TE0 and TM0 modes at 1550nm and 2000nm bands. Figure 4 (a) shows the optical field propagation simulated in the TE0 mode at 1550nm, (b) shows the optical field propagation simulated in the TM0 mode at 1550nm, (c) shows the optical field propagation simulated in the TE0 mode at 2000nm, and (d) shows the optical field propagation simulated in the TM0 mode at 2000nm.

[0027] The experimental test results for this structure are as follows: Figure 5As shown. In the experiment, by selecting different polarizations in different bands, the transmission spectra of TE0 and TM0 modes in the 1550nm and 2000nm bands were obtained by experimentally measuring them at different ports. Figure 5 In the diagram, (a) shows the transmission spectrum of the TE0 mode in the 1550nm band measured at different ports, (b) shows the transmission spectrum of the TM0 mode in the 1550nm band measured at different ports, (c) shows the transmission spectrum of the TE0 mode in the 2000nm band measured at different ports, and (d) shows the transmission spectrum of the TM0 mode in the 2000nm band measured at different ports. It can be seen that the TE0 and TM0 modes in the 1550nm and 2000nm bands are separated using this invention.

[0028] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A dual-band polarization beam splitter on a silicon substrate, characterized in that, The system includes a dual-band mode multiplexer (1), a first Bragg grating (2), and a second Bragg grating (3) cascaded in sequence. The dual-band mode multiplexer (1) includes two tapered waveguides of different widths, with the wider waveguide being the bus waveguide and the narrower waveguide being the access waveguide. The two ends of the access waveguide are separated from the bus waveguide by S-bend waveguides, and the bus waveguide is directly connected to the first Bragg grating (2). The dual-band mode multiplexer (1) is used to convert the backward propagation TE1 mode of the first and second bands into the TE0 mode in the access waveguide. The first Bragg grating (2) is used to convert the forward propagation TE0 mode of the first band into the backward propagation TE1 mode, and the second Bragg grating (3) is used to convert the forward propagation TE0 mode of the second band into the backward propagation TE1 mode. The first Bragg grating (2) and the second Bragg grating (3) form a phase difference of π by etching two rows of holes inside the multimode waveguide and staggering the two rows of holes by half a period.

2. The on-chip dual-band polarization beamsplitter according to claim 1, characterized in that, During operation, the TE0 mode of the first and second bands is input from the bus waveguide of the dual-band mode multiplexer (1), and after passing through the dual-band mode multiplexer (1), it is converted into the backward transmission TE1 mode by the first Bragg grating (2) and the second Bragg grating (3), and then converted into the TE0 mode in the access waveguide by the dual-band mode multiplexer (1), and output through the S-bend waveguide; the TM0 mode of the first and second bands is input from the bus waveguide of the dual-band mode multiplexer (1), directly passes through the dual-band mode multiplexer (1), the first Bragg grating (2) and the second Bragg grating (3), and is finally output.

3. The on-chip dual-band polarization beamsplitter according to claim 1, characterized in that, The hole can be rectangular or circular.

4. The on-chip dual-band polarization beamsplitter according to claim 1, characterized in that, The first Bragg grating (2) and the second Bragg grating (3) have different Bragg periods, corresponding to the first band and the second band respectively.

5. The on-chip dual-band polarization beamsplitter according to claim 4, characterized in that, The first band is the 1550nm band, and the second band is the 2000nm band.

6. The on-chip dual-band polarization beamsplitter according to claim 1, characterized in that, The period and width of the two end holes of the first Bragg grating (2) and the second Bragg grating (3) gradually decrease.

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