Broadband and low-loss compact silicon-based polarizer based on sub-wavelength grating assisted total reflection angle mirror

Abstract

The invention relates to a broadband and low-loss compact silicon-based polarizing device based on a sub-wavelength grating assisted total reflection angle mirror, which is characterized in that the silicon-based polarizing device comprises a substrate layer, an oxide lower cladding and a silicon waveguide layer from bottom to top, the silicon waveguide layer comprises a nonlinear conical structure and a cascade angle mirror silicon waveguide from left to right in sequence, and four groups of silicon sub-wavelength gratings and a nonlinear conical structure. The scheme has the characteristics of compact size, high extinction ratio, large bandwidth and low insertion loss, and is suitable for various photon integrated loop waveguide functional devices based on a TE mode in the fields of optical communication, laser radar and the like.

Description

Technical Field

[0001] The present invention belongs to the technical fields of integrated photonics and optical communication, and particularly relates to a broadband, low-loss, compact silicon-based polarization device based on a sub-wavelength grating-assisted total internal reflection angle mirror, and more particularly to a broadband, low-loss, compact silicon-based polarizer. Background Art

[0002] In a Photonic Integrated Circuit (PIC) system, polarization management devices are core components for realizing efficient optical signal regulation, and their performance directly affects key indicators in applications such as optical communication and lidar. Silicon-based polarizers have attracted much attention due to their advantages such as compatibility with CMOS processes and easy integration.

[0003] Traditional silicon-based polarizers mostly rely on strongly birefringent waveguides or asymmetric directional couplers (ADCs) to achieve polarization selection. For example, polarizers based on shallow-etched waveguides can filter modes through strong polarization-dependent losses, but the device size is large (length ≥ 100 μm), making it difficult to meet the requirements of high-density integration. While the ADC-based scheme can reduce the size, it has problems such as narrow bandwidth (<100 nm) and high wavelength sensitivity, which limit its application in broadband systems. Xu Hongnan et al. proposed a cascaded total internal reflection angle mirror structure polarizer in 2021, which selectively reflects TE / TM modes. However, the extinction ratio of a single angle mirror structure is limited, and multiple stages need to be cascaded to improve the performance, resulting in an increase in the total device length (>80 μm) and high insertion loss (IL > 1.2 dB). At the same time, this design is sensitive to the tolerance of the tilt angle θ (±5°), and process errors are likely to cause performance degradation.

[0004] In summary, existing silicon-based polarizers are difficult to simultaneously consider key indicators such as extinction ratio, bandwidth, size, and process tolerance, which restricts their application in high-density integrated chips. There is an urgent need for an innovative design that can reduce the process complexity through structural optimization while ensuring low loss and wide bandwidth, and achieve a compact layout. Summary of the Invention

[0005] The object of the present invention is to provide a silicon integrated photonics integrated waveguide polarization management device based on an SOI platform to solve the problems of low extinction ratio, large size, complex process, and limited bandwidth existing in existing waveguide polarizers. The present invention combines the respective advantages of three structures: adiabatic coupling, cascaded reflection, and subwavelength filtering, and innovatively proposes a polarization control structure based on "nonlinear adiabatic tapered waveguide - cascaded total reflection angle mirror - chirped subwavelength grating (SWG)". The angle mirror structure is adopted to utilize the total reflection effect to reduce the loss of TE light, ensuring manufacturing friendliness. At the same time, a silicon subwavelength grating layer is introduced to enhance the birefringence effect between TE and TM, breaking through the minimum size limit of the subwavelength structure and the overall size limit of the total reflection structure, being friendly to process manufacturing, and having the characteristics of compact size, high extinction ratio, large bandwidth, and low insertion loss, and is suitable for various PIC waveguide polarization management devices based on the TE mode in the fields of optical communication and lidar, etc.

[0006] To solve the problems existing in existing waveguide polarizers, the solution of the present invention is as follows:

[0007] The silicon waveguide and grating layer of the silicon-based polarizer with the total reflection angle mirror as the structure include three structures: 1. The adiabatic tapered waveguide is used to transmit TE and TM mode light; 2. The total reflection angle mirror reflects TE mode light and scatters TM mode light through the way of strong birefringence; 3. The chirped subwavelength grating enhances the refractive index difference between TE and TM mode light.

[0008] The broadband, low-loss and compact silicon-based polarizing device based on a subwavelength grating-assisted total reflection angle mirror includes a silicon substrate layer, an oxide undercladding layer and a silicon waveguide layer from bottom to top.

[0009] The material of the oxide undercladding layer of the polarizer is silicon dioxide, and the refractive index should be less than that of the silicon in the polarizer. The thickness of the undercladding layer is at least 2μm and provides certain optical isolation and mechanical support.

[0010] The adiabatic tapered waveguide adopts a tapered structure with a nonlinearly varying width to achieve the adiabatic transition between the input fundamental mode and the slab mode, reduce the mode conversion loss, and at the same time optimize the waveguide width to ensure the near-lossless transmission of the TE mode.

[0011] The cascaded total reflection angle mirror is based on the strong birefringence characteristic of the silicon waveguide. By optimizing the angle θ of the angle mirror, the TE mode undergoes total internal reflection, while the TM mode is scattered because it does not meet the total reflection critical angle condition, significantly improving the single-stage extinction ratio. The cascaded structure further enhances the ability to suppress the TM mode. Setting the waveguide gap distance effectively avoids the interference between scattered TMs.

[0012] Integrate chirped SWGs with gradually decreasing duty cycles outside the angular mirror reflection interface, use the equivalent medium theory to regulate the equivalent refractive index, enhance the scattering loss of the TM mode, and optimize parameters such as the duty cycle and the minimum period, while achieving broadband, high extinction ratio, and low insertion loss at the same time.

[0013] The waveguide layer adopts a full etching process, and the waveguide and the sub-wavelength grating are completely etched through.

[0014] Compared with the existing inventions, the present invention has the following advantages and positive effects:

[0015] The present invention is based on a total reflection cascaded angular mirror with a micron-scale size and has low requirements for micro-nano process manufacturing. The present invention adopts the standard SOI process, with a minimum feature size ≥ 210 nm and the parameter tolerance relaxed to ±50 nm, greatly reducing the manufacturing difficulty and cost.

[0016] Compared with the basic structure, the present invention introduces a chirped sub-wavelength grating to improve the extinction ratio of the unit structure and significantly reduces the overall size of the polarizer. The total device length ≤ 45 μm, which is reduced by 45% compared with other structures.

[0017] Through comprehensive optimization of the structural parameters, the present invention obtains an extremely low insertion loss while having a high extinction ratio.

[0018] In this example, the silicon-based polarizer is suitable for CMOS-compatible processes, can be integrated with existing semiconductor processes, and uses existing manufacturing technologies to further reduce costs and improve manufacturing reliability. The present invention breaks through the limitations of traditional polarizers that rely on multiple cascades and complex structures. Due to the relatively large size of the total reflection angular mirror, the characteristic size of the sub-wavelength grating structure interacting with it increases to 210 nm, greatly reducing the process difficulty. This solution provides an efficient and reliable technical solution for polarization control in integrated photon systems, is suitable for high-density photon integration chips, and meets the polarization management requirements of core devices such as optical communication and lidar. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] By reading the detailed description of the selected embodiments below, other features, advantages, and benefits of the present invention will become clear to those skilled in the art. The drawings are only used to further specifically describe the detailed description of the selected embodiments and are not considered to be a limitation of the present invention.

[0020] Figure 1 A 3D schematic diagram of a broadband, low-loss, compact silicon-based polarizing device based on a total reflection angular mirror assisted by a sub-wavelength grating provided for a specific embodiment of the present invention;

[0021] Figure 2 A top view schematic diagram of a broadband, low-loss, compact silicon-based polarizing device based on a total reflection angular mirror assisted by a sub-wavelength grating provided for a specific embodiment of the present invention;

[0022] Figure 3 Optical simulation diagram of the TE mode of a broadband, low-loss and compact silicon-based polarization device based on a subwavelength grating-assisted total reflection angle mirror provided for the specific embodiment of the present invention near the wavelength of 1550 nm;

[0023] Figure 4 Optical simulation diagram of the TM mode of a broadband, low-loss and compact silicon-based polarization device based on a subwavelength grating-assisted total reflection angle mirror of the present invention near the wavelength of 1550 nm;

[0024] Figure 5 Transmittance of the TE mode light of a broadband, low-loss and compact silicon-based polarization device based on a subwavelength grating-assisted total reflection angle mirror of the present invention in the wavelength range of 1470 - 1675 nm.

[0025] Figure 6 Transmittance of the TM mode light of a broadband, low-loss and compact silicon-based polarization device based on a subwavelength grating-assisted total reflection angle mirror of the present invention in the wavelength range of 1470 - 1675 nm.

[0026] In the figure: 10, substrate layer; 20, oxide undercladding; 30, nonlinear tapered structure; 40, cascaded angle mirror silicon waveguide; 50, silicon subwavelength grating. Specific Embodiment

[0027] The present invention will be further clarified below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the following specific embodiments are only used to illustrate the present invention and not to limit the scope of the present invention.

[0028] Embodiment: Figure 1 Shown is a 3D structure diagram of a broadband, low-loss and compact silicon-based polarizer based on a subwavelength grating-assisted total reflection angle mirror provided in this example, including, from bottom to top, a substrate layer 10, an oxide undercladding 20, and a silicon waveguide layer (30, 40 and 50). The silicon waveguide layer, from left to right, is successively a nonlinear tapered structure 30, a cascaded angle mirror silicon waveguide structure 40, 4 groups of silicon subwavelength gratings 50, and a nonlinear tapered structure 30.

[0029] Figure 1The underlying layer 10 is made of silicon and serves as a support for the entire chip. The oxide cladding layer 20 is made of silicon dioxide, and its refractive index is less than that of the material used for the silicon waveguide layer. In this preferred example, the thickness of the oxide undercladding layer 20 is 2 μm, and the thickness of the silicon waveguide layer is 0.22 μm. The nonlinear adiabatic tapered waveguide structure 30 is connected to both ends of the cascaded corner mirror silicon waveguide 40 and is used to transmit optical signals with TE and TM polarizations. In this preferred example, the silicon subwavelength grating 50 is close to the reflection interface of the four cascaded corner mirror silicon waveguides 40 and is perpendicularly aligned with the reflection interface. The silicon waveguide and grating layer of the silicon-based polarization device includes three structures: an adiabatic tapered waveguide, a total reflection corner mirror, and a chirped subwavelength grating; among them, the adiabatic tapered waveguide is used to transmit optical signals in TE and TM modes; the total reflection corner mirror reflects TE-mode light and scatters TM-mode light through the strong birefringence of the silicon waveguide; the chirped subwavelength grating enhances the refractive index difference between TE and TM mode lights. The adiabatic tapered waveguide adopts a tapered structure with a nonlinearly varying width to achieve an adiabatic transition between the input fundamental mode and the slab mode, reduce the mode conversion loss, and optimize the waveguide width to ensure near-lossless transmission of the TE mode.

[0030] Based on the strong birefringence characteristic of the silicon waveguide, the cascaded total reflection corner mirror makes the TE mode undergo total internal reflection by optimizing the corner mirror inclination angle θ, while the TM mode is scattered because it does not meet the total reflection critical angle condition, significantly improving the single-stage extinction ratio. The cascaded structure further enhances the ability to suppress the TM mode. Setting the waveguide gap distance effectively avoids the interference between scattered TMs. A chirped SWG with a gradually decreasing duty cycle is integrated outside the corner mirror reflection interface. Using the equivalent medium theory to regulate the equivalent refractive index, it enhances the scattering loss of the TM mode. At the same time, parameters such as the duty cycle and the minimum period are optimized to achieve both broadband and high extinction ratio and low insertion loss simultaneously.

[0031] As a preferred embodiment, as Figure 1 shown, the waveguide silicon reflection layer 30 adopts a single full etching process, and the thicknesses of the waveguide and the subwavelength grating are equal and both are completely etched through, simplifying the processing technology.

[0032] Figure 2It is a top view of a planar broadband, low-loss, compact silicon-based polarizer based on a subwavelength grating-assisted total reflection angle mirror, and the arrow indicates the direction of light propagation therein. During operation, TE and TM signals enter the nonlinear adiabatic tapered waveguide simultaneously, with the width continuously varying between 0.4 μm and 4 μm and the shape being parabolic. Subsequently, the TE light continuously enters the silicon waveguide under the action of total reflection at the reflection interface of the silicon waveguide, while the TM light does not satisfy the total reflection condition and enters the silicon subwavelength grating through scattering. Each group of silicon subwavelength gratings includes 3 subwavelength grating bars. The grating period of the silicon subwavelength grating is fixed at 0.3 μm, the duty cycle varies between 0.2 and 0.36, the bottom angle θ2 is 16°, and introducing a chirped subwavelength structure is more conducive to the scattering of TM light into free space. D0 is 4 μm and D1 is 3.5 μm to prevent the scattered TM from coupling back into the waveguide again. Thanks to the above SWG structure, the cascading times are only 4, significantly reducing the device size. The device length is 44 μm and the minimum feature size is 210 nm. Through collaborative optimization, the device achieves an insertion loss < -0.75 dB, an extinction ratio > 20 dB, and a bandwidth > 200 nm in the 1550 nm band.

[0033] Figure 3 Shown is the simulation result of the optical field intensity distribution of the TE incident light at a wavelength of 1550 nm for this preferred embodiment. It can be seen that most of the TE light is totally reflected through the waveguide at the reflection interface, and the light in other directions has less loss.

[0034] Figure 4 Shown is the simulation result of the optical field intensity distribution of the TM incident light at a wavelength of 1550 nm for this preferred embodiment. It can be seen that most of the TM light is dissipated within 4 groups of subwavelength gratings and cannot be stably transmitted in the silicon waveguide.

[0035] As Figure 5 、 6 Shown, in the TE mode, the insertion loss in the wavelength range of 1470 - 1675 nm is between 0.4 and 0.55 dB; in the TM mode, the insertion loss in the wavelength range of 1470 - 1675 nm is between 20 and 26 dB. Therefore, within a 200 nm bandwidth, the polarization-dependent loss of TE / TM is above 20 dB.

[0036] It should be noted that the above embodiments are not used to limit the protection scope of the present invention, and equivalent transformations or substitutions made on the basis of the above technical solutions all fall within the claims of the present invention.

Claims

1. A broadband, low-loss, compact silicon-based polarizing device based on a sub-wavelength grating-assisted total reflection corner mirror, characterized in that: The silicon-based polarizing device includes, from bottom to top, a substrate layer, an oxide lower cladding layer, and a silicon waveguide layer, wherein the silicon waveguide layer includes, from left to right, a nonlinear conical structure, a cascaded corner mirror silicon waveguide, four groups of silicon sub-wavelength gratings, and a nonlinear conical structure.

2. A broadband, low-loss, compact silicon-based polarizing device based on a sub-wavelength grating-assisted total reflection corner mirror according to claim 1, characterized in that: The substrate layer is made of silicon and is used to support the entire chip.

3. A broadband, low-loss, compact silicon-based polarizing device based on a sub-wavelength grating-assisted total reflection corner mirror according to claim 1, characterized in that: The oxide lower cladding material of the polarizer is silicon dioxide, and the refractive index should be smaller than the material silicon in the polarizer. The thickness of the oxide lower cladding is at least 2 μm and provides optical isolation and mechanical support. The thickness of the silicon waveguide layer is 0.22 μm.

4. A broadband, low-loss, compact silicon-based polarizing device based on a sub-wavelength grating-assisted total reflection corner mirror according to claim 1, characterized in that: The nonlinear conical structure is connected to two ends of the cascaded corner mirror silicon waveguide structure for transmitting TE and TM polarized optical signals. The silicon subwavelength grating is close to the four cascaded corner mirror silicon waveguide reflection interfaces and is vertically aligned with the reflection interfaces.

5. A broadband, low-loss, compact silicon-based polarizing device based on a sub-wavelength grating-assisted total reflection corner mirror according to claim 1, characterized in that: The nonlinear tapered structure is an adiabatic tapered waveguide structure with a nonlinearly gradient width, which achieves a nearly lossless transition between the input fundamental mode and the slab mode.

6. A broadband, low-loss, compact silicon-based polarizing device based on a sub-wavelength grating-assisted total reflection corner mirror according to claim 1, characterized in that: The cascaded corner mirror silicon waveguide is based on the strong birefringence characteristics of silicon waveguide and optimizes the corner mirror tilt angle θ to achieve total reflection of TE mode light at four reflection interfaces.

7. A broadband, low-loss, compact silicon-based polarizing device based on a sub-wavelength grating-assisted total reflection corner mirror according to claim 1, characterized in that: The four groups of silicon subwavelength gratings are completely consistent and adopt a chirped design, that is, the period of the subwavelength grating is fixed and the duty cycle varies gradually. A chirped subwavelength grating with a gradually decreasing duty cycle is integrated on the outer side of the corner mirror reflection interface to expand the refractive index difference between TE and TM mode light, further enhancing the birefringence characteristics of the waveguide.

8. A broadband, low-loss, compact silicon-based polarizing device based on a sub-wavelength grating-assisted total reflection corner mirror according to claim 1, characterized in that: The thickness of the waveguide layer is completely consistent and a full etching process is used. The nonlinear conical structure, cascaded corner mirror silicon waveguide and 4 groups of sub-wavelength gratings are completely etched through.

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