A tm mode polarizer based on a curved narrow waveguide

Abstract

The application discloses a TM mode polarizer based on a curved narrow waveguide, which comprises a single-mode input waveguide, an input end conversion waveguide, a curved narrow waveguide, an output end conversion waveguide and a single-mode output waveguide along the light transmission direction. The input light is first converted into the mode supported by the narrow waveguide by the input end conversion waveguide, then the input TE mode is leaked and the input TM mode is passed by reasonably selecting the bending radius and width of the curved narrow waveguide, and finally the TM mode supported by the single-mode waveguide is converted by the output end conversion waveguide, so that the polarizer function of filtering the TE mode and passing the TM mode is realized. The application comprises, in the direction perpendicular to the light transmission plane, an upper cladding layer, a core layer, a buried oxygen layer and a substrate in sequence. The application can realize a high-performance TM mode polarizer in a large optical bandwidth due to the wavelength-insensitive structure, and has important practical application value in the fields of on-chip optical communication, optical sensing and spectroscopy.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of integrated optoelectronic devices, in particular to a TM mode polarizer based on curved narrow waveguide. BACKGROUND

[0002] Due to the advantages of CMOS compatibility, high integration and strong light field confinement, on-chip silicon-based high-performance integrated devices have always been a research hotspot. Because of the high refractive index contrast between silicon and silicon dioxide, silicon optical devices exhibit strong mode field confinement, thereby achieving a compact footprint. However, this also makes the silicon waveguide have a serious waveguide birefringence, and the silicon optical device has strong polarization dependence, so most devices can only work in a specific polarization state. Therefore, the polarizer that can filter out another polarized light by using a single polarized light is of great significance in the on-chip optical circuit.

[0003] The currently proposed polarizer schemes can be mainly divided into three types: reflective, absorptive and leaky. The reflective polarizer is generally based on Bragg reflection, which reflects a certain polarized light back to the incident end, and the other polarized light is output from the output end, and the length is usually compact. Because of the strong wavelength dependence of Bragg reflection, the reflective polarizer is limited by the principle and cannot achieve ultra-large bandwidth. In addition, the reflective polarizer cannot be directly connected with the light source, and some researchers have proposed using a tapered multi-mode Bragg grating as a polarizer, which reflects a certain polarized light in the form of a high-order mode back to the incident port, and then filters out the high-order mode through a mode multiplexer to reduce the reflection, but the length is usually long, which is not conducive to the realization of dense integration. The absorptive polarizer realizes the absorption of a certain polarized light by introducing metal or two-dimensional materials, which can achieve a small footprint, but the loss of the required polarized light is generally large, the working bandwidth is small, and new materials are introduced, which greatly increases the difficulty of device fabrication. The leaky polarizer usually adopts a directional coupler structure, and the directional coupler polarizer generally has a narrow bandwidth, and some researchers have proposed cascading directional couplers as polarizers to improve the bandwidth, but this inevitably increases the length of the device.

[0004] The currently proposed high-performance polarizer is usually a TE polarizer. Due to the waveguide cross-sectional shape, the TE mode field is more compact and more difficult to leak from the waveguide than the TM mode field, so it is more difficult to realize a TM polarizer than a TE polarizer. Since TM mode is widely used in optical sensing and optical communication, a high-performance TM polarizer has important practical significance. SUMMARY

[0005] The present application proposes a TM mode polarizer based on a curved narrow waveguide, which can achieve an insertion loss of less than 1 dB and a polarization extinction ratio of more than 20 dB in a bandwidth range of 1260-1580 nm, and has a simple structure, low reflection and CMOS compatibility.

[0006] The present invention is achieved through the following technologies:

[0007] A TM mode polarizer based on a curved narrow waveguide comprises, in order along the light transmission direction, a single-mode input waveguide, an input-end conversion waveguide, a curved narrow waveguide, an output-end conversion waveguide, and a single-mode output waveguide. Input light is first converted into a mode supported by the narrow waveguide by the input-end conversion waveguide. Then, by selecting the bending radius and width of the curved narrow waveguide, the input TE mode is leaked while the input TM mode passes through. Finally, the polarizer is converted into a TM mode supported by the single-mode waveguide by the output-end conversion waveguide, thereby achieving the polarizer function of filtering out the TE mode and passing the TM mode. The TM mode polarizer comprises, in order perpendicular to the light transmission plane, an upper cladding layer, a core layer, a buried oxide layer, and a substrate. The curved shape of the curved narrow waveguide can be selected to be a circle, an Euler curve, a Bezier curve, an interpolation curve, or a combination of the above curves.

[0008] The materials that can be used for the upper cladding layer of the TM mode polarizer include air, silicon dioxide, or polymer materials.

[0009] The core layer materials that can be used for the TM mode polarizer include silicon, gallium nitride, indium phosphide, lithium niobate, or polymer materials.

[0010] The input end conversion waveguide can be a linear tapered waveguide or a nonlinear tapered waveguide.

[0011] The operating bandwidth of the TM mode polarizer includes the optical communication band, the infrared band or the visible light band.

[0012] The width of the single-mode input waveguide and the single-mode output waveguide are both set to 430 nm. The input-end conversion waveguide is a linear tapered waveguide with a tapered length of 5 µm. The output-end conversion waveguide is symmetrical with the input-end conversion waveguide. The curved narrow waveguide adopts an Euler curve, which is a gradually narrow waveguide with a width of 0.11 µm at infinite curvature and 0.13 µm at minimum curvature. The minimum curvature radius is set to 5 µm.

[0013] Compared with the existing technology, the present invention has the following beneficial effects:

[0014] The embodiments of the present invention adopt a silicon photonics platform, which has many advantages such as low loss, high integration density and CMOS compatibility; however, the present invention can be implemented on a variety of integrated optical platforms, and the applicable platforms are not limited to the silicon photonics platform adopted in the embodiments, but also include platforms such as lithium niobate and silicon nitride.

[0015] The present invention exploits the inherent characteristic that different polarizations experience different losses through a curved narrow waveguide, which is less dependent on the wavelength of the incident light itself. This allows for polarization across a wide wavelength range. The operating bandwidth is not limited to the optical communication waveguides in the embodiments, but also encompasses infrared and visible light bands.

[0016] The present invention does not introduce metal or two-dimensional materials to absorb a certain polarized light and does not require a special process flow, so it has high CMOS compatibility and is convenient for large-scale manufacturing.

[0017] The present invention has a simple structure and only requires one etching step, which greatly simplifies the production process and improves practicality.

[0018] Any technical solution of the present invention may not necessarily achieve all of the above beneficial effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a schematic diagram of the structure of a TM mode polarizer based on a curved narrow waveguide;

[0020] In the figure, there are single-mode input waveguide 1, input end conversion waveguide 2, curved narrow waveguide 3, output end conversion waveguide 4, and output single-mode waveguide 5; the dotted lines are for clearer distinction of the parts and do not mean that they are invisible or unprotected.

[0021] Figure 2 Schematic diagram of the waveguide cross section of the TM mode polarizer based on a curved narrow waveguide;

[0022] In the figure, there are upper cladding layer 6, waveguide layer 7, buried oxide layer 8, and substrate 9.

[0023] Figure 3 are the insertion loss and polarization extinction ratio in the embodiment.

[0024] Figure 4 1 is a diagram of the light field transmission of TM and TE modes in a curved narrow waveguide in an embodiment. DETAILED DESCRIPTION

[0025] First, the mechanism of the present invention is explained. Along the light transmission direction, there is a single-mode input waveguide, an input-end conversion waveguide, a curved narrow waveguide, an output-end conversion waveguide, and a single-mode output waveguide. The input light is first converted by the input-end conversion waveguide into a mode supported by the narrow waveguide. Then, by properly selecting the bending radius and width of the curved narrow waveguide, the input TE mode leaks, while the input TM mode passes through. Finally, the output-end conversion waveguide converts the light into the TM mode supported by the single-mode waveguide, thereby filtering out the TE mode and performing the polarizer function of the TM mode. By properly selecting the bending radius and width of the curved narrow waveguide, due to the difference in polarization direction, the TE mode has much greater loss than the TM mode, resulting in leakage of the TE mode, while the TM mode is confined in the curved narrow waveguide and finally output from the output end. Example

[0026] Figure 1 Schematic diagram of the structure of a TM mode polarizer based on a curved narrow waveguide.

[0027] Along the transmission direction of light, there are single-mode input waveguide 1, input end conversion waveguide 2, curved narrow waveguide 3, output end conversion waveguide 4, and single-mode output waveguide 5. Figure 2 This is a schematic cross-sectional diagram of a TM-mode polarizer waveguide based on a curved narrow waveguide. Perpendicular to the light transmission plane, the following are the upper cladding layer 6, waveguide layer 7, buried oxide layer 8, and substrate 9. In this embodiment, a silicon photonic platform with a waveguide layer 7 of 340 nm thickness is used, and the upper cladding layer 6 is silicon dioxide.

[0028] The width of both the single-mode input and output waveguides is set to 430 nm, allowing the waveguides to support only the fundamental TE and TM modes. The width conversion waveguide at the input is a linear tapered waveguide. The longer the tapered length, the better the conversion performance for the TM mode and the lower the loss. Here, the length is set to 5 µm to achieve near-adiabatic properties. The conversion waveguide at the output is completely symmetrical with the input conversion waveguide, with the same parameters. To achieve a low-loss connection between the curved waveguide and the straight waveguide, the curved narrow waveguide is shaped like an Euler curve. This curve allows for a gradual change in the radius of curvature between infinity and a finite value, making it ideal for low-loss connections between straight and curved waveguides. To further increase the device's optimization freedom, the curved narrow waveguide is a tapered narrow waveguide. At infinite curvature, the width is set to 0.11 µm. At minimum curvature, to reduce TM mode loss, the width is set to 0.13 µm, with a minimum radius of curvature of 5 µm. In a curved narrow waveguide, since the electric field vibration direction of TE polarization is along the device surface, the confinement is weak and the bending loss is high; while the electric field vibration direction of TM polarization is perpendicular to the device surface, the confinement is strong and the bending loss is relatively low. It is based on this principle that the TE mode leaks into the cladding and the TM mode passes through with low loss.

[0029] Figure 3 The following simulations show the insertion loss and polarization extinction ratio of a TM polarizer under the aforementioned conditions. They demonstrate that within the 1260-1580 nm optical communication band, the device achieves insertion loss below 1 dB and a polarization extinction ratio exceeding 20 dB. The device is virtually lossless at short wavelengths, while achieving an ultra-high polarization extinction ratio exceeding 40 dB at long wavelengths. Furthermore, the device exhibits a simple structure, low reflection, and a CMOS-compatible fabrication process, requiring only a single etching step. Figure 4 Figure 2 shows the optical field transmission diagram of TM and TE modes in a bent narrow waveguide at an operating wavelength of 1550 nm. The TM mode (left part) passes through with low loss, while the TE mode (right part) leaks into the cladding.

[0030] The embodiments described above may be further combined or replaced, and the embodiments are merely descriptions of embodiments of the present invention and do not limit the concept and scope of the present invention. Various changes and improvements made to the technical solutions of the present invention by persons skilled in the art without departing from the design concept of the present invention are within the scope of protection of the present invention. The scope of protection of the present invention is defined by the appended claims and any equivalents thereof.

Claims

1. A TM mode polarizer based on a curved narrow waveguide, characterized by: Along the transmission direction of light, there are single-mode input waveguide, input end conversion waveguide, curved narrow waveguide, output end conversion waveguide, and single-mode output waveguide in order; the input light is first converted into a mode supported by the narrow waveguide by the input end conversion waveguide, and then by selecting the bending radius and width of the curved narrow waveguide, the input TE mode is leaked, while the input TM mode passes through, and finally converted into the TM mode supported by the single-mode waveguide through the output end conversion waveguide, thereby realizing the polarizer function of filtering out the TE mode and passing through the TM mode; the TM mode polarizer is perpendicular to the direction of the light transmission plane and is composed of the upper cladding layer, the core layer, the buried oxide layer and the substrate in order; the width of the single-mode input waveguide and the single-mode output waveguide are both set to 430 nm, the input end conversion waveguide is a linear tapered waveguide, the length of the tapered is set to 5 µm, the output end conversion waveguide is symmetrical with the input end conversion waveguide, the shape of the curved narrow waveguide adopts the Euler curve, which is a narrow waveguide with a gradually changing width. The width is set to 0.11 µm at the infinite radius of curvature and 0.13 at the minimum radius of curvature. µm, and the minimum curvature radius is set to 5 µm.

2. A TM mode polarizer based on a curved narrow waveguide according to claim 1, characterized in that: The materials that can be used for the upper cladding layer of the TM mode polarizer include air, silicon dioxide, or polymer materials.

3. The TM mode polarizer based on a curved narrow waveguide according to claim 1, characterized in that: The core layer materials that can be used for the TM mode polarizer include silicon, gallium nitride, indium phosphide, lithium niobate, or polymer materials.

4. The TM mode polarizer based on a curved narrow waveguide according to claim 1, characterized in that: The input end conversion waveguide can be a linear tapered waveguide or a nonlinear tapered waveguide.

5. The TM mode polarizer based on a curved narrow waveguide according to claim 1, characterized in that: The operating bandwidth of the TM mode polarizer includes the optical communication band, the infrared band or the visible light band.

Images