Polarization rotation beam splitter structure based on lithium niobate film

Abstract

The invention discloses a polarization rotation beam splitter structure based on a lithium niobate film, belongs to the technical field of optical waveguides, and particularly relates to an integrated optical device for realizing polarization rotation and beam splitting of optical signals. The main structure is two linear tapered waveguides and an asymmetric coupler, the structure utilizes the excellent electro-optical characteristics of a lithium niobate film, and through the design of the linear tapered waveguides and an asymmetric directional coupling unit, when light is transmitted along the tapered waveguides which are gradually increased, due to the hybridization between modes, the light transmission efficiency is improved, and the light transmission efficiency is improved. The TM0 mode of narrow band import emission is gradually converted into the high-order TE1 mode, the high-order TE1 mode is coupled through the asymmetric coupler and converted into the TE0 mode to be output, efficient polarization rotation and beam splitting functions between the TM mode and the TE mode are achieved, and potential application prospects are achieved. The polarization rotation beam splitter not only realizes simplification of a device structure, is expected to be applied to efficient polarization multiplexing and polarization modulation, but also provides flexibility for manufacturing a large-scale optical switch array and an optical interconnection network.

Description

Technical Field

[0001] The invention belongs to the technical field of optical waveguides, and in particular relates to a polarization rotation beam splitter structure based on a lithium niobate film. Background Art

[0002] Polarization rotation devices are a type of optical component that changes the polarization state of the mode transmitted in the waveguide through an on-chip optical waveguide device. It can convert the input TE polarization state mode into the output TM mode, or convert the input TM polarization state mode into the output TE mode. Using polarization rotation devices, the polarization component of the light wave can be controlled on the chip, thereby realizing a series of functional applications related to optical signal processing such as polarization multiplexing, polarization splitting, coherent modulation and detection.

[0003] Polarization state, as one of the most basic degrees of freedom of optical signals, has a wide range of applications in optical communications, optical computing, optical sensing, and optical measurement. Compared with spatial optical paths, on-chip integrated optical paths have the advantages of high integration, high controllability, and high efficiency. Therefore, the manipulation of the polarization state of optical signals in integrated optical waveguides is an indispensable part of the research and application based on optical chips. Among them, TE mode and TM mode are a pair of mutually orthogonal linear polarization state modes in optical waveguides. In many applications, it is necessary to realize the polarization rotation transformation from TE mode to TM mode, or from TM mode to TE mode, so integrated polarization rotation control devices are very important. Lithium Niobate (LN) is selected as the manufacturing platform for electro-optical switches mainly because lithium niobate has excellent electro-optic coefficient, good optical quality, and stable chemical properties, which make it an ideal material for high-performance electro-optic modulation. The high electro-optic coefficient of lithium niobate means that a small electric field can cause a significant change in the refractive index, thereby achieving efficient optical signal modulation. In addition, lithium niobate has low optical loss, which is conducive to maintaining the integrity of optical signals and improving transmission efficiency. Summary of the invention

[0004] In order to realize the rotation of the optical mode, the present invention proposes a polarization rotation beam splitter structure based on lithium niobate film, the purpose of which is to realize the rotation of the TM-TE mode with a ridge waveguide on a lithium niobate platform and output it in TE0 mode. As the fundamental mode, the TE0 mode has the lowest loss and the best transmission characteristics. Therefore, outputting in TE0 mode is crucial to ensure the stability and reliability of optical signals in long-distance transmission. The structure realizes the rotation and beam splitting of the optical mode by precisely designing the dimensions of the linear tapered waveguide.

[0005] The technical solution of the present invention is as follows:

[0006] A polarization rotator based on lithium niobate film comprises two tapered waveguides, taper1 and taper2, and an asymmetric directional coupler. The materials of the polarization rotator include a substrate layer, a lower cladding layer, a waveguide core layer and an upper cladding layer. The waveguide core layer is made of lithium niobate film.

[0007] A polarization rotator based on lithium niobate film provides a TM-TE mode polarization rotation, comprising an input waveguide region, a polarization rotation region, and an output waveguide region.

[0008] Furthermore, a silicon dioxide layer is disposed on the substrate layer, a lithium niobate film is disposed on the silicon dioxide layer, a ridge waveguide is disposed on the lithium niobate film, and a protective layer of silicon dioxide is disposed on the ridge waveguide.

[0009] Furthermore, the thickness of the silicon dioxide layer is 2 μm, the thickness of the lithium niobate film is 600 nm, the height of the ridge waveguide is 350 nm, and the thickness of the slab is 250 nm.

[0010] Furthermore, the input waveguide region is a ridge waveguide with uniform width, the polarization rotation region is two linear tapered waveguides, and the output waveguide region is composed of an asymmetric directional coupler connected to an S-bend waveguide.

[0011] Furthermore, the widths of the two tapered waveguides of the polarization rotator are sequentially increased.

[0012] Furthermore, the input waveguide is used to input the TE fundamental mode polarized light and the TM fundamental mode polarized light into the polarization rotator, the TE fundamental mode polarized light is directly output from the output waveguide region through two linear tapered waveguides, the TM polarized light enters the polarization rotation region through the input waveguide region, realizes the conversion from TM0 mode to TE1 mode, and is coupled to another waveguide through an asymmetric directional coupler and output from the S-bend waveguide in TE0 mode.

[0013] The beneficial effects of the present invention are:

[0014] The present invention adopts a thin-film lithium niobate optical waveguide slab area made of thin-film lithium niobate material, fully utilizes the birefringence characteristics of the thin-film lithium niobate material in the waveguide structure design, and compensates for the structural birefringence, thereby realizing the fundamental mode polarization rotation function in a single waveguide. Compared with similar devices, it not only has a simpler waveguide structure, but also has good application prospects in high-speed and large-capacity optical interconnection and optical communication applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a schematic structural diagram of the polarization rotator based on lithium niobate film of the present invention.

[0016] Figure 2 It is a longitudinal cross-sectional view of the polarization rotator structure based on lithium niobate film of the present invention.

[0017] Figure 3 FIG. 1 is a top view of the polarization rotator structure based on lithium niobate film according to the present invention.

[0018] Figure 4 A transformation relationship diagram of the effective refractive index of a ridge waveguide width mode according to the present invention;

[0019] Figure 5 This is a propagation simulation diagram of the present invention working in TE0 mode.

[0020] Figure 6 This is a propagation simulation diagram of the present invention working in TM0 mode. Specific implementation plan

[0021] In order to more clearly understand the above-mentioned purpose, features and advantages of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict. In addition, the present invention can also be implemented in other ways different from those described herein, and therefore, the protection scope of the present invention is not limited by the specific embodiments disclosed below.

[0022] The present invention proposes a polarization rotation beam splitter structure based on lithium niobate film, such as Figure 1 As shown, the structure consists of a group of asymmetric directional couplers, two cascaded tapered waveguides, and a bent waveguide. The device is based on Z-cut LNOI material. The input optical signal TE0 mode is input from Port 1, passes through waveguide I, and is output from Port 2 in TEO mode. The input optical signal TM0 mode is input from Port 1, is converted into TE1 mode through the tapered waveguide in waveguide II, is coupled to waveguide II through the asymmetric directional coupler, and is output from Port 3.

[0023] like Figure 2 As shown, the substrate layer 1, the lower cladding layer 2, the waveguide core layer 3 and the upper cladding layer 4 are arranged in sequence, and the upper cladding layer 4 is outside the outside air; the waveguide core layer 3 is made of thin film lithium niobate. The total thickness of the waveguide core layer is 600nm, the thickness h1 of the ridge is 350nm, and the thickness h2 of the plate is 250nm.

[0024] like Figure 3 As shown, a top view of a polarization rotation beam splitter structure based on lithium niobate film is shown, in which the entrance width w1 of the first tapered waveguide segment in waveguide I, the exit width w2 of the first tapered waveguide segment, the exit width w3 of the second tapered waveguide, and the width w4 of waveguide II are determined according to the effective refractive index mixing region.

[0025] like Figure 4As shown, specifically, when determining the width, it is necessary to satisfy: w1 < w2 < w3 < w4, and w2 and w3 are in the mixed region within the dashed line of the effective refractive index, while w1 and w4 are outside the effective refractive index mixed region; according to the differences in the four upper ridge widths, the actual mode conversion efficiency of the polarization rotation unit is different, thereby affecting the length of the polarization rotation unit. In this example, w1 = 1 μm, w2 = 2.2 μm, w3 = 2.4 μm, and w4 = 1.14 μm.

[0026] Specifically, the input waveguide is the first linear tapered waveguide for the transition from a single-mode waveguide to the input end of the polarization rotation unit, gradually changing from a width of 1 μm to 2.2 μm, and the length L1 is 19 μm; the second linear tapered waveguide for the transition of the polarization rotation unit, gradually changing from a width of 2.2 μm to 2.4 μm, and the length L2 is 190 μm; the two tapered waveguide segments are connected in sequence, with different lengths and the widths increasing sequentially.

[0027] According to the above scheme of the polarization rotation beam splitter structure based on lithium niobate thin film, the parameters of the waveguide are designed within the specified range. Based on the beam propagation method, the present invention is verified by simulation.

[0028] As Figure 5 shown, due to the anisotropy of the lithium niobate material, the refractive index of lithium niobate n 0 = 2.21 (in the x direction of the crystal body), n e = 2.14 (in the z direction of the crystal). Through the finite element / finite difference time domain algorithm, the optical field transmission diagram of the TE0 mode is obtained. The TE0 mode is input from the Port1 port and directly outputs from the Port2 through two linear tapered waveguides.

[0029] As Figure 6 shown, through the finite element / finite difference time domain algorithm, the optical field transmission diagram of the TM0 mode is obtained. The TM0 mode is input from the Port1 port, evolves into the TE1 mode in the linear tapered waveguide, the TE1 mode couples into the waveguide II in the asymmetric directional coupler, evolves into the TE0 mode, and finally the TM0 mode input from the Port1 port outputs as the TE0 mode from the Port3 port.

[0030] As mentioned above, it is only a preferred specific embodiment of the present invention for the patent, but the protection scope of the present invention for the patent is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention for the patent should be covered within the protection scope of the present invention for the patent.

Claims

1. The present invention proposes a deflection rotating beam splitter structure based on lithium niobate film, characterized in that: The polarization rotation beam splitter is realized by cascading two different linear tapered waveguides to realize the rotation of the mode, and the asymmetric directional coupling unit uses a ridged S-bend waveguide and a ridged straight waveguide to connect the output port to realize the mode splitting. The deflection rotation beam splitter structure based on lithium niobate film includes a substrate, a silicon dioxide layer is provided on the substrate, a lithium niobate film layer is provided on the silicon dioxide layer, a ridge waveguide is provided on the lithium niobate film layer, and a protective layer is provided on the ridge waveguide and the lithium niobate film layer.

2. The deflection rotation beam splitter structure based on lithium niobate film according to claim 1, characterized in that: The thickness of the substrate is 10 μm, the thickness of the silicon dioxide layer is 10 μm, the thickness of the lithium niobate film layer is 600 nm, the height of the ridge waveguide is 350 nm, and the thickness of the protective layer is 500 nm.

3. The deflection rotation beam splitter structure based on lithium niobate film according to claim 1, characterized in that: The widths of the two tapered waveguides of the polarization rotation unit increase successively: the width of the input waveguide is 1μm, the width of the first tapered waveguide increases from 1μm to 2.2μm, the width of the second tapered waveguide increases from 2.2μm to 2.4μm, the width of the connecting curved waveguide part of the asymmetric directional coupler is 1.2μm, and the width of the connecting tapered coupler part is 2.4μm.

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