A lithium niobate thin-film MMI polarizer / splitter

Abstract

This application provides a lithium niobate thin-film MMI polarizer and beam splitter, comprising: a waveguide layer, a waveguide protection layer, and a silicon substrate; the waveguide layer includes: a proton exchange waveguide, an MMI, and a transition waveguide, which are integrated on the same lithium niobate thin-film chip; the transition waveguide has a double-layer tapered structure to meet the thermal insulation transmission requirements and is used for coupling between the proton exchange waveguide and the MMI; the MMI includes: an input waveguide unit, a multimode interference region, and an output waveguide unit; the input and output waveguides connected to the multimode interference region both adopt tapered structures to meet the thermal insulation transmission requirements; this invention solves the problem of efficient coupling between the proton exchange waveguide and the MMI and realizes the function of polarization and beam splitting.

Description

Technical Field

[0001] This application relates to the field of optoelectronic technology, specifically to a lithium niobate thin-film MMI polarizer / beam splitter. Background Technology

[0002] Lithium niobate thin-film optical waveguides possess the high electro-optic coefficient, acousto-optic coefficient, and good nonlinear effects of traditional lithium niobate waveguides, enabling highly integrated optical paths and thus becoming the most promising optoelectronic device platform. Among many optical functions, beam splitting and combining, and polarization are the two most fundamental functions of integrated optoelectronic platforms. To achieve beam splitting and combining, multimode interference couplers (MMIs) based on lithium niobate ridge waveguides are typically used, offering advantages such as wavelength insensitivity, small device structure, large process tolerance, and low loss. Beam polarization can also be achieved through specific structural designs, separating TE and TM light in the waveguide. However, relying solely on an MMI structure to simultaneously achieve beam splitting and polarization functions leads to design complexity and inter-functional interference, resulting in a reduction in individual performance metrics. For example, the polarization extinction ratio achieved by existing MMIs for polarization is only around 20-25 dB, exhibiting high polarization noise, which severely impacts device performance and consequently affects the optical path quality of the entire integrated optical module.

[0003] Traditional proton-exchange waveguides fabricated with lithium niobate inherently only support TE or TM modes, achieving very high polarization extinction ratios, typically 30-50 dB. Furthermore, proton-exchange waveguides are usually diffused waveguides, forming a small refractive index difference with the lithium niobate substrate, resulting in a large mode size during optical field propagation within the waveguide. This large mode field facilitates coupling with external optical fibers but hinders large-scale integration of optical devices. In contrast, ridge waveguides effectively reduce bending losses and improve the integration of optical devices, not only reducing the size of optical modes but also effectively lowering the half-wave voltage of lithium niobate electro-optic modulators. Therefore, this patent proposes a scheme that simultaneously implements proton-exchange waveguides and ridge waveguides on the same platform, leveraging the advantages of both to improve the optical path transmission efficiency and functional performance of lithium niobate chips.

[0004] However, due to the difference in waveguide dimensions, efficient coupling between proton exchange waveguides and lithium niobate thin-film ridge waveguides is difficult. The mode field area of ​​lithium niobate thin-film ridge waveguides typically does not exceed 1 μm. 2 The mode area of ​​a proton-exchanged waveguide is 3-4 times that of a ridge waveguide. Direct coupling between them results in very high coupling loss. Therefore, achieving efficient coupling between them is crucial for ensuring beam transmission quality. Current research on mode conversion is limited to the connection between fiber and waveguide; efficient beam transmission between an on-chip proton-exchanged waveguide and a ridge waveguide remains a pressing issue. Summary of the Invention

[0005] The problem addressed by this application is the efficient coupling between proton-switched optical waveguides and MMIs, achieving both high-standard polarization and efficient beam splitting capabilities, which is beneficial for the large-scale application of integrated lithium niobate thin-film waveguide devices.

[0006] After extensive simulation calculations and other verification work, the present invention finally obtained the following structural scheme:

[0007] like Figure 1 As shown, a lithium niobate thin-film MMI polarizing beam splitter includes:

[0008] Silicon substrate 101, waveguide protective layers 102 and 104, waveguide layer 103;

[0009] like Figure 2 As shown, the waveguide layer 103 includes a proton exchange waveguide 1031, an MMI, and transition waveguides 1032 and 1033;

[0010] The waveguide layer 103 is integrated on the same lithium niobate thin film chip;

[0011] The waveguide protection layer includes a lower cladding layer 102 and an upper cladding layer 104 of the waveguide layer;

[0012] Preferably, the waveguide protective layers 102 and 104 are silicon oxide layers.

[0013] Preferably, the proton exchange waveguide 1031 is prepared by a gas-phase proton exchange method.

[0014] Preferably, the MMI is a 1×2 MMI, and its imaging principle is based on the self-image effect of a multimode waveguide;

[0015] like Figure 4 As shown, the MMI includes an input waveguide unit I, a multimode interference region II, and an output waveguide unit III; the input waveguide unit I includes a first input tapered waveguide 1034; the output waveguide unit III includes a first output tapered waveguide 1036, a first output single-mode waveguide 1037, a second output tapered waveguide 1038, and a second output single-mode waveguide 1039; the input waveguide unit I, the multimode interference region II, and the output waveguide unit III are connected in sequence;

[0016] The first input waveguide 1034 is a tapered ridge waveguide. The linear tapered structure satisfies the thermal transmission condition, which can reduce the loss caused by mode conversion between narrow and wide waveguides, and at the same time reduce the mode phase difference of self-image and improve image quality.

[0017] Preferably, the first input waveguide 1034 is centrally symmetrical.

[0018] The first and second output waveguides 1036 and 1038 of the MMI are tapered ridge waveguides, and the two output waveguides are distributed symmetrically in the MMI multimode waveguide region.

[0019] Preferably, the length of the MMI multimode interference region 1035 is...

[0020]

[0021] For TE light, σ = 0; for TM light, σ = 1; W M n is the width of the multimode interference region. r n is the core refractive index. s denoted as cladding refractive index.

[0022] Preferably, the width of the single-mode waveguide should be less than the maximum waveguide width that satisfies the single-mode condition, the length of the tapered waveguide is 40-70 μm, the width of the multimode interference region is 5-7 μm, the length is 15-20 μm, and the spacing between the two output waveguides is 0.8-1.2 μm.

[0023] Preferably, the MMI waveguide structure is a ridge waveguide with an etching rate between 0.4 and 0.6.

[0024] Preferably, the MMI and the proton exchange waveguide 1031 are prepared by gas-phase proton exchange simultaneously;

[0025] Preferably, the MMI is fabricated using electron beam lithography (RIE) and inductively coupled plasma etching (ICP);

[0026] For an input optical field, the phase difference between the two output optical fields of the 1×2MMI is 0, that is, the two output optical fields are in phase.

[0027] Preferably, before fabricating the aforementioned transition waveguides 1032 and 1033, the lithium niobate film between the proton exchange waveguide 1031 and the MMI is etched away using inductively coupled plasma etching (ICP).

[0028] Preferably, the transition waveguides 1032 and 1033 are made of silicon oxynitride, and the refractive index is modulated between the refractive indices of the silicon oxide layer and the waveguide layer.

[0029] Preferably, the transition waveguides 1032 and 1033 are designed as a double-layer conical structure; the wide end of the lower conical waveguide 1033 is connected to the proton exchange waveguide 1031, the two waveguides have the same thickness, the width is wider than the width of the proton exchange waveguide 1031, and the tip part covers the first input conical waveguide 1034 of the MMI, with a tip width of less than 2.5 μm; the wide end of the upper conical waveguide 1032 has the same width as the wide end of the lower conical waveguide 1033, the waveguide tip ends at the first input conical waveguide 1034 of the MMI, and the tip width is less than 300 nm; the left side of the upper conical waveguide 1032 covers the proton exchange waveguide 1031.

[0030] Preferably, the design of the 1032 and 1033 transition waveguide tapered structures satisfies the thermal transmission requirements.

[0031] Preferably, the transition waveguides 1032 and 1033 are constructed using plasma-enhanced chemical vapor deposition (PECVD), with the reaction equation: N2 + NH3 + N2O + SiH4 → SiON. x +(N2+H2), the refractive index of silicon oxynitride is controlled by adjusting the ratio of N2O and SiH4, and the double-layer conical structure is prepared by electron beam lithography (RIE) and inductively coupled plasma etching (ICP). The double-layer conical structure is prepared by a two-step lithography process. Figure 6 This is a schematic diagram of each cross-section after the invention is completed.

[0032] The propagation of the optical field within the waveguide is simulated through simulation, such as... Figure 7 As shown, the basic process of optical field transmission is as follows: Light enters from the gas-phase proton exchange waveguide 1031, and the output light is linearly polarized light (TE or TM). This beam first passes through transition waveguides 1032 and 1033. Through the design of the transition waveguide structure dimensions and the control of the refractive index, the coupling efficiency between the proton exchange waveguide 1031 and the transition waveguides 1032 and 1033 is maximized. The beam is then coupled from the proton exchange waveguide 1031 into the transition waveguide. As the width of the upper tapered waveguide 1032 gradually narrows, the optical field... The beam is gradually compressed to the lower tapered waveguide 1033, completing the vertical mode field conversion. It then couples to the first input tapered ridge waveguide 1034, which has a higher refractive index. Within the tapered waveguide, it satisfies the adiabatic transmission condition and couples to the multimode interference region 1035. Multimode interference occurs based on the self-image effect. At the position where the first double self-image is generated, the beam splits into two beams at the end of the multimode interference region 1035 and couples to the corresponding two tapered output waveguides 1036 and 1038. Finally, it is output through two single-mode waveguides 1037 and 1039. Thus, the process of converting the large-mode field beam output from the proton exchange waveguide 1031 into two small-mode field beams is completed, realizing the function of beam splitting.

[0033] The advantages and positive effects of this invention are as follows:

[0034] It can achieve efficient coupling between proton exchange waveguide and MMI, realize stable and efficient beam transmission, and play a high standard of polarization and beam splitting function; it abandons the traditional method of setting a separate mode converter for proton exchange waveguide, and integrates the mode conversion structure and waveguide structure into the same lithium niobate chip, realizing device miniaturization, higher integration, and more suitable for mass production. Attached Figure Description

[0035] Figure 1 A cross-sectional schematic diagram of an embodiment of the present invention is shown;

[0036] Figure 2 A two-dimensional top view schematic diagram of an embodiment of the present invention is shown;

[0037] Figure 3 A three-dimensional view of the connection between the transition waveguide and the MMI in an embodiment of the present invention is shown;

[0038] Figure 4 A two-dimensional top view of the MMI in an embodiment of the present invention is shown;

[0039] Figure 5 A complete process flow diagram of the preparation according to an embodiment of the present invention is shown;

[0040] Figure 6 The following are schematic diagrams of different cross-sections after the fabrication of embodiments of the present invention.

[0041] Figure 7 A schematic diagram of the transmission of the optical field in the waveguide according to an embodiment of the present invention is shown;

[0042] Figure 8 The diagram shows beam output patterns with different cross-sections according to embodiments of the present invention. Detailed Implementation

[0043] To better understand the present invention, the present invention will be further described below in conjunction with the embodiments and accompanying drawings.

[0044] Figure 1 A cross-sectional schematic diagram of an embodiment of the present invention is shown, as follows: Figure 1 As shown, a lithium niobate thin-film MMI polarizer includes: a Si substrate 101, a SiO2 lower cladding layer 102, a waveguide layer 103 located on the lower cladding layer, and a SiO2 upper cladding layer 104. Figure 2 A two-dimensional top view schematic diagram of an embodiment of the present invention is shown. The waveguide layer includes a proton exchange waveguide 1031, an upper transition waveguide 1032, a lower transition waveguide 1033, and an MMI. Figure 3 A three-dimensional view of the connection between the transition waveguide and the MMI in an embodiment of the present invention is shown. Figure 4A two-dimensional top view of an MMI in an embodiment of the present invention is shown. The MMI includes an input waveguide unit I, a multimode interference region II, and an output waveguide unit III. The input waveguide unit I includes a first input tapered ridge waveguide 1034; the output waveguide unit II includes a first output tapered ridge waveguide 1036, a first output single-mode ridge waveguide 1037, a second output tapered ridge waveguide 1038, and a second output single-mode ridge waveguide 1039; the input waveguide unit I, the multimode interference region II, and the output waveguide unit III are connected sequentially.

[0045] In this embodiment, the base material used is an x-cut lithium niobate crystal, and the proton exchange-prepared lithium niobate optical waveguide only supports TE mode transmission.

[0046] The lithium niobate film has a thickness of 0.6 μm. The proton exchange waveguide 1031 employs a gas-phase proton exchange method, which exhibits a high refractive index change and, compared to traditional proton exchange, a narrower waveguide width, eliminates the need for annealing, and causes less damage to the crystal structure. In this embodiment, the proton exchange waveguide 1031 has a width of 2 μm, and the exchange depth is equal to the thickness of the lithium niobate film (0.6 μm), resulting in a refractive index contrast of 0.05.

[0047] The proton exchange waveguide 1031 is coupled to a 1×2 MMI via transition waveguides 1032 and 1033. In this embodiment, the first input tapered waveguide 1034 is centrally symmetrically input. The width of the tip of waveguide 1034 is 0.8 μm, the width of the wide end is 1.6 μm, the length is 40 μm, and the taper change rate is 20 nm / μm. The first and second output tapered waveguides 1036 and 1038 have the same dimensions, but the wide end and the tip are reversed. Connected to the first input tapered waveguide 1034 is a multimode interference region 1035 with a width W of 5 μm and a length L of 15.5 μm. The beam is split into two beams at the end of the multimode interference region 1035 at the position where the first double self-image is generated. These beams are coupled to the first and second output tapered waveguides 1036 and 1038, and then output through single-mode waveguides 1037 and 1039. The distance between the two output waveguides is 1.2 μm. The 1×2 MMI is a ridge waveguide structure with a total thickness of 0.6 μm for the lithium niobate film and an etching depth of 0.3 μm. The MMI and the proton exchange waveguide 1031 undergo gas-phase proton exchange simultaneously. After the proton exchange is completed, the MMI waveguide design pattern is obtained by electron beam lithography (EBL) and inductively coupled plasma etching (ICP).

[0048] Transition waveguides 1032 and 1033 achieve efficient coupling between the proton exchange waveguide 1031 and the MMI. In this embodiment, the transition waveguide is a double-layer conical structure made of silicon oxynitride with a refractive index of 1.8. It is connected to the output end of the proton exchange waveguide 1031. The lower conical waveguide 1033 has a width of 2.7 μm at its wide end, a tip width of 2 μm, a length L2 of 20 μm, a taper rate of 35 nm / μm, and a thickness of 0.6 μm. The tip of this conical waveguide is deposited on the first input conical waveguide 1032, covering a length L3 of 10 μm. The upper conical waveguide 1032 has the same width at its wide end as the waveguide 1033 (2.7 μm) and a thickness of 0.3 μm. Its tip terminates at the input of the first conical waveguide 1034 on the right side, with a width of 200 nm. Furthermore, the upper conical waveguide 1032 covers the upper layer of the proton exchange waveguide 1031 on its left side.

[0049] During the fabrication of the transition waveguide, a 0.6 μm lithium niobate film between the proton exchange waveguide and the MMI was first etched using ICP technology. After etching, the transition waveguide (silicon oxynitride) was deposited using PECVD in a mixed gas environment of NH3, N2O, and SiH4 at 300 °C. The reaction equation is N2 + NH3 + N2O + SiH4 → SiON. x The refractive index of silicon oxynitride is controlled by adjusting the ratio of N2O and SiH4. An upper tapered waveguide 1032 is obtained using electron beam lithography (EBL) and inductively coupled plasma etching (ICP). Similarly, a lower tapered waveguide 1033 is etched using EBL and ICP. After the waveguide layers are fabricated, a silicon oxide film is deposited on the surface of the lithium niobate crystal as a protective layer 104. Finally, the wafer end faces are polished using chemical mechanical polishing (CMP). The complete fabrication process of this embodiment is as follows: Figure 5 As shown, Figure 6 This is a schematic diagram of the structure of different cross sections after the preparation of the embodiments of the present invention.

[0050] The above process can complete the fabrication of the lithium niobate thin-film MMI polarizer beam splitter described in this application. Simulations were performed to demonstrate the propagation of the optical field within the waveguide in this embodiment. Figure 7As shown, the light beam is coupled from the proton exchange waveguide 1031 into the transition waveguides (silicon oxynitride) 1032 and 1033. As the width of the upper tapered waveguide 1032 gradually narrows, the light field is gradually compressed into the lower tapered waveguide 1033, completing the mode field conversion in the vertical direction. Then, it is coupled to the first input tapered ridge waveguide 1034, which has a higher refractive index. Within the tapered waveguide, it satisfies the adiabatic transmission condition and couples to the multimode interference region 1035. Multimode interference occurs based on the self-image effect. At the position where the first double self-image is generated, the beam splits into two beams at the end of the multimode interference region 1035 and couples to the corresponding two tapered output waveguides 1036 and 1038. Finally, it is output through two single-mode waveguides 1037 and 1039, completing the conversion from a mode field area of ​​1.5 μm. 2 up to 0.7um 2 The conversion, Figure 8 The diagram shows the beam output pattern of different cross-sections in this embodiment. The phase difference between the two output light fields of the 1×2 MMI is 0, meaning that the two output light fields are in phase and both are in TE mode. The output field energy is retained at approximately 98%, the splitting ratio is 1:1, which can achieve equal distribution of optical power between the two output light fields, and the extinction ratio reaches approximately 40dB.

[0051] The embodiments of the present invention have been described in detail above, but the content described is only a preferred embodiment of the present invention and should not be considered as limiting the scope of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of this patent.

Claims

1. A lithium niobate thin-film MMI polarizer / splitter, comprising: Silicon substrate, waveguide protective layer, waveguide layer; The waveguide layer includes a proton exchange waveguide, an MMI, and a transition waveguide; The waveguide layer is integrated on the same lithium niobate thin film chip; The transition waveguide material is silicon oxynitride, and the refractive index is modulated between the refractive indices of the silicon oxide layer and the waveguide layer. The waveguide is designed as a double-layered conical structure; The wide end of the lower tapered waveguide is connected to the proton exchange waveguide. The two waveguides have the same thickness, but the width is wider than that of the proton exchange waveguide. The tip of the waveguide covers the first input tapered waveguide of the MMI. The width of the upper tapered waveguide is the same as that of the lower tapered waveguide, and the waveguide tip ends at the first input tapered waveguide of the MMI. The upper conical waveguide overlaps the proton exchange waveguide on its left side; The conical structure design meets the requirements for heat transfer.

2. The lithium niobate thin-film MMI polarizer and beam splitter according to claim 1, characterized in that: The waveguide protection layer includes a lower cladding layer and an upper cladding layer of the waveguide layer; The waveguide protective layer is a silicon oxide layer.

3. The lithium niobate thin-film MMI polarizer according to claim 1, characterized in that: The proton-exchange waveguide is fabricated using a gas-phase proton exchange method.

4. A lithium niobate thin-film MMI polarizer and beam splitter according to claim 3, characterized in that: The MMI is a 1×2 MMI, and its imaging principle is based on the self-image effect of multimode waveguides. Includes input waveguide unit, multimode interference region, and output waveguide unit; The input waveguide unit includes a first input tapered waveguide, which is centrally symmetrical for input. The output waveguide unit includes a first output tapered waveguide, a first output single-mode waveguide, a second output tapered waveguide, and a second output single-mode waveguide; The input waveguide unit, the multimode interference region, and the output waveguide unit are connected in sequence. The conical structure design meets the requirements for heat transfer.

5. A lithium niobate thin-film MMI polarizer and beam splitter according to claim 4, characterized in that... : The MMI and the proton exchange waveguide are simultaneously prepared by gas-phase proton exchange. The fabrication was completed using electron beam lithography (RIE) and inductively coupled plasma etching (ICP). The waveguide structure is a ridge waveguide.

6. A lithium niobate thin-film MMI polarizer according to claim 1, characterized in that: Before fabricating the transition waveguide, the lithium niobate film between the proton exchange waveguide and the MMI is etched away using inductively coupled plasma etching (ICP).

7. A lithium niobate thin-film MMI polarizer and beam splitter according to claim 1, characterized in that: The transition waveguide was fabricated using plasma-enhanced chemical vapor deposition (PECVD); The fabrication was completed using electron beam lithography (RIE) and inductively coupled plasma etching (ICP). The double-layered conical structure is completed through a two-step photolithography process.

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