Dual-ladder waveguide mode spot converter and method of manufacture

Abstract

This disclosure provides a dual-step waveguide mode converter, comprising: a substrate (1), a lower cladding layer (2), a thin-film lithium niobate waveguide layer (3), a first dielectric loading layer (4), a second dielectric loading layer (5), and an upper cladding layer (6) arranged sequentially from bottom to top; wherein, the thin-film lithium niobate waveguide layer (3) comprises a plurality of waveguide segments connected in sequence, each waveguide segment being a straight waveguide of equal width and thickness, the width and thickness between adjacent waveguide segments decreasing abruptly along the optical transmission direction, and the centerlines of each waveguide segment being aligned; the first dielectric loading layer (4) is a silicon nitride layer covering the thin-film lithium niobate waveguide layer (3), and its refractive index is lower than that of the thin-film lithium niobate waveguide layer (3); the second dielectric loading layer (5) is a silicon oxynitride layer disposed above the first dielectric loading layer (4), and its refractive index is between that of the first dielectric loading layer (4) and the refractive index of the optical fiber core. This disclosure also provides a method for fabricating a dual-step waveguide mode converter.

Description

Technical Field

[0001] This disclosure relates to the field of photonic integration technology, specifically to a double-step waveguide mode converter and its fabrication method. Background Technology

[0002] Thin-film lithium niobate photonic integrated chips have become a key platform in the field of high-speed optical communication due to their excellent electro-optic performance. However, the mode field diameter of thin-film lithium niobate waveguides is usually less than 1 μm, while the mode field diameter of standard single-mode fiber is about 9-10.5 μm. There is a serious mode field mismatch between the two, with direct coupling loss as high as 10 dB or more.

[0003] Existing technologies generally follow the design principle of "the finer the tip, the lower the loss," leading to an irreconcilable contradiction between process tolerance and coupling loss. How to achieve low-loss coupling while maintaining a large tip width (≥300nm) remains a technical challenge that TFLN platforms urgently need to solve. Summary of the Invention

[0004] In view of the above problems, this disclosure provides a double-step waveguide mode converter and its fabrication method.

[0005] According to a first aspect of this disclosure, a dual-step waveguide mode converter is provided, comprising: a substrate, a lower cladding layer, a thin-film lithium niobate waveguide layer, a first dielectric loading layer, a second dielectric loading layer, and an upper cladding layer, arranged sequentially from bottom to top; wherein, the thin-film lithium niobate waveguide layer comprises a plurality of waveguide segments connected in sequence, each waveguide segment being a straight waveguide of equal width and thickness, the width and thickness between adjacent waveguide segments decreasing abruptly along the optical transmission direction, and the centerlines of each waveguide segment being aligned; the first dielectric loading layer is a silicon nitride layer covering the thin-film lithium niobate waveguide layer, and its refractive index is lower than that of the thin-film lithium niobate waveguide layer; the second dielectric loading layer is a silicon oxynitride layer disposed above the first dielectric loading layer, and its refractive index is between that of the first dielectric loading layer and the refractive index of the optical fiber core.

[0006] According to embodiments of this disclosure, the width difference between adjacent waveguide segments is 50-200 nm, and the thickness difference is 30-100 nm; the length of each waveguide segment is 50-100 μm.

[0007] According to embodiments of this disclosure, the width of the waveguide segment at the end along the optical transmission direction is greater than 500 nm.

[0008] According to embodiments of this disclosure, the refractive index of the first dielectric loading layer is 1.8-2.2.

[0009] According to embodiments of this disclosure, the refractive index of the second dielectric loading layer is 1.5-1.7.

[0010] According to embodiments of this disclosure, the thin-film lithium niobate waveguide layer is an X-cut or Z-cut single-crystal lithium niobate thin film.

[0011] According to embodiments of this disclosure, the substrate is silicon or quartz, and both the lower cladding layer and the upper cladding layer are silicon dioxide layers.

[0012] According to a second aspect of this disclosure, a fabrication method is provided for use in the dual-step waveguide mode converter described in the first aspect, comprising the following steps: depositing a lower cladding layer on the substrate; fabricating a thin-film lithium niobate waveguide layer on the lower cladding layer; etching the thin-film lithium niobate waveguide layer through multiple selective etching processes to form a plurality of waveguide segments, wherein the width and thickness of each waveguide segment decrease in a stepwise manner along the optical transmission direction, and the centerlines of each waveguide segment are aligned along the same straight line; depositing a first dielectric loading layer on the thin-film lithium niobate waveguide layer; depositing a second dielectric loading layer on the first dielectric loading layer; and depositing an upper cladding layer on the second dielectric loading layer.

[0013] According to embodiments of this disclosure, the step of etching the thin-film lithium niobate waveguide layer through multiple selective etching processes to form multiple waveguide segments includes: defining the width of each waveguide segment through photolithography, such that the width of each waveguide segment decreases in a stepwise manner along the light transmission direction, and the center lines of each waveguide segment are aligned along the same straight line; and performing a stepped thickness reduction on each waveguide segment, such that the thickness of each waveguide segment decreases in a stepwise manner along the light transmission direction.

[0014] According to embodiments of this disclosure, the method further includes polishing the end face of the dual-step waveguide mode converter.

[0015] The dual-step waveguide mode converter and its fabrication method disclosed herein have at least the following beneficial effects:

[0016] 1. The end width is greater than 500nm, which is far greater than the limit of DUV lithography (about 250nm), and mass production can be carried out without electron beam lithography, significantly improving the yield;

[0017] 2. By controlling the width and thickness steps in a dual-dimensional manner, mode mismatch is generated at the interface between each segment, which stimulates the transfer of energy to the upper layer;

[0018] 3. No resonant structure, bandwidth is determined by material dispersion, and can cover the C+L band (1530-1625nm).

[0019] 4. No ultra-fine tips, no suspended structure, high mechanical stability;

[0020] 5. Both the width and thickness dimensions can be optimized independently, enabling coupling strength distributions that are impossible with traditional tapered shapes;

[0021] 6. With the center lines of each segment aligned, the first dielectric loading layer does not need to be segmented and patterned, reducing the difficulty of photolithography and improving the process yield. Attached Figure Description

[0022] The foregoing contents, as well as other objects, features, and advantages of this disclosure, will become clearer from the following description of embodiments with reference to the accompanying drawings, in which:

[0023] Figure 1 A schematic diagram of a dual-step waveguide mode converter according to an embodiment of the present disclosure is shown.

[0024] Figure 2 This schematically illustrates the mode field distribution of the output end face of a double-step waveguide mode converter simulated using the finite-difference time-domain method according to an embodiment of the present disclosure.

[0025] Figure 3 This schematically illustrates the electric field distribution when an optical signal, simulated using the finite-difference time-domain method according to an embodiment of the present disclosure, propagates along the propagation direction within a two-step waveguide mode converter;

[0026] Figure 4 A flowchart illustrating a method for fabricating a dual-step waveguide mode converter according to an embodiment of the present disclosure is shown. Detailed Implementation

[0027] The embodiments of the present disclosure will now be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the disclosure. In the following detailed description, numerous specific details are set forth to provide a thorough understanding of the embodiments of the present disclosure for ease of explanation. However, it will be apparent that one or more embodiments may be practiced without these specific details. Furthermore, descriptions of well-known structures and techniques are omitted in the following description to avoid unnecessarily obscuring the concepts of the present disclosure.

[0028] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit this disclosure. The terms “comprising,” “including,” etc., as used herein indicate the presence of the stated features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0029] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein are to be interpreted in a manner consistent with the context of this specification, and not in an idealized or overly rigid way.

[0030] When using expressions such as "at least one of A, B and C", they should generally be interpreted in accordance with the meaning that is commonly understood by those skilled in the art (e.g., "a system having at least one of A, B and C" should include, but is not limited to, a system having A alone, a system having B alone, a system having C alone, a system having A and B, a system having A and C, a system having B and C, and / or a system having A, B and C, etc.).

[0031] Figure 1 This is a schematic diagram of the structure of the dual-step waveguide mode converter provided in this embodiment.

[0032] This embodiment provides a two-step waveguide mode converter. For example... Figure 1 As shown, the mode converter includes, from bottom to top, a substrate 1, a lower cladding layer 2, a thin-film lithium niobate waveguide layer 3, a first dielectric loading layer 4, a second dielectric loading layer 5, and an upper cladding layer 6.

[0033] Substrate 1 provides mechanical support and can be made of silicon or quartz with a thickness of 500-1000 μm. Lower cladding 2 is disposed on substrate 1, made of silicon dioxide with a thickness of 2-5 μm and a low refractive index, and is used to confine the light field within the waveguide layer and ensure adequate isolation from the substrate.

[0034] The material of the thin-film lithium niobate waveguide layer 3 is an X-cut or Z-cut single-crystal lithium niobate thin film. For example... Figure 1 As shown, the thin-film lithium niobate waveguide layer 3 includes multiple waveguide segments (e.g., 7, 8, 9, 10, 11) connected sequentially along the light transmission direction (from left to right in the figure). Each waveguide segment is a straight waveguide of equal width and thickness, and the width and thickness between adjacent waveguide segments decrease abruptly along the light transmission direction. The centerlines of all waveguide segments are aligned along the same straight line to ensure that the light does not shift laterally during propagation, reducing additional losses.

[0035] In this embodiment, the thin-film lithium niobate waveguide layer 3 preferably comprises five waveguide segments. Along the light transmission direction, the width and thickness of these five waveguide segments decrease progressively. For example, the first waveguide segment 7 (near the input side) has a width of 0.9-1.1 μm and a thickness of 450-550 nm; the second waveguide segment 8 has a width of 0.8-1.0 μm and a thickness of 400-500 nm; the third waveguide segment 9 has a width of 0.7-0.9 μm and a thickness of 350-450 nm; the fourth waveguide segment 10 has a width of 0.6-0.8 μm and a thickness of 300-400 nm; and the fifth waveguide segment 11 (near the chip end face) has a width of 0.5-0.7 μm and a thickness of 250-350 nm. The width difference between adjacent segments is 50-200 nm, and the thickness difference is 30-100 nm. The lengths of each waveguide segment can be equal or unequal, but preferably all are 50-100 μm.

[0036] In a specific example, the widths of the first to fifth segments are 1.0 μm, 0.9 μm, 0.8 μm, 0.7 μm, and 0.6 μm, respectively; the thicknesses are 500 nm, 450 nm, 400 nm, 350 nm, and 300 nm, respectively; and the lengths are 80 μm, 70 μm, 90 μm, 80 μm, and 80 μm, respectively. It should be noted that the width of the fifth segment (the waveguide segment at the end along the light transmission direction) is greater than 500 nm, for example, 0.6 μm. This is much larger than the limit linewidth of deep ultraviolet lithography (approximately 250 nm), thus providing good process tolerance.

[0037] The first dielectric loading layer 4 is a silicon nitride (Si3N4) layer, continuously covering the thin-film lithium niobate waveguide layer 3. Its refractive index (1.8-2.2) is lower than that of the thin-film lithium niobate waveguide layer 3. The thickness of this layer is 100-300 nm. The first dielectric loading layer 4 is used at the interface between each waveguide segment, where the radiated light excited due to mode mismatch can be captured and guided forward by this layer.

[0038] The second dielectric loading layer 5 is a silicon oxynitride (SiON) layer, disposed above the first dielectric loading layer 4, with a refractive index (1.5-1.7) between that of the first dielectric loading layer 4 and the fiber core. The thickness of this layer is 2-4 μm. The function of the second dielectric loading layer 5 is to continue capturing the upwardly transferred optical energy at subsequent inter-segment interfaces and to form a large mode field at the end that matches the mode field of a standard single-mode fiber.

[0039] The upper cladding layer 6 covers the second dielectric loading layer 5. It is made of silicon dioxide and has a thickness of 1-3 μm, and is used to protect the entire device structure.

[0040] In the dual-step waveguide mode converter provided in this embodiment, the optical signal is input from the left end in the form of the TE fundamental mode. During propagation, mode mismatch occurs at each inter-segment interface due to the step changes in width and thickness, thereby exciting some of the optical energy upward to the dielectric loading layer. As propagation continues, the optical energy is gradually transferred from the high-refractive-index thin-film lithium niobate waveguide layer 3 to the first dielectric loading layer 4, and finally all of it is transferred to the second dielectric loading layer 5, which has the refractive index closest to that of the optical fiber.

[0041] Figure 2 This is a schematic diagram of the mode field distribution at the output end face of the mode converter simulated using the finite-difference time-domain method in this embodiment. Figure 3 This is a schematic diagram of the electric field distribution when an optical signal simulated using the finite-difference time-domain method is transmitted along the propagation direction within the mode converter, as provided in this embodiment.

[0042] refer to Figures 1-3During operation, the optical signal is input from the left and enters segment 7 in the form of the TE fundamental mode. Within segment 7, the optical field is highly confined within the lithium niobate ridge, with a mode field diameter of approximately 1 μm. At the interface between segment 7 and segment 8, a first mode mismatch occurs due to a jump in width from 1.0 μm to 0.9 μm and in thickness from 500 nm to 450 nm. A small amount of energy is excited and radiated upwards, captured by the first dielectric loading layer 4 and guided forward. Within segment 8, the optical field propagates simultaneously within both the lithium niobate and the first dielectric loading layer. At the interface between segment 8 and segment 9, a second mode mismatch occurs due to a jump in width from 0.9 μm to 0.8 μm and in thickness from 450 nm to 400 nm, again causing upward radiation of excited energy. Within segment 9, the optical field begins to transfer from the first dielectric loading layer 8 to the second dielectric loading layer 9. At the interface between segments 9 and 10, the width jumps from 0.8 μm to 0.7 μm and the thickness jumps from 400 nm to 350 nm, resulting in a third mode mismatch, which excites energy upwards. At the interface between segments 10 and 11, the width jumps from 0.7 μm to 0.6 μm and the thickness jumps from 350 nm to 300 nm, resulting in a fourth mode mismatch, which excites most of the remaining energy upwards. Within segment 11, the optical field is entirely guided by the second dielectric loading layer 5, forming a circularly symmetric mode field with a diameter of approximately 8 μm at the chip end face, which is highly matched to the standard single-mode fiber.

[0043] like Figure 2 As shown, at the chip facet, the optical field is confined within the second dielectric loading layer 5, forming a circularly symmetric mode field with a diameter of approximately 8-10 μm, which highly matches the mode field of a standard single-mode fiber. Simulation results show that at a wavelength of 1550 nm, the coupling efficiency of this mode converter with a standard single-mode fiber can reach 99.23%, with a coupling loss as low as 0.034 dB / facet, and it has excellent alignment tolerance, with a 1 dB alignment tolerance greater than 2.2 μm in both the y and z directions.

[0044] Another embodiment provides a method for fabricating a double-step waveguide mode converter, which can be used to fabricate such... Figure 1 The pattern converter shown.

[0045] Figure 4 This is a schematic flowchart of the preparation method provided in this embodiment.

[0046] like Figure 4 As shown, the preparation method provided in this embodiment includes operations S1 to S6.

[0047] Step S1: Provide a substrate and deposit a lower cladding layer.

[0048] Specifically, substrate 1 can be a 4-inch quartz wafer or a silicon wafer. Then, a layer of silicon dioxide is deposited on substrate 1 as a lower cladding layer 2 by plasma-enhanced chemical vapor deposition (PECVD), the thickness of which is preferably 2-5 μm.

[0049] Step S2: Prepare a thin-film lithium niobate waveguide layer on the lower cladding.

[0050] S2 includes S21 to S23.

[0051] S21, a thin film of lithium niobate is formed on the silicon dioxide undercoat 2 by ion slicing technology, with a thickness of, for example, 300-600 nm.

[0052] S22. Multiple waveguide segments are patterned on a thin-film lithium niobate film using deep ultraviolet (DUV) lithography. The width of each waveguide segment decreases abruptly along the light transmission direction, and the centerlines of each waveguide segment are aligned along the same straight line without lateral offset. For example, five waveguide segments can be defined with widths of 1.0 μm, 0.9 μm, 0.8 μm, 0.7 μm, and 0.6 μm, respectively.

[0053] S23 uses multiple selective etching processes to thin each waveguide segment in a stepped manner, resulting in a distribution where the thickness of each waveguide segment decreases in a stepped manner along the optical transmission direction.

[0054] by Figure 2 Taking the five waveguide segments shown as an example, the specific process of step-by-step thickness reduction includes:

[0055] First etching: Etching is performed in all waveguide regions to form a base thickness, such as 500 nm;

[0056] Second etching; etching is performed in the second to the last segment (e.g., segment 8 to segment 11) region to thin the first thickness (e.g., 50 nm) to make the thickness of these segments 450 nm;

[0057] The third etching; etching is performed in the region from the third segment to the last segment (e.g., segment 9 to segment 11), and then the thickness is reduced a second time (e.g., 50 nm) to make the thickness of these segments 400 nm;

[0058] Fourth etching: Etching is performed in the region from the fourth segment to the last segment (e.g., segment 10 to segment 11), and then the thickness is thinned a third time (e.g., 50 nm) to make the thickness of these segments 350 nm.

[0059] The fifth etching step involves etching only in the final segment (e.g., segment 11) and then thinning the segment by a fourth thickness (e.g., 50 nm) to make the segment 300 nm thick.

[0060] Through the above-mentioned multiple selective etching processes, a stepped structure with progressively decreasing thickness along the light transmission direction is formed, and the width of each segment has been determined by step S21, thereby achieving a step change in both width and thickness.

[0061] Step S3: Deposit the first medium loading layer.

[0062] On the patterned thin-film lithium niobate waveguide layer 3, a silicon nitride layer 4 is continuously deposited by PECVD as the first dielectric loading layer 4, with a preferred thickness of 100-300 nm. This layer continuously covers the entire waveguide segment without the need for patterning etching.

[0063] Step S4: Deposit the second medium loading layer.

[0064] On the first dielectric loading layer 4, a layer of silicon oxynitride is continuously deposited by PECVD as the second dielectric loading layer 5, the thickness of which is preferably 2-4 μm. This layer also covers continuously and does not require patterning.

[0065] Step S5: Deposit the cladding layer.

[0066] On the second dielectric loading layer 5, a layer of silicon dioxide is deposited by PECVD as an upper cladding layer 6, the thickness of which is preferably 1-3 μm, for the purpose of protecting the device.

[0067] Step S6: Polish the end face.

[0068] Finally, the chip's end face (i.e., the input / output end face) is mechanically polished to obtain a smooth coupling end face, thereby reducing scattering loss when the optical fiber is connected to the chip.

[0069] By following the steps described above, a double-step waveguide mode converter with high coupling efficiency and large process tolerance can be fabricated.

[0070] Those skilled in the art will understand that the features described in the various embodiments of this disclosure can be combined and / or combined in various ways, even if such combinations or combinations are not explicitly described in this disclosure. In particular, the features described in the various embodiments of this disclosure can be combined and / or combined in various ways without departing from the spirit and teachings of this disclosure. All such combinations and / or combinations fall within the scope of this disclosure.

[0071] The embodiments of this disclosure have been described above. However, these embodiments are for illustrative purposes only and are not intended to limit the scope of this disclosure. Although various embodiments have been described above, this does not mean that the measures in the various embodiments cannot be used advantageously in combination. Various substitutions and modifications can be made by those skilled in the art without departing from the scope of this disclosure, and all such substitutions and modifications should fall within the scope of this disclosure.

Claims

1. A dual-step waveguide mode converter, characterized in that, include: The following layers are arranged sequentially from bottom to top: substrate (1), lower cladding (2), thin-film lithium niobate waveguide layer (3), first dielectric loading layer (4), second dielectric loading layer (5), and upper cladding (6); The thin-film lithium niobate waveguide layer (3) includes multiple waveguide segments connected in sequence. Each waveguide segment is a straight waveguide with equal width and thickness. The width and thickness between adjacent waveguide segments decrease in a stepwise manner along the optical transmission direction, and the center lines of each waveguide segment are aligned. The first dielectric loading layer (4) is a silicon nitride layer that covers the thin-film lithium niobate waveguide layer (3), and its refractive index is lower than that of the thin-film lithium niobate waveguide layer (3). The second dielectric loading layer (5) is a silicon oxynitride layer, which is disposed above the first dielectric loading layer (4), and its refractive index is between the refractive index of the first dielectric loading layer (4) and the refractive index of the fiber core.

2. The dual-step waveguide mode converter according to claim 1, characterized in that, The width difference between adjacent waveguide segments is 50-200 nm, and the thickness difference is 30-100 nm; the length of each waveguide segment is 50-100 μm.

3. The dual-step waveguide mode converter according to claim 1, characterized in that, The width of the waveguide segment at the end along the optical transmission direction is greater than 500 nm.

4. The dual-step waveguide mode converter according to claim 1, characterized in that, The refractive index of the first dielectric loading layer (4) is 1.8-2.2; the refractive index of the second dielectric loading layer (5) is 1.5-1.

7.

5. The dual-step waveguide mode converter according to claim 1, characterized in that, The thin-film lithium niobate waveguide layer (3) is an X-cut or Z-cut single-crystal lithium niobate thin film.

6. The dual-step waveguide mode converter according to claim 1, characterized in that, The substrate (1) is silicon or quartz, and the lower cladding layer (2) and the upper cladding layer (6) are both silicon dioxide layers.

7. A method for fabricating a double-step waveguide mode converter according to any one of claims 1 to 6, characterized in that, Includes the following steps: A lower cladding layer (2) is deposited on the substrate (1); A thin-film lithium niobate waveguide layer (3) is prepared on the lower cladding layer (2); The thin-film lithium niobate waveguide layer (3) is etched by multiple selective etching processes to form multiple waveguide segments. The width and thickness of each waveguide segment decrease in a stepwise manner along the light transmission direction, and the center lines of each waveguide segment are aligned along the same straight line. A first dielectric loading layer (4) is deposited on the thin-film lithium niobate waveguide layer (3); A second dielectric loading layer (5) is deposited on the first dielectric loading layer (4); An upper cladding layer (6) is deposited on the second medium loading layer (5).

8. The preparation method according to claim 7, characterized in that, The process of etching the thin-film lithium niobate waveguide layer (3) through multiple selective etching processes to form multiple waveguide segments includes: The width of each waveguide segment is defined by photolithography, so that the width of each waveguide segment decreases in a stepwise manner along the light transmission direction, and the center lines of each waveguide segment are aligned along the same straight line. Each waveguide segment is thinned in a stepped manner, so that the thickness of each waveguide segment decreases in a stepped manner along the optical transmission direction.

9. The preparation method according to claim 7, characterized in that, Also includes: The end face of the double-step waveguide mode converter is polished.

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