Spot size converter based on proton exchange and lithium niobate thin film

Abstract

The application discloses a mode spot converter based on proton exchange and a lithium niobate film, which comprises a lithium niobate film, a proton exchange waveguide, a silicon dioxide waveguide, wherein the lithium niobate film comprises a stepped film section, a flat film section and a last film section; the stepped film section has multiple steps, the tread of each step is a plane, and the kick of each step is a concave curve; the tread of the highest step of the stepped film section is flush with the top surface of the flat film section; and the last film section forms an LN ridge waveguide; the proton exchange waveguide is formed on the stepped film section and the flat film section of the lithium niobate film; the stepped film section forms a stepped section of the proton exchange waveguide, and the flat film section forms a transition section of the proton exchange waveguide; and the silicon dioxide waveguide serves as an upper cladding layer of the stepped section of the proton exchange waveguide. The application is used for end face coupling between a chip and an external optical fiber, and has the advantages of high transmission efficiency, small light leakage and low transmission loss.

Description

Proton exchange and lithium niobate thin film-based modulus converter Technical Field

[0001] This invention relates to the field of speckle converters, specifically a speckle converter based on proton exchange and lithium niobate thin films. Background Technology

[0002] Optical communication devices are the core foundation of optical communication systems and crucial components of optical transmission systems. Photonic integrated circuit (PIC) technology represents the mainstream development direction for future optical devices and has been a focus of industry attention and research in recent years. Compared to currently widely used discrete components, PIC technology offers significant advantages in terms of size, power consumption, cost, and reliability, making it the mainstream development direction for future optical devices.

[0003] Lithium niobate (LNOI) crystal is a synthetically produced multifunctional material with excellent electro-optic, acousto-optic, and nonlinear optical properties. It exhibits high transmittance in both the visible and near-infrared bands and is widely used in integrated optics. Due to the high refractive index difference between lithium niobate and silicon dioxide, photonic devices fabricated based on LNOI have seen significant improvements in integration density and device performance, making multi-device integration using LNOI as a platform possible.

[0004] Gas-phase proton exchange, developed from the proton exchange method, is a chemical reaction between lithium niobate and a proton source. At a certain temperature, hydrogen ions (H+) in the proton source replace lithium ions (Li+) in lithium niobate, thereby changing the refractive index of lithium niobate and forming a waveguide.

[0005] However, to effectively utilize thin-film lithium niobate integrated photonic devices, the on-chip waveguide must be connected to other devices in the system in a suitable manner. This connection is known as the I / O (Input / Output) of the optical chip, which is the interface that couples light out of the chip or onto the chip.

[0006] Taking the coupling of a thin-film lithium niobate single-mode waveguide with a common SMF-28 single-mode fiber in photonic systems as an example, the core size of the SMF-28 fiber is an order of magnitude larger than that of a typical thin-film lithium niobate single-mode waveguide. This mode size mismatch results in significant insertion loss, preventing efficient coupling of off-chip optical signals with the on-chip waveguide. To reduce the coupling loss between the thin-film lithium niobate waveguide and the fiber, there are generally two methods: vertical coupling primarily using grating couplers, and edge coupling primarily using mode converters.

[0007] A significant drawback of grating couplers is their dispersive nature, which limits their coupling efficiency and operating bandwidth, and often makes them polarization / wavelength sensitive. In contrast, fiber-to-chip edge couplers (mode field converters) can significantly improve coupling efficiency and achieve a wider operating bandwidth while maintaining low polarization dependence by matching the mode field dimensions of the fiber and the chip edge.

[0008] Various mode converters, such as those based on inverted taper, multi-tip taper, multi-taper, and subwavelength grating metamaterials, have been extensively studied for efficient coupling between lensed fibers and waveguides. However, the performance of these mode converter structures often encounters limitations when it comes to coupling standard single-mode fiber (SMF) with a large mode field diameter (MFD) to a silicon waveguide. Specifically, at the chip endface, the mode field extends symmetrically around the core structure. When attempting to couple an SMF to a chip, the excessively large optical mode field diameter allows light to easily leak from the buried oxide layer (BOX) into the silicon substrate, thus affecting various aspects of the mode converter's performance. Summary of the Invention

[0009] This invention provides a mode spot converter based on proton exchange and lithium niobate thin film to solve the problem of light leakage in existing mode spot converters when used in SMF coupled chips with large optical mode field diameters.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0011] A mode converter based on proton exchange and lithium niobate thin films includes:

[0012] The lithium niobate film 1 comprises three continuous segments along its length: a stepped film segment 2, a flat film segment 3, and a final film segment 4. The stepped film segment 2 has multiple steps, with each step's tread surface 201 being a plane and each step's riser surface 202 being a concave curved surface. The tread surface 201 of the highest step in the stepped film segment 2 is flush with the top surface of the flat film segment 3. The final film segment 4 forms an LN ridge waveguide 5, with the top surface of the LN ridge waveguide 5 being flush with the top surface of the flat film segment 3.

[0013] A proton exchange waveguide 6 is formed on the stepped thin film segment 2 and the flat thin film segment 3 of the lithium niobate thin film 1. The proton exchange waveguide 6 is continuous. The exchange depth of the proton exchange waveguide 6 at each position of the stepped thin film segment 2 and the flat thin film segment 3 is equal to the film thickness at the corresponding position of the stepped thin film segment 2 and the flat thin film segment 3. A proton exchange waveguide stepped segment 601 is formed from the proton exchange waveguide portion of the stepped thin film segment (2). The proton exchange waveguide stepped segment 601 has multiple steps, and the number of steps of the proton exchange waveguide stepped segment 601 is consistent with the number of steps of the stepped thin film segment 2. A proton exchange waveguide transition segment (602) is formed from the proton exchange waveguide portion of the flat thin film segment 3, and the end of the proton exchange waveguide transition segment 602 is connected to the beginning of the LN ridge waveguide 5 of the last thin film segment 4.

[0014] The silicon dioxide waveguide 7 serves as the upper cladding of the proton exchange waveguide step segment 601 in the proton exchange waveguide 6.

[0015] Furthermore, in the stepped film segment 2, the width of each step is equal;

[0016] In the stepped membrane segment 2, except for the tread of the highest step, the treads 201 of the other steps are of equal length, and the tread length of the highest step is greater than the tread length of the other steps.

[0017] Furthermore, in the stepped film segment 2, the concave surfaces that serve as the kick surfaces 202 in each step are all concave surfaces determined by the same quadratic function.

[0018] Furthermore, in the proton exchange waveguide step segment 601, the width of each step is equal;

[0019] In the proton exchange waveguide step segment 601, except for the tread surface of the highest step, the tread surfaces 6011 of the other steps are of equal length, and the tread surface length of the highest step is greater than the tread surface length of the other steps.

[0020] Furthermore, in the proton exchange waveguide stepped segment 601, the shape of the kick surface 6012 of each step is a concave surface determined by a quadratic function that is the same as the concave surface in each step of the stepped thin film segment 2.

[0021] Furthermore, in the proton exchange waveguide step segment 601, the width of each step segment is 6 micrometers.

[0022] Furthermore, in the stepped film segment 2, the length of the kick surface 202 of each step is the same and is greater than 100 micrometers;

[0023] In the proton exchange waveguide stepped section 601, the length of the kick surface 6012 of each step is consistent with the length of the kick surface 202 of each step in the stepped thin film section 2.

[0024] Furthermore, the proton exchange waveguide transition section 602 includes two continuous sections, namely a tapered adiabatic transition section 6021 and a straight transition section 6022, the width of the straight transition section 6022 being consistent with the width of the narrow end of the tapered adiabatic transition section 6021.

[0025] The width of the wide end of the conical adiabatic transition section 6021 in the proton exchange waveguide transition section 602 is consistent with the width of each step in the proton exchange waveguide stepped section 601, and the wide end of the conical adiabatic transition section 6021 in the proton exchange waveguide transition section 602 is continuous and integral with the highest step in the proton exchange waveguide stepped section 601.

[0026] The end of the straight transition section 6022 in the proton exchange waveguide transition section 602 is connected to the beginning of the LN ridge waveguide 5 of the last thin film section 4 as the end of the proton exchange waveguide transition section 602.

[0027] Furthermore, the LN ridge waveguide 5 includes two continuous segments, namely a tapered ridge waveguide segment 501 and a straight ridge waveguide segment 502, the width of the straight ridge waveguide segment 502 being consistent with the width of the narrow end of the tapered ridge waveguide segment 501.

[0028] The width of the wide end of the tapered ridge waveguide segment 501 in the LN ridge waveguide 5 is greater than the width of the end of the proton exchange waveguide transition segment 602. The wide end of the tapered ridge waveguide segment 501 in the LN ridge waveguide 5 serves as the beginning of the LN ridge waveguide 5 and is connected to the end of the proton exchange waveguide transition segment 602.

[0029] Furthermore, the stepped thin film segment 2 has five steps, thereby the proton exchange waveguide stepped segment 601 has a five-step section.

[0030] Furthermore, the silicon dioxide waveguide 7 comprises three silicon dioxide layers, with the uppermost silicon dioxide layer having the lowest refractive index and the lowermost silicon dioxide layer having the highest refractive index.

[0031] Furthermore, both the proton exchange waveguide transition section 602 in the proton exchange waveguide 6 and the LN ridge waveguide 5 use air as the upper cladding.

[0032] This invention combines a proton-exchange waveguide formed using a proton exchange process with an LN ridge waveguide formed using a ridge waveguide etching process, achieving extremely low coupling loss and good polarization characteristics. Innovatively, it proposes using a proton-exchange waveguide as an intermediate transition waveguide between the silicon dioxide waveguide and the LN ridge waveguide, supplemented by a stepped structure of a portion of the proton-exchange waveguide. This achieves a gradual transition in the waveguide height direction, thus enabling a gradual shift from the silicon dioxide waveguide mode to the proton-exchange waveguide mode, minimizing mode abrupt changes, improving coupling efficiency, and effectively reducing loss. The width of the proton-exchange waveguide stepped segment is 6 micrometers, significantly reducing the difficulty of photolithography. Simultaneously, the kicker length of each step in the proton-exchange waveguide stepped segment is greater than 100 micrometers, ensuring a spacing greater than 100 micrometers between any two adjacent steps. This reduces the precision requirements for overlay and allows for mass production using low-cost fabrication processes.

[0033] In this invention, the silicon dioxide waveguide has a relatively weak light confinement capability, allowing for a larger mode field diameter to match the larger 6.5μm optical fiber, thereby significantly reducing the mode adaptation loss between the chip and the fiber. Simultaneously, the silicon dioxide waveguide of this invention utilizes multiple waveguide layers with varying refractive indices to modulate the shape of the modes within the silicon dioxide waveguide, making the modes closer to those in the proton-exchange waveguide beneath the lithium niobate film. This facilitates the conversion of modes within the silicon dioxide waveguide to modes within the proton-exchange waveguide.

[0034] In this invention, the proton-exchange waveguide has natural polarization characteristics, which can better maintain the polarization stability in the subsequent structure and ensure low-loss transmission. The tapered adiabatic transition section in the proton-exchange waveguide transition section can initially compress the mode field size to 4μm. The tapered ridge waveguide section of the subsequent LN ridge waveguide further compresses the mode field size to 1μm, and through adiabatic transmission, it can reduce high-order mode leakage and reduce loss.

[0035] Therefore, this invention is used for end-face coupling between a chip and an external optical fiber, enabling mode field conversion between a large-mode-field diameter optical fiber and a ridge waveguide structure. It has extremely low coupling loss and good polarization characteristics, and also has the advantages of high transmission efficiency, low optical leakage, and low transmission loss. Attached Figure Description

[0036] Figure 1 is a three-dimensional structural diagram of an embodiment of the present invention.

[0037] Figure 2 is a schematic diagram of the three-dimensional structure after removing the silicon dioxide waveguide in an embodiment of the present invention.

[0038] Figure 3 is a top view of the structure of an embodiment of the present invention, wherein: a is a top view of the overall structure, and b is a top view after removing the silicon dioxide waveguide.

[0039] Figure 4 is a front cross-sectional view of the proton exchange waveguide stepped section in an embodiment of the present invention.

[0040] Figure 5 is a front sectional view of the overall structure of an embodiment of the present invention.

[0041] Figure 6 shows the coupling efficiency curve of the present invention in the C-band.

[0042] Figure 7 shows the electric field distribution diagram of the present invention.

[0043] Figure 8 is a diagram showing the optical mode distribution at the input end of this invention.

[0044] Figure 9 is a diagram showing the output optical mode distribution of this invention.

[0045] Explanation of reference numerals in the attached figures: 1-Lithium niobate thin film, 2-Stepped thin film segment, 201-Tread of stepped thin film segment, 202-Kick of stepped thin film segment, 3-Smooth thin film segment, 4-Last thin film segment, 5-LN ridge waveguide, 501-Conical ridge waveguide segment, 502-Straight ridge waveguide segment, 6-Proton exchange waveguide, 601-Stepped segment of proton exchange waveguide, 6011-Tread of stepped segment of proton exchange waveguide, 6012-Kick of stepped segment of proton exchange waveguide, 602-Transition segment of proton exchange waveguide, 6021-Conical adiabatic transition segment, 6022-Straight transition segment, 7-Silicon dioxide waveguide, 8-Substrate, 9-Silicon dioxide insulating layer. Detailed Implementation

[0046] To enable those skilled in the art to better understand the present invention, the embodiments will be described in detail below with reference to the accompanying drawings and examples. This will allow for a full understanding of how the present invention uses technical means to solve technical problems and achieve corresponding technical effects, and to facilitate its implementation. The embodiments of the present invention and the various features within them can be combined with each other without conflict, and all resulting technical solutions are within the protection scope of the present invention.

[0047] Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0048] It should be noted that the terms "comprising" and "having" and any variations thereof in the specification, claims, and accompanying drawings of this invention are intended to cover non-exclusive inclusion.

[0049] As shown in Figures 1, 2, 3, 4, and 5, this embodiment discloses a mode converter based on proton exchange and a lithium niobate thin film, including a silicon substrate 8, a silicon dioxide insulating layer 9, a lithium niobate thin film 1, and a silicon dioxide waveguide 7. The silicon dioxide insulating layer 9 is disposed on the silicon substrate 8 as a buried oxide layer, and the lithium niobate thin film 1 is disposed on the silicon dioxide insulating layer 9 as a core layer.

[0050] The long side of the lithium niobate film 1 is oriented in the left-right direction in Figure 3, with the direction from left to right being the length direction of the lithium niobate film 1. The lithium niobate film 1 consists of three continuous segments along its length: the initial stepped film segment 2, the middle flat film segment 3, and the final film segment 4.

[0051] Among them, the stepped thin film segment 2 is a stepped structure consisting of five steps formed on the basis of the initial film thickness of 600 nm of the lithium niobate thin film 1. The height between the highest and lowest points of each step is 120 nm, and the film thickness of the highest step is 600 nm. The tread surface 201 of each step is a horizontal plane, and the riser surface 202 of each step is a concave surface determined by the same quadratic function, preferably z = Ax. 2 The function, where A is the function coefficient, A=150, is formed by polishing the concave surface.

[0052] In the stepped film segment 2, the width of each step is equal to 50 micrometers, and the length L3 of the riser 202 (i.e., the length of the riser 202 in the left-right direction) of each step is equal to greater than 100 micrometers. In this embodiment, L3 is preferably 150 micrometers. In the stepped film segment 2, except for the tread of the highest step, the length L4 of the tread 201 of the other four steps is equal to 90 micrometers, and the length of the tread of the highest step is greater than the length of the tread 201 of the other steps. Thus, the stepped film segment 2 gradually increases in height from left to right.

[0053] To maintain the initial thickness of 600nm of the lithium niobate film 1, the flat film segment 3 has a continuous and integral structure with the highest step in the stepped film segment 2 and the left side of the flat film segment 3 along its length direction. The tread surface 201 of the highest step in the stepped film segment 2 is flush with the top surface of the flat film segment 3.

[0054] The final thin film segment 4 is formed by shallow etching of the LN ridge waveguide 5 on the basis of the initial thickness of the lithium niobate thin film 1 (600 nm). The etching depth is half of the initial thickness of the lithium niobate thin film 1 (600 nm). That is, the remaining part of the final thin film segment 4 after etching, excluding the LN ridge waveguide 5, has a film thickness of 300 nm. The height of the LN ridge waveguide 5 itself (i.e., the height between the top surface of the LN ridge waveguide 5 and the top surface of the remaining part of the final thin film segment 4) is 300 nm. The flat thin film segment 3 is continuous and integral with the overall final thin film segment 4 on the left side of the length direction.

[0055] The LN ridge waveguide 5 comprises two continuous segments along its length: a tapered ridge waveguide segment 501 and a straight ridge waveguide segment 502. The width W5 of the straight ridge waveguide segment 502 is the same as the narrow end of the tapered ridge waveguide segment 501, both being 1.2 micrometers. The width W4 of the wide end of the tapered ridge waveguide segment 501 is 4.6 micrometers, thus the tapered ridge waveguide segment 501 of the LN ridge waveguide 5 exhibits a gradually decreasing width along its length. The wide end of the tapered ridge waveguide segment 501 serves as the beginning of the LN ridge waveguide 5, the narrow end of the tapered ridge waveguide segment 501 is continuous with the beginning of the straight ridge waveguide segment 502, and the end of the straight ridge waveguide segment 502 serves as the end of the LN ridge waveguide 5. Furthermore, the beginning of the LN ridge waveguide 5 is located on the right side of the length direction of the flat film segment 3, the end of the LN ridge waveguide 5 is flush with the right side of the length direction of the etched last film segment 4, and the overall top surface of the LN ridge waveguide 5 is flush with the top surface of the flat film segment 3.

[0056] A continuous proton exchange waveguide 6 is formed in the stepped thin film segment 2 and the flat thin film segment 3 through a proton exchange process. A proton exchange waveguide stepped segment 601 is formed from the proton exchange waveguide portion of the stepped thin film segment 2, and a proton exchange waveguide transition segment 602 is formed from the proton exchange waveguide portion of the flat thin film segment 3. The proton exchange waveguide stepped segment 601 has multiple steps, and the number of steps is consistent with that of the stepped thin film segment 2; the proton exchange waveguide stepped segment 601 has five steps.

[0057] The exchange depth of the proton exchange waveguide 6 at each location in the stepped thin film segment 2 and the flat thin film segment 3 is equal to the film thickness at the corresponding location in the stepped thin film segment 2 and the flat thin film segment 3. That is, the film thickness of the proton exchange waveguide portion formed on the tread surface 201 of each step in the stepped thin film segment 2 is the same as the film thickness of the tread surface 201 of each step in the stepped thin film segment 2; the film thickness of the proton exchange waveguide portion formed on each location on the kick surface 202 of each step in the stepped thin film segment 2 is the same as the film thickness of the kick surface 202 of each step in the stepped thin film segment 2; and the film thickness of the proton exchange waveguide portion formed on the flat thin film segment 3 is the same as the film thickness of the flat thin film segment 3, both being 600 nm.

[0058] In the proton exchange waveguide stepped segment 601, the width W2 of each step is equal, all being 6 micrometers. Except for the tread surface 6011 of the highest step, the lengths of the tread surfaces 6011 of the other four steps in the proton exchange waveguide stepped segment 601 are equal to the lengths L4 of the tread surfaces 201 of the other steps in the stepped thin film segment 2, all being 90 micrometers. The length of the tread surface of the highest step in the proton exchange waveguide stepped segment 601 is greater than the lengths of the tread surfaces 6011 of the other steps. Furthermore, the shape of the kick surfaces 6012 of each step in the proton exchange waveguide stepped segment 601 is a concave surface determined by the same quadratic function that defines the concave surface of the kick surface 202 in each step of the stepped thin film segment 2. The lengths of the kick surfaces 6012 of each step in the proton exchange waveguide stepped segment 601 are equal to the lengths L3 of the kick surfaces 202 of each step in the stepped thin film segment 2, both being 150 micrometers.

[0059] The proton exchange waveguide transition section 602 comprises two continuous segments along its length: a tapered adiabatic transition section 6021 and a straight transition section 6022. The width W3 of the straight transition section 6022 is the same as the width of the narrow end of the tapered adiabatic transition section 6021, both being 4 micrometers. This makes the width W3 of the straight transition section 6022 smaller than the width W4 of the wide end of the tapered ridge waveguide section 501 in the LN ridge waveguide 5.

[0060] In the proton exchange waveguide transition section 602, the width of the wide end of the tapered adiabatic transition section 6021 is the same as the width W2 of each step in the proton exchange waveguide stepped section 601, both being 6 micrometers. Specifically, the wide end of the tapered adiabatic transition section 6021 in the proton exchange waveguide transition section 602 is continuous with the highest step in the proton exchange waveguide stepped section 601. The beginning of the straight transition section 6022 is continuous with the narrow end of the tapered adiabatic transition section 6021. The end of the straight transition section 6022 serves as the end of the proton exchange waveguide transition section 602, and is located on the right side of the length direction of the flat thin film section 3. Furthermore, the end of the straight transition section 6022 connects to the beginning of the LN ridge waveguide 5 of the last thin film section 4.

[0061] The silicon dioxide waveguide 7 is entirely overlaid on the proton exchange waveguide step segment 601 of the proton exchange waveguide 6, serving as the upper cladding of the proton exchange waveguide step segment 601. The overall width W1 of the silicon dioxide waveguide 7 is 7.1 micrometers, and the overall length L1 of the silicon dioxide waveguide 7 is 1250 micrometers. The silicon dioxide waveguide 7 serves as the coupling initiation segment between the fiber and the chip, and its dimensions should match those of the fiber, allowing for a mode field diameter of 6.5 μm.

[0062] The silicon dioxide waveguide 7 consists of three layers of silicon dioxide with different thicknesses and refractive indices. The uppermost silicon dioxide layer has the lowest refractive index, and the lowermost silicon dioxide layer has the highest refractive index. In this embodiment, the thickness of the uppermost silicon dioxide layer is 1.8 micrometers, and its refractive index for a light source with a wavelength of 1550 nm is 1.43; the thickness of the middle silicon dioxide layer is 2 micrometers, and its refractive index for a light source with a wavelength of 1550 nm is 1.45; the thickness of the lowermost silicon dioxide layer is 2.4 micrometers, and its refractive index for a light source with a wavelength of 1550 nm is 1.46.

[0063] In the proton exchange waveguide 6, the proton exchange waveguide transition section 602 and the LN ridge waveguide 5 of the last thin film section 4 both use air as the upper cladding.

[0064] Figure 6 shows the coupling efficiency curve in the C-band of this embodiment. The coupling efficiency from the 6.5-micron mode field diameter fiber to the proton exchange waveguide transition section 602 reaches over 95% within the C-waveguide. The mode overlap integral between the proton exchange waveguide transition section 602 and the tapered ridge waveguide section 501 of the LN ridge waveguide 5 is 0.993124. The overall device insertion loss is 0.1847 dB / face under a 1550 nm wavelength light source.

[0065] The electric field distribution diagram of this embodiment is shown in Figure 7. The left side is the fiber input, and the mode field size at the fiber output is about 6.5 μm. The optical mode distribution immediately after entering the silicon dioxide waveguide 7 is shown in Figure 8. At the concave curved surface 6012 of the first step of the proton exchange waveguide step 601, the light is rapidly coupled into the proton exchange waveguide step 601, but some of it still diffuses within the silicon dioxide waveguide 7. As the proton exchange waveguide step 601 extends along the step in the length direction, the portion of light diffused in the silicon dioxide waveguide 7 gradually couples into the proton exchange waveguide step 601. After the proton exchange waveguide step 601, the light almost completely enters the proton exchange waveguide transition section 602. The electric field distribution diagram of the optical mode is shown in Figure 9, realizing the coupling from the optical fiber to the lithium niobate waveguide. Subsequently, as the shape of the lithium niobate waveguide changes, the optical mode is further compressed.

[0066] The light in the optical fiber first enters the silica waveguide 7. The silica waveguide 7 has a weaker ability to confine the light, which can achieve a larger mode field diameter, matching the larger size of the optical fiber with a mode field diameter of 6.5μm. At the same time, the silica waveguide 7 with multiple layers of different refractive index distributions further adjusts the mode shape, pulling the mode center to a position close to the lower proton exchange waveguide step segment 601, thereby improving the efficiency of mode conversion into the proton exchange waveguide 6.

[0067] Most existing lithium niobate mode converters employ an inverted conical structure with an extremely narrow conical waveguide in the width direction to achieve high-efficiency mode size conversion. This invention, however, uses a conical waveguide in the height direction and a proton exchange waveguide 6 with a multi-layered stepped structure to achieve high-efficiency mode conversion, enabling a gradual transition from the silica waveguide 7 mode to the proton exchange waveguide 6 mode, minimizing mode abrupt changes and reducing coupling loss. Simultaneously, the proton exchange waveguide 6 possesses natural polarization characteristics, effectively maintaining polarization stability in subsequent structures and ensuring low-loss transmission. Furthermore, the conical adiabatic transition section 6021 of the proton exchange waveguide transition section 602 can initially compress the mode field size to 4 μm; the conical ridge waveguide section 501 of the subsequent LN ridge waveguide 5 further compresses the mode field size to 1 μm, and through adiabatic transmission, it reduces higher-order mode leakage and lowers loss. This achieves high-efficiency coupling from the optical fiber to the lithium niobate waveguide.

[0068] Furthermore, the overlay precision requirement for the pattern converter structure of the present invention is low, and it can be mass-produced using a low-cost manufacturing process.

[0069] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings. These embodiments are merely descriptions of preferred embodiments and are not intended to limit the scope or concept of the invention. The specific technical features described in the above embodiments can be combined in any suitable manner without contradiction. Such combinations, as long as they do not violate the spirit of the present invention, should also be considered as part of this disclosure. To avoid unnecessary repetition, the present invention will not further describe the various possible combinations.

[0070] This invention is not limited to the specific details of the above embodiments. Within the scope of the technical concept of this invention and without departing from the design idea of ​​this invention, all modifications and improvements made by those skilled in the art to the technical solutions of this invention should fall within the protection scope of this invention. The technical content for which protection is sought in this invention has been fully described in the claims.

Claims

1. A mode converter based on proton exchange and lithium niobate thin film, characterized in that, include: A lithium niobate thin film (1) comprises three continuous segments along its length: a stepped thin film segment (2), a flat thin film segment (3), and a final thin film segment (4). The stepped thin film segment (2) has multiple steps, with each step's tread (201) being a plane and each step's kick surface (202) being a concave surface. The tread (201) of the highest step in the stepped thin film segment (2) is flush with the top surface of the flat thin film segment (3). The final thin film segment (4) forms an LN ridge waveguide (5), with the top surface of the LN ridge waveguide (5) flush with the top surface of the flat thin film segment (3). A proton exchange waveguide (6) is formed in the stepped thin film segment (2) and the flat thin film segment (3) of the lithium niobate thin film (1), and the proton exchange waveguide (6) is continuous. The proton exchange waveguide (6) is located in the stepped thin film segment (2). The exchange depth at each position of the flat thin film segment (3) is equal to the film thickness at the corresponding position of the stepped thin film segment (2) and the flat thin film segment (3); a proton exchange waveguide stepped segment (601) is formed by the proton exchange waveguide portion of the stepped thin film segment (2), the proton exchange waveguide stepped segment (601) has multiple steps, and the number of steps of the proton exchange waveguide stepped segment (601) is consistent with the number of steps of the stepped thin film segment (2); a proton exchange waveguide transition segment (602) is formed by the proton exchange waveguide portion of the flat thin film segment (3), and the end of the proton exchange waveguide transition segment (602) is connected to the beginning of the LN ridge waveguide (5) of the last thin film segment (4); a silicon dioxide waveguide (7) serves as the upper cladding of the proton exchange waveguide stepped segment (601) in the proton exchange waveguide (6).

2. The mode converter based on proton exchange and lithium niobate thin film according to claim 1, characterized in that, In the stepped film segment (2), the width of each step is equal; in the stepped film segment (2), except for the tread of the highest step, the length of the tread (201) of the other steps is equal, and the length of the tread of the highest step is greater than the length of the tread (201) of the other steps; and in the stepped film segment (2), the concave surfaces that serve as the kick surfaces (202) in each step are all concave surfaces determined by the same quadratic function.

3. The mode converter based on proton exchange and lithium niobate thin film according to claim 2, characterized in that, In the proton exchange waveguide stepped section (601), the width of each step is equal; in the proton exchange waveguide stepped section (601), except for the tread of the highest step, the treads (6011) of the other steps are of equal length, and the tread length of the highest step is greater than the tread length of the other steps; and in the proton exchange waveguide stepped section (601), the shape of the kick surface (6012) of each step is a concave surface determined by a quadratic function that determines the concave surface of each step in the stepped thin film section (2).

4. The mode converter based on proton exchange and lithium niobate thin film according to claim 3, characterized in that, In the proton exchange waveguide stepped section (601), the width of each step is 6 micrometers.

5. The mode converter based on proton exchange and lithium niobate thin film according to claim 3, characterized in that, In the stepped thin film segment (2), the length of the kick surface (202) of each step is the same and is greater than 100 micrometers; in the proton exchange waveguide stepped segment (601), the length of the kick surface (6012) of each step is consistent with the length of the kick surface (202) of each step in the stepped thin film segment (2).

6. The mode converter based on proton exchange and lithium niobate thin film according to claim 1, characterized in that, The proton exchange waveguide transition section (602) comprises two continuous sections, namely a conical adiabatic transition section (6021) and a straight transition section (6022). The width of the straight transition section (6022) is the same as the width of the narrow end of the conical adiabatic transition section (6021). The width of the wide end of the conical adiabatic transition section (6021) in the proton exchange waveguide transition section (602) is the same as the width of each step in the proton exchange waveguide stepped section (601). The wide end of the conical adiabatic transition section (6021) in the proton exchange waveguide transition section (602) is continuous with the highest step in the proton exchange waveguide stepped section (601). The end of the straight transition section (6022) in the proton exchange waveguide transition section (602) is connected to the beginning of the LN ridge waveguide (5) of the last thin film section (4).

7. The mode converter based on proton exchange and lithium niobate thin film according to claim 1, characterized in that, The LN ridge waveguide (5) comprises two continuous segments, namely a tapered ridge waveguide segment (501) and a straight ridge waveguide segment (502). The width of the straight ridge waveguide segment (502) is the same as the width of the narrow end of the tapered ridge waveguide segment (501). The width of the wide end of the tapered ridge waveguide segment (501) in the LN ridge waveguide (5) is greater than the width of the end of the proton exchange waveguide transition segment (602). The wide end of the tapered ridge waveguide segment (501) in the LN ridge waveguide (5) serves as the beginning of the LN ridge waveguide (5) and is connected to the end of the proton exchange waveguide transition segment (602).

8. The mode converter based on proton exchange and lithium niobate thin film according to any one of claims 1-7, characterized in that, The stepped thin film segment (2) has five steps, thereby the proton exchange waveguide stepped segment (601) has a five-step section.

9. The mode converter based on proton exchange and lithium niobate thin film according to any one of claims 1-7, characterized in that, The silicon dioxide waveguide (7) comprises three silicon dioxide layers, with the uppermost silicon dioxide layer having the lowest refractive index and the lowermost silicon dioxide layer having the highest refractive index.

10. The mode converter based on proton exchange and lithium niobate thin film according to any one of claims 1-7, characterized in that, The proton exchange waveguide transition section (602) in the proton exchange waveguide (6) and the LN ridge waveguide (5) both use air as the upper cladding.

Images