Low-coupling-loss spot size converter based on thin-film lithium niobate platform

Abstract

The invention relates to the technical field of optical fiber sensors, and discloses a low-coupling-loss spot size converter based on a film lithium niobate platform, which comprises a substrate, a lower cladding and a lithium niobate film layer located on the lower cladding, the lithium niobate film layer comprises a first waveguide part, a second waveguide part, a third waveguide part and a lens body which are connected in sequence, the lens body is located on the lower cladding, the lens layer is connected with the lens body, the first waveguide part and the second waveguide part are coated with the lens layer, and the first end of the first waveguide part is connected with the lens body; light is sequentially transmitted in the lens body, the first waveguide part, the second waveguide part and the third waveguide part, and in the light transmission direction, the transverse sizes of the first waveguide part, the second waveguide part and the third waveguide part are sequentially increased, and the transverse size of the lens layer is sequentially reduced, so that mode field conversion between the single-mode optical fiber and the ridge waveguide structure is realized; the structure size is smaller, the coupling loss is lower, and the matching between the waveguide and an optical fiber with a larger mode field diameter can be efficiently realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of optical fiber sensor, in particular to a low coupling loss mode spot converter based on a thin film lithium niobate platform. BACKGROUND

[0002] With the development of information society, people's production and life require higher speed and quantity of data transmission, and higher frequency of electrical interconnection, which causes problems such as limited bandwidth, signal distortion and greater interference, and the realization of ultra-high-speed circuit has encountered a bottleneck. However, the emergence of optical communication has broken through this bottleneck, making long-distance high-speed communication possible, greatly facilitating people's production and life, and therefore, optical communication has become one of the most important technological advances in the twenty-first century.

[0003] Optical communication devices are the core foundation of optical communication systems and important devices of optical transmission systems. The technology is a strategic high-tech with foresight, leadership and exploration in the field of optical communication, and represents the level of a country in the field of optical communication.

[0004] PIC (Photonic Integrated Circuit) is the mainstream development direction of future optical devices, and has been the focus of attention and research in the industry in recent years. Compared with the discrete components widely used at present, PIC has obvious advantages in size, power consumption, cost and reliability, and is the mainstream development direction of future optical devices.

[0005] Lithium niobate crystal is a kind of artificial synthetic multifunctional material, which has excellent electro-optic, acousto-optic and nonlinear optical properties, and has high transmittance in the visible and near-infrared wavebands, and is widely used in integrated optics. Due to the high refractive index difference between lithium niobate and silicon dioxide, the integration level and device performance of photonic devices based on LNOI material have been greatly improved, and it has become possible to realize multi-device integration based on LNOI platform.

[0006] However, in order to effectively utilize thin film lithium niobate integrated photonic devices, it is necessary to connect the on-chip waveguide with other devices in the system through a suitable way. This connection is the I / O (Input / Output) of the optical chip, that is, the light is coupled out of the chip or coupled to the interface on the chip. Taking the coupling between thin film lithium niobate single mode waveguide and common SMF-28 single mode fiber in photonic system as an example. The core size of SMF-28 fiber is one order of magnitude larger than that of typical thin film lithium niobate single mode waveguide, and the mismatch of mode size causes huge insertion loss in coupling, which makes the off-chip optical signal unable to be coupled with the on-chip waveguide efficiently. In order to reduce the coupling loss between thin film lithium niobate waveguide and optical fiber, there are generally two ways, one is vertical coupling mode mainly based on grating coupler, and the other is edge coupling mode mainly based on mode spot converter.

[0007] One significant drawback of grating couplers is that their operation principle has a dispersive nature, which limits their coupling efficiency and operational bandwidth, and often makes them polarization / wavelength sensitive. In contrast, fiber-chip edge couplers (mode field transformers) can significantly improve the coupling efficiency, achieve a wider operational bandwidth, and maintain a lower polarization dependence by matching the mode field size of the fiber and the chip edge.

[0008] Various mode field transformers, such as devices based on inverse taper, multi-tipped taper, multi-taper, and sub-wavelength grating metamaterial, have been widely studied for efficient coupling between lensed fibers and waveguides. However, when it comes to coupling standard single-mode fibers (SMFs) with large MFD to silicon waveguides, these structures often encounter limitations. Specifically, at the chip facet, the mode field expands symmetrically around the core structure. When trying to couple the SMF with the chip, due to the large optical mode field, light is easily leaked from the buried oxide layer (BOX) to the Si substrate, thereby affecting the performance of the mode field transformer.

[0009] Moreover, current mainstream mode field transformers (such as step-type, multi-tipped taper, etc.) usually achieve mode matching only through single-dimensional waveguide geometry regulation (such as optimizing the transverse width or longitudinal etching depth). This asymmetric design method fails to simultaneously regulate the two-dimensional localization factor of the waveguide cross-section, resulting in strict adiabatic conditions for the mode evolution process, forcing the length of the conversion region to expand and increasing the structure size. In addition, it also leads to intensified evanescent field leakage and high-order mode excitation, reducing the coupling efficiency. SUMMARY

[0010] In view of the deficiencies in the prior art, the purpose of the present application is to provide a low-coupling-loss mode field transformer based on a thin-film lithium niobate platform for end-face coupling of a chip and an external optical fiber, achieving more efficient and lower-loss transmission. The mode field transformer is designed by combining a graded-index lens with a multi-layer inverse taper structure, achieving extremely low coupling loss and small device size. In addition, the method and structure design of transitioning from a triangular waveguide cross-section to a trapezoidal waveguide cross-section are innovatively proposed, which simultaneously realizes slow linear adiabatic transitions in the waveguide width and height, improving the coupling efficiency of the device.

[0011] To achieve the above purpose, the present application provides the following technical solutions:

[0012] The low-coupling-loss mode field transformer based on a thin-film lithium niobate platform comprises:

[0013] a substrate, the substrate is made of a silicon sheet as a whole;

[0014] a lower cladding layer located on the substrate, the lower cladding layer is made of silicon dioxide;

[0015] a lithium niobate thin film layer located on the lower cladding layer;

[0016] The lithium niobate thin film layer comprises:

[0017] The first waveguide part, the second waveguide part and the third waveguide part are connected in sequence,

[0018] The lens body is located on the lower cladding layer,

[0019] The lens layer is connected with the lens body, and the lens layer is coated on the first waveguide part and the second waveguide part, and the first end of the first waveguide part is connected with the lens body.

[0020] The light is transmitted in the lens body, the first waveguide part, the second waveguide part and the third waveguide part in sequence, and in the direction of light transmission, the lateral dimensions of the first waveguide part, the second waveguide part and the third waveguide part increase in sequence, and the lateral dimensions of the lens layer decrease in sequence, so as to realize the mode field conversion between the single-mode optical fiber and the ridge waveguide structure.

[0021] In the application, preferably, the lens body adopts a plurality of layers of gradient refractive index material, the first end of the second lens layer (8) is connected with the second end of the bottommost layer of the lens body, and the refractive index is consistent with that of the bottommost layer of the lens body. The parabolic variation law of the refractive index of the material with the thickness of the lens is determined according to the existing formula, and the material is designed by layering according to the root mean square calculation method, and the refractive index of each layer is calculated. The number of layers can be set to five.

[0022] In the application, preferably, the first waveguide part is a single-layer lithium niobate inverse conical structure, and the cross-sectional dimension of the first waveguide part linearly increases from the first end to the second end in the direction of light propagation.

[0023] In the application, preferably, the cross section of the first end of the first waveguide part is triangular, and the cross section of the second end of the first waveguide part is trapezoidal. The sidewall of the first waveguide part has a certain inclination angle, and the first end serves as the starting position and has a smaller starting base width, and the cross section is triangular. The base width and the height of the first waveguide part linearly increase with the length of the conical shape until the height reaches a set value, and then the cross section of the first waveguide part changes to trapezoidal, so as to facilitate the mode field matching with the third waveguide part. The simultaneous linear tapering increase of the height and the width can effectively improve the coupling efficiency and accelerate the mode field compression. The smaller starting width and height are helpful to match the larger mode field output by the lens part.

[0024] In the application, preferably, the second waveguide part is a double-layer lithium niobate inverse conical structure, which comprises a first waveguide layer and a second waveguide layer, and the first waveguide layer is arranged on the second waveguide layer. The thicknesses of the first waveguide layer and the second waveguide layer are the same, and are each half of the thickness of the second end of the first waveguide part. The first waveguide layer and the second waveguide layer are connected with the first waveguide part, and the first waveguide layer and the second waveguide layer adopt the same sidewall inclination angle as the first waveguide part.

[0025] In the application, preferably, the top width of the first end of the first waveguide layer is consistent with the top width of the second end of the first waveguide part, the bottom width of the second waveguide layer is consistent with the bottom width of the second end of the first waveguide part, and the top width of the second end of the second waveguide layer is greater than the bottom width of the second end of the first waveguide layer.

[0026] In the application, preferably, the cross sections of the first waveguide layer and the second waveguide layer are both trapezoidal, and the widths of the first waveguide layer and the second waveguide layer increase linearly along the direction of light propagation, and the width of the second end of the second waveguide layer is consistent with the width of the second end of the lens layer.

[0027] In the application, preferably, the width, thickness and refractive index of the first end of the lens layer covered by the first waveguide part and the second waveguide part are consistent with the width, thickness and refractive index of the bottom layer of the lens body, and the width of the second end of the lens layer is consistent with the width of the second end of the second waveguide part. The first end of the lens layer is connected with the bottom layer of the lens body, and the lens layer is made of silicon oxynitride material, which continuously narrows in width and gradually decreases in thickness along the direction of light propagation.

[0028] In the application, preferably, the third waveguide part is a double-layer lithium niobate ridge waveguide structure, and the inclination angle of the side wall of the third waveguide part is consistent with that of the second waveguide part. The third waveguide part includes a third waveguide layer and a fourth waveguide layer, the third waveguide layer is arranged on the fourth waveguide layer, the third waveguide layer is connected with the first waveguide layer, the fourth waveguide layer is connected with the second waveguide layer, and the thicknesses of the third waveguide layer and the fourth waveguide layer are consistent with the thicknesses of the first waveguide layer and the second waveguide layer.

[0029] In the application, preferably, the first waveguide part, the first waveguide layer and the third waveguide layer are a lithium niobate central core layer connected together, and the second waveguide layer and the fourth waveguide layer are lithium niobate waveguide layers connected together.

[0030] In the application, preferably, the width of the first end of the third waveguide layer is the same as the width of the second section of the first waveguide layer, and the overall width of the fourth waveguide layer is consistent with the width of the lower cladding layer.

[0031] Compared with the prior art, the application has the following beneficial effects:

[0032] The application adopts the design of combining the gradient refractive index lens with the inverse taper structure, and the structure of the transition from the triangular waveguide section to the trapezoidal waveguide section realizes the linear taper transition in two dimensions of the waveguide width and height. In the inverse taper structure alone, in order to realize the compression conversion of a larger mode field, a longer adiabatic transition is usually needed to reduce the loss. The design of combining the lens layer with the inverse taper waveguide structure can accelerate the process of mode field compression, and the lens part can quickly compress the longitudinal height of the mode field to less than one half, so that the subsequent inverse taper waveguide only needs to convert the compressed mode field into a mode field size matched with the waveguide. The fused structure has smaller structure size and lower coupling loss, and can efficiently realize the matching between the waveguide and the optical fiber with a larger mode field diameter.

[0033] The mode spot converter of the application realizes the optical field transmittance of ≥95% (corresponding to an ultra-low coupling loss of 0.2 dB) for TE polarized light at a working wavelength of 1550 nm; when working in the TM polarization state, the coupling loss difference with the TE mode is controlled within 2%, verifying the strong polarization insensitivity of the device. At the same time, the fiber-chip alignment tolerance is ≥±1.5 μm, the overall size of the device is compressed to within 8 μm x 500 μm, and the efficient coupling between the high-confinement thin-film lithium niobate waveguide with a ridge width of 1.2 μm and the large-diameter optical fiber is successfully realized, with the three core advantages of low transmission loss, sub-millimeter compact configuration and excellent tolerance, providing a key I / O solution for high-density photonic integrated systems. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 The application provides a top view schematic diagram of a low-coupling-loss mode spot converter based on a thin-film lithium niobate platform.

[0035] Figure 2 The application provides a three-dimensional structure schematic diagram of a low-coupling-loss mode spot converter based on a thin-film lithium niobate platform.

[0036] Figure 3 The application provides side view sectional views of a low-coupling-loss mode spot converter based on a thin-film lithium niobate platform.

[0037] Figure 4 The application provides a partial structure enlarged view of a first waveguide part of a low-coupling-loss mode spot converter based on a thin-film lithium niobate platform.

[0038] Figure 5 The application provides a refractive index gradient curve of a lens body of a low-coupling-loss mode spot converter based on a thin-film lithium niobate platform.

[0039] Figure 6A schematic diagram of the relationship curve of the fiber and the lens body y direction alignment tolerance of the low coupling loss mode spot converter based on the thin film lithium niobate platform according to the present application.

[0040] Figure 7 A schematic diagram of the relationship curve of the fiber and the lens body z direction alignment tolerance of the low coupling loss mode spot converter based on the thin film lithium niobate platform according to the present application.

[0041] Figure 8 A schematic diagram of the relationship curve of the transmittance and the tip width of the first waveguide section of the low coupling loss mode spot converter based on the thin film lithium niobate platform according to the present application.

[0042] Figure 9 A schematic diagram of the relationship curve of the transmittance and the sidewall angle of the first waveguide section of the low coupling loss mode spot converter based on the thin film lithium niobate platform according to the present application.

[0043] Figure 10 A transmission mode field diagram of the low coupling loss mode spot converter based on the thin film lithium niobate platform according to the present application at the working wavelength of 1550 nm for TE polarized light.

[0044] Figure 11 Different cross-section position mode field diagrams of the low coupling loss mode spot converter based on the thin film lithium niobate platform according to the present application at the working wavelength of 1550 nm for TE polarized light.

[0045] In the drawings: 1 - substrate, 2 - lower cladding layer, 3 - lithium niobate thin film layer, 4 - first waveguide section, 5 - second waveguide section, 6 - third waveguide section, 7 - lens body, 8 - lens layer, 9 - optical fiber. DETAILED DESCRIPTION

[0046] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0047] It should be noted that when a component is referred to as being "fixed" to another component, it can be directly on the other component or there can be intervening components. When a component is referred to as being "connected" to another component, it can be directly connected to the other component or there can be intervening components. When a component is referred to as being "disposed" on another component, it can be directly on the other component or there can be intervening components. The terms "vertical", "horizontal", "left", "right", and similar expressions used herein are for illustrative purposes only.

[0048] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description herein is for describing particular embodiments only and is not intended to be limiting of the application. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0049] Referring to Figure 1 A preferred embodiment of the present application provides a low coupling loss mode spot converter based on a thin film lithium niobate platform, comprising a substrate 1 made of silicon sheet; a lower cladding layer 2 made of silicon dioxide, which is located on the substrate 1; a lithium niobate thin film layer 3, which is located on the lower cladding layer 2; wherein the lithium niobate thin film layer 3 comprises: a first waveguide part 4, a second waveguide part 5 and a third waveguide part 6 connected in sequence, a lens body 7 located on the lower cladding layer 2, a lens layer 8 connected with the lens body 7, the lens layer 8 covering the first waveguide part 4 and the second waveguide part 5, and the first end of the first waveguide part 4 being connected with the lens body 7; light is transmitted in the lens body 7, the first waveguide part 4, the second waveguide part 5 and the third waveguide part 6 in sequence, and in the direction of light transmission, the lateral dimensions of the first waveguide part 4, the second waveguide part 5 and the third waveguide part 6 increase in sequence, the side wall inclination angles of the first waveguide part 4, the second waveguide part 5 and the third waveguide part 6 are uniform, and the lateral dimensions of the lens layer 8 decrease in sequence.

[0050] Specifically, as shown in Figure 1 and 2 , the input optical fiber 9 adopts a SMF standard single-mode optical fiber, and the coupling surface of the device is arranged in the yz direction, and the transmission direction of the light is the x direction; after the light is coupled by the optical fiber, it first enters the SiON graded refractive index lens body 7, and the mode field is compressed slightly; then it enters the first waveguide part 4 of the single-layer LN (Lithium niobate, lithium niobate crystal) inverse taper structure, the starting first end of the first waveguide part 4 is a waveguide with a triangular cross section, as shown in Figure 3 (1), which can realize simultaneous adiabatic transition in two directions and speed up the mode field compression process; the second end of the first waveguide part 4 is a waveguide with a trapezoidal cross section, as shown in Figure 3 (2), which enters the second waveguide part 5, and the double-layer LN inverse taper structure of the second waveguide part 5 is mainly used for the transition of the LN single-layer waveguide to the LN ridge waveguide, to suppress the leakage of high-order modes and reduce the loss.

[0051] As shown in Figure 3 , the third waveguide part 6 is a waveguide with a rectangular cross section, as shown in Figure 3(1) is the first end of the first waveguide section 4, i.e. the starting position, and a triangular side view cross section is formed due to the side wall of the first waveguide section 4 having a certain angle and the waveguide bottom width being small at this time; as the waveguide bottom width gradually increases, the height of the triangular cross section waveguide also increases simultaneously, and when the width increases to a certain value, the cross section of the waveguide becomes trapezoidal and the height no longer changes, and the cross section of the second end of the first waveguide section 4 is shown in (2) of Figure 3 ; Figure 3 (3) is a side view cross section of the second waveguide section 5, and the width of the lower second waveguide layer 52 increases at a speed greater than the width increase speed of the upper first waveguide layer 51, so as to facilitate the transition from the single-layer waveguide to the ridge waveguide structure.

[0052] As shown in Figure 3 and Figure 4 , the waveguide cross section of the single-layer first waveguide section 4 has a transition process from a triangular shape to a trapezoidal shape, so the top width of the first waveguide section 4 is 0 in the first half of the structure, and the top width increases with the increase of the waveguide bottom width when the cross section shape of the first waveguide section 4 changes from a triangular shape to a trapezoidal shape in the second half of the structure; in addition, after the waveguide top width increases to greater than 0, i.e. when the cross section shape of the first waveguide section 4 changes from a triangular shape to a trapezoidal shape, the height of the first waveguide section 4 no longer changes.

[0053] As shown in Figure 5 , the refractive index variation law of the SiON graded refractive index lens body 7 is shown by the black solid line in the figure; the average algorithm is used to obtain the refractive index values corresponding to each segment when the lens body 7 is divided into 8 layers, which is shown by the black dotted line; for the convenience of process, the standard deviation algorithm is used to obtain the refractive index values corresponding to each segment when the lens is divided into 5 segments, which is shown by the black dashed line.

[0054] As shown in Figure 2 , 6 and 7, Figure 6 , the transmittance curve when the fiber center and the SiON lens center are misaligned in the y direction, Figure 7 , the transmittance curve when the fiber center and the SiON lens center are misaligned in the z direction, the 0.5 dB alignment tolerance in the y direction is about ±1.5 μm, and the 0.5 dB alignment tolerance in the z direction is about -1.7 μm-0.7 μm, and the loss is lowest when -0.5 μm.

[0055] As shown in Figure 3 , 4 , 8 and 9, Figure 8 , the transmittance curve when the tip width Wtip changes, Figure 9 , the transmittance curve when the waveguide side wall angle Angle changes, and as shown in the figure, when the tip width is 0.2 μm and the side wall angle is 60°-80°, the overall structure transmittance can be kept relatively high.

[0056] As shown in Figure 10 When inputting TE polarized light of 1550nm wavelength, the transmission mode field of the light in the structure and the overall structure transmittance can reach above 0.95, and the ultra-low coupling loss is about 0.2dB.

[0057] As shown in Figure 11 When inputting TE polarized light of 1550nm wavelength, the mode field schematic diagram of different positions. Figure 11 (a) is the mode field diagram of the coupling surface of the optical fiber 9 to the lens body 7, and the mode field diameter is about 6.5μm; Figure 11 (b) is the mode field diagram of the light after passing through the lens body 7, the transverse mode field diameter is basically unchanged, and the longitudinal mode field diameter is compressed to about half; Figure 11 (c) is the mode field diagram of the third waveguide part 6, the transverse mode field diameter is about 1.2μm, the longitudinal mode field diameter is about 0.6μm, and the light is basically bound in the ridge waveguide.

[0058] Therefore, the low coupling loss mode spot converter based on the thin film lithium niobate platform can realize ultra-low coupling loss and high mode conversion efficiency, and has the advantages of small size and low polarization sensitivity, and can be applied to the edge coupling of optical fibers and chips to realize low-loss optical field coupling.

[0059] The above description is a detailed description of the preferred embodiments of the application, but the embodiments are not used to limit the scope of the patent application of the application, and any equivalent changes or modifications made under the technical spirit of the application should belong to the patent scope covered by the application.

Claims

1. A low coupling loss mode spot converter based on thin film lithium niobate platform, characterized in that, The application relates to a lithium niobate waveguide lens, which comprises the following parts: a substrate (1); a lower cladding layer (2) on the substrate (1); a lithium niobate thin film layer (3) on the lower cladding layer (2); wherein the lithium niobate thin film layer (3) comprises: a first waveguide section (4), a second waveguide section (5) and a third waveguide section (6) connected in sequence, a lens body (7) on the lower cladding layer (2), a lens layer (8) connected with the lens body (7), the lens layer (8) covering the first waveguide section (4) and the second waveguide section (5), and the first end of the first waveguide section (4) being connected with the lens body (7); light is transmitted in the lens body (7), the first waveguide section (4), the second waveguide section (5) and the third waveguide section (6) in sequence, and in the direction of light transmission, the lateral dimensions of the first waveguide section (4), the second waveguide section (5) and the third waveguide section (6) increase in sequence, and the lateral dimensions of the lens layer (8) decrease in sequence.

2. The low coupling loss mode spot converter based on thin film lithium niobate platform of claim 1, wherein, The lens body (7) is made of multilayer gradient refractive index material, the first end of the second lens layer (8) is connected with the bottom layer of the lens body (7), and the refractive index is consistent with the bottom layer of the lens body (7).

3. The low coupling loss mode spot converter based on thin film lithium niobate platform of claim 2, wherein, The first waveguide section (4) is a single-layer lithium niobate inverse conical structure, and the cross-sectional dimension of the first waveguide section (4) linearly increases from the first end to the second end in the direction of light propagation.

4. The low coupling loss mode spot converter based on thin film lithium niobate platform of claim 3, wherein, The cross section of the first end of the first waveguide section (4) is triangular, and the cross section of the second end of the first waveguide section (4) is trapezoidal.

5. The low coupling loss mode spot converter based on thin film lithium niobate platform of claim 2, wherein, The second waveguide section (5) is a double-layer lithium niobate inverse conical structure, comprising a first waveguide layer (51) and a second waveguide layer (52), the first waveguide layer (51) is arranged on the second waveguide layer (52), the thicknesses of the first waveguide layer (51) and the second waveguide layer (52) are the same, and the thicknesses of the first waveguide layer (51) and the second waveguide layer (52) are each half of the thickness of the second end of the first waveguide section (4).

6. The low coupling loss mode spot converter based on thin film lithium niobate platform of claim 5, wherein, The top width of the first end of the first waveguide layer (51) is consistent with the top width of the second end of the first waveguide section (4), the bottom width of the second waveguide layer (52) is consistent with the bottom width of the second end of the first waveguide section (4), and the top width of the second end of the second waveguide layer (52) is greater than the bottom width of the second end of the first waveguide layer (51).

7. The low coupling loss mode spot converter based on thin film lithium niobate platform of claim 1, wherein, The cross sections of the first waveguide layer (51) and the second waveguide layer (52) are trapezoidal.

8. The low coupling loss mode spot converter based on thin film lithium niobate platform of claim 1, wherein, The width, thickness and refractive index of the first end of the lens layer (8) covered by the first waveguide section (4) and the second waveguide section (5) are consistent with the width, thickness and refractive index of the bottom layer of the lens body (7), and the width of the second end of the lens layer (8) is consistent with the width of the second end of the second waveguide section (5).

9. The low coupling loss mode spot converter based on thin film lithium niobate platform of claim 1, wherein, The third waveguide part (6) is a double-layer lithium niobate ridge waveguide structure, including a third waveguide layer (61) and a fourth waveguide layer (62), the third waveguide layer (61) is arranged on the fourth waveguide layer (62), the third waveguide layer (61) is connected with the first waveguide layer (51), the fourth waveguide layer (62) is connected with the second waveguide layer (52), the thickness of the third waveguide layer (61) and the fourth waveguide layer (62) is consistent with the thickness of the first waveguide layer (51) and the second waveguide layer (52).

10. The low coupling loss mode spot converter based on thin film lithium niobate platform of claim 9, wherein, The width of the first end of the third waveguide layer (61) is the same as the width of the second section of the first waveguide layer (51), and the overall width of the fourth waveguide layer (62) is consistent with the width of the lower cladding layer (2).

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