A tunable mode converter based on lithium niobate thin film and a preparation method thereof

Abstract

This invention discloses a tunable mode converter based on lithium niobate thin film and its fabrication method, relating to the field of integrated optics technology. The mode converter includes a lithium niobate thin film substrate on an insulator, a grating-coupled input terminal, an input terminal tapered graded waveguide, an input terminal ridge-type lithium niobate waveguide, a bent waveguide, a V-shaped silicon array mode conversion region, an output terminal ridge-type lithium niobate waveguide, an output terminal tapered graded waveguide, a grating-coupled output terminal, and a silicon nitride stress compensation layer covering the bent waveguide. Employing an asymmetric V-shaped silicon array and bent waveguide structure, the periodic arrangement biased towards one side of the waveguide breaks the efficiency bottleneck of traditional symmetrical gratings, achieving higher mode coupling strength, solving the phase mismatch problem caused by fixed periods, and improving bandwidth transmission capability. Furthermore, the device fabrication process is compatible with traditional CMOS device fabrication processes, has low cost, and is suitable for mass industrial production.

Description

Technical Field

[0001] This invention relates to the field of integrated optics technology, and in particular to a tunable mode converter based on lithium niobate thin film and its fabrication method. Background Technology

[0002] The rapid development of silicon-based optoelectronic technology has placed higher demands on communication networks. Compared to traditional electrical interconnects, optical interconnects offer significant advantages in terms of loss, delay, bandwidth, and electromagnetic interference immunity, perfectly meeting the needs of current communication networks. Mode division multiplexing (MDF) technology treats each orthogonal mode as an independent channel, greatly improving the transmission capacity of optical interconnect networks and providing a solution for high-capacity optical interconnects. Lithium niobate optical waveguides, with their good physical and chemical stability, large electro-optic coefficient, and excellent switching speed, have become an important transmission medium for optical interconnect networks.

[0003] Patent application number 201711129678.X discloses a wavelength division multiplexing (WDM) and mode division multiplexing (MDM) hybrid multiplexer that integrates photonic crystals and nanowire waveguides. It utilizes the coupling effect between microcavities and waveguides to achieve the filtering function for light waves of specific frequencies. However, the structure of the above patent is complex and the photonic crystal fabrication is sensitive, resulting in limited bandwidth. Patent application number 201910841331.0 discloses a method and system for optimizing the parameters of a conical structure for optical mode division multiplexers. This method for optimizing the parameters of a conical structure relies on genetic algorithms to improve process tolerance, but it cannot solve the dynamic tuning problem. Patent application number 202310951412.2 discloses a curved waveguide for high-efficiency optical wave transmission, its design method, and its fabrication process. The cross-section of the 90° right-angle curved waveguide is designed as a stepped hexagonal cross-section or an asymmetric semi-circular cross-section, which can reduce the radiation loss of the curved waveguide to the surrounding cladding, thereby effectively improving the transmission efficiency of the light wave. However, the above structure lacks mode conversion function and has high processing costs. Summary of the Invention

[0004] To address the above technical problems, this invention provides a tunable mode converter based on a lithium niobate thin film, comprising a lithium niobate on an insulator thin film substrate, on which a thin-film lithium niobate waveguide is disposed. The thin-film lithium niobate waveguide includes a grating-coupled input terminal, an input terminal tapered gradient waveguide, an input terminal ridge-type lithium niobate waveguide, a bent waveguide, an output terminal ridge-type lithium niobate waveguide, an output terminal tapered gradient waveguide, and a grating-coupled output terminal, arranged sequentially along the light propagation direction. A silicon nitride stress compensation layer is covered on the bent waveguide. A V-shaped silicon array mode conversion region is provided on the surface of the bent waveguide. The V-shaped silicon array mode conversion region includes a plurality of V-shaped silicon atoms arranged at equal intervals along the bending direction of the bent waveguide, with the pointed bottom portion of the V-shaped silicon atoms facing the light propagation direction and its opening angle being 60° to 120°.

[0005] The technical solution further defined in this invention is:

[0006] Furthermore, the curved waveguide consists of a circular arc with a total bending angle of 90° to 180°, and the period Λ of the V-shaped silicon array on its surface varies with the radius of curvature of the curved waveguide, satisfying the following equation:

[0007]

[0008] Where Λ0 is the design period of the straight waveguide, R0 is the reference radius of curvature, and R represents the radius of curvature of the curved waveguide.

[0009] As described above, in a tunable mode converter based on lithium niobate thin film, the deposition position offset Δx of the V-shaped silicon array mode conversion region is inversely proportional to the radius of curvature R, specifically expressed as:

[0010] Δx = 0.3R·(λ / 1550nm)

[0011] Where λ is the operating wavelength, and the offset direction is towards the outside of the curved waveguide.

[0012] As described above, a tunable mode converter based on lithium niobate thin film has a curved waveguide whose radius of curvature is fitted by a third-order Bezier curve. The curved waveguide has a height of 50–500 nm, a ridge width of 0.5–10 μm, and a waveguide sidewall angle of 0°–30°.

[0013] As described above, a tunable mode converter based on a lithium niobate thin film has air trenches etched on the outer side of the curved waveguide to increase the refractive index difference.

[0014] As described above, in a tunable mode converter based on lithium niobate thin film, there is no offset at the connection between the bent waveguide and the input ridge-type lithium niobate optical waveguide and the output ridge-type lithium niobate optical waveguide.

[0015] As described above, a tunable mode converter based on lithium niobate thin film has an offset at the connection between the bent waveguide and the input ridge lithium niobate optical waveguide and the output ridge lithium niobate optical waveguide.

[0016] As described above, a tunable mode converter based on lithium niobate thin film replaces the V-shaped silicon array mode conversion region with a rectangular antimony selenide array mode conversion region. The rectangular antimony selenide array mode conversion region includes several rectangular antimony selenides that are equally spaced along the bending direction of the curved waveguide, and the length direction of each rectangular antimony selenide coincides with the width direction of the curved waveguide.

[0017] As described above, a tunable mode converter based on lithium niobate thin film has one or more bent waveguides. When multiple bent waveguides are set, they are connected in a multi-stage cascade manner.

[0018] The present invention also provides a preparation method of a tunable mode converter based on a lithium niobate thin film, comprising the following steps:

[0019] S1. Preparation of a lithium niobate thin film ridge waveguide: Provide a lithium niobate thin film substrate on insulator. After cleaning successively with acetone, ethanol and deionized water, purge the surface of the substrate with a nitrogen gun; After cleaning, use a magnetron sputtering instrument to deposit a metal chromium film on the surface of the lithium niobate thin film as a mask layer. Through photolithography, development and chemical etching processes, transfer the optical waveguide pattern on the mask plate to the chromium film; Then soak the sample in a proton source at 200 - 300 °C for proton exchange for 5 - ३0 mins. H ions in the proton source solution react with Li ions in the lithium niobate crystal. H ions replace part of the Li ions in the lithium niobate crystal and diffuse into the crystal, forming H

[0021] Li 1-x NbO3 crystal layer, where the value range of x is 0 < x < 1, indicating the relative concentration of H x Li1 -x in the NbO3 crystal layer; After proton exchange, clean the substrate after proton exchange successively with solvents such as acetone, ethanol and deionized water. Subsequently, use inductively coupled plasma to etch the substrate to remove the proton-exchanged part of the lithium niobate thin film; After etching, remove the remaining chromium of the proton exchange barrier layer. Thus, the production of the lithium niobate thin film ridge waveguide is completed;

[0020] S2. Grating etching at the input and output ends: On the processed waveguide device, use a focused ion beam method to etch a grating structure at the input and output ends. The shape of the grating is a rectangular structure. The selected ion beam current and ion dose are 50 - 300 pA and 0.5 - 3 nC / μm respectively;

[0021] S3. Preparation of tapered gradient waveguides at the input and output ends: Use laser direct writing to prepare tapered gradient waveguides. The laser is a femtosecond laser with a wavelength of 600 - 1550 nm, a pulse width of 50 - 500 fs, a repetition frequency of 1 - 10 kHz. The width at the input end gradually changes from 1 - 10 μm to the width at the output end of 0.1 - 5 μm, and the taper rate is 0.1 - 10 μm / mm;

[0022] S4. Preparation of a V-shaped silicon array: After the tapered gradient waveguides are prepared, clean the surface of the lithium niobate thin film successively by ultrasonic cleaning with acetone, isopropyl alcohol and deionized water to remove surface contaminants; Then use H plasma treatment with a power of 50 - 200 W and a time of 1 - 10 mins; Then use low-temperature plasma-enhanced chemical vapor deposition or magnetron sputtering to deposit a silicon thin film on the surface of the lithium niobate waveguide at a temperature below 300 °C, a thickness of 0.5 - 5 μm, a power of 100 - 500 W, and a chamber vacuum degree below 1×10 -6Next, a layer of AZ1505 positive photoresist with a thickness of 1–5 μm is coated onto the silicon thin film. The photoresist is then lithographically patterned using ultraviolet (UV) lithography or electron beam lithography, with a UV exposure dose of 40–200 mJ / cm². 2 The development time is 30-60 seconds; then, the silicon film is etched by dry etching or wet etching, and the sample stage angle is adjusted to form a V-shaped silicon sidewall; finally, residual photoresist is removed by oxygen plasma ashing and acetone cleaning.

[0023] S5. Preparation of silicon nitride stress compensation layer: A silicon nitride layer with a thickness of 10-100 nm is deposited on the surface of the curved waveguide. Low-temperature plasma-enhanced chemical vapor deposition or magnetron sputtering is used to offset the stress gradient of thin-film lithium niobate and reduce the loss of the curved waveguide.

[0024] S6. Metal electrode preparation: Titanium is evaporated by electron beam as an adhesion layer with a thickness of 10-50 nm, followed by gold deposition with a thickness of 10-500 nm.

[0025] The beneficial effects of this invention are:

[0026] (1) In this invention, an asymmetric V-shaped silicon array and a curved waveguide structure are adopted. By periodically arranging the waveguide side, the efficiency bottleneck of the traditional symmetric grating is broken, higher mode coupling strength is achieved, the phase mismatch problem caused by fixed period is solved, and the bandwidth transmission capability is improved.

[0027] (2) In this invention, the conversion efficiency can be dynamically adjusted by applying voltage through the bent Ti / Au electrode design; the stress of the lithium niobate film can be reduced by low-temperature deposition of silicon nitride layer, and high-temperature process damage can be avoided.

[0028] (3) In this invention, the tunable mode converter based on lithium niobate thin film is applicable to wavelengths covering 1460-1625nm (0 to L band), and also supports multi-level device integration (size <500×500μm). 2 It is suitable for high-density photonic integrated circuits. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the overall structure of the present invention;

[0030] Figure 2 This is a schematic diagram of the structure of the grating coupling input terminal in an embodiment of the present invention;

[0031] Figure 3 This is a schematic diagram of the silicon nitride stress compensation layer in an embodiment of the present invention;

[0032] Figure 4 This is a schematic diagram of the structure of a device in an embodiment of the present invention where there is an offset between the curved waveguide and the straight waveguide;

[0033] Figure 5 This is a schematic diagram of the structure of a device with a curved waveguide having a different radius of curvature in an embodiment of the present invention;

[0034] Figure 6 This is a schematic diagram of the structure of the antimony selenide rectangular array mode conversion region in an embodiment of the present invention;

[0035] Figure 7 This is a schematic diagram of the structure of a device with a two-stage cascaded bent waveguide in an embodiment of the present invention;

[0036] Figure 8 This is a schematic diagram of the device testing apparatus in an embodiment of the present invention;

[0037] Figure 9 This is a schematic diagram of the device testing process in an embodiment of the present invention.

[0038] Among them, 1. Lithium niobate thin film substrate on insulator; 2. Thin film lithium niobate waveguide; 21. Gradient-coupled input end; 22. Input end tapered graded waveguide; 23. Input end ridge type lithium niobate waveguide; 24. Bending waveguide; 25. Output end ridge type lithium niobate waveguide; 26. Output end tapered graded waveguide; 27. Gradient-coupled output end; 3. Silicon nitride stress compensation layer; 4. V-shaped silicon array mode conversion region; 5. Air trench; 6. Antimony selenide rectangular array mode conversion region. Detailed Implementation

[0039] This embodiment provides a tunable mode converter based on lithium niobate thin film, such as... Figures 1 to 3 As shown, the substrate includes a lithium niobate-on-insulator (LNOI) thin film substrate. The LNOI thin film substrate 1 is mainly composed of a silicon substrate, an intermediate silicon dioxide insulating layer, and a top lithium niobate thin film. A thin film lithium niobate waveguide 2 is provided on the LNOI thin film substrate 1. The thin film lithium niobate waveguide 2 includes a grating-coupled input terminal 21, an input terminal tapered gradient waveguide 22, an input terminal ridge lithium niobate waveguide 23, a bent waveguide 24, an output terminal ridge lithium niobate waveguide 25, an output terminal tapered gradient waveguide 26, and a grating-coupled output terminal 27 arranged sequentially along the light propagation direction. A silicon nitride stress compensation layer 3 is covered on the bent waveguide 24.

[0040] like Figure 1 and Figure 4As shown, the connection between the curved waveguide 24 and the input ridge lithium niobate optical waveguide 23 and the output ridge lithium niobate optical waveguide 25 can be without offset or with a certain offset. The curved waveguide 24 is often connected to the straight waveguide. In the straight waveguide, the mode field is symmetrically distributed around the waveguide center. However, in the curved waveguide 24 with the same width, the mode field distribution changes. This causes mode mismatch during the transmission of the optical signal from the straight waveguide to the curved waveguide 24 with the same width, resulting in power loss. In order to reduce the transition loss between the straight waveguide and the curved waveguide 24, a certain offset can be introduced at the input and output ports of the straight waveguide and the curved waveguide 24.

[0041] Air trenches 5 are etched on the outer side of the bent waveguide 24 to increase the refractive index difference. This is mainly to significantly reduce the radiation loss when light propagates at the bend of the waveguide. This is one of the key technologies for designing and manufacturing compact optical paths in integrated optics (especially silicon photonics). By utilizing the huge refractive index difference between air and waveguide material, a strong optical barrier is formed in the outer region of the bent waveguide 24 where light leakage is most likely to occur. This strongly reflects the light field that is trying to radiate back into the waveguide, thereby greatly reducing bending loss and making it possible to design smaller, higher-density photonic integrated chips. It has the following advantages: 1. Enhanced optical field confinement and suppression of radiation loss; 2. Reduced effective bending radius; 3. Improved flexibility and performance of waveguide design; 4. Optimization of specific modes.

[0042] like Figure 5 As shown, the curved waveguide 24 is composed of a circular arc with a total bending angle of 90° to 180°. The radius of curvature of the curved waveguide 24 is obtained by fitting a third-order Bezier curve. The height of the curved waveguide 24 is 50 to 500 nm, the ridge width is 0.5 to 10 μm, and the waveguide sidewall angle is 0° to 30°.

[0043] The curved waveguide 24 has a V-shaped silicon array mode conversion region 4 on its surface. The V-shaped silicon array mode conversion region 4 includes several V-shaped silicon cells that are equally spaced along the bending direction of the curved waveguide 24, with the pointed bottom of the V-shaped silicon cells facing the direction of light propagation. The opening angle of the V-shaped silicon cells is 60° to 120°. The period Λ of the V-shaped silicon array on the surface of the curved waveguide 24 varies with the radius of curvature of the curved waveguide 24, satisfying the following equation:

[0044]

[0045] Where Λ0 is the design period of the straight waveguide, R0 is the reference curvature radius, and R represents the curvature radius of the curved waveguide.

[0046] The deposition position offset Δx of the V-shaped silicon array mode conversion region 4 is inversely proportional to the radius of curvature R, and the specific expression is as follows:

[0047] Δx = 0.3R·(λ / 1550nm)

[0048] Where λ is the operating wavelength, and the offset direction is towards the outside of the curved waveguide 24.

[0049] The V-shaped silicon array on the surface of the curved waveguide 24 can significantly change the material refractive index distribution in the mode conversion region, resulting in a significant change in the input mode field after entering the conversion region. For example, when the input TE0 mode enters the conversion region, due to the large difference in material refractive index and structural asymmetry, the input TE0 mode will be split into two TE0 mode beams. One TE0 mode is located in the region without silicon metasurface, and the other TE0 mode is located in the region with silicon metasurface. The mode propagation constants corresponding to the two regions are different. Then, through optical transmission, the TE0 modes in the two regions will gradually accumulate a phase difference. When the phase difference reaches 180°, the modes in the two regions are combined to form the TE1 mode.

[0050] A synergistic effect can be formed between the curved waveguide 24 and the V-shaped silicon array. First, in the curved waveguide 24, the curvature of the light propagation path leads to an increase in the outer optical path and a decrease in the inner optical path, forming an equivalent refractive index gradient. This asymmetry causes the electric field distribution of the fundamental mode (TE0) to shift towards the inner side of the bend, providing conditions for the excitation of higher-order modes (TE1). Then, the V-shaped silicon array is asymmetrically arranged along the curved waveguide, and its high refractive index introduces periodic refractive index perturbations. The phase matching condition is satisfied through Bragg scattering, which promotes the transfer of TE0 mode energy to TE1 mode. The period of the V-shaped silicon array gradually changes along the bending direction, dynamically adjusting the propagation constant difference to ensure efficient coupling over a wide wavelength range. Finally, the curved waveguide structure forces the light field to concentrate inward, reducing radiation loss on the outer side, while enhancing the mode overlap integral in the V-shaped silicon array region, thereby improving coupling efficiency.

[0051] like Figure 6 As shown, the V-shaped silicon array mode conversion region 4 above the curved waveguide 24 can be replaced by a rectangular antimony selenide array mode conversion region 6. The rectangular antimony selenide array mode conversion region 6 includes several rectangular antimony selenides that are equally spaced along the bending direction of the curved waveguide 24, and the length direction of each rectangular antimony selenide coincides with the width direction of the curved waveguide 24.

[0052] The number of bent waveguides 24 can be set to one or more. When multiple bent waveguides 24 are set, they are connected in a multi-stage cascade manner. Adjacent converters are connected through 180° bent waveguides 24, such as... Figure 7 As shown, this is a tunable mode converter with two-stage cascaded curved waveguides 24. Based on this, more curved waveguides 24, such as three-stage and four-stage, can be cascaded to achieve multi-stage mode tuning. Furthermore, the silicon nitride stress compensation layer 3 covering the outside of the curved waveguide 24 can be replaced with a silicon dioxide cladding, and then silicon nitride is embedded in the silicon dioxide cladding as stress compensation.

[0053] This embodiment also provides a method for fabricating a tunable mode converter based on lithium niobate thin film, including the following steps:

[0054] S1. Fabrication of a thin-film lithium niobate ridge waveguide: A lithium niobate-on-insulator (LNOI) thin-film substrate is provided and sequentially cleaned with acetone, ethanol, and deionized water. The substrate surface is then purged with a nitrogen gun. After cleaning, a chromium film is deposited on the surface of the thin-film lithium niobate using a magnetron sputtering apparatus as a mask layer. The waveguide pattern on the mask is transferred onto the chromium film through photolithography, development, and chemical etching processes. The sample is then immersed in a proton source (stearic acid) at 200°C for proton exchange for 20 minutes. H+ ions in the proton source solution exchange with Li+ ions in the lithium niobate crystal, with H+ ions replacing some of the Li+ ions in the lithium niobate crystal and diffusing into the crystal to form H+ ions. 0.5 Li 0.5 The NbO3 crystal layer was then used. After proton exchange, the substrate was cleaned sequentially with solvents such as acetone, ethanol, and deionized water. Subsequently, inductively coupled plasma etching was used to remove the proton-exchanged portion of the thin-film lithium niobate. After etching, the residual chromium in the proton exchange barrier layer was removed, thus completing the fabrication of the thin-film lithium niobate ridge waveguide. The curved waveguide 24 has a radius of curvature of 150 μm, the ridge waveguide has a width of 2 μm, the input width of the tapered waveguide is 5 μm, and a 0.5 μm offset is set between the curved waveguide 24 and the straight lithium niobate waveguide.

[0055] S2. Grating Etching at Input and Output Ends: On the fabricated waveguide device, a grating structure is formed at the input and output ends by focusing an ion beam. The grating is rectangular in shape. To ensure that the grating structure is sufficiently fine, the selected ion beam current and ion dose are 100 pA and 0.5 nC / μm, respectively.

[0056] S3. Fabrication of tapered graded waveguides at input and output ends: Tapered graded waveguides are fabricated using laser direct writing. The laser wavelength is a 1550nm femtosecond laser with a pulse width of 50fs and a repetition frequency of 1kHz. The width at the input end is 5μm, which is gradually reduced to 2μm at the output end, with a gradient rate of 1μm / mm. In addition to femtosecond laser direct writing, electron beam lithography and grayscale exposure methods can also be used.

[0057] S4. Fabrication of the V-shaped silicon array: After the tapered gradient waveguide was fabricated, the surface of the thin-film lithium niobate was ultrasonically cleaned sequentially with acetone, isopropanol, and deionized water to remove surface contaminants. Then, oxygen plasma treatment at 100W power for 5 minutes was used to improve surface hydrophilicity. Next, a silicon thin film was deposited on the lithium niobate waveguide surface using low-temperature plasma-enhanced chemical vapor deposition or magnetron sputtering at a temperature below 300℃, a thickness of 0.5μm, a power of 200W, and a chamber vacuum level below 1×10⁻⁶. -6 Next, a layer of AZ1505 positive photoresist with a thickness of 2 μm is coated on the silicon thin film. The photoresist is then lithographically patterned using ultraviolet (UV) lithography or electron beam lithography, with a UV exposure dose of 100 mJ / cm². 2 The development time is 30s; then, the silicon thin film is etched by dry etching or wet etching, and the sample stage angle is adjusted to form a V-shaped silicon sidewall; finally, residual photoresist is removed by oxygen plasma ashing and acetone cleaning; the period of the V-shaped silicon array is 10μm and the opening angle is 90°.

[0058] S5. Preparation of silicon nitride stress compensation layer 3: A silicon nitride layer with a thickness of 100 nm is deposited on the surface of the bent waveguide 24. Low-temperature plasma-enhanced chemical vapor deposition is used to offset the stress gradient of thin-film lithium niobate and reduce the loss of the bent waveguide 24.

[0059] S6. Metal electrode preparation: Titanium is evaporated by electron beam as an adhesion layer with a thickness of 50 nm, followed by gold deposition with a thickness of 100 nm.

[0060] Based on the above embodiments, some modifications were made to form three other different methods for fabricating tunable mode converters based on lithium niobate thin films. In comparative method one, the V-shaped silicon array mode conversion region 4 is not set. In comparative method two, the bent waveguide 24 is not set. In comparative method three, there is no offset between the bent waveguide 24 and the straight waveguide.

[0061] Comparison Method 1:

[0062] Step 1: Fabrication of a thin-film lithium niobate ridge waveguide: A lithium niobate-on-insulator (LNOI) thin-film substrate is provided and sequentially cleaned with acetone, ethanol, and deionized water. The substrate surface is then purged with a nitrogen gun. After cleaning, a chromium film is deposited on the surface of the thin-film lithium niobate using a magnetron sputtering apparatus as a mask layer. The waveguide pattern on the mask is transferred onto the chromium film through photolithography, development, and chemical etching processes. The sample is then immersed in a proton source (stearic acid) at 200°C for proton exchange for 20 minutes. H+ ions in the proton source solution exchange with Li+ ions in the lithium niobate crystal, with H+ ions replacing some of the Li+ ions in the lithium niobate crystal and diffusing into the crystal to form H+ ions. 0.5 Li0.5 The NbO3 crystal layer was then used. After proton exchange, the substrate was cleaned sequentially with solvents such as acetone, ethanol, and deionized water. Subsequently, inductively coupled plasma etching was used to remove the proton-exchanged portion of the thin-film lithium niobate. After etching, the residual chromium in the proton exchange barrier layer was removed, thus completing the fabrication of the thin-film lithium niobate ridge waveguide. The curved waveguide 24 has a radius of curvature of 150 μm, the ridge waveguide has a width of 2 μm, and the input width of the tapered waveguide is 5 μm.

[0063] Step 2: Input and output grating etching: On the fabricated waveguide device, a focused ion beam is used to etch grating structures at the input and output ends. The gratings are rectangular in shape. To ensure that the grating structure is sufficiently fine, the selected ion beam current and ion dose are 100 pA and 0.5 nC / μm, respectively.

[0064] Step 3: Fabrication of tapered graded waveguides at the input and output ends: Tapered graded waveguides are fabricated using laser direct writing with a 1550nm femtosecond laser, a pulse width of 50fs, a repetition frequency of 1kHz, and a gradient from 5μm at the input end to 2μm at the output end at a rate of 1μm / mm. In addition to femtosecond laser direct writing, electron beam lithography and grayscale exposure methods can also be used.

[0065] Step 4: Fabrication of silicon nitride stress compensation layer 3: A silicon nitride layer with a thickness of 100 nm is deposited on the surface of the bent waveguide 24. Low-temperature plasma-enhanced chemical vapor deposition is used to offset the stress gradient of the thin film lithium niobate and reduce the loss of the bent waveguide 24.

[0066] Step 5: Metal electrode preparation: Titanium is evaporated by electron beam as an adhesion layer with a thickness of 50 nm, followed by gold deposition with a thickness of 100 nm.

[0067] Comparison Method Two:

[0068] Step 1: Fabrication of a thin-film lithium niobate ridge waveguide: A lithium niobate-on-insulator (LNOI) thin-film substrate is provided and sequentially cleaned with acetone, ethanol, and deionized water. The substrate surface is then purged with a nitrogen gun. After cleaning, a chromium film is deposited on the surface of the thin-film lithium niobate using a magnetron sputtering apparatus as a mask layer. The waveguide pattern on the mask is transferred onto the chromium film through photolithography, development, and chemical etching processes. The sample is then immersed in a proton source (stearic acid) at 200°C for proton exchange for 20 minutes. H+ ions in the proton source solution exchange with Li+ ions in the lithium niobate crystal, with H+ ions replacing some of the Li+ ions in the lithium niobate crystal and diffusing into the crystal to form H+ ions. 0.5 Li 0.5The NbO3 crystal layer was then used. After proton exchange, the substrate was cleaned sequentially with solvents such as acetone, ethanol, and deionized water. Subsequently, inductively coupled plasma etching was used to remove the proton-exchanged portion of the thin-film lithium niobate. After etching, the residual chromium in the proton exchange barrier layer was removed, thus completing the fabrication of the thin-film lithium niobate ridge waveguide. The ridge waveguide has a width of 2 μm, and the input end of the tapered waveguide has a width of 5 μm.

[0069] Step 2: Input and output grating etching: On the fabricated waveguide device, a focused ion beam is used to etch grating structures at the input and output ends. The gratings are rectangular in shape. To ensure that the grating structure is sufficiently fine, the selected ion beam current and ion dose are 100 pA and 0.5 nC / μm, respectively.

[0070] Step 3: Fabrication of tapered graded waveguides at the input and output ends: Tapered graded waveguides are fabricated using laser direct writing with a 1550nm femtosecond laser, a pulse width of 50fs, a repetition frequency of 1kHz, and a gradient from 5μm at the input end to 2μm at the output end at a rate of 1μm / mm. In addition to femtosecond laser direct writing, electron beam lithography and grayscale exposure methods can also be used.

[0071] Step 4: Fabrication of the V-shaped silicon array: After the tapered waveguide is fabricated, the surface of the thin-film lithium niobate is ultrasonically cleaned sequentially with acetone, isopropanol, and deionized water to remove surface contaminants. Then, oxygen plasma treatment at 100W power for 5 minutes is used to improve surface hydrophilicity. Next, a silicon thin film is deposited on the lithium niobate waveguide surface using low-temperature plasma-enhanced chemical vapor deposition or magnetron sputtering at a temperature below 300℃, a thickness of 0.5 μm, a power of 200W, and a chamber vacuum level below 1×10⁻⁶. -6 Next, a layer of AZ1505 positive photoresist with a thickness of 2 μm is coated on the silicon thin film. The photoresist is then lithographically patterned using ultraviolet (UV) lithography or electron beam lithography, with a UV exposure dose of 100 mJ / cm². 2 The development time is 30s; then the silicon film is etched by dry etching or wet etching, and the sample stage angle is adjusted to form a V-shaped silicon sidewall; finally, residual photoresist is removed by oxygen plasma ashing and acetone cleaning; the period of the V-shaped silicon array is 10μm and the opening angle is 90°.

[0072] Step 5: Preparation of silicon nitride stress compensation layer 3: A silicon nitride layer with a thickness of 100 nm is deposited on the surface of the curved waveguide using low-temperature plasma-enhanced chemical vapor deposition to offset the stress gradient of the thin film lithium niobate.

[0073] Step 6: Metal electrode preparation: Titanium is evaporated by electron beam as an adhesion layer with a thickness of 50 nm, followed by gold deposition with a thickness of 100 nm.

[0074] Comparison Method 3:

[0075] Step 1: Fabrication of a thin-film lithium niobate ridge waveguide: A lithium niobate-on-insulator (LNOI) thin-film substrate is provided and sequentially cleaned with acetone, ethanol, and deionized water. The substrate surface is then purged with a nitrogen gun. After cleaning, a chromium film is deposited on the surface of the thin-film lithium niobate using a magnetron sputtering apparatus as a mask layer. The waveguide pattern on the mask is transferred onto the chromium film through photolithography, development, and chemical etching processes. The sample is then immersed in a proton source (stearic acid) at 200°C for proton exchange for 20 minutes. H+ ions in the proton source solution exchange with Li+ ions in the lithium niobate crystal, with H+ ions replacing some of the Li+ ions in the lithium niobate crystal and diffusing into the crystal to form H+ ions. 0.5 Li 0.5 The NbO3 crystal layer was then used. After proton exchange, the substrate was cleaned sequentially with solvents such as acetone, ethanol, and deionized water. Subsequently, inductively coupled plasma etching was used to remove the proton-exchanged portion of the thin-film lithium niobate. After etching, the residual chromium in the proton exchange barrier layer was removed, thus completing the fabrication of the thin-film lithium niobate ridge waveguide. The curved waveguide 24 has a radius of curvature of 150 μm, the ridge waveguide has a width of 2 μm, and the input width of the tapered waveguide is 5 μm.

[0076] Step 2: Input and output grating etching: On the fabricated waveguide device, a focused ion beam is used to etch grating structures at the input and output ends. The gratings are rectangular in shape. To ensure that the grating structure is sufficiently fine, the selected ion beam current and ion dose are 100 pA and 0.5 nC / μm, respectively.

[0077] Step 3: Fabrication of tapered graded waveguides at the input and output ends: Tapered graded waveguides are fabricated using laser direct writing with a 1550nm femtosecond laser, a pulse width of 50fs, a repetition frequency of 1kHz, and a gradient from 5μm at the input end to 2μm at the output end at a rate of 1μm / mm. In addition to femtosecond laser direct writing, electron beam lithography and grayscale exposure methods can also be used.

[0078] Step 4: Fabrication of the V-shaped silicon array: After the tapered waveguide is fabricated, the surface of the thin-film lithium niobate is ultrasonically cleaned sequentially with acetone, isopropanol, and deionized water to remove surface contaminants. Then, oxygen plasma treatment at 100W power for 5 minutes is used to improve surface hydrophilicity. Next, a silicon thin film is deposited on the lithium niobate waveguide surface using low-temperature plasma-enhanced chemical vapor deposition or magnetron sputtering at a temperature below 300℃, a thickness of 0.5 μm, a power of 200W, and a chamber vacuum level below 1×10⁻⁶. -6 Next, a layer of AZ1505 positive photoresist with a thickness of 2 μm is coated on the silicon thin film. The photoresist is then lithographically patterned using ultraviolet (UV) lithography or electron beam lithography, with a UV exposure dose of 100 mJ / cm². 2 The development time is 30s; then the silicon film is etched by dry etching or wet etching, and the sample stage angle is adjusted to form a V-shaped silicon sidewall; finally, residual photoresist is removed by oxygen plasma ashing and acetone cleaning; the period of the V-shaped silicon array is 10μm and the opening angle is 90°.

[0079] Step 5: Fabrication of silicon nitride stress compensation layer 3: A silicon nitride layer with a thickness of 100 nm is deposited on the surface of the bent waveguide 24. Low-temperature plasma-enhanced chemical vapor deposition is used to offset the stress gradient of the thin film lithium niobate and reduce the loss of the bent waveguide 24.

[0080] Step 6: Metal electrode preparation: Titanium is evaporated by electron beam as an adhesion layer with a thickness of 50 nm, followed by gold deposition with a thickness of 100 nm.

[0081] Use such as Figure 8 The device testing apparatus shown is used for testing, and a schematic diagram of the testing process is as follows. Figure 9 As shown in Table 1 below, the performance of the method in this embodiment is compared with that of comparative methods one to three. The comparison results show that the tunable mode converter based on lithium niobate thin film prepared by the method in this embodiment can achieve higher maximum conversion efficiency and wider 3dB bandwidth, and can also achieve broadband mode conversion, which has significant advantages over other methods.

[0082] Table 1

[0083]

[0084] This embodiment employs an asymmetric V-shaped silicon array and a bent waveguide 24. By periodically arranging the waveguide to one side, it breaks through the efficiency bottleneck of traditional symmetric gratings, achieves higher mode coupling strength, solves the phase mismatch problem caused by fixed period, and improves bandwidth transmission capability. In addition, the device fabrication process is compatible with traditional CMOS device fabrication processes, has low cost, and is suitable for mass industrial production.

[0085] In addition to the embodiments described above, the present invention may have other implementations. All technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope claimed by the present invention.

Claims

1. A tunable mode converter based on lithium niobate thin film, characterized in that: The system includes a lithium niobate thin film substrate on an insulator (1), a thin film lithium niobate waveguide (2) on the lithium niobate thin film substrate (1), the thin film lithium niobate waveguide (2) includes a grating-coupled input terminal (21), an input terminal tapered gradient waveguide (22), an input terminal ridge lithium niobate waveguide (23), a bent waveguide (24), an output terminal ridge lithium niobate waveguide (25), an output terminal tapered gradient waveguide (26), and a grating-coupled output terminal (27) arranged sequentially along the light propagation direction. The bent waveguide (24) is covered with a silicon nitride stress compensation layer (3). The surface of the bent waveguide (24) is provided with a V-shaped silicon array mode conversion region (4). The V-shaped silicon array mode conversion region (4) includes several V-shaped silicons arranged at equal intervals along the bending direction of the bent waveguide (24), and the pointed bottom part of the V-shaped silicon faces the light propagation direction, and its opening angle is 60° to 120°. The curved waveguide (24) consists of a circular arc with a total bending angle of 90° to 180°, and its surface has a V-shaped silicon array periodicity. As the radius of curvature of the bent waveguide (24) changes, the following equation is satisfied: ； in, The design period for the straight waveguide is R0, which is the reference radius of curvature, and R represents the radius of curvature of the curved waveguide (24). The radius of curvature of the curved waveguide (24) is obtained by fitting a third-order Bezier curve.

2. The tunable mode converter based on lithium niobate thin film according to claim 1, characterized in that: The deposition position offset of the V-shaped silicon array mode conversion region (4) It is inversely proportional to the radius of curvature R, and the specific expression is: ； in, The working wavelength is offset towards the outside of the curved waveguide (24).

3. A tunable mode converter based on lithium niobate thin film according to claim 1, characterized in that: The curved waveguide (24) has a height of 50-500 nm, a ridge width of 0.5-10 μm, and a waveguide sidewall angle of 0°-30°.

4. A tunable mode converter based on lithium niobate thin film according to claim 1, characterized in that: The curved waveguide (24) has air grooves (5) etched on its outer side to increase the refractive index difference.

5. A tunable mode converter based on lithium niobate thin film according to claim 1, characterized in that: There is no offset at the connection between the curved waveguide (24) and the input ridge lithium niobate optical waveguide (23) and the output ridge lithium niobate optical waveguide (25).

6. A tunable mode converter based on lithium niobate thin film according to claim 1, characterized in that: There is an offset at the connection between the curved waveguide (24) and the input ridge lithium niobate optical waveguide (23) and the output ridge lithium niobate optical waveguide (25).

7. A tunable mode converter based on lithium niobate thin film according to claim 1, characterized in that: The V-shaped silicon array mode conversion region (4) is replaced with a rectangular antimony selenide array mode conversion region (6). The rectangular antimony selenide array mode conversion region (6) includes several rectangular antimony selenides that are equally spaced along the bending direction of the curved waveguide (24), and the length direction of each rectangular antimony selenide coincides with the width direction of the curved waveguide (24).

8. A tunable mode converter based on lithium niobate thin film according to claim 1, characterized in that: The number of the curved waveguides (24) is set to one or more. When multiple curved waveguides (24) are set, each curved waveguide (24) is connected by a multi-level cascade method.

9. A method for fabricating a tunable mode converter based on a lithium niobate thin film as described in any one of claims 1-8, characterized in that: Includes the following steps: S1. Preparation of thin-film lithium niobate ridge waveguide: Provide a lithium niobate thin-film substrate on insulator (1). After cleaning it successively with acetone, ethanol and deionized water, blow-dry the surface of the substrate with a nitrogen gun. After cleaning, use a magnetron sputtering instrument to deposit a metal chromium film on the surface of the thin-film lithium niobate as a mask layer. Through photolithography, development and chemical etching processes, transfer the optical waveguide pattern on the mask to the chromium film. Then soak the sample in a proton source at 200 - 300 °C for 5 - 30 mins for proton exchange. H ions in the proton source solution react with Li ions in the lithium niobate crystal, and H ions replace some Li ions in the lithium niobate crystal and diffuse into the crystal to form H Li 1-x NbO3 crystal layer, where the value range of x is 0 < x < 1, indicating the relative concentration of H x Li1 - x ions in the H - Li1 6 2 NbO3 crystal layer; After completing the proton exchange, clean the substrate after proton exchange successively with solvents such as acetone, ethanol and deionized water. Subsequently, use inductively coupled plasma to etch the substrate to remove the proton-exchanged part of the thin-film lithium niobate. After etching, remove the remaining chromium of the proton exchange blocking layer. Thus, the production of the thin-film lithium niobate ridge waveguide is completed; S2. Input and output grating etching: On the fabricated waveguide device, a grating structure is etched at the input and output ends using a focused ion beam. The grating is rectangular in shape, and the selected ion beam current and ion dose are 50-300 pA and 0.5-3 nC / μm, respectively. S3. Fabrication of tapered waveguides at input and output ends: Tapered waveguides are fabricated using a laser direct-write method. The laser wavelength is a femtosecond laser with a wavelength of 600–1550 nm, a pulse width of 50–500 fs, a repetition frequency of 1–10 kHz, and the width at the input end is tapered from 1–10 μm to 0.1–5 μm at the output end, with a tapering rate of 0.1–10 μm / mm. S4. Fabrication of V-shaped silicon array: After the tapered gradient waveguide is fabricated, the surface of the thin-film lithium niobate is ultrasonically cleaned with acetone, isopropanol and deionized water in sequence to remove surface contaminants. Then, H plasma treatment was used at a power of 50–200 W for 1–10 mins; subsequently, a silicon thin film was deposited on the lithium niobate waveguide surface using low-temperature plasma-enhanced chemical vapor deposition or magnetron sputtering at a temperature below 300 °C, a thickness of 0.5–5 μm, a power of 100–500 W, and a chamber vacuum level below 1 × 10⁻⁶. - 6 Next, a layer of AZ1505 positive photoresist with a thickness of 1–5 μm is coated onto the silicon thin film. The photoresist is then lithographically patterned using ultraviolet (UV) lithography or electron beam lithography, with a UV exposure dose of 40–200 mJ / cm². 2 The development time is 30-60 seconds; then, the silicon film is etched by dry etching or wet etching, and the sample stage angle is adjusted to form a V-shaped silicon sidewall; finally, residual photoresist is removed by oxygen plasma ashing and acetone cleaning. S5. Preparation of silicon nitride stress compensation layer (3): A silicon nitride layer with a thickness of 10-100 nm is deposited on the surface of the bent waveguide (24). Low-temperature plasma-enhanced chemical vapor deposition or magnetron sputtering is used to offset the stress gradient of thin film lithium niobate and reduce the loss of bent waveguide (24). S6. Metal electrode preparation: Titanium is evaporated by electron beam as an adhesion layer with a thickness of 10-50 nm, followed by gold deposition with a thickness of 10-500 nm.

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