Tunable mode converter based on lithium niobate film and preparation method

Abstract

The invention discloses a tunable mode converter based on a lithium niobate film and a preparation method, and relates to the technical field of integrated optics. The mode converter comprises a lithium niobate film substrate on an insulator, a grating coupling input end, an input end conical gradual change optical waveguide, an input end ridge-type lithium niobate optical waveguide, a bent waveguide, a V-shaped silicon array mode conversion area, an output end ridge-type lithium niobate optical waveguide and an output end conical gradual change optical waveguide. The grating is coupled with an output end; the silicon nitride stress compensation layer covers the bent waveguide; a structural mode of an asymmetric V-shaped silicon array and a bent waveguide is adopted, through periodic arrangement towards one side of the waveguide, the efficiency bottleneck of a traditional symmetric grating is broken, higher mode coupling strength is achieved, the problem of phase mismatch caused by a fixed period is solved, the bandwidth transmission capacity is improved, in addition, the device process is compatible with a traditional CMOS device preparation process, and the device is suitable for large-scale production. And the method is low in cost and suitable for large-scale industrial production and preparation.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of integrated optics, in particular to a tunable mode converter based on lithium niobate thin film and a preparation method. BACKGROUND

[0002] The booming development of silicon-based optoelectronic technology puts forward higher requirements for communication networks. Compared with traditional electrical interconnection, optical interconnection has significant advantages in terms of loss, delay, bandwidth and electromagnetic resistance, which meets the current demand of communication network. Mode division multiplexing technology regards each orthogonal mode as an independent channel, which greatly improves the transmission capacity of optical interconnection network and brings a solution to large-capacity optical interconnection. Lithium niobate optical waveguide has good physical and chemical stability, large electro-optic coefficient and excellent switching speed, and becomes an important transmission medium for optical interconnection network.

[0003] The patent with application number 201711129678.X discloses a wave division mode division hybrid multiplexer integrated with photonic crystal and nanowire waveguide, which realizes the download filtering function of specific frequency light wave by using the coupling effect of microcavity and waveguide, but the structure of the above patent is complex and the preparation of photonic crystal is sensitive, and the bandwidth is limited; the patent with application number 201910841331.0 discloses a tapered structure parameter optimization method and system for optical mode division multiplexer, which depends on genetic algorithm to improve process tolerance and cannot solve the problem of dynamic tuning; the patent with application number 202310951412.2 discloses a curved waveguide for high-efficiency optical wave transmission, design method and manufacturing process, the cross section of 90° right angle curved waveguide is designed as stepped hexagonal cross section or asymmetric semicircular cross section, which can reduce the radiation loss of curved waveguide to the surrounding cladding, thereby effectively improving the transmission efficiency of optical wave, but the above structure lacks mode conversion function and has high processing cost. SUMMARY

[0004] In order to solve the above technical problems, the present application provides a tunable mode converter based on lithium niobate thin film, which comprises an insulator-on-lithium niobate thin film substrate, a thin film lithium niobate waveguide is arranged on the insulator-on-lithium niobate thin film substrate, the thin film lithium niobate waveguide comprises a grating coupling input end, an input end tapered gradually changing optical waveguide, an input end ridge type lithium niobate optical waveguide, a curved waveguide, an output end ridge type lithium niobate optical waveguide, an output end tapered gradually changing optical waveguide and a grating coupling output end arranged in sequence along the light propagation direction, and a silicon nitride stress compensation layer is arranged on the curved waveguide; a V-shaped silicon array mode conversion area is arranged on the surface of the curved waveguide, the V-shaped silicon array mode conversion area comprises a plurality of V-shaped silicons arranged at equal intervals along the bending direction of the curved waveguide, and the tip part of the V-shaped silicon faces the direction of light propagation, and the opening angle thereof is 60°-120°.

[0005] The further defined technical solution of the present application is:

[0006] Further, the curved waveguide is composed of a circular arc, the total bending angle is 90°-180°, and the V-shaped silicon array period Λ on the surface of the curved waveguide changes with the curvature radius of the curved waveguide, and satisfies the following formula:

[0007]

[0008] Wherein, 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.

[0009] The deposition position offset Δx of the V-shaped silicon array mode conversion area of the tunable mode converter based on the lithium niobate thin film is inversely proportional to the curvature radius R, and the specific expression is:

[0010] Δx=0.3R·(λ / 1550nm)

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

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

[0013] The curved waveguide of the tunable mode converter based on the lithium niobate thin film is etched with an air groove on the outside for increasing the refractive index difference.

[0014] The curved waveguide of the tunable mode converter based on the lithium niobate thin film is not offset at the connection with the input end ridge type lithium niobate optical waveguide and the output end ridge type lithium niobate optical waveguide.

[0015] The curved waveguide of the tunable mode converter based on the lithium niobate thin film has an offset amount at the connection with the input end ridge type lithium niobate optical waveguide and the output end ridge type lithium niobate optical waveguide.

[0016] The V-shaped silicon array mode conversion area of the tunable mode converter based on the lithium niobate thin film is replaced by an antimony selenide rectangular array mode conversion area, the antimony selenide rectangular array mode conversion area includes a plurality of rectangular antimony selenides arranged at equal intervals 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] The number of the curved waveguide of the tunable mode converter based on the lithium niobate thin film is set to one or more, and when multiple curved waveguides are set, the curved waveguides are connected by a multi-stage cascade mode.

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

[0019] S1. Preparation of thin-film lithium niobate ridge waveguide: Provide a lithium niobate thin film substrate on an insulator, clean it with acetone, ethanol and deionized water in sequence, and then purge the substrate surface with a nitrogen gun; after cleaning, use a magnetron sputtering device to plate a metal chromium film on the surface of the lithium niobate thin film as a mask layer, and transfer the optical waveguide pattern on the mask to the chromium film through photolithography, development and chemical etching processes; then immerse the sample in a proton source at 200-300°C for 5-30 minutes for proton exchange, and the H ions in the proton source solution exchange with the Li ions in the lithium niobate crystal. The H ions replace some of the Li ions in the lithium niobate crystal and diffuse into the crystal to form H x Li 1-x NbO3 crystal layer, where the value of x ranges from 0 <x<1,表示H x Li1 -x The relative concentration of H ions in the NbO3 crystal layer; after proton exchange is completed, the proton-exchanged substrate is cleaned using solvents such as acetone, ethanol, and deionized water, and then the substrate is etched using inductively coupled plasma to remove the proton-exchanged portion of the thin-film lithium niobate; after etching, the residual chromium in the proton-exchange barrier layer is removed, thus completing the fabrication of the thin-film lithium niobate ridge waveguide;

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

[0021] S3. Preparation of tapered waveguides at the input and output ends: The tapered waveguides were prepared by laser direct writing. A femtosecond laser with a laser wavelength of 600-1550 nm, a pulse width of 50-500 fs, a repetition rate of 1-10 kHz, and a gradient from 1-10 μm at the input end to 0.1-5 μm at the output end. The gradient rate was 0.1-10 μm / mm.

[0022] S4. Preparation of V-shaped silicon array: After the tapered waveguide is prepared, the surface of the thin-film lithium niobate is ultrasonically cleaned with acetone, isopropyl alcohol, and deionized water in sequence to remove surface contaminants. Then, H plasma is used for treatment at a power of 50-200W for 1-10 minutes. Then, a silicon thin film is deposited on the surface of the lithium niobate waveguide 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 100-500W, and a chamber vacuum of less than 1×10 -6Torr; then a layer of AZ1505 positive photoresist is coated on the silicon film with a thickness of 1-5 microns, and the photoresist is subjected to photolithography by using ultraviolet lithography or electron beam lithography, the ultraviolet exposure dose is 40-200 mJ / cm 2 , the developing time is 30-60 seconds; then the silicon film is etched by using dry etching or wet etching, and the sample table angle is adjusted to form a V-shaped silicon sidewall; finally, the residual photoresist is removed through oxygen plasma ashing and acetone cleaning;

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

[0024] S6, metal electrode preparation: titanium is evaporated by electron beam evaporation as an adhesion layer with a thickness of 10-50 nm, and then a gold layer is evaporated.

[0025] The beneficial effects of the present application are:

[0026] (1) In the present application, the structure of asymmetric V-shaped silicon array and curved waveguide is adopted, the periodic arrangement on one side of the waveguide is used to break the efficiency bottleneck of the traditional symmetric grating, realize higher mode coupling strength, solve the phase mismatching problem caused by fixed period, and improve the bandwidth transmission capacity;

[0027] (2) In the present application, the curved Ti / Au electrode design is used to support the application of voltage to dynamically adjust the conversion efficiency; the low-temperature deposition of the silicon nitride layer can reduce the stress of the lithium niobate film and avoid damage caused by high-temperature process;

[0028] (3) In the present application, the tunable mode converter based on the lithium niobate film is applicable to wavelengths covering 1460-1625 nm (O to L waveband), and in addition, supports multi-stage device integration (size < 500*500 microns 2 ), and is suitable for high-density photonic integrated circuits. BRIEF DESCRIPTION OF DRAWINGS

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

[0030] Figure 2 is a schematic diagram of the structure of the grating coupling input end in the embodiment of the present application;

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

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

[0033] Figure 5 Schematic diagram of the structure of a device having curved waveguides with different curvature radii according to an embodiment of the present invention;

[0034] Figure 6 Schematic diagram of the structure of the antimony selenide rectangular array mode conversion area in an embodiment of the present invention;

[0035] Figure 7 Schematic diagram of the structure of a device with two-stage cascaded curved waveguides in an embodiment of the present invention;

[0036] Figure 8 Schematic diagram of a device testing apparatus according to an embodiment of the present invention;

[0037] Figure 9 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. Grating coupling input end; 22. Tapered gradient optical waveguide at input end; 23. Ridge type lithium niobate optical waveguide at input end; 24. Bend waveguide; 25. Ridge type lithium niobate optical waveguide at output end; 26. Tapered gradient optical waveguide at output end; 27. Grating coupling 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 DESCRIPTION

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

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

[0041] Air grooves 5 are also etched on the outside of the curved waveguide 24 to increase the refractive index difference. This is mainly to significantly reduce the radiation loss of light when propagating 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 area of ​​the curved waveguide 24 where light leakage is most likely to occur, which strongly reflects the light field trying to radiate back into the waveguide, thereby greatly reducing the bending loss and making it possible to design smaller and higher-density photonic integrated chips. It has the following advantages: 1. Enhanced light field confinement and suppressed 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 curvature radius of the curved waveguide 24 is fitted by 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] A V-shaped silicon array mode conversion region 4 is provided on the surface of the curved waveguide 24. The V-shaped silicon array mode conversion region 4 includes a plurality of V-shaped silicon elements spaced evenly apart along the curved direction of the curved waveguide 24. The pointed bottom portion of the V-shaped silicon elements faces the direction of light propagation, and the opening angle thereof is 60° to 120°. The period Λ of the V-shaped silicon array on the surface of the curved waveguide 24 varies with the curvature radius of the curved waveguide 24, satisfying the following equation:

[0044]

[0045] Wherein, 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 24.

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

[0047] Δx=0.3R·(λ / 1550nm)

[0048] Here, λ is the operating wavelength, and the offset direction is toward 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 zone, thereby causing the input mode field to undergo significant changes after entering the conversion zone. For example, when the input TE0 mode enters the conversion zone, 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 of which is located in an area without a silicon metasurface, and the other is located in an area with a silicon metasurface. The mode propagation constants corresponding to the two areas are different. Then, through optical transmission, the TE0 modes of the two areas will gradually accumulate a phase difference. When the phase difference reaches 180°, the modes of the two areas are synthesized 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 causes the outer optical path to increase and the inner optical path to decrease, forming an equivalent refractive index gradient; this asymmetry causes the electric field distribution of the fundamental mode (TE0) to shift toward the inner side of the bend, providing conditions for the excitation of the higher-order mode (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 met 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 gather inward, reducing the outer radiation loss, while enhancing the mode overlap integral of the V-shaped silicon array area and improving the 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 with an antimony selenide rectangular array mode conversion region 6. The antimony selenide rectangular array mode conversion region 6 includes a plurality of rectangular antimony selenides arranged at equal intervals along the curved 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 the curved waveguides 24 can be set to one or more. When multiple curved waveguides 24 are provided, each curved waveguide 24 is connected in a multi-stage cascade manner. Adjacent converters are connected via 180° curved waveguides 24. Figure 7 As shown, a tunable mode converter of a two-stage cascaded curved waveguide 24 is provided. On this basis, three-stage, four-stage, and other more curved waveguides 24 can be cascaded to achieve multi-stage mode tuning. The silicon nitride stress compensation layer 3 covering the outer side of the curved waveguide 24 can also be replaced with a silicon dioxide cladding, and then silicon nitride is embedded in the silicon dioxide cladding as stress compensation.

[0053] The embodiment also provides a preparation method of the tunable mode converter based on the lithium niobate thin film, comprising the following steps:

[0054] S1, thin film lithium niobate ridge waveguide preparation: a piece of lithium niobate on insulator (LNOI) substrate is provided, and after being cleaned by acetone, ethanol and deionized water in sequence, the surface of the substrate is blown by a nitrogen gun; after cleaning, a metal chromium film is plated on the surface of the thin film lithium niobate as a mask layer by using a magnetron sputtering instrument, and a light waveguide pattern on the mask plate is transferred to the chromium film by photoetch, development and chemical etching process; then the sample is soaked in a proton source (stearic acid) at 200 DEG C for 20 mins for proton exchange, and an exchange reaction occurs between H ions in the proton source solution and Li ions in the lithium niobate crystal, the H ions replace part of the Li ions in the lithium niobate crystal and diffuse into the crystal to form an H 0.5 Li 0.5 NbO3 crystal layer; after completing the proton exchange, the substrate after the proton exchange is cleaned by using solvents such as acetone, ethanol and deionized water in sequence, and then the substrate is etched by using inductive coupled plasma to remove the proton exchanged part of the thin film lithium niobate; after etching, the chromium remaining in the proton exchange blocking layer is removed, and thus the fabrication of the thin film lithium niobate ridge waveguide is completed; wherein the curvature radius of the curved waveguide 24 is 150 μm, the width of the ridge waveguide is 2 μm, the input end width of the tapered waveguide is 5 μm, and a 0.5 μm offset is arranged between the curved waveguide 24 and the lithium niobate straight waveguide.

[0055] S2, input and output end grating etching: a grating structure is etched on the input and output ends of the processed waveguide device by using a focused ion beam, and the shape of the grating is a rectangular structure; in order to ensure that the grating structure is fine enough, the selected ion beam current and ion dose are 100 pA and 0.5 nC / μm respectively.

[0056] S3, input and output end tapered waveguide preparation: a tapered waveguide is prepared by using a laser direct writing method, a femtosecond laser with a wavelength of 1550 nm is used, the pulse width is 50 fs, the repetition frequency is 1 kHz, the input end width is 5 μm and gradually changes to the output end width of 2 μm, and the gradual change rate is 1 μm / mm; in addition to the femtosecond laser direct writing, electron beam lithography and gray exposure methods can also be used.

[0057] S4, V-shaped silicon array preparation: after the preparation of the tapered gradient waveguide is completed, the surface of the thin film lithium niobate is ultrasonically cleaned with acetone, isopropyl alcohol and deionized water in sequence to remove surface contaminants; then oxygen plasma treatment is used, the power is 100 W, and the time is 5 mins to improve the surface hydrophilicity; then low temperature plasma enhanced chemical vapor deposition or magnetron sputtering is used to deposit a silicon film on the surface of the lithium niobate waveguide, the temperature is lower than 300℃, the thickness is 0.5μm, the power is 200 W, and the chamber vacuum degree is lower than 1x10 -6 Torr; then a layer of AZ1505 positive photoresist is coated on the silicon film, the thickness is 2μm, ultraviolet lithography or electron beam lithography is used for photoresist lithography, the ultraviolet exposure dose is 100mJ / cm 2 , and the developing time is 30s; then dry etching or wet etching is used to etch the silicon film, the sample stage angle is adjusted to form a V-shaped silicon sidewall; finally, oxygen plasma ashing and acetone cleaning are used to remove the residual photoresist; 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 layer of silicon nitride layer with a thickness of 100nm is deposited on the surface of the curved waveguide 24, and low temperature plasma enhanced chemical vapor deposition method is used to offset the stress gradient of the thin film lithium niobate and reduce the loss of the curved waveguide 24.

[0059] S6, metal electrode preparation: titanium is evaporated by electron beam evaporation as an adhesion layer with a thickness of 50nm, and then gold layer is evaporated with a thickness of 100nm.

[0060] Based on the above embodiment method, three different preparation methods of lithium niobate thin film based tunable mode converter are formed by making some changes; in the comparative method one, the V-shaped silicon array mode conversion area 4 is not set; in the comparative method two, the curved waveguide 24 is not set; in the comparative method three, there is no offset between the curved waveguide 24 and the straight waveguide.

[0061] Comparative method one:

[0062] Step one, preparation of thin film lithium niobate ridge waveguide: provide a lithium niobate on insulator (LNOI) substrate, clean it with acetone, ethanol and deionized water in sequence, and then blow the surface of the substrate with a nitrogen gun; after cleaning, a metal chromium film is plated on the surface of the thin film lithium niobate using a magnetron sputtering instrument as a mask layer, the waveguide pattern on the mask is transferred to the chromium film through lithography, development and chemical etching process; then the sample is soaked in a proton source (stearic acid) at 200℃ for 20 mins, the H ions in the proton source solution and the Li ions in the lithium niobate crystal undergo exchange reaction, the H ions replace part of the Li ions in the lithium niobate crystal and diffuse into the crystal, forming a H 0.5 Li0.5 NbO3 crystal layer; after the proton exchange is completed, the substrate after the proton exchange is cleaned with solvents such as acetone, ethanol, deionized water, and the like, and then the substrate is etched by inductively coupled plasma to remove the proton exchanged part of the thin film lithium niobate; after the etching is completed, the chromium remaining in the proton exchange blocking layer is removed, and thus the fabrication of the thin film lithium niobate ridge waveguide is completed; the curvature radius of the curved waveguide 24 is 150 μm, the width of the ridge waveguide is 2 μm, and the input end width of the tapered waveguide is 5 μm.

[0063] Step two, input and output end grating etching: after the waveguide device is processed, a focused ion beam is used to etch grating structures at the input and output ends. The shape of the grating is a rectangular structure. In order to ensure that the grating structure is fine enough, the ion beam current and ion dose selected are 100 pA and 0.5 nC / μm, respectively.

[0064] Step three, input and output end tapered waveguide preparation: a femtosecond laser with a wavelength of 1550 nm is used to prepare the tapered waveguide. The pulse width is 50 fs, the repetition frequency is 1 kHz, the input end width is 5 μm, the output end width is 2 μm, and the taper rate is 1 μm / mm. In addition to femtosecond laser direct writing, electron beam lithography and gray exposure methods can also be used.

[0065] Step four, preparation of the silicon nitride stress compensation layer 3: a layer of silicon nitride with a thickness of 100 nm is deposited on the surface of the curved 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 curved waveguide 24.

[0066] Step five, metal electrode preparation: titanium is used as an adhesion layer by electron beam evaporation, with a thickness of 50 nm, and then a gold layer is evaporated, with a thickness of 100 nm.

[0067] Comparative method two:

[0068] Step one, thin film lithium niobate ridge waveguide preparation: a piece of lithium niobate thin film on insulator (LNOI) substrate is provided, and the substrate surface is cleaned with acetone, ethanol and deionized water in turn, and then blown with a nitrogen gun; after cleaning, a metal chromium film is coated on the surface of the thin film lithium niobate as a mask layer using a magnetron sputtering instrument, and the light waveguide pattern on the mask is transferred to the chromium film through photoetch, development and chemical etching process; then the sample is soaked in a proton source (stearic acid) at 200°C for 20 mins, and a proton exchange reaction occurs between H ions in the proton source solution and 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 to form H 0.5 Li 0.5Nb03 crystal layer; after the proton exchange is completed, the substrate after the proton exchange is cleaned with solvents such as acetone, ethanol and deionized water in sequence, and then the substrate is etched by inductive coupling plasma, and the lithium niobate film is removed from the proton exchange part; after etching is completed, the chromium remaining in the proton exchange blocking layer is removed, and thus the fabrication of the thin-film lithium niobate ridge waveguide is completed; wherein the width of the ridge waveguide is 2μm, and the input end width of the tapered waveguide is 5μm.

[0069] Step two, input and output end grating etching: a grating structure is etched on the input and output ends of the processed waveguide device by using a focused ion beam, and the shape of the grating is a rectangular structure; in order to ensure that the grating structure is fine enough, the ion beam current and ion dose selected are 100pA and 0.5nC / μm respectively.

[0070] Step three, input and output end tapered waveguide preparation: a tapered waveguide is prepared by using a laser direct writing method, a femtosecond laser with a wavelength of 1550nm, a pulse width of 50fs and a repetition frequency of 1kHz; the input end width is gradually changed to 5μm, and the output end width is gradually changed to 2μm; the gradual change rate is 1μm / mm; in addition to the femtosecond laser direct writing, electron beam lithography and gray exposure methods can also be used.

[0071] Step four, V-shaped silicon array preparation: after the tapered waveguide preparation is completed, the surface of the thin-film lithium niobate is ultrasonically cleaned with acetone, isopropyl alcohol and deionized water in sequence to remove surface contaminants; then the surface hydrophilicity is improved by using oxygen plasma treatment with a power of 100W for 5mins; then a silicon film is deposited on the surface of the lithium niobate waveguide by using low-temperature plasma-enhanced chemical vapor deposition or magnetron sputtering, with a temperature lower than 300℃, a thickness of 0.5μm, a power of 200W and a chamber vacuum degree lower than 1×10 -6 Torr; then a layer of AZ1505 positive photoresist with a thickness of 2μm is coated on the silicon film, and the photoresist is lithographed by using ultraviolet lithography or electron beam lithography, with an ultraviolet exposure dose of 100mJ / cm 2 , and a developing time of 30s; then the silicon film is etched by using dry etching or wet etching, and the sample table angle is adjusted to form a V-shaped silicon sidewall; finally, the 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 five, preparation of silicon nitride stress compensation layer 3: a layer of silicon nitride with a thickness of 100nm is deposited on the surface of the bending waveguide by using low-temperature plasma-enhanced chemical vapor deposition to offset the stress gradient of the thin-film lithium niobate.

[0073] Step six, metal electrode preparation: titanium is evaporated by electron beam as an adhesion layer, with a thickness of 50 nm, followed by gold layer evaporation, with a thickness of 100 nm.

[0074] Comparative method three:

[0075] Step one, thin film lithium niobate ridge waveguide preparation: provide a piece of lithium niobate on insulator (LNOI) substrate, after cleaning with acetone, ethanol and deionized water in turn, the substrate surface is blown by nitrogen gun; after cleaning, a metal chromium film is plated on the surface of the thin film lithium niobate as a mask layer using a magnetron sputtering instrument, the photomask pattern on the mask is transferred to the chromium film through photoetch, development and chemical etching process; then the sample is soaked in a proton source (stearic acid) at 200℃ for 20 mins, the H ions in the proton source solution and the Li ions in the lithium niobate crystal exchange, the H ions replace part of the Li ions in the lithium niobate crystal and diffuse into the crystal, forming a H 0.5 Li 0.5 NbO3 crystal layer; after completing the proton exchange, the substrate after proton exchange is cleaned with acetone, ethanol, deionized water and other solvents in turn, and then the substrate is etched by inductively coupled plasma to remove the proton exchanged part of the thin film lithium niobate; after etching, the chromium remaining in the proton exchange barrier layer is removed, thus the fabrication of the thin film lithium niobate ridge waveguide is completed; the curvature radius of the curved waveguide 24 is 150μm, the width of the ridge waveguide is 2μm, and the input end width of the tapered waveguide is 5μm.

[0076] Step two, input and output grating etching: after the waveguide device is processed, the grating structure is etched on the input and output ends by focused ion beam method, the shape of the grating is rectangular structure, in order to ensure that the grating structure is fine enough, the selected ion beam current and ion dose are 100pA and 0.5nC / μm respectively.

[0077] Step three, input and output tapered waveguide preparation: laser direct writing method is used to prepare tapered waveguide, femtosecond laser with wavelength of 1550nm, pulse width of 50fs and repetition frequency of 1kHz, the input end width is gradually changed to 5μm, the output end width is gradually changed to 2μm, and the gradual change rate is 1μm / mm. In addition to femtosecond laser direct writing, electron beam lithography and gray exposure method can also be used.

[0078] Step four, V-shaped silicon array preparation: after the preparation of the tapered gradient waveguide, the surface of the thin film lithium niobate is ultrasonically cleaned with acetone, isopropyl alcohol and deionized water in sequence to remove surface contaminants; then oxygen plasma treatment is used, the power is 100W, and the time is 5mins to improve the surface hydrophilicity; then low temperature plasma enhanced chemical vapor deposition or magnetron sputtering is used to deposit a silicon film on the surface of the lithium niobate waveguide, the temperature is lower than 300℃, the thickness is 0.5μm, the power is 200W, and the chamber vacuum degree is lower than 1x10 -6 Torr; then a layer of AZ1505 positive photoresist is coated on the silicon film, the thickness is 2μm, ultraviolet lithography or electron beam lithography is used for photoetching, the ultraviolet exposure dose is 100mJ / cm 2 , the developing time is 30s; then dry etching or wet etching is used to etch the silicon film, the sample stage angle is adjusted to form a V-shaped silicon sidewall; finally, oxygen plasma ashing and acetone cleaning are used to remove the residual photoresist; the period of the V-shaped silicon array is 10μm, and the opening angle is 90°.

[0079] Step five, preparation of silicon nitride stress compensation layer 3: a layer of silicon nitride layer with a thickness of 100nm is deposited on the surface of the curved waveguide 24, which is used to offset the stress gradient of the thin film lithium niobate and reduce the loss of the curved waveguide 24 by using low temperature plasma enhanced chemical vapor deposition method.

[0080] Step six, metal electrode preparation: titanium is evaporated by electron beam evaporation as an adhesion layer with a thickness of 50nm, and then gold layer is evaporated with a thickness of 100nm.

[0081] The device is tested using a device test apparatus as shown in Figure 8 , and the schematic diagram during the test is shown in Figure 9 , the performance of the method of the embodiment is compared with the comparative methods one to three, and the comparison results are shown in Table 1, and it can be known from the comparison that the tunable mode converter based on lithium niobate thin film prepared by the method of the embodiment can realize higher maximum conversion efficiency and wider 3dB bandwidth, and can realize the conversion of wide spectrum mode, which has significant advantages compared with other methods.

[0082] Table 1

[0083]

[0084] In this embodiment, the structure mode of asymmetric V-shaped silicon array and curved waveguide 24 is adopted, the periodic arrangement on one side of the deflection waveguide is used to break the efficiency bottleneck of the traditional symmetric grating, realize higher mode coupling strength, solve the phase mismatching problem caused by fixed period, and improve the bandwidth transmission capacity. In addition, the device process preparation is compatible with the traditional CMOS device preparation process, the cost is low, and it is suitable for mass industrial production and preparation.

[0085] In addition to the embodiments described above, the present application can have other embodiments. Any technical solutions formed by equivalent replacement or equivalent transformation shall fall within the protection scope of the present application.

Claims

1. A tunable mode converter based on lithium niobate thin film, characterized by: The invention comprises a lithium niobate thin film substrate (1) on an insulator, a thin film lithium niobate waveguide (2) is provided on the lithium niobate thin film substrate (1), the thin film lithium niobate waveguide (2) comprises a grating coupling input end (21), an input end tapered gradient optical waveguide (22), an input end ridge type lithium niobate optical waveguide (23), a curved waveguide (24), an output end ridge type lithium niobate optical waveguide (25), an output end tapered gradient optical waveguide (26) and a grating coupling output end (27) arranged in sequence along the light propagation direction, and the curved waveguide (24) is covered with a silicon nitride stress compensation layer (3); a V-shaped silicon array mode conversion region (4) is provided on the surface of the curved waveguide (24), and the V-shaped silicon array mode conversion region (4) comprises a plurality of V-shaped silicons arranged at equal intervals along the bending direction of the curved waveguide (24), and the pointed bottom portion of the V-shaped silicon faces the direction of light propagation, and the opening angle thereof is 60° to 120°.

2. The tunable mode converter based on lithium niobate thin film according to claim 1, characterized in that: The curved waveguide (24) is composed 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 changes with the curvature radius of the curved waveguide (24), satisfying the following formula: 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 (24).

3. The tunable mode converter based on lithium niobate thin film according to claim 1, characterized in that: The deposition position offset Δx of the V-shaped silicon array mode conversion region (4) is inversely proportional to the curvature radius R, and the specific expression is: Δx=0.3R·(λ / 1550nm) Wherein, λ is the operating wavelength, and the offset direction is toward the outside of the curved waveguide (24).

4. The tunable mode converter based on lithium niobate thin film according to claim 1, characterized in that: The curvature radius 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°.

5. The tunable mode converter based on lithium niobate thin film according to claim 1, characterized in that: An air groove (5) for increasing the refractive index difference is etched on the outer side of the curved waveguide (24).

6. The 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-end ridge-type lithium niobate optical waveguide (23) and the output-end ridge-type lithium niobate optical waveguide (25).

7. The 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), the input-end ridge-type lithium niobate optical waveguide (23), and the output-end ridge-type lithium niobate optical waveguide (25).

8. The 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 by an antimony selenide rectangular array mode conversion region (6), wherein the antimony selenide rectangular array mode conversion region (6) comprises a plurality of rectangular antimony selenides arranged at equal intervals along the bending direction of the bending waveguide (24), and the length direction of each rectangular antimony selenide coincides with the width direction of the bending waveguide (24).

9. The 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 a plurality of curved waveguides (24) are provided, the curved waveguides (24) are connected in a multi-stage cascade manner.

10. A method for preparing a tunable mode converter based on lithium niobate thin film, characterized in that: The following steps are involved: 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 substrate surface 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 minutes for proton exchange. The H+ ions in the proton source solution react with the Li+ ions in the lithium niobate crystal. The H+ ions replace some of the Li+ ions in the lithium niobate crystal and diffuse into the crystal, forming H x 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 HxLi1−xNbO3 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 processed waveguide device, a focused ion beam is used to etch a rectangular grating structure at the input and output ends. The selected ion beam current and ion dose are 50-300pA and 0.5-3nC / μm respectively. S3. Preparation of tapered waveguides at the input and output ends: The tapered waveguides were prepared by laser direct writing. A femtosecond laser with a laser wavelength of 600-1550 nm, a pulse width of 50-500 fs, a repetition rate of 1-10 kHz, and a gradient from 1-10 μm at the input end to 0.1-5 μm at the output end. The gradient rate was 0.1-10 μm / mm. S4. V-shaped silicon array preparation: After the tapered waveguide is prepared, the surface of the thin film lithium niobate is ultrasonically cleaned using acetone, isopropyl alcohol, and deionized water in sequence to remove surface contaminants; Then, H plasma treatment is performed with a power of 50 to 200 W for 1 to 10 minutes. Then, a silicon thin film is deposited on the surface of the lithium niobate waveguide using low-temperature plasma-enhanced chemical vapor deposition or magnetron sputtering at a temperature below 300°C, a thickness of 0.5 to 5 μm, a power of 100 to 500 W, and a chamber vacuum of less than 1×10 - 6 Torr; then a layer of AZ1505 positive photoresist with a thickness of 1 to 5 μm is coated on the silicon film, and the photoresist is photolithographically processed using UV lithography or electron beam lithography with a UV exposure dose of 40 to 200 mJ / cm 2 , the development time is 30 to 60 seconds; then the silicon film is etched by dry etching or wet etching, and the angle of the sample stage is adjusted to form a V-shaped silicon sidewall; finally, the residual photoresist is removed by oxygen plasma ashing and acetone cleaning; S5. Preparation of silicon nitride stress compensation layer (3): depositing a silicon nitride layer with a thickness of 10 to 100 nm on the surface of the curved waveguide (24) by low-temperature plasma enhanced chemical vapor deposition or magnetron sputtering to offset the stress gradient of the thin film lithium niobate and reduce the loss of the curved waveguide (24); S6. Preparation of metal electrodes: Titanium is evaporated by electron beam as an adhesion layer with a thickness of 10 to 50 nm, followed by evaporation of a gold layer with a thickness of 10 to 500 nm.

Images