Ion implantation type lithium niobate electro-optic modulator and method for manufacturing the same

Abstract

The application discloses a lithium niobate electro-optic modulator of ion implantation type and a preparation method thereof, relates to the technical field of integrated photonics, and comprises a thin film lithium niobate substrate, a ridge type broadened waveguide region, an ion implantation type grating arranged on the ridge type broadened waveguide region, a ridge type single-mode waveguide region, a tapered waveguide region, a 1*2 Mach-Zehnder structure optical splitter, a passive bias waveguide region, a heating electrode arranged directly above the passive bias waveguide region, modulation electrodes arranged on both sides of the ridge type single-mode waveguide region and a 1*2 Mach-Zehnder structure optical combiner. The lithium niobate electro-optic modulator provided by the application enhances the modulation depth of the lithium niobate electro-optic modulator by integrating the ion implantation type grating, the passive bias waveguide and the heating electrode, expands the wavelength tuning range of the modulator, reduces the insertion loss of the modulator, is suitable for high-density photon chips, and meets the requirements of multiple scenes such as high-speed communication, quantum sensing and laser radar.

Description

Technical Field

[0001] This invention relates to the field of integrated photonics technology, and in particular to an ion-implanted lithium niobate electro-optic modulator and its fabrication method. Background Technology

[0002] Traditional lithium niobate-based Mach-Zehnder electro-optic intensity modulators typically employ electro-optic biasing, which involves applying a DC voltage to the electrodes to introduce a fixed phase difference between the upper and lower arms of the Mach-Zehnder line, thereby controlling the bias point. However, due to the charge relaxation effect in lithium niobate crystals, lithium niobate-based Mach-Zehnder electro-optic intensity modulators exhibit significant bias point drift under constant or low-frequency applied electric fields.

[0003] For Mach-Zehnder electro-optic intensity modulators based on thin-film lithium niobate, researchers have proposed using thermo-optical biasing instead of electro-optical biasing, thereby fundamentally avoiding the severe bias point drift problem introduced by electro-optical biasing. Patent application number 201110316901.8, "Reflective Sagnac Interferometric Fiber Optic Current Sensor," is based on a Sagnac interferometer and uses a multifunctional lithium niobate device (Mach-Zehnder phase adjustment system) to achieve current sensing; however, this scheme relies on fiber optic circulators and polarization-maintaining transmission cables, resulting in a complex optical path. Patent application number 202410471943.6, "A Thin-Film Lithium Niobate Y-Branch Modulator with High Polarization Extinction Ratio," uses mode hybridization (TE / TM conversion) and single-mode bent waveguides to filter out TM polarized light; however, this scheme relies on a metal absorption layer to eliminate leakage light, making the process complex. The patent application number 202411803655.2, entitled "A method for preparing a thin-film lithium niobate electro-optic modulator and an electro-optic modulator", uses a silicon substrate to etch a thick grating layer and bond a lithium niobate thin film. However, this method relies on high-precision photolithography and chemical mechanical polishing, and the grating duty cycle error is greater than ±5%.

[0004] In summary, existing technologies for fabricating lithium niobate electro-optic modulators suffer from drawbacks such as complex processes, low device integration, low modulation efficiency, lack of dynamic tuning capabilities, and reliance solely on static structural design. Summary of the Invention

[0005] To address the above technical problems, this invention provides an ion-implanted lithium niobate electro-optic modulator, comprising a thin-film lithium niobate substrate and a lithium niobate waveguide structure disposed on the thin-film lithium niobate substrate. The lithium niobate waveguide structure includes a mode converter, a 1×2 Mach-Zehnder optical beamsplitter, a passive bias waveguide region, a waveguide modulation region, a 1×2 Mach-Zehnder optical beam combiner, and the mode converter arranged sequentially along the light propagation direction. The structures of the two mode converters are symmetrical to each other along the light propagation direction. The 1×2 Mach-Zehnder optical beamsplitter and the 1×2 Mach-Zehnder optical beam combiner are connected by two parallel ridge-type single-mode waveguides.

[0006] The mode converter includes an ion-implanted grating with dual gradient characteristics. The duty cycle increases gradually from the input to the output of the mode converter along the light propagation direction, and the injection depth increases gradually from the mode converter region to the waveguide modulation region.

[0007] The passive bias waveguide region includes upper and lower arms, which are part of two ridge-type single-mode waveguides. The lower arm includes a section with a width of m. p +∆m p And the length is d p A waveguide of length d, and through t A tapered waveguide with a width of m p The two waveguides are connected, and the upper arm is m wide. p By adjusting the length d of the asymmetric structure p +2d t A phase difference is introduced between the upper and lower arms;

[0008] A heating electrode is located directly above the optical waveguide in the passive bias waveguide region; modulation electrodes are located on both sides of the ridge-type single-mode waveguide in the width direction in the waveguide modulation region.

[0009] The technical solution further defined in this invention is:

[0010] Furthermore, the thin-film lithium niobate substrate is provided with a silicon substrate, a gold reflective layer, an intermediate buried oxide layer, and a lithium niobate thin film from bottom to top.

[0011] As described above, an ion-implanted lithium niobate electro-optic modulator has a mode converter with an inverted conical structure. The mode converter on the side of the 1×2 Mach-Zehnder structure optical beam splitter includes a ridge-type broadening waveguide and a tapered waveguide arranged sequentially along the light propagation direction, and an ion-implanted grating is set on the ridge-type broadening waveguide. The mode converter is made of any one of lithium niobate, silicon nitride, polymer materials, and silicon dioxide. The width of the input end of the mode converter is set to 1–10 μm, the width of the output end is set to 0.5–2 μm, and the length of the cone region is set to 10–200 μm.

[0012] As described above, in an ion-implanted lithium niobate electro-optic modulator, the duty cycle in the ion-implanted grating increases linearly from 20% at the input of the mode converter to 60% at the output along the light propagation direction, with a gradient rate of 10–50% / mm; the implantation depth increases from 100 nm in the mode converter region to 300 nm in the waveguide modulation region, with a gradient rate of 50–200 nm / mm.

[0013] As described above, in an ion-implanted lithium niobate electro-optic modulator, in the passive bias waveguide region, m p The width ranges from 0.1 to 10 μm, and the ∆m p The width increment ranges from 0.01 to 10 μm.

[0014] As described above, in an ion-implanted lithium niobate electro-optic modulator, the heating electrode is made of aluminum, and a connection of thickness t is established between the heating electrode and the optical waveguide. b The heating electrode is isolated by a silica buffer layer. The thickness of the heating electrode is 100-400 nm and the length is 0.1-10 mm. The thickness of the silica buffer layer is 0.1-10 μm.

[0015] As described above, an ion-implanted lithium niobate electro-optic modulator has a fixed spacing of 0.1–10 μm and a thickness of 100–1200 nm for the modulation electrodes.

[0016] This invention also provides a method for fabricating an ion-implanted lithium niobate electro-optic modulator, comprising the following steps:

[0017] S1. Preparation of thin-film lithium niobate substrate: First, bathe the thin-film lithium niobate substrate in a glass cleaning solution prepared by sulfuric acid, potassium dichromate and deionized water in a certain proportion for 1-20 minutes; then rinse it with deionized water, and place the rinsed substrate in an acetone solution at 70°C and an ethanol solution at 80°C for 1-30 minutes each; then blow the surface of the cleaned thin-film lithium niobate substrate with a nitrogen gun, and place it on a hot plate at 60°C to dry it completely; finally, visually inspect it with an optical microscope to ensure that the substrate is completely clean.

[0018] S2. Fabrication of the mode converter: First, the corresponding mask pattern is drawn using software. The mode converter is then fabricated using a combination of proton exchange and dry etching. After fabrication, a polymethyl methacrylate resist with a thickness of 100–300 nm is coated onto the surface of the ridge-widened waveguide. The grating period is defined as 100–500 nm, and the duty cycle is 20%–60%. The grating is then fabricated by ion implantation into the ridge-widened waveguide. The ion implantation type is helium ions or oxygen ions, the implantation energy is 100–200 keV, and the implantation dose is 1 × 10⁻⁶. 14 ~1×10 15 ions / cm2 The injection angle is 5° to 20° for tilted injection; finally, annealing is performed at a temperature of 200 to 600°C for 10 to 60 seconds; through periodic mask design, an ion-implanted grating structure with refractive index modulation is formed in the injection area.

[0019] S3. Fabrication of nanostructures on thin-film lithium niobate substrates: First, a layer of metallic chromium with stable physicochemical properties and a smooth surface, greater than 200 nm thick, is sputtered onto the surface of a thin-film lithium niobate substrate using magnetron sputtering to prevent the patterned area from being subjected to proton exchange and wet etching. Next, electron beam lithography is used to etch the chromium-coated lithium niobate substrate surface using AZ5214 photoresist. The photoresist is spin-coated in three stages for 10–90 s, followed by soft baking in an oven. Then, hard contact exposure and development processes are used to transfer the mask pattern onto the photoresist. Next, a mixed solution of cerium ammonium nitrate and nitric acid is used to wet-etch a portion of the chromium mask, and acetone is used to remove residual photoresist, completing the transfer of the mask pattern onto the metallic chromium film. Finally, a lithium niobate waveguide structure is fabricated using proton exchange combined with wet etching or dry etching.

[0020] S4. Fabrication of heating electrodes: First, a silicon dioxide layer of 100–900 nm thickness is sputtered onto the wafer surface using an RF magnetron sputtering instrument. Then, a photoresist pattern is fabricated for the silicon dioxide layer window. The silicon dioxide is etched using a buffered silicon dioxide etching solution. The silicon dioxide in the modulator region is not covered by photoresist and is therefore removed. The silicon dioxide across the waveguide region for the heating electrodes and modulation electrodes is covered by photoresist and is therefore retained. A photoresist pattern for the heating electrodes is fabricated. Then, a layer of aluminum of 100–900 nm thickness is deposited onto the wafer surface using a thermal evaporation deposition instrument. Finally, the heating electrodes are stripped off in an acetone solution.

[0021] S5. Preparation of modulation electrode: The photoresist pattern of the modulation electrode is prepared, and then a layer of aluminum with a thickness of 1-5 μm is deposited on the wafer surface using a thermal evaporation coating instrument. Finally, the modulation electrode is stripped out in acetone solution.

[0022] Furthermore, in step S2, the mode converter is set to any one of a broadening mode converter, a biconical mode converter, and a stepped mode converter, and the ion implantation grating is located on the ridge-type broadening waveguide, with an ion implantation dose range of 1×10⁻⁶. 14 ~5×10 15 ions / cm 2 The implanted ions are helium ions or oxygen ions, the implantation energy is 10 to 500 keV, and the ion beam density is kept constant or gradually increased.

[0023] As described above, in the fabrication method of an ion-implanted lithium niobate electro-optic modulator, the conical sidewall of the mode converter is formed by tilted reactive ion etching with an tilt angle of 0.5° to 2° and a sidewall roughness of less than 10 nm. The etching gas is a CF4 / CHF3 mixed gas in a ratio of 1:2, and the radio frequency power is 150-250 W.

[0024] The beneficial effects of this invention are:

[0025] In this invention, the modulation depth of the lithium niobate electro-optic modulator is enhanced by integrating an ion-implanted grating, a passive bias waveguide, and a heating electrode, thereby expanding the wavelength tuning range of the modulator and reducing its insertion loss. The passive bias waveguide suppresses DC drift in the modulator, effectively solving the polarization crosstalk problem caused by bending in traditional devices. Standard semiconductor processes such as ion implantation and chemical mechanical polishing are employed to avoid complex etching steps, improving grating fabrication yield and reducing device fabrication costs. Furthermore, this embodiment supports integration on various substrates such as silicon and gallium arsenide, making it suitable for high-density photonic chips and meeting the needs of multiple scenarios including high-speed communication, quantum sensing, and lidar. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the structure of the thin-film lithium niobate substrate in an embodiment of the present invention;

[0027] Figure 2 This is a schematic diagram of the lithium niobate waveguide structure in an embodiment of the present invention;

[0028] Figure 3 This is a schematic diagram illustrating the transfer of a photomask pattern to photoresist in an embodiment of the present invention;

[0029] Figure 4 This is a schematic diagram illustrating the transfer of a photoresist pattern onto a chromium film in an embodiment of the present invention;

[0030] Figure 5 This is a schematic diagram illustrating the removal of photoresist from the chromium film in an embodiment of the present invention;

[0031] Figure 6 This is a schematic diagram of proton exchange on a thin-film lithium niobate substrate in an embodiment of the present invention;

[0032] Figure 7 This is a schematic diagram of dry etching of a thin-film lithium niobate substrate in an embodiment of the present invention;

[0033] Figure 8 This is a schematic diagram of sputtering a silicon dioxide thin film on the surface of a thin-film lithium niobate substrate in an embodiment of the present invention;

[0034] Figure 9 This is a schematic diagram of the fabrication of a heating electrode in the bias region in an embodiment of the present invention;

[0035] Figure 10 This is a schematic diagram of the fabrication of a modulation electrode in the modulation region in an embodiment of the present invention;

[0036] Figure 11 This is a schematic diagram of the electro-optic response testing device for a lithium niobate electro-optic modulator in an embodiment of the present invention;

[0037] Figure 12 This is a schematic diagram of the curves obtained from the electro-optic response test in an embodiment of the present invention;

[0038] Figure 13 This is a physical diagram of the electro-optic modulator in an embodiment of the present invention.

[0039] Among them, 1. Thin-film lithium niobate substrate; 11. Silicon substrate; 12. Gold reflective layer; 13. Intermediate buried oxide layer; 14. Lithium niobate thin film; 2. Lithium niobate waveguide structure; 21. Mode converter; 211. Ridge-type broadened waveguide region; 212. Tapered graded waveguide region; 213. Ion-implanted grating; 22. 1×2 Mach-Zehnder structure optical beamsplitter; 23. Passive bias waveguide region; 24. Waveguide modulation region; 25. 1×2 Mach-Zehnder structure optical beam combiner; 26. Ridge-type single-mode waveguide; 3. Heating electrode; 4. Modulation electrode; 5. Chromium film; 6. Photoresist; 7. Proton exchange solution; 8. Silicon dioxide passivation layer. Detailed Implementation

[0040] This embodiment provides an ion-implanted lithium niobate electro-optic modulator, including a thin-film lithium niobate substrate and a lithium niobate waveguide structure disposed on the thin-film lithium niobate substrate, such as... Figure 1 As shown, the thin-film lithium niobate substrate is provided with a silicon substrate, a gold reflective layer, an intermediate buried oxide layer and a lithium niobate thin film from bottom to top.

[0041] like Figure 2 As shown, the lithium niobate waveguide structure includes a mode converter, a 1×2 Mach-Zehnder optical beam splitter, a passive bias waveguide region, a waveguide modulation region, a 1×2 Mach-Zehnder optical beam combiner, and a mode converter arranged sequentially along the light propagation direction. The structures of the two mode converters are symmetrical to each other along the light propagation direction. The 1×2 Mach-Zehnder optical beam splitter and the 1×2 Mach-Zehnder optical beam combiner are connected by two parallel ridge-type single-mode waveguides.

[0042] like Figure 2As shown, the mode converter is designed as an inverted conical structure. The mode converter on the side of the 1×2 Mach-Zehnder structure optical beam splitter includes a ridge-type broadening waveguide and a tapered gradient waveguide arranged sequentially along the optical propagation direction, and an ion-implanted grating is set on the ridge-type broadening waveguide. The material of the mode converter is any one of lithium niobate, silicon nitride, polymer materials (such as SU-8, polyimide) and silicon dioxide. The width of the input end of the mode converter is set to 1 to 10 μm, the width of the output end is set to 0.5 to 2 μm, and the length of the cone region is set to 10 to 200 μm.

[0043] The grating structure fabricated by ion implantation on the surface of the mode converter has dual gradient characteristics. First, there is the duty cycle gradient, which increases linearly from 20% at the input end of the mode converter to 60% at the output end along the light propagation direction, with a gradient rate of 10–50% / mm. Second, there is the implantation depth gradient, which increases from 100 nm deep in the mode converter region to 300 nm deep in the waveguide modulation region, with a gradient rate of 50–200 nm / mm.

[0044] The passive bias waveguide region includes upper and lower arms, which are part of two ridge-type single-mode waveguides. The lower arm includes a section with a width of m. p +∆m p And the length is d p A waveguide of length d, and through t A tapered waveguide with a width of m p The two waveguides are connected, and the upper arm is m wide. p By adjusting the length d of the asymmetric structure p +2d t A phase difference is introduced between the upper and lower arms; where m p The width ranges from 0.1 to 10 μm, and the ∆m p The width increment ranges from 0.01 to 10 μm.

[0045] A heating electrode is located directly above the optical waveguide within the passive bias waveguide region. The heating electrode is made of aluminum, and a connection of thickness t is established between the heating electrode and the optical waveguide. b The heating electrode is isolated by a silica buffer layer. The thickness of the heating electrode is 100-400 nm and the length is 0.1-10 mm. The thickness of the silica buffer layer is 0.1-10 μm.

[0046] Modulation electrodes are provided on both sides of the ridge-type single-mode waveguide in the width direction within the waveguide modulation region. The fixed spacing between the modulation electrodes is 0.1 to 10 μm, and the thickness is 100 to 1200 nm.

[0047] This embodiment also provides a method for fabricating an ion-implanted lithium niobate electro-optic modulator, including the following steps:

[0048] S1. Preparation of thin-film lithium niobate substrate: First, the thin-film lithium niobate substrate is bathed in a glass cleaning solution prepared by sulfuric acid, potassium dichromate and deionized water in a certain proportion for 10 minutes to remove larger organic matter, dust and particles from the substrate surface; then, it is rinsed with deionized water, and the rinsed substrate is placed in an acetone solution at 70°C and an ethanol solution at 80°C for 10 minutes each; then, the surface of the cleaned thin-film lithium niobate substrate is blown with a nitrogen gun and placed on a hot plate at 60°C to dry it completely; finally, it is visually inspected with an optical microscope to ensure that the substrate is completely clean.

[0049] S2. Fabrication of the mode converter: First, the corresponding mask pattern is drawn using software. The mode converter is then fabricated using a combination of proton exchange and dry etching. After fabrication, a polymethyl methacrylate (PMMA) resist with a thickness of 300 nm is coated onto the surface of the ridge-widened waveguide. The grating period is defined as 100 nm, and the duty cycle as 20%. The grating is then fabricated by ion implantation into the ridge-widened waveguide. The ion implantation type is helium ions, the implantation energy is 100 keV, and the implantation dose is 1 × 10⁻⁶. 15 ions / cm 2 The injection angle is 7° tilted injection. After helium ions are injected into the lithium niobate lattice, lattice damage or doping is caused, and the refractive index of the material is locally changed. Finally, annealing is performed at 200℃ for 20s. Through periodic mask design, an ion-implanted grating structure with refractive index modulation is formed in the injection area.

[0050] The mode converter is configured as any one of a broadened mode converter, a biconical mode converter, and a stepped mode converter. The ion implantation grating is located on the ridge-type broadened waveguide, and the ion implantation dose range is 1×10⁻⁶. 14 ~5×10 15 ions / cm 2 The implanted ions are helium ions or oxygen ions, the implantation energy is 10 to 500 keV, and the ion beam current density can be kept constant or gradually increased.

[0051] The tapered sidewalls of the pattern converter are formed by tilted reactive ion etching with an angle of 0.5° to 2° and a sidewall roughness of less than 10 nm. The etching gas is a CF4 / CHF3 mixture (ratio 1:2) and the radio frequency power is 150-250W.

[0052] S3, Preparation of nanostructures on the surface of thin-film lithium niobate substrates; such as Figure 3As shown, firstly, a 300nm thick layer of stable and smooth chromium metal with good physicochemical properties was sputtered onto the surface of a thin-film lithium niobate substrate using magnetron sputtering as a mask to prevent the patterned area from being subjected to proton exchange and wet etching. Next, electron beam lithography was used to etch the chromium-coated lithium niobate substrate surface using AZ5214 photoresist. After three-stage spin coating for 30 seconds, the substrate was soft-baked in an oven. Figure 4 As shown, a hard-contact exposure and development process is then used to transfer the mask pattern onto the photoresist; as... Figure 5 As shown, a mixed solution of cerium ammonium nitrate and nitric acid was then used to wet-etch a portion of the chromium mask, and acetone was used to remove residual photoresist, thus completing the transfer of the mask pattern onto the chromium thin film; as shown. Figures 6 to 7 As shown, the lithium niobate waveguide structure was finally prepared by proton exchange wet etching or dry etching.

[0053] S4. Preparation of heating electrode: such as Figure 8 As shown, firstly, a 300nm thick silicon dioxide layer is sputtered onto the wafer surface using an RF magnetron sputtering instrument; then, a photoresist pattern is fabricated for the silicon dioxide layer windows, and the silicon dioxide is etched using a buffered silicon dioxide etchant. The silicon dioxide in the modulator region is not obstructed by photoresist and is therefore removed, while the silicon dioxide across the waveguide region for the heating and modulation electrodes is obstructed by photoresist and is therefore retained; as shown... Figure 9 As shown, a photoresist pattern for the heating electrode is fabricated, and then a 300nm thick layer of aluminum is deposited on the wafer surface using a thermal evaporation coating instrument. Finally, the heating electrode is stripped out in an acetone solution.

[0054] S5. Preparation of the modulation electrode: such as Figure 10 As shown, a photoresist pattern for the modulation electrode is fabricated, and then a 1μm thick layer of aluminum is deposited on the wafer surface using a thermal evaporation coating instrument. Finally, the modulation electrode is stripped out in an acetone solution.

[0055] Based on the above embodiments, some modifications were made to form three other different methods for fabricating ion-implanted lithium niobate electro-optic modulators. In comparative method one, no ion-implanted grating structure was set; in comparative method two, no passive bias structure was set; and in comparative method three, no thermo-optic bias structure was set.

[0056] Comparison Method 1:

[0057] Step 1: Preparation of thin-film lithium niobate substrate: First, bathe the thin-film lithium niobate substrate in a glass cleaning solution prepared by sulfuric acid, potassium dichromate and deionized water in a certain proportion for 10 minutes to remove larger organic matter, dust and particles from the substrate surface; then rinse it with deionized water, and place the rinsed substrate in an acetone solution at 70℃ and an ethanol solution at 80℃ for 10 minutes each; next, blow the cleaned surface of the thin-film lithium niobate substrate with a nitrogen gun, and place it on a hot plate at 60℃ to dry it completely; finally, visually inspect it with an optical microscope to ensure that the substrate is completely clean.

[0058] Step 2: Fabrication of grating-type pattern converter: First, the corresponding mask pattern is drawn using software, and the broadened pattern converter is fabricated by combining proton exchange and dry etching.

[0059] Step 3: Fabrication of nanostructures on thin-film lithium niobate substrates; First, a 300nm thick layer of stable and smooth metallic chromium is sputtered onto the surface of a thin-film lithium niobate substrate using magnetron sputtering to prevent the patterned area from being subjected to proton exchange and wet etching; Next, electron beam lithography is used to etch the chromium-coated lithium niobate substrate surface using AZ5214 photoresist, which is applied in three stages by spin coating for 30 seconds and then soft-baked in an oven. Hard contact exposure and development processes are then used to transfer the mask pattern onto the photoresist; Next, a mixed solution of cerium ammonium nitrate and nitric acid is used to wet-etch a portion of the chromium mask, and acetone is used to remove residual photoresist, completing the transfer of the mask pattern onto the metallic chromium film; Finally, a lithium niobate waveguide structure is fabricated using proton exchange combined with wet etching or dry etching.

[0060] Step 4: Fabrication of heating electrodes: First, a 300nm thick silicon dioxide layer is sputtered on the wafer surface using an RF magnetron sputtering instrument; then, a photoresist pattern is fabricated for the silicon dioxide layer window, and the silicon dioxide is etched using a buffered silicon dioxide etching solution. The silicon dioxide in the modulator region is not covered by photoresist, so it is removed. The silicon dioxide across the waveguide region for the heating electrodes and modulation electrodes is covered by photoresist, so it is retained; a photoresist pattern for the heating electrodes is fabricated, and then a 300nm thick aluminum layer is deposited on the wafer surface using a thermal evaporation deposition instrument. Finally, the heating electrodes are stripped off in an acetone solution.

[0061] Step 5: Create the photoresist pattern for the modulation electrode, then use a thermal evaporation coating machine to deposit a 1μm thick layer of aluminum on the wafer surface, and finally peel off the modulation electrode in an acetone solution.

[0062] Comparison Method Two:

[0063] Step 1: Preparation of thin-film lithium niobate substrate: First, bathe the thin-film lithium niobate substrate in a glass cleaning solution prepared by sulfuric acid, potassium dichromate and deionized water in a certain proportion for 10 minutes to remove larger organic matter, dust and particles from the substrate surface; then rinse it with deionized water, and place the rinsed substrate in an acetone solution at 70℃ and an ethanol solution at 80℃ for 10 minutes each; next, blow the cleaned surface of the thin-film lithium niobate substrate with a nitrogen gun, and place it on a hot plate at 60℃ to dry it completely; finally, visually inspect it with an optical microscope to ensure that the substrate is completely clean.

[0064] Step 2: Fabrication of the mode converter: First, the corresponding mask pattern is drawn using software. The mode converter is then fabricated using a combination of proton exchange and dry etching. After fabrication, a polymethyl methacrylate (PMMA) resist with a thickness of 300 nm is coated onto the surface of the ridge-widened waveguide. The grating period is defined as 100 nm, and the duty cycle is 20%. The grating is then fabricated by ion implantation into the ridge-widened waveguide. The ion implantation type is helium ions, the implantation energy is 100 keV, and the implantation dose is 1 × 10⁻⁶. 15 ions / cm 2 The injection angle is 7° tilted injection. After helium ions are injected into the lithium niobate lattice, lattice damage or doping is caused, and the refractive index of the material is locally changed. Finally, annealing is performed at 200℃ for 20s. Through periodic mask design, an ion-implanted grating structure with refractive index modulation is formed in the injection area.

[0065] Step 3: Fabrication of nanostructures on thin-film lithium niobate substrates; First, a 300nm thick layer of stable and smooth metallic chromium is sputtered onto the surface of a thin-film lithium niobate substrate using magnetron sputtering to prevent the patterned area from being subjected to proton exchange and wet etching; Next, electron beam lithography is used to etch the chromium-coated lithium niobate substrate surface using AZ5214 photoresist. After spin coating in three stages for 30 seconds, the substrate is soft-baked in an oven. Then, hard contact exposure and development processes are used to transfer the mask pattern onto the photoresist, removing the passive bias waveguide pattern; Then, a mixed solution of cerium ammonium nitrate and nitric acid is used to wet-etch part of the chromium mask, and acetone is used to remove the residual photoresist, completing the transfer of the mask pattern onto the metallic chromium film; Finally, the lithium niobate waveguide structure is fabricated by proton exchange combined with wet etching or dry etching.

[0066] Step 4: Fabrication of heating electrodes: First, a 300nm thick silicon dioxide layer is sputtered on the wafer surface using an RF magnetron sputtering instrument; then, a photoresist pattern is fabricated for the silicon dioxide layer window, and the silicon dioxide is etched using a buffered silicon dioxide etching solution. The silicon dioxide in the modulator region is not covered by photoresist, so it is removed. The silicon dioxide across the waveguide region for the heating electrodes and modulation electrodes is covered by photoresist, so it is retained; a photoresist pattern for the heating electrodes is fabricated, and then a 300nm thick aluminum layer is deposited on the wafer surface using a thermal evaporation deposition instrument. Finally, the heating electrodes are stripped off in an acetone solution.

[0067] Step 5: Preparation of modulation electrode: Create photoresist pattern for modulation electrode, then deposit a 1μm thick layer of aluminum on the wafer surface using a thermal evaporation coating instrument, and finally peel off the modulation electrode in acetone solution.

[0068] Comparison Method 3:

[0069] Step 1: Preparation of thin-film lithium niobate substrate: First, bathe the thin-film lithium niobate substrate in a glass cleaning solution prepared by sulfuric acid, potassium dichromate and deionized water in a certain proportion for 10 minutes to remove larger organic matter, dust and particles from the substrate surface; then rinse it with deionized water, and place the rinsed substrate in an acetone solution at 70℃ and an ethanol solution at 80℃ for 10 minutes each; next, blow the cleaned surface of the thin-film lithium niobate substrate with a nitrogen gun, and place it on a hot plate at 60℃ to dry it completely; finally, visually inspect it with an optical microscope to ensure that the substrate is completely clean.

[0070] Step 2: Fabrication of the mode converter: First, the corresponding mask pattern is drawn using software. The mode converter is then fabricated using a combination of proton exchange and dry etching. After fabrication, a polymethyl methacrylate (PMMA) resist with a thickness of 300 nm is coated onto the surface of the ridge-widened waveguide. The grating period is defined as 100 nm, and the duty cycle is 20%. The grating is then fabricated by ion implantation into the ridge-widened waveguide. The ion implantation type is helium ions, the implantation energy is 100 keV, and the implantation dose is 1 × 10⁻⁶. 15 ions / cm 2 The injection angle is 7° tilted injection. After helium ions are injected into the lithium niobate lattice, lattice damage or doping is caused, and the refractive index of the material is locally changed. Finally, annealing is performed at 200℃ for 20s. Through periodic mask design, an ion-implanted grating structure with refractive index modulation is formed in the injection area.

[0071] Step 3: Fabrication of nanostructures on thin-film lithium niobate substrates; First, a 300nm thick layer of stable and smooth metallic chromium is sputtered onto the surface of a thin-film lithium niobate substrate using magnetron sputtering to prevent the patterned area from being subjected to proton exchange and wet etching; Next, electron beam lithography is used to etch the chromium-coated lithium niobate substrate surface using AZ5214 photoresist, which is applied in three stages by spin coating for 30 seconds and then soft-baked in an oven. Hard contact exposure and development processes are then used to transfer the mask pattern onto the photoresist; Next, a mixed solution of cerium ammonium nitrate and nitric acid is used to wet-etch a portion of the chromium mask, and acetone is used to remove residual photoresist, completing the transfer of the mask pattern onto the metallic chromium film; Finally, a lithium niobate waveguide structure is fabricated using proton exchange combined with wet etching or dry etching.

[0072] Step 4: Preparation of modulation electrode: Create photoresist pattern for modulation electrode, then deposit a 1μm thick layer of aluminum on the wafer surface using a thermal evaporation coating instrument, and finally peel off the modulation electrode in acetone solution.

[0073] The performance of the method in this embodiment is compared with that of comparative methods one to three, as shown in Table 1 below. The comparison shows that the lithium niobate electro-optic modulator prepared by the method in this embodiment has a greater modulation depth, a wider wavelength tuning range, a larger 3dB bandwidth, a lower insertion loss, and a higher polarization extinction ratio. All parameters are superior to those of the lithium niobate electro-optic modulators prepared based on comparative methods one to three.

[0074] Table 1

[0075]

[0076] like Figure 11 As shown, the electro-optic response of the lithium niobate electro-optic modulator in this embodiment was tested, and the test results are as follows. Figure 12 As shown, from Figure 12 As can be seen from the S11 curve, the overall return loss of the modulator prepared in this embodiment is less than -17dB within the frequency range of 0.1 to 40 GHz. S11 is close to 0dB (or <-20dB) in the DC to 1 GHz band, indicating that the electrode impedance is well matched with the driving source (usually 50Ω) and the reflection loss is low. The S21 curve also clearly shows that if the RF signal starts directly from DC, the 3dB modulation bandwidth of the device is about 35 GHz to 40 GHz. If the RF signal frequency starts from 4.8 GHz, the 3dB modulation bandwidth of the device is much greater than 40 GHz. It should be noted that currently, the device modulation bandwidth is limited by the testing capabilities of laboratory equipment, rather than the performance of the prepared device itself.

[0077] like Figure 13The image shown is a physical diagram of the electro-optic modulator in this embodiment. By integrating an ion-implanted grating, a passive bias waveguide, and a heating electrode, the modulation depth of the lithium niobate electro-optic modulator is enhanced, expanding the wavelength tuning range of the modulator and reducing its insertion loss. The passive bias waveguide suppresses the DC drift of the modulator, effectively solving the polarization crosstalk problem caused by bending in traditional devices. Standard semiconductor processes such as ion implantation and chemical mechanical polishing are used to avoid complex etching steps, improve the grating fabrication yield, and reduce device fabrication costs. At the same time, this embodiment also supports integration on various substrates such as silicon and gallium arsenide, making it suitable for high-density photonic chips and meeting the needs of various scenarios such as high-speed communication, quantum sensing, and lidar.

[0078] This embodiment provides a thin-film lithium niobate electro-optic modulator with low driving voltage, large modulation depth, low mode mismatch loss, support for dynamic wavelength compensation, and strong adaptability to ambient temperature fluctuations. This enables the lithium niobate electro-optic modulator to be used in demanding applications such as 5G optical modules and quantum integration, thereby solving a series of problems mentioned in the background art.

[0079] This embodiment focuses on a thin-film lithium niobate electro-optic modulator. It employs ion-implanted ridge-type broadened waveguides to fabricate grating coupling devices, eliminating the need for complex fiber structures and resulting in a more compact device suitable for high-speed optical communication chip integration. This embodiment uses a passive bias waveguide to stabilize the operating point, combined with thermo-optical tuning electrodes to dynamically compensate for polarization drift, eliminating the need for additional filter structures and simplifying the process. This embodiment directly forms a graded grating through ion implantation, achieving a grating period error of less than ±2nm, controllable duty cycle gradient, avoiding etching damage, and providing a larger tuning range for the refractive index modulation depth compared to the aforementioned patents.

[0080] 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. An ion-implanted lithium niobate electro-optic modulator, characterized in that: The invention includes a thin-film lithium niobate substrate and a lithium niobate waveguide structure disposed on the thin-film lithium niobate substrate. The lithium niobate waveguide structure includes a mode converter, a 1×2 Mach-Zehnder optical beam splitter, a passive bias waveguide region, a waveguide modulation region, a 1×2 Mach-Zehnder optical beam combiner, and a mode converter arranged sequentially along the light propagation direction. The structures of the two mode converters are symmetrical to each other along the light propagation direction. The 1×2 Mach-Zehnder optical beam splitter and the 1×2 Mach-Zehnder optical beam combiner are connected by two parallel ridge-type single-mode waveguides. The mode converter includes an ion-implanted grating with dual gradient characteristics. The duty cycle increases gradually from the input to the output of the mode converter along the light propagation direction, and the injection depth increases gradually from the mode converter region to the waveguide modulation region. The passive bias waveguide region includes upper and lower arms, which are part of two ridge-type single-mode waveguides. The lower arm includes a section with a width of m. p +∆m p And the length is d p A waveguide of length d, and through t A tapered waveguide with a width of m p The two waveguides are connected, and the upper arm is m wide. p By adjusting the length d of the asymmetric structure p +2d t A phase difference is introduced between the upper and lower arms; A heating electrode is provided directly above the optical waveguide in the passive bias waveguide region; modulation electrodes are provided on both sides of the ridge-type single-mode waveguide in the width direction in the waveguide modulation region. The mode converter is designed as an inverted conical structure. The mode converter on the side of the optical beam splitter with the 1×2 Mach-Zehnder structure includes a ridge-type broadening waveguide and a tapered waveguide arranged sequentially along the optical propagation direction, and an ion-implanted grating is set on the ridge-type broadening waveguide. In ion-implanted gratings, the duty cycle increases linearly from 20% at the input of the mode converter to 60% at the output along the light propagation direction, with a gradient rate of 10–50% / mm; the implantation depth increases from 100 nm in the mode converter region to 300 nm in the waveguide modulation region, with a gradient rate of 50–200 nm / mm.

2. The ion-implanted lithium niobate electro-optic modulator according to claim 1, characterized in that: The thin-film lithium niobate substrate is provided with a silicon substrate, a gold reflective layer, an intermediate buried oxide layer and a lithium niobate thin film in sequence from bottom to top.

3. The ion-implanted lithium niobate electro-optic modulator according to claim 1, characterized in that: The material of the speckle converter is set to any one of lithium niobate, silicon nitride, polymer material and silicon dioxide. The width of the input end of the speckle converter is set to 1 to 10 μm, the width of the output end is set to 0.5 to 2 μm, and the length of the cone region is set to 10 to 200 μm.

4. The ion-implanted lithium niobate electro-optic modulator according to claim 1, characterized in that: In the passive bias waveguide region, m p The width ranges from 0.1 to 10 μm, and the ∆m p The width increment ranges from 0.01 to 10 μm.

5. The ion-implanted lithium niobate electro-optic modulator according to claim 1, characterized in that: The heating electrode is made of aluminum, and a connection of thickness t is used between the heating electrode and the optical waveguide. b The heating electrode is isolated by a silica buffer layer. The thickness of the heating electrode is 100-400 nm and the length is 0.1-10 mm. The thickness of the silica buffer layer is 0.1-10 μm.

6. The ion-implanted lithium niobate electro-optic modulator according to claim 1, characterized in that: The modulation electrodes have a fixed spacing of 0.1–10 μm and a thickness of 100–1200 nm.

7. A method for fabricating an ion-implanted lithium niobate electro-optic modulator as described in any one of claims 1-6, characterized in that: Includes the following steps: S1. Preparation of thin-film lithium niobate substrate: First, bathe the thin-film lithium niobate substrate in a glass cleaning solution prepared by sulfuric acid, potassium dichromate and deionized water in a certain proportion for 1-20 minutes; then rinse it with deionized water, and place the rinsed substrate in an acetone solution at 70°C and an ethanol solution at 80°C for 1-30 minutes each; then blow the surface of the cleaned thin-film lithium niobate substrate with a nitrogen gun, and place it on a hot plate at 60°C to dry it completely; finally, visually inspect it with an optical microscope to ensure that the substrate is completely clean. S2. Fabrication of the mode converter: First, the corresponding mask pattern is drawn using software. The mode converter is then fabricated using a combination of proton exchange and dry etching. After fabrication, a polymethyl methacrylate resist with a thickness of 100–300 nm is coated onto the surface of the ridge-widened waveguide. The grating period is defined as 100–500 nm, and the duty cycle is 20%–60%. The grating is then fabricated by ion implantation into the ridge-widened waveguide. The ion implantation type is helium ions or oxygen ions, the implantation energy is 100–200 keV, and the implantation dose is 1 × 10⁻⁶. 14 ~1×10 15 ions / cm 2 The injection angle is 5° to 20° with tilted injection; finally, annealing is performed at a temperature of 200 to 600° for 10 to 60 seconds. Through periodic mask design, an ion-implanted grating structure with refractive index modulation is formed in the injection region; S3. Fabrication of nanostructures on thin-film lithium niobate substrates: First, a layer of metallic chromium with a thickness greater than 200 nm, possessing stable physicochemical properties and a smooth surface, is sputtered onto the surface of a thin-film lithium niobate substrate using magnetron sputtering as a mask to prevent the patterned area from being subjected to proton exchange and wet etching. Then, electron beam lithography is used to etch the surface of the thin-film lithium niobate substrate covered with the chromium film. The photoresist is AZ5214. After spin coating in three stages for 10–90 s, the substrate is soft-baked in an oven. Finally, hard contact exposure and development processes are used to transfer the mask pattern onto the photoresist. Then, a mixed solution of cerium ammonium nitrate and nitric acid was used to perform wet etching on part of the chromium mask, and acetone was used to remove the residual photoresist, thus transferring the mask pattern onto the metal chromium thin film; finally, the lithium niobate waveguide structure was prepared by proton exchange wet etching or dry etching. S4. Fabrication of heating electrodes: First, a silicon dioxide layer of 100–900 nm thickness is sputtered onto the wafer surface using an RF magnetron sputtering instrument. Then, a photoresist pattern is fabricated for the silicon dioxide layer window. The silicon dioxide is etched using a buffered silicon dioxide etching solution. The silicon dioxide in the modulator region is not covered by photoresist and is therefore removed. The silicon dioxide across the waveguide region for the heating electrodes and modulation electrodes is covered by photoresist and is therefore retained. A photoresist pattern for the heating electrodes is fabricated. Then, a layer of aluminum of 100–900 nm thickness is deposited onto the wafer surface using a thermal evaporation deposition instrument. Finally, the heating electrodes are stripped off in an acetone solution. S5. Preparation of modulation electrode: The photoresist pattern of the modulation electrode is prepared, and then a layer of aluminum with a thickness of 1-5 μm is deposited on the wafer surface using a thermal evaporation coating instrument. Finally, the modulation electrode is stripped out in acetone solution.

8. The method for fabricating an ion-implanted lithium niobate electro-optic modulator according to claim 7, characterized in that: In step S2, the mode converter is set to any one of a broadened mode converter, a biconical mode converter, and a stepped mode converter. The ion implantation grating is located on the ridge-type broadened waveguide, and the ion implantation dose range is 1×10⁻⁶. 14 ~5×10 15 ions / cm 2 The implanted ions are helium ions or oxygen ions, the implantation energy is 10 to 500 keV, and the ion beam density is kept constant or gradually increased.

9. The method for fabricating an ion-implanted lithium niobate electro-optic modulator according to claim 7, characterized in that: The tapered sidewall of the mode converter is formed by tilted reactive ion etching with an tilt angle of 0.5° to 2° and a sidewall roughness of less than 10 nm. The etching gas is a CF4 / CHF3 mixture in a ratio of 1:2 and the radio frequency power is 150-250 W.

Images