Optical phased array radiation antenna based on LNOI and preparation method thereof

Abstract

The invention discloses an LNOI-based optical phased array radiation antenna and a preparation method thereof, the antenna takes an LNOI as a platform, silicon nitride columns cover an input waveguide of the antenna to form a silicon nitride grating, the silicon nitride columns are distributed in the middle or two sides of the input waveguide along a guided wave transmission direction, direct etching of lithium niobate is avoided, and the performance of the antenna is improved. The radiation area of the radiation antenna can be reduced, so that the radiation envelope is expanded, and the far-field scanning range is increased; meanwhile, the widths of the silicon nitride columns are gradually changed, the effect of uniform radiation is achieved, the effective radiation length of the radiation antenna is prolonged, and the scanning precision is improved.

Description

Technical Field

[0001] The present invention relates to the field of optical communications, and in particular to an optical phased array radiating antenna based on LNOI and a preparation method thereof. Background Art

[0002] Beam steering scanning technology achieves spatial scanning or dynamic pointing by controlling the direction of a beam's propagation. Its core goal is to adjust the beam's propagation path with high precision and speed to meet scanning requirements in various scenarios. Compared to traditional millimeter-wave radar, lidar operates at a shorter wavelength, offers higher resolution, a longer detection range, and enhanced anti-interference capabilities. Optical phased arrays (OPAs) can be used in all-solid-state lidar, replacing traditional mechanical lidars. These technologies offer a wide scanning range, high stability, and rapid beam steering.

[0003] Lithium niobate (LNOI) crystals offer a range of advantages, including a broadband optically transparent window covering the visible, near-infrared, and extremely long mid-infrared wavelengths; a high refractive index, allowing thin-film LNOI to form high-contrast optical waveguides on a variety of substrates; a non-centrosymmetric crystal with a large nonlinear coefficient, making it widely used in nonlinear frequency conversion and photon pair generation; an extremely high linear electro-optic coefficient, making it a preferred material for electro-optic modulators; and excellent acousto-optic, piezoelectric, and thermo-optical properties, as well as stable physical and chemical properties. However, as an inert material, LNOI presents challenges for direct etching of LNOI radiating antennas. Summary of the Invention

[0004] Purpose of the invention: The purpose of the present invention is to provide an optical phased array radiating antenna based on LNOI and its preparation method, which avoids directly etching lithium niobate to form a grating, while reducing loss, expanding the radiation envelope, and increasing the scanning range.

[0005] Technical solution: The LNOI-based optical phased array radiating antenna described in the present invention includes: an LNOI and a silicon nitride grating layer covering it. The LNOI includes a lithium niobate waveguide layer, an input waveguide is etched on the lithium niobate waveguide layer, and a silicon nitride grating layer is formed by covering the surface of the input waveguide with silicon nitride pillars. When the guided wave is transmitted through the input waveguide, it radiates into free space through the silicon nitride grating layer, thereby forming a scanning beam.

[0006] Furthermore, the silicon nitride columns are distributed in the middle of the input waveguide along the waveguide transmission direction, or the silicon nitride columns are symmetrically distributed on both sides of the input waveguide along the waveguide transmission direction.

[0007] Furthermore, the side length of the silicon nitride column perpendicular to the waveguide transmission direction is equal to its width, and the width of each silicon nitride column is the same.

[0008] Furthermore, the side length of the silicon nitride column perpendicular to the waveguide transmission direction is its width. The width of the silicon nitride column is designed to be gradually changed along the waveguide transmission direction. The calculation method of the width of the silicon nitride column is as follows:

[0009] according to The radiation coefficient of the silicon nitride column at each position is calculated, and the width value corresponding to the radiation coefficient is calculated based on the cubic polynomial fitted between the width of the silicon nitride column and the radiation coefficient, thereby obtaining the width of the silicon nitride column at each position; where x is the position of the silicon nitride column on the input waveguide surface, and L is the length from the first silicon nitride column to the last silicon nitride column.

[0010] Furthermore, when the radiation coefficient α(x) reaches its maximum value, the width of the silicon nitride column at position x also reaches its maximum value, and the widths of the silicon nitride columns after position x in the waveguide transmission direction are all at their maximum values.

[0011] Furthermore, the LNOI further comprises, from bottom to top, a supporting substrate layer of silicon material and a buffer layer of silicon dioxide material.

[0012] Furthermore, the silicon nitride grating layer is also covered with an upper cladding layer of silicon dioxide material.

[0013] The method for preparing the LNOI-based optical phased array radiating antenna of the present invention comprises the following steps:

[0014] Step 1: Etching the lithium niobate waveguide layer of LNOI to form an input waveguide;

[0015] Step 2: depositing a silicon nitride film on the input waveguide and writing a pattern of silicon nitride columns, and forming silicon nitride columns by etching the silicon nitride film.

[0016] Furthermore, step 2 specifically includes the following steps:

[0017] Step 2.1, depositing a silicon nitride film on the input waveguide by plasma enhanced chemical vapor deposition, spin coating a photoresist, and writing a pattern of silicon nitride pillars on the photoresist by electron beam lithography;

[0018] Step 2.2, developing, depositing a chromium film on the surface of the photoresist using electron beam evaporation, and transferring the pattern of the silicon nitride pillars onto the chromium film;

[0019] In step 3.3, the silicon nitride film is etched using plasma gas, and the residual chromium mask is removed using a cerium ammonium nitrate solution to obtain silicon nitride pillars.

[0020] The computer-readable storage medium of the present invention stores a computer program, and when the computer program is executed by a processor, the method for preparing the LNOI-based optical phased array radiating antenna is implemented.

[0021] Beneficial effects: Compared with the prior art, the advantages of the present invention are:

[0022] (1) The optical phased array antenna based on LNOI materials of the present invention can weakly couple the input TE0 to free space, forming a high-precision scanning beam in the far field. By combining the superior electro-optical effect of lithium niobate with the large refractive index contrast of LNOI, it has a wide optical transparency window and very low loss.

[0023] (2) The present invention etches silicon nitride by heterogeneously integrating a silicon nitride grating layer on LNOI, thereby avoiding direct etching of lithium niobate, overcoming the problem that lithium niobate has a small line width and is difficult to directly etch into a grating, and improving the degree of freedom in design.

[0024] (3) The present invention freely etches the covering silicon nitride grating layer, which can reduce the structure of the radiating antenna and thus expand the radiation envelope, increase the far-field scanning range, and reduce the power attenuation of the main lobe scanning beam at large scanning angles.

[0025] (4) Based on the relationship between the width of the silicon nitride column and its radiation coefficient, the present invention adopts a gradient design for the width of the silicon nitride, thereby changing the radiation coefficient of the radiating antenna at different positions, achieving a uniform radiation effect, extending the effective radiation length of the radiating antenna, and improving scanning accuracy.

[0026] (5) This invention can promote the future integration, miniaturization and low-loss development of OPA and enrich the research system of OPA. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a structural diagram of the optical phased array radiating antenna according to embodiment 1 of the present invention.

[0028] Figure 2 This is a cross-sectional view of the optical phased array radiating antenna according to embodiment 1 of the present invention.

[0029] Figure 3 Schematic diagram of the input waveguide and silicon nitride grating layer structure of Example 1 of the present invention.

[0030] Figure 4 This is a top view of the input waveguide and silicon nitride grating layer structure according to embodiment 1 of the present invention.

[0031] Figure 5 This is the far-field diffraction distribution diagram of Example 1 of the present invention.

[0032] Figure 6 This is a structural diagram of the optical phased array radiating antenna according to embodiment 2 of the present invention.

[0033] Figure 7This is a cross-sectional view of the optical phased array radiating antenna according to embodiment 2 of the present invention.

[0034] Figure 8 Schematic diagram of the input waveguide and silicon nitride grating layer structure of embodiment 2 of the present invention.

[0035] Figure 9 FIG. 1 is a top view of the input waveguide and silicon nitride grating layer structure according to embodiment 2 of the present invention.

[0036] Figure 10 This is the far-field diffraction distribution diagram of Example 2 of the present invention.

[0037] Figure 11 FIG. 1 is a top view of the input waveguide and silicon nitride grating layer structure according to embodiment 3 of the present invention.

[0038] Figure 12 This is a radiation power distribution diagram of Example 3 of the present invention.

[0039] Figure 13 This is a top view of the input waveguide and silicon nitride grating layer structure according to embodiment 4 of the present invention.

[0040] Figure 14 This is a radiation power distribution diagram of Example 4 of the present invention. DETAILED DESCRIPTION

[0041] The present invention proposes an optical phased array radiating antenna based on LNOI and a method for fabricating the same. An input waveguide based on the LNOI platform is selected, and the core material is lithium niobate. Silicon nitride is deposited on the input waveguide using plasma-enhanced chemical vapor deposition, followed by photoresist coating, electron beam lithography, development to form a hard mask, and etching. Finally, the mask is removed by wet processing to form a silicon nitride grating layer on the input waveguide. The waveguide (in this embodiment, the waveguide is the TE0 mode) is coupled to free space through a silicon nitride grating column heterogeneously integrated on the lithium niobate waveguide, forming a scanning beam in the far field. This structure allows the silicon nitride grating layer covering the lithium niobate to be freely etched, thereby reducing the radiation area of the radiating antenna to obtain a larger radiation envelope, reducing the power attenuation of the main lobe scanning beam during larger scans, and thus increasing the lateral scanning range. Furthermore, the width of the silicon nitride grating column can be designed with a gradient structure to adjust the radiation coefficient of the radiating antenna, extend the effective length of the radiation, and improve the scanning resolution.

[0042] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0043] Example 1

[0044] like Figure 1As shown, the optical phased array radiating antenna of this embodiment includes a supporting substrate layer 1, a buffer layer 2, a lithium niobate waveguide layer 3, a silicon nitride grating layer 4 and an upper cladding layer 5. The supporting substrate layer 1, the buffer layer 2 and the lithium niobate waveguide layer 3 form a LNOI platform. The silicon nitride columns in the silicon nitride grating layer 4 cover the input waveguide of the LNOI platform, and the area above the radiating antenna is free space. Figure 2 FIG. 4 is a cross-sectional view of the optical phased array radiating antenna according to this embodiment.

[0045] In this embodiment, the length of each structure in the optical phased array radiating antenna is measured along the TE0 waveguide transmission direction, and the width of each structure in the optical phased array radiating antenna is measured perpendicular to the TE0 waveguide transmission direction. The supporting substrate layer 1 is made of silicon with a thickness of 500 μm, which serves as a support. The buffer layer 2 is made of silicon dioxide with a thickness of 2 μm, which prevents light from leaking into the silicon substrate. The lithium niobate waveguide layer 3 is made of lithium niobate material, and its input waveguide has a width of 1 μm, a thickness of 600 nm, and an etching depth of 300 nm. The upper cladding layer 5 is also made of silicon dioxide with a thickness of 2 μm. The refractive index difference between the upper cladding layer 5 and the lithium niobate waveguide layer 3 is large, which can confine light to the input waveguide. The upper cladding layer 5 can also be omitted when actually manufacturing the optical phased array radiating antenna.

[0046] like Figure 3 It is a schematic structural diagram of the input waveguide of the lithium niobate waveguide layer 3 and the silicon nitride grating layer 4. Figure 4 for Figure 3 In this embodiment, a silicon nitride grating is formed by several silicon nitride pillars of identical structure. The silicon nitride pillars are distributed along the length of the input waveguide in the middle of the input waveguide. The silicon nitride pillars have a period of 800 nm, a duty cycle of 50%, a thickness of 150 nm, and a width w of 100 nm.

[0047] like Figure 5 The figure shows the far-field diffraction distribution diagram of the optical phased array radiating antenna of this embodiment. By reducing the radiation area, the smaller the radiation area, the larger the far-field radiation envelope, and a wide range of scanning can be achieved.

[0048] Example 2

[0049] like Figure 6 As shown, the optical phased array radiating antenna of this embodiment includes a supporting substrate layer 1, a buffer layer 2, a lithium niobate waveguide layer 3, a silicon nitride grating layer 4 and an upper cladding layer 5. The structures of the supporting substrate layer 1, the buffer layer 2, the lithium niobate waveguide layer 3 and the upper cladding layer 5 are the same as those in Example 1 and are not described again in this embodiment. Figure 7 FIG. 4 is a cross-sectional view of the optical phased array radiating antenna of this embodiment.

[0050] like Figure 8FIG. 1 is a schematic structural diagram of the input waveguide of the lithium niobate waveguide layer 3 and the silicon nitride grating layer 4 of this embodiment. Figure 9 for Figure 7 A top view of the silicon nitride grating. Several identical silicon nitride pillars form a silicon nitride grating, symmetrically distributed along the length of the input waveguide. The silicon nitride pillars have an 800nm period, a 50% duty cycle, a thickness of 150nm, and a width w of 100nm.

[0051] like Figure 10 The figure shows the far-field diffraction distribution diagram of the optical phased array radiating antenna of this embodiment. By reducing the radiation area, the smaller the radiation area, the larger the far-field radiation envelope, and a wide range of scanning can be achieved.

[0052] Example 3

[0053] For a radiating antenna with a uniform structure grating, the near-field intensity of the light field decreases with the increase of the propagation length and eventually tends to zero. The radiation coefficient of the radiating antenna is defined as α, which decays exponentially. In this case, light is radiated from the starting position of the radiating antenna, but no light is radiated from the ending position, causing the effective radiation length of the radiating antenna to be less than the actual length of the grating, thereby reducing the longitudinal far-field scanning accuracy. Based on Example 1, this embodiment makes a gradual design for the width of the silicon nitride column. By changing the width of the silicon nitride grating at different positions, the radiation coefficient of the position is changed, thereby achieving uniform radiation, and avoiding the exponential decay of light intensity under the uniform structure radiating antenna, thereby extending the effective radiation length of the radiating antenna and improving the scanning accuracy.

[0054] Uniform radiation requires that the radiation coefficient of the radiating antenna satisfies the following formula: L is the length from the first silicon nitride column to the last silicon nitride column, x is the position of the silicon nitride grating, and α(x) is the radiation coefficient of the radiating antenna at different positions. The radiation coefficient of the radiating antenna of the present invention is related to the width of the silicon nitride column. The larger the width, the larger the radiation coefficient. By fitting the width of the silicon nitride column and the radiation coefficient with a cubic polynomial, the fitting relationship between the width w of the silicon nitride column and the radiation coefficient α in Example 1 is obtained as follows: α = -6.6396e-15w3 + 1.0972e-11w2 + -1.2775e-09w + 4.9583e-08. By designing the width of the silicon nitride grating at different positions according to the radiation coefficient formula and the fitting relationship, the effect of uniform radiation can be achieved, which extends the effective radiation length of the radiating antenna. As Figure 11The figure shows a top view of the input waveguide and silicon nitride grating layer 4 in this embodiment. The starting position of the silicon nitride column is 0, and the period of the silicon nitride grating is 800 nm. Therefore, the positions of the silicon nitride columns are 0 nm, 800 nm, 1600 nm, etc., respectively. The length of the radiating antenna is set as required, and the position of the silicon nitride grating is substituted into the above radiation coefficient formula to obtain the corresponding radiation coefficient at different silicon nitride grating positions. The width of the silicon nitride grating is designed based on the radiation coefficient using the fitting relationship, and the width ranges from 200 to 1000 nm. The width of the silicon nitride grating at different positions is designed to be gradually changed. When the radiation coefficient reaches the maximum value, the silicon nitride grating width is also designed to be the maximum value of 1000 nm and remains unchanged.

[0055] like Figure 12 The figure shows the radiation power distribution diagram of the optical phased array radiating antenna in this embodiment, which achieves 1 mm uniform radiation.

[0056] Example 4

[0057] This embodiment, based on Example 2, incorporates a gradient design for the width of the silicon nitride pillars. By varying the width of the silicon nitride grating at different locations, the emissivity at that location is varied, thereby achieving uniform radiation. The design method for the silicon nitride pillar width is the same as in Example 3. In Example 2, the fitting relationship between the silicon nitride pillar width w and the emissivity α is: α = 9.2171e-15w3 + -2.6779e-12w2 + 4.9341e-10w + 6.4467e-09.

[0058] like Figure 13 The figure shows a top view of the input waveguide and silicon nitride grating layer 4 in this embodiment. The silicon nitride pillars have an 800nm period, a 50% duty cycle, a thickness of 150nm, and a width range of 100-500nm. When the emissivity reaches its maximum value, the width of the silicon nitride pillars on both sides of the remaining positions remains unchanged at 500nm.

[0059] like Figure 14 The figure shows the radiation power distribution diagram of the optical phased array radiating antenna in this embodiment, which achieves 1 mm uniform radiation.

[0060] Example 5

[0061] The method for preparing the LNOI-based optical phased array radiating antenna of this embodiment is used to prepare the optical phased array radiating antenna described in Examples 1 to 4, and the method includes the following steps.

[0062] Step 1: Using an x-cut lithium niobate thin film wafer as a substrate, a complete substrate layer 1 and a buffer layer 2 as well as a lithium niobate planar waveguide covered on the buffer layer 2 are obtained.

[0063] Step 2: Ma-N2405 photoresist is spin-coated onto the cleaned lithium niobate plate, and the lithium niobate waveguide pattern is written using electron beam lithography (EBL, ELS-F125, Elionix). Subsequently, inductively coupled plasma etching (ICP, Oxford Plasma Pro 100 Cobra 300) is performed to a depth of 300 nm. After cleaning the lithium niobate sidewalls and removing the mask, the input waveguide is obtained, thus forming lithium niobate waveguide layer 3.

[0064] In step 3, a 150nm thick amorphous silicon nitride film is deposited on the input waveguide using plasma-enhanced chemical vapor deposition (PECVD) technology and a photoresist (PMMA glue) is spin-coated. A silicon nitride grating pattern is then inscribed on the photoresist using electron beam lithography. The silicon nitride grating consists of several silicon nitride columns. After development is complete, a layer of chromium film is deposited on the photoresist surface using electron beam evaporation technology as a hard mask. The photoresist is then stripped and the silicon nitride grating pattern is transferred to the chromium film. Finally, trifluoromethane (CHF3) and sulfur hexafluoride (SF6) are used as plasma etching gases (Plasma Pro100Cobra300, Oxford Instruments), and the remaining chromium mask is removed using ammonium cerium nitrate solution to prepare the silicon nitride grating layer 4.

[0065] In step 4, silicon dioxide with a thickness of 2 μm is deposited on the silicon nitride grating layer 4 using PECVD technology to obtain an upper cladding layer 5, thereby obtaining an optical phased array radiating antenna based on LNOI material.

Claims

1. An optical phased array radiating antenna based on LNOI, characterized in that: include: An LNOI and a silicon nitride grating layer (4) covered thereon, wherein the LNOI comprises a lithium niobate waveguide layer (3), etching is performed on the lithium niobate waveguide layer (3) to form an input waveguide, and silicon nitride columns are covered on the surface of the input waveguide to form a silicon nitride grating layer (4); when the guided wave is transmitted through the input waveguide, it is radiated into free space through the silicon nitride grating layer, thereby forming a scanning light beam.

2. The LNOI-based optical phased array radiating antenna according to claim 1, characterized in that: The silicon nitride columns are distributed in the middle of the input waveguide along the waveguide transmission direction, or the silicon nitride columns are symmetrically distributed on both sides of the input waveguide along the waveguide transmission direction.

3. The LNOI-based optical phased array radiating antenna according to claim 2, characterized in that: The side length of the silicon nitride column perpendicular to the waveguide transmission direction is its width, and the width of each silicon nitride column is the same.

4. The LNOI-based optical phased array radiating antenna according to claim 2, characterized in that: The side length of the silicon nitride column perpendicular to the waveguide transmission direction is its width. The width of the silicon nitride column is designed to be gradually changed along the waveguide transmission direction. The calculation method of the width of the silicon nitride column is as follows: according to Calculating the radiation coefficient of the silicon nitride column at each position, and calculating the width value corresponding to the radiation coefficient according to a cubic polynomial fitted between the width of the silicon nitride column and the radiation coefficient, thereby obtaining the width of the silicon nitride column at each position; Where x is the position of the silicon nitride column on the input waveguide surface, and L is the length from the first silicon nitride column to the last silicon nitride column.

5. The LNOI-based optical phased array radiating antenna according to claim 4, characterized in that: When the radiation coefficient α(x) reaches its maximum value, the width of the silicon nitride column at position x also reaches its maximum value, and the widths of the silicon nitride columns after position x in the waveguide transmission direction are all at their maximum values.

6. The LNOI-based optical phased array radiating antenna according to claim 1, characterized in that: The LNOI further comprises, from bottom to top, a supporting substrate layer (1) of silicon material and a buffer layer (2) of silicon dioxide material.

7. The LNOI-based optical phased array radiating antenna according to claim 1, characterized in that: The silicon nitride grating layer (4) is also covered with an upper cladding layer (5) of silicon dioxide material.

8. A method for preparing the LNOI-based optical phased array radiating antenna according to any one of claims 1 to 7, characterized in that: The following steps are involved: Step 1: Etching the lithium niobate waveguide layer of LNOI to form an input waveguide; Step 2: depositing a silicon nitride film on the input waveguide and writing a pattern of silicon nitride columns, and forming silicon nitride columns by etching the silicon nitride film.

9. The method for preparing the LNOI-based optical phased array radiating antenna according to claim 8, characterized in that: Step 2 specifically includes the following steps: Step 2.1, depositing a silicon nitride film on the input waveguide by plasma enhanced chemical vapor deposition, spin coating a photoresist, and writing a pattern of silicon nitride pillars on the photoresist by electron beam lithography; Step 2.2, developing, depositing a chromium film on the surface of the photoresist using electron beam evaporation, and transferring the pattern of the silicon nitride pillars onto the chromium film; In step 3.3, the silicon nitride film is etched using plasma gas, and the residual chromium mask is removed using a cerium ammonium nitrate solution to obtain silicon nitride pillars.

10. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by a processor, the method for preparing the LNOI-based optical phased array radiating antenna according to claim 8 is implemented.

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