A thin film lithium niobate optical isolator based on acousto-optic scattering and a preparation method thereof

Abstract

The application belongs to the technical field of integrated optics, and discloses a thin film lithium niobate optical isolation device based on acousto-optic scattering and a preparation method thereof. The device comprises, from bottom to top, a substrate (1), an acoustic isolation layer, a thin film lithium niobate layer (3) and a fan-shaped electro-acoustic interdigital transducer (4) in sequence. The thin film lithium niobate layer (3) is provided with a mode multiplexer, a mode demultiplexer and a bus ridge waveguide (6). The interdigital transducer of the fan-shaped electro-acoustic interdigital transducer (4) is distributed along an arc line, and the whole is in a fan shape. The application improves the structure of the active device, adopts the thin film lithium niobate layer integrated with the bus ridge waveguide and a pair of mode (de)multiplexers, cooperates with the fan-shaped electro-acoustic interdigital transducer, interacts with the optical field through the acoustic field excited by the electrode of the fan-shaped electro-acoustic interdigital transducer, can accurately control the direction of the acoustic field and the acousto-optic phase matching condition, realizes one-way phase matching, and achieves the non-reciprocal optical isolation function.

Description

Technical Field

[0001] This invention belongs to the field of integrated optical technology, and more specifically, relates to a thin-film lithium niobate optical isolator based on acousto-optic scattering and its fabrication method. Background Technology

[0002] In recent years, thin-film lithium niobate has been widely used in integrated optical devices due to its large second-order nonlinear coefficient, electro-optic coefficient, piezoelectric response, and wide optical transparency window. It also holds promise as a monolithic integrated optical chip platform combining active and passive optical devices, attracting widespread attention from researchers both domestically and internationally. With the surge of research on the interaction between light and sound fields in integrated optics, thin-film lithium niobate exhibits superior piezoelectric properties compared to other piezoelectric thin-film materials such as aluminum nitride and gallium arsenide. Simultaneously, its ultra-low optical loss (less than 0.2 dB / m) makes it stand out among numerous material systems. Therefore, its low optical loss and efficient electro-acoustic conversion make thin-film lithium niobate an ideal material for fabricating high-performance acousto-optic devices.

[0003] In acousto-optic scattering, based on the phase matching process of optical modes, it can be divided into intra-mode scattering and inter-mode scattering. For inter-mode scattering involving phonons, a large phonon wave vector propagating along the waveguide direction is required to satisfy the phase matching of different optical modes. Therefore, the magnitude and propagation direction of the applied sound field wave vector need to be precisely controlled to achieve complete inter-mode scattering phase matching. Acousto-optic inter-mode scattering can directly realize energy conversion and frequency transformation between modes, and can be widely used in frequency shifting and non-reciprocal optical transmission, making it a very meaningful research direction. Achieving more efficient acousto-optic scattering and more accurate phase matching are key research objectives.

[0004] Fabrication of thin-film lithium niobate waveguides is also a challenge. Due to the characteristics of lithium niobate, the etching ratio of typical photoresist masks to lithium niobate is generally not very high, making it difficult to achieve a large etching depth while ensuring the accuracy of the fabricated lines. Therefore, the fabrication of low-loss optical waveguides is also a key challenge in the fabrication of high-performance acousto-optic devices. Summary of the Invention

[0005] To address the aforementioned deficiencies or improvement needs of existing technologies, the present invention aims to provide a thin-film lithium niobate optical isolation device based on acousto-optic scattering and its fabrication method. This active device employs a thin-film lithium niobate layer integrating a bus ridge waveguide and a pair of mode (de)multiplexers, coupled with a fan-shaped electro-acoustic interdigital transducer. The acoustic field excited by the electrodes of the fan-shaped electro-acoustic interdigital transducer interacts with the optical field, enabling precise control of the acoustic field direction and acousto-optic phase matching conditions, achieving unidirectional phase matching and non-reciprocal optical isolation. Furthermore, the present invention optimizes the device fabrication process, resulting in a low-loss thin-film lithium niobate waveguide, further enhancing device performance.

[0006] To achieve the above objectives, according to one aspect of the present invention, a thin-film lithium niobate optical isolation device based on acousto-optic scattering is provided, characterized in that it comprises, from bottom to top, a substrate (1), an acoustic isolation layer, a thin-film lithium niobate layer (3), and a fan-shaped electroacoustic interdigital transducer (4); wherein,

[0007] The thin-film lithium niobate layer (3) is provided with a mode multiplexer, a mode demultiplexer and a bus ridge waveguide (6). The bus ridge waveguide (6) is coupled to the mode multiplexer and the mode demultiplexer to multiplex different optical field modes into the bus ridge waveguide (6) or demultiplex different optical field modes from the bus ridge waveguide (6).

[0008] The interdigitated fingers of the fan-shaped electroacoustic interdigital transducer (4) are distributed along an arc. The arc length corresponding to the outermost set of interdigitated fingers is greater than the arc length corresponding to the innermost set of interdigitated fingers, thus making the whole fan-shaped; and the divergence direction of the fan shape is towards the bus ridge waveguide (6).

[0009] The fan-shaped electroacoustic interdigital transducer (4) is used to excite a sound field in the thin-film lithium niobate layer (3) under the drive of an external radio frequency signal, which enables different optical field modes transmitted unidirectionally in the bus ridge waveguide (6) to interact with the sound field, and finally realizes that different optical field modes are routed from different ports of the mode multiplexer and the mode demultiplexer, thus realizing a unidirectional optical transmission optical isolator.

[0010] As a further preferred embodiment of the present invention, the total divergence angle of the fan-shaped electroacoustic interdigital transducer (4) is 0.1° to 10°.

[0011] As a further preferred embodiment of the present invention, the center emission direction of the excitation sound field of the fan-shaped electroacoustic interdigital transducer (4) forms an angle with the cross section of the bus ridge waveguide (6), and this angle is denoted as θ.

[0012] The total divergence angle of the fan-shaped electroacoustic interdigital transducer (4) is denoted as 2α;

[0013] Then, the angle θ' between the emission angle of the fan-shaped electroacoustic interdigital transducer (4) and the cross section of the bus ridge waveguide (6) is in the range [θ-α, θ+α].

[0014] As a further preferred embodiment of the present invention, the total thickness of the thin film lithium niobate layer (3) is 300 nanometers to 1000 nanometers, and the mode multiplexer, mode demultiplexer and bus ridge waveguide (6) disposed therein are formed by etching, with an etching depth of 150 nanometers to 500 nanometers.

[0015] The bus ridge waveguide (6) has a trapezoidal cross-section and a waveguide sidewall tilt angle of 50 to 70 degrees; the bus ridge waveguide (6) is a multimode waveguide and the upper width of the trapezoidal cross-section is 1800 nanometers to 5000 nanometers.

[0016] As a further preferred embodiment of the present invention, the metal material selected for the fan-shaped electroacoustic interdigital transducer (4) is gold, copper or aluminum, with a thickness of 50 nanometers to 1000 nanometers;

[0017] There are multiple fan-shaped electroacoustic interdigital transducers (4), and adjacent fan-shaped electroacoustic interdigital transducers (4) share an electrode contact plate; preferably, the bus ridge waveguide (6) is U-shaped folded, the number of fan-shaped electroacoustic interdigital transducers (4) is even, half of the total number of fan-shaped electroacoustic interdigital transducers (4) have the fan-shaped divergence direction facing one side of the U-shape of the bus ridge waveguide (6), and the other half of the total number of fan-shaped electroacoustic interdigital transducers (4) have the fan-shaped divergence direction facing the other side of the U-shape of the bus ridge waveguide (6);

[0018] The output aperture of a single fan-shaped electroacoustic interdigital transducer (4) is more than 10 micrometers, the period length is equal to the ridge width of the bus waveguide (6), and the number of periods is more than 2.

[0019] As a further preferred embodiment of the present invention, when the thin film lithium niobate layer (3) is an X-cut lithium niobate crystal, the emission direction of the sound field center of the fan-shaped interdigital transducer (4) forms a positive 30-degree angle with the Y-axis of the crystal.

[0020] When the thin film lithium niobate layer (3) is made of Y-cut lithium niobate crystal, the emission direction of the sound field center of the fan-shaped interdigital transducer (4) is at an angle of ±45 degrees to the Z-axis of the crystal.

[0021] When the thin-film lithium niobate layer (3) is made of Z-cut lithium niobate crystal, the emission direction of the sound field center of the fan-shaped interdigital transducer (4) is at an angle of 0 degrees or ±60 degrees to the Y-axis of the crystal.

[0022] As a further preferred embodiment of the present invention, the acoustic isolation layer is a buried oxide layer (2), and the acoustic field mode excited by the fan-shaped electroacoustic interdigital transducer (4) in the thin film lithium niobate layer (3) is surface acoustic wave;

[0023] Alternatively, the acoustic isolation layer may be an air layer, and the acoustic field mode excited by the fan-shaped electroacoustic interdigital transducer (4) in the thin-film lithium niobate layer (3) may be a Lamb wave.

[0024] As a further preferred embodiment of the present invention, the mode multiplexer has the same structure as the mode demultiplexer, wherein the mode multiplexer has an upper port and a lower port, which are used to input the fundamental mode and the higher-order mode, respectively, and the optical coupling input to the upper port or the lower port is vertical coupling or horizontal coupling.

[0025] As a further preferred embodiment of the present invention, the upper cladding of the thin-film lithium niobate optical isolator based on acousto-optic scattering is an air upper cladding;

[0026] Alternatively, the thin-film lithium niobate photoisolator based on acousto-optic scattering may also have an upper cladding layer, the material of which is silicon oxide, silicon nitride, or HSQ type photoresist.

[0027] According to another aspect of the present invention, the present invention provides a method for fabricating the above-mentioned thin-film lithium niobate optical isolator based on acousto-optic scattering, characterized by comprising the following steps:

[0028] Step 1: Prepare a substrate consisting of a substrate (1), an acoustic isolation layer, and a thin film of lithium niobate from bottom to top. Use electron beam evaporation to grow metallic chromium with a thickness of 80 nanometers to 160 nanometers on the substrate.

[0029] Step 2: Spin-coat electron beam adhesive onto a chromium-plated substrate. The thickness of the electron beam adhesive is 300 nanometers to 500 nanometers. After spin-coating, place the substrate on a heating plate at a preset heating temperature of 150 to 180 degrees Celsius and let it stand for at least 1 minute.

[0030] Step 3: Expose the substrate using an electron beam exposure system, then develop and fix it, and use oxygen plasma to remove the residual adhesive from the developed substrate to obtain a waveguide pattern.

[0031] Step 4: After etching the substrate using chlorine-based inductively coupled plasma etching, the remaining electron beam resist is removed to obtain a chromium layer pattern.

[0032] Step 5: After etching the substrate with fluorine-based ICP, the remaining chromium metal is removed to obtain a thin film lithium niobate layer with a mode multiplexer, a mode demultiplexer and a bus ridge waveguide (6).

[0033] Step six: Clean the substrate;

[0034] Step 7: Perform thermal annealing on the substrate. Specifically, under normal pressure in an oxygen atmosphere, heat the substrate from room temperature to a preset holding temperature at a rate of 1 to 5 degrees Celsius per minute, and hold it at that temperature for more than 1 hour; then gradually cool it down from the holding temperature at a rate of 0.5 to 2 degrees Celsius per minute back to room temperature; wherein the preset holding temperature is between 500 degrees Celsius and 1100 degrees Celsius.

[0035] Step 8: First, spin-coat the substrate with electron beam resist. The thickness of the electron beam resist is 300 nm to 500 nm. After spin-coating, place the substrate on a heating plate at a preset heating temperature of 100 to 200 degrees Celsius and let it stand for at least 1 minute. Then, expose the substrate with an electron beam exposure system, followed by development and fixing. Oxygen plasma is used to remove the residual resist from the development process to obtain a fan-shaped electroacoustic interdigital transducer pattern.

[0036] Step 9: Place the above substrate in an electron beam evaporation system to deposit the metal of the fan-shaped electroacoustic interdigital transducer, and use a stripping process to remove the remaining electron beam adhesive and metal film, thereby obtaining a thin-film lithium niobate optical isolation device based on acousto-optic scattering.

[0037] Preferably, in step two, the electron beam adhesive is ARP6200;

[0038] In step three, the electron beam exposure system is a VISTEC EBPG 5000PES;

[0039] In step six, the cleaning specifically involves placing the substrate in a mixed solution obtained by mixing ammonia solution, hydrogen peroxide solution, and deionized water in a volume ratio of 1:1:5 and allowing it to stand for 1 to 15 minutes at a temperature of 60 to 80 degrees Celsius. The concentration of the ammonia solution is 25 wt% to 28 wt%, and the concentration of the hydrogen peroxide solution is 30 wt%.

[0040] Compared with existing technologies, the acousto-optic scattering-based thin-film lithium niobate optical isolator of this invention, through the above-described technical solution, sets up a bus ridge waveguide, a mode multiplexer, and a demultiplexer within the thin-film lithium niobate layer, and a fan-shaped electroacoustic interdigital transducer above the thin-film lithium niobate layer. Driven by an external radio frequency signal, the electroacoustic interdigital transducer excites an acoustic field within the thin-film lithium niobate layer through the piezoelectric effect, ultimately achieving direction-dependent acousto-optic interaction in the low-loss waveguide of the thin-film lithium niobate layer. The mode (de)multiplexer enables the (de)multiplexing of different optical field modes, achieving optical isolation of light in specific directions. Due to the acousto-optic scattering effect, the bus ridge waveguide supports the transmission of different modes and input / output from different ports. Taking a traditional mode (de)multiplexer with a top and bottom port design as an example, non-reciprocal optical isolation can be achieved when using only two specific ports. However, the fan-shaped electroacoustic transducer, through angle optimization and divergent emission of the acoustic field, makes phase matching easier to achieve.

[0041] Unlike existing heterogeneous integration schemes where the optical waveguide and piezoelectric thin film are separated into two layers (for example, our previous Chinese patent application CN114815332A uses silicon as the optical waveguide, while the sound field is generated by a piezoelectric aluminum scandium nitride layer; the sound and light fields are guided by two different materials, making it a relatively complex heterogeneous integration scheme), the intensity of acousto-optic interaction is limited. This invention utilizes thin-film lithium niobate to enhance the intensity of acousto-optic interaction within the same material system. The thin-film lithium niobate layer in this invention incorporates a bus ridge waveguide, a mode multiplexer, and a demultiplexer, serving both as an optical waveguide for guiding light and actively generating a sound field under the action of electrodes, resulting in high acousto-optic interaction intensity.

[0042] This invention utilizes the fan-shaped design of a fan-shaped electroacoustic interdigital transducer to precisely control the direction of the sound field and the acousto-optic phase matching conditions, achieving unidirectional phase matching and non-reciprocal optical isolation. The fan-shaped design of the electroacoustic interdigital transducer ensures that the exit angle of the excited sound field forms a certain angle range with the cross-section of the bus ridge waveguide in the thin-film lithium niobate layer (i.e., the expanded interval [θ-α, θ+α] in the later embodiments, rather than a fixed angle value). Since the longitudinal wave vector component of the sound field along the waveguide direction should ideally satisfy the difference in longitudinal propagation constant between the fundamental mode transmitted light in the bus ridge waveguide and the higher-order optical modes after acousto-optic scattering, the exit angle range of the fan-shaped sound field in this invention makes phase matching of different optical field modes during acousto-optic scattering easier. If a traditional electroacoustic interdigital transducer (non-fan-shaped, with interdigits distributed along a straight line segment) is used, the output direction of the excitation sound field of the electroacoustic interdigital transducer forms a fixed angle (θ) with the cross section of the bus ridge waveguide. This ensures that the longitudinal component of the acoustic wave vector satisfies the difference in longitudinal propagation constant between the fundamental mode transmitted light in the bus ridge waveguide and the higher-order optical modes after acousto-optic scattering. This achieves quasi-phase matching of different optical field modes during acousto-optic scattering (that is, the size of the angle is determined by the difference in longitudinal propagation constant between the fundamental mode transmitted light input to the bus ridge waveguide and the higher-order optical modes after acousto-optic scattering, used to compensate for the phase mismatch of different optical field modes during acousto-optic scattering). However, due to process errors in the actual etching process of the thin-film lithium niobate layer to prepare the waveguide, the deviation of the waveguide geometric parameters will cause fluctuations in this constant difference, affecting the quasi-phase matching.

[0043] Furthermore, by preferably controlling the total divergence angle of the fan-shaped electroacoustic interdigital transducer to 0.1° to 10°, this invention can compensate for errors in the waveguide fabrication process and avoid the energy dissipation of the acoustic field caused by an excessively large sound field divergence angle, which would affect the efficiency of acousto-optic scattering. This reduces the requirements for quasi-phase matching in acousto-optic scattering fabrication and improves the fabrication yield of the device. This invention preferably uses a structure in which multiple fan-shaped electroacoustic interdigital transducers share a common electrode contact plate and are arranged vertically on both sides of a U-shaped waveguide. This significantly extends the length of the acousto-optic interaction and overcomes the problem of the output aperture of a single electroacoustic interdigital transducer being limited by fabrication process and electrical impedance matching.

[0044] Furthermore, this invention optimizes the device fabrication method by employing electron beam resist (e.g., ARP6200.13) in conjunction with electron beam exposure and a double-mask etching process. This ensures the line accuracy and good sidewall morphology of the thin-film lithium niobate waveguide, resulting in a low-loss thin-film lithium niobate waveguide. After the waveguide fabrication is completed, electrode patterns are exposed and fabricated, ultimately yielding the target device.

[0045] In summary, this invention achieves unidirectional single-sideband acousto-optic scattering by electrically driving thin-film lithium niobate, enabling optical isolation devices with low transmission loss and high isolation, which have important applications in integrated optics. Attached Figure Description

[0046] Figure 1 A three-dimensional model diagram of the thin-film lithium niobate optical isolator structure provided by the present invention.

[0047] Figure 2 This is a schematic diagram illustrating the forward transmission working principle of the acousto-optic scattering optical isolator according to an embodiment of the present invention.

[0048] Figure 3 This is a schematic diagram illustrating the reverse transmission working principle of the acousto-optic scattering optical isolator according to an embodiment of the present invention.

[0049] Figure 4 This is a wave vector matching diagram for unidirectional acousto-optic scattering according to an embodiment of the present invention.

[0050] Figure 5 This is a schematic diagram illustrating the working principle of the fan-shaped electroacoustic interdigital transducer according to an embodiment of the present invention.

[0051] Figure 6 The transmission spectrum of the thin-film lithium niobate optical waveguide resonator prepared in Example 1 is shown.

[0052] Figure 7 This is a performance comparison chart of a traditional interdigital transducer (non-sectoral) and a sectoral electroacoustic interdigital transducer.

[0053] Figure 1 The meanings of the reference numerals in the figures are as follows: 1-Silicon substrate, 2-Buried oxide layer, 3-Thin lithium niobate layer, 4-Fan-shaped electroacoustic interdigital transducer, 5-Mode (de)multiplexer, 6-Bus ridge waveguide (i.e., thin lithium niobate bus ridge waveguide). Detailed Implementation

[0054] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0055] Example 1

[0056] The thin-film lithium niobate optical isolator provided by this invention has the following three-dimensional structure diagram: Figure 1As shown, the structure includes a silicon substrate 1, a buried oxide layer 2, a thin-film lithium niobate layer 3, and a fan-shaped electroacoustic interdigital transducer 4. The thin-film lithium niobate layer 3 contains a pair of mode (de)multiplexers 5 (hereinafter referred to as the first mode (de)multiplexer and the second mode (de)multiplexer, respectively; similar to conventional methods, the multiplexers are used for input, and the demultiplexers are used for output), and a bus ridge waveguide 6. The refractive index of the thin-film lithium niobate layer is greater than that of the buried oxide layer and the air cladding, confining the optical field modes within the lithium niobate waveguide.

[0057] In this embodiment, the thin-film lithium niobate optical isolator is prepared according to the following steps:

[0058] Step 1: Prepare a substrate consisting of a silicon substrate 1, a buried oxide layer 2, and a thin film of lithium niobate from bottom to top. Use electron beam evaporation to grow a 160-nanometer-thick layer of metallic chromium on the substrate.

[0059] Step 2: Spin-coat electron beam adhesive onto a chromium-plated substrate. The thickness of the electron beam adhesive is 500 nanometers. After spin-coating, place the substrate on a heating plate at a preset heating temperature of 150 degrees Celsius and let it stand for 1 minute.

[0060] Step 3: Expose the substrate using an electron beam exposure system, then develop and fix it, and use oxygen plasma to remove the residual adhesive from the developed substrate to obtain a waveguide pattern.

[0061] Step 4: After etching the substrate using chlorine-based inductively coupled plasma etching, the remaining electron beam resist is removed to obtain a chromium layer pattern.

[0062] Step 5: After etching the substrate with fluorine-based ICP, the remaining chromium metal is removed to obtain a thin film lithium niobate layer with a mode multiplexer, a mode demultiplexer and a bus ridge waveguide.

[0063] Step six: Clean the substrate;

[0064] Step 7: Perform thermal annealing on the substrate. Specifically, under normal pressure in an oxygen atmosphere, heat the substrate from room temperature to a preset holding temperature at a rate of 1 degree Celsius per minute and hold it for 2 hours; then gradually cool it down from the holding temperature at a rate of 2 degrees Celsius per minute back to room temperature; wherein, the preset holding temperature is 500 degrees Celsius.

[0065] The low-loss microring waveguide resonator prepared through steps one to seven can achieve a load Q value as high as 1.1 × 10⁻⁶. 6 ,like Figure 6 As shown, the corresponding waveguide linear loss can be calculated to be approximately 0.2 dB / cm.

[0066] Step 8: Perform the electron beam lithography operation from Steps 2 and 3 on the substrate again to form a fan-shaped interdigital transducer pattern. Specifically: spin-coat the substrate with electron beam resist, the thickness of which is 500 nanometers. After spin-coating, place the substrate on a heating plate at a preset heating temperature of 150 degrees Celsius for 1 minute. Then, expose the substrate with an electron beam exposure system, followed by development and fixing. Oxygen plasma is used to remove residual resist from the development process to obtain the fan-shaped interdigital transducer pattern.

[0067] Step 9: Place the above substrate in an electron beam evaporation system to deposit the metal of the fan-shaped electroacoustic interdigital transducer, and use a stripping process to remove the remaining electron beam adhesive and metal film, thereby obtaining a thin-film lithium niobate optical isolation device based on acousto-optic scattering.

[0068] In step two, the electron beam adhesive is ARP6200.13;

[0069] In step three, the electron beam exposure system is a VISTEC EBPG 5000PES;

[0070] In step six, the cleaning specifically involves placing the substrate in a mixed solution obtained by mixing ammonia solution, hydrogen peroxide solution, and deionized water in a volume ratio of 1:1:5 and letting it stand for 10 minutes at a temperature of 60 degrees Celsius; wherein the concentration of the ammonia solution is 25wt%-28wt% and the concentration of the hydrogen peroxide solution is 30wt%.

[0071] Driven by an external electrical signal, the periodic structure of the fan-shaped electroacoustic interdigital transducer can form a periodic electric field in the thin-film lithium niobate, which in turn excites the sound field through the piezoelectric effect. The excited sound field is emitted from near the electrode and transmitted to the thin-film lithium niobate waveguide, where the acousto-optic scattering process is finally realized. Different optical field modes are multiplexed or demultiplexed through different ports of the mode (demultiplexer), thus forming a unidirectional optical isolation characteristic between specific input and output ports.

[0072] In the selection of sound field modes, the energy of the low-order modes of surface acoustic waves is mainly concentrated in the thin-film lithium niobate layer; while Lamb waves are released through the acoustic isolation layer (air layer), and the energy is completely confined to the thin-film lithium niobate layer during propagation, so that the sound field energy can be effectively utilized to carry out acousto-optic scattering in the lithium niobate waveguide.

[0073] In this embodiment, the forward and reverse propagating light participating in acousto-optic scattering to achieve optical isolation exhibit different optical field modes in the thin-film lithium niobate waveguide, namely, the forward input light is the fundamental mode (TE) of the transverse electric mode. 00The reverse-transmitted light, after undergoing acousto-optic scattering, transforms into a first-order transverse electric mode (TE) in the waveguide. 10 In this embodiment, the forward transmission schematic diagram, reverse transmission schematic diagram, and acousto-optic wave vector matching diagram of the optical isolator based on intermodal acousto-optic scattering are respectively shown in the figure. Figure 2 , Figure 3 and Figure 4 As shown. When the optical field is input from the upper port of the first mode multiplexer, the mode entering the thin-film lithium niobate waveguide is the fundamental mode TE. 00 ,like Figure 2 As shown. At this time, the sound field excited by the external radio frequency signal in the fan-shaped electroacoustic transducer is consistent with the propagation direction of the optical field in the waveguide, as indicated by the dispersion curve of the optical field mode ( Figure 4 This demonstrates that the longitudinal propagation constants of the fundamental mode and the first-order mode are significantly different, and the sound field mode propagating in the same direction as the light field cannot satisfy the phase-matching condition for intermodal acousto-optic scattering. Therefore, the forward-propagating fundamental mode light field does not interact with the sound field excited by the electrodes, but is normally propagated to the second-mode (de)multiplexer and output from its upper port. When the light field is input from the upper port of the second-mode multiplexer (i.e., reflected back), as... Figure 3 As shown, since the fundamental mode and the first-order mode of the reverse propagation satisfy the phase matching of intermodal scattering after the intervention of the reverse propagation acoustic field ( Figure 4 ), fundamental mode TE 00 The energy of the light field can be fully converted into the first-order mode TE. 10 The first-order mode obtained during the acousto-optic scattering process is no longer output from the upper port when it reaches the first-mode (demultiplexer) in the reverse direction. Instead, it is mainly output from the lower port, thus realizing the function of optical isolation for optical reverse transmission.

[0074] In this embodiment, at a preset optical wavelength of 1550 nm, the refractive index of the X-cut thin-film lithium niobate waveguide layer exhibits birefringence. The in-plane refractive indices of the lithium niobate crystal in the Y and Z directions are 2.21 and 2.14, respectively, and the refractive index of the silicon dioxide buried oxide layer is 1.45. Therefore, the optical field mode can be confined within the lithium niobate waveguide. The total cross-sectional thickness of the thin-film lithium niobate waveguide, the first mode (demultiplexer), and the second mode (demultiplexer) is 600 nm, and the etching depth is 300 nm. The ridge widths of the multimode waveguide and the single-mode waveguide are 2200 nm and 900 nm, respectively. The thickness of the silicon dioxide buried oxide layer is 2 μm.

[0075] In this embodiment, the fan-shaped electroacoustic interdigital transducers are all made of aluminum, with a thickness of 100 nanometers, a period length of 2200 nanometers, and 20 pairs of interdigital bars per period. The output aperture (i.e., the length radiated onto the waveguide) of a single interdigital transducer is 200 micrometers. The angle θ between the center emission angle of the fan-shaped transducer and the perpendicular direction of the bus ridge waveguide is 5.5 degrees. Figure 5As shown, the total divergence angle 2α of the fan-shaped structure is approximately 2.0 degrees. Thus, the angle θ' between the emission angle of the fan-shaped electroacoustic interdigital transducer and the vertical direction of the bus ridge waveguide is in the range [θ-α, θ+α]. θ' is also the total acoustic field emission range (correspondingly, in this embodiment, the total acoustic field emission range θ' is 4.5 degrees to 6.5 degrees; since the longitudinal wave vector component of the acoustic field along the waveguide direction should satisfy the difference in longitudinal propagation constant between the fundamental mode transmitted light in the bus ridge waveguide and the higher-order optical mode after acousto-optic scattering as much as possible, this fan-shaped design in this invention can effectively cover the fluctuation of the difference in optical mode propagation constant caused by process errors in the fabrication of the bus ridge waveguide). In the plane of the X-cut thin-film lithium niobate, the central acoustic field emission angle direction of the fan-shaped electroacoustic interdigital transducer can be at a 30-degree angle to the crystal Y-axis to obtain the maximum electromechanical coupling coefficient. When the radio frequency driving frequency of the interdigital transducer is around 3 GHz, it can excite a strong acoustic wave, generate a strong strain in the thin-film lithium niobate waveguide, and obtain a good acousto-optic scattering effect.

[0076] Example 2

[0077] The thin-film lithium niobate optical isolator provided by this invention has the following three-dimensional structure diagram: Figure 1 As shown, the structure includes a silicon substrate 1, a buried oxide layer 2, a thin-film lithium niobate layer 3, and a fan-shaped electroacoustic interdigital transducer 4. The thin-film lithium niobate layer 3 contains a pair of mode (de)multiplexers 5 (hereinafter referred to as the first mode (de)multiplexer and the second mode (de)multiplexer, respectively; similar to conventional methods, the multiplexers are used for input, and the demultiplexers are used for output), and a bus ridge waveguide 6. The refractive index of the thin-film lithium niobate layer is greater than that of the buried oxide layer and the air cladding, confining the optical field modes within the lithium niobate waveguide.

[0078] In this embodiment, the thin-film lithium niobate optical isolator is prepared according to the following steps:

[0079] Step 1: Prepare a substrate consisting of a silicon substrate 1, a buried oxide layer 2, and a thin film of lithium niobate from bottom to top. Use electron beam evaporation to grow a metallic chromium layer with a thickness of 80 nanometers on the substrate.

[0080] Step 2: Spin-coat electron beam adhesive onto a chromium-plated substrate. The thickness of the electron beam adhesive is 300 nanometers. After spin-coating, place the substrate on a heating plate at a preset heating temperature of 180 degrees Celsius and let it stand for 1 minute.

[0081] Step 3: Expose the substrate using an electron beam exposure system, then develop and fix it, and use oxygen plasma to remove the residual adhesive from the developed substrate to obtain a waveguide pattern.

[0082] Step 4: After etching the substrate using chlorine-based inductively coupled plasma etching, the remaining electron beam resist is removed to obtain a chromium layer pattern.

[0083] Step 5: After etching the substrate with fluorine-based ICP, the remaining chromium metal is removed to obtain a thin film lithium niobate layer with a mode multiplexer, a mode demultiplexer and a bus ridge waveguide.

[0084] Step six: Clean the substrate;

[0085] Step 7: Perform thermal annealing on the substrate. Specifically, under normal pressure in an oxygen atmosphere, heat the substrate from room temperature to a preset holding temperature at a rate of 5 degrees Celsius per minute and hold it for 1 hour; then gradually cool it down from the holding temperature at a rate of 0.5 degrees Celsius per minute back to room temperature; wherein, the preset holding temperature is 1100 degrees Celsius.

[0086] Step 8: Perform the electron beam lithography operation from Steps 2 and 3 on the substrate again to form a fan-shaped interdigital transducer pattern. Specifically: spin-coat the substrate with electron beam resist, the thickness of which is 300 nanometers. After spin-coating, place the substrate on a heating plate at a preset heating temperature of 180 degrees Celsius for 1 minute. Then, expose the substrate with an electron beam exposure system, followed by development and fixing. Oxygen plasma is used to remove the residual resist from the development process to obtain the fan-shaped interdigital transducer pattern.

[0087] Step 9: Place the above substrate in an electron beam evaporation system to deposit the metal of the fan-shaped electroacoustic interdigital transducer, and use a stripping process to remove the remaining electron beam adhesive and metal film, thereby obtaining a thin-film lithium niobate optical isolation device based on acousto-optic scattering.

[0088] In step two, the electron beam adhesive is ARP6200.13;

[0089] In step three, the electron beam exposure system is a VISTEC EBPG 5000PES;

[0090] In step six, the cleaning specifically involves placing the substrate in a mixed solution obtained by mixing ammonia solution, hydrogen peroxide solution, and deionized water in a volume ratio of 1:1:5 and letting it stand for 1 minute at a temperature of 80 degrees Celsius; wherein the concentration of the ammonia solution is 25wt%-28wt% and the concentration of the hydrogen peroxide solution is 30wt%.

[0091] Comparative Example 1

[0092] To further illustrate the design advantages of the sector-shaped electroacoustic interdigital transducer, based on the above embodiment 1, the inventors also simulated the theoretical acousto-optical scattering efficiency of a traditional non-sector-shaped interdigital transducer and the sector-shaped electroacoustic interdigital transducer designed in this invention. The fixed angles of the non-sector-shaped electroacoustic interdigital transducer are 4.5 degrees, 5.5 degrees, and 6.5 degrees, respectively, while the divergence angle of the sector-shaped electroacoustic interdigital transducer is not fixed, θ' is 4.5 degrees to 6.5 degrees. Figure 7 As shown, it can be observed that the non-fan-shaped electroacoustic interdigital transducer is greatly affected by the fixed angle. Except for the 5.5-degree acoustic field exit angle, which just satisfies the peak modulation efficiency in the 1550nm band of interest, the peak efficiency at the other two angles is far from the C-band of optical communication. In contrast, the fan-shaped electroacoustic interdigital transducer designed based on this application exhibits high and flat efficiency across the entire band. In the 1550nm band, its efficiency is nearly 8 dB higher than that of the non-fan-shaped interdigital transducers with fixed angles of 4.5 degrees and 6.5 degrees.

[0093] The above embodiments are merely examples. The cladding material of the optical waveguide in this invention is not limited to air cladding, but can also be silicon oxide, silicon nitride, or HSQ photoresist, etc., as long as it does not have an adverse effect on the optical field transmission.

[0094] In this invention, the electrode configuration of the fan-shaped electroacoustic transducer is not limited to the three-electrode contact plate configuration in the embodiment, i.e., the ground-signal-ground (GSG) configuration. It can also be a five-electrode contact plate GGSSG configuration or a seven-electrode contact plate GGSSG configuration, and so on, as long as it satisfies the condition of multiple interdigital transducers connected in parallel.

[0095] In addition, the above embodiment takes the acoustic isolation layer between the substrate and the thin film lithium niobate layer as a buried oxide layer as an example. In this case, the sound field mode excited by the fan-shaped electroacoustic interdigital transducer 4 in the thin film lithium niobate layer 3 is surface acoustic wave. Similar to other existing technologies, in addition to the buried oxide layer, the acoustic isolation layer can also be an air layer (i.e., the thin film lithium niobate layer is suspended). In this case, the sound field mode excited by the fan-shaped electroacoustic interdigital transducer 4 in the thin film lithium niobate layer 3 is Lamb wave.

[0096] In addition, Figure 1 In addition to the pattern (demultiplexer) being located on the same side of the device and the bus ridge waveguide employing a U-shaped fold design, the pattern (demultiplexer) can also be located on opposite sides of the device, one on the left and one on the right. Correspondingly, the bus ridge waveguide can adopt a linear design (in this case, only the fan-shaped electro-acoustic interdigital transducers facing the bus ridge waveguide can be retained). The number of fan-shaped electro-acoustic interdigital transducers can also vary, being greater than or equal to one. The electrode configuration of the corresponding fan-shaped electro-acoustic transducers can also be flexibly changed (for example, when the number of fan-shaped electro-acoustic interdigital transducers is one, using...). Figure 1The electrode configuration of the sector-shaped electroacoustic transducer shown is a three-electrode contact plate configuration. In addition to two PADs being connected to the interdigital pinions, the other PAD can be suspended. Of course, the more sector-shaped electroacoustic interdigital transducers are connected in parallel, the longer the acousto-optic interaction region in the thin-film lithium niobate waveguide will be. This can also improve the efficiency of mode conversion in the waveguide to a certain extent, further enhancing the performance of the non-reciprocal optical isolator.

[0097] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A thin-film lithium niobate non-reciprocal optical isolator based on acousto-optic scattering, characterized in that, From bottom to top, it includes a substrate (1), an acoustic isolation layer, a thin-film lithium niobate layer (3), and a fan-shaped electroacoustic interdigital transducer (4); among which, The thin-film lithium niobate layer (3) is provided with a mode multiplexer, a mode demultiplexer and a bus ridge waveguide (6). The bus ridge waveguide (6) is coupled to the mode multiplexer and the mode demultiplexer to multiplex different optical field modes into the bus ridge waveguide (6) or demultiplex different optical field modes from the bus ridge waveguide (6). The interdigitated fingers of the fan-shaped electroacoustic interdigital transducer (4) are distributed along an arc. The arc length corresponding to the outermost set of interdigitated fingers is greater than the arc length corresponding to the innermost set of interdigitated fingers, thus making the whole fan-shaped; and the divergence direction of the fan shape is towards the bus ridge waveguide (6). There are multiple fan-shaped electroacoustic interdigital transducers (4), and two adjacent fan-shaped electroacoustic interdigital transducers (4) share an electrode contact plate; the bus ridge waveguide (6) is U-shaped folded, and there are an even number of fan-shaped electroacoustic interdigital transducers (4). Half of the total number of fan-shaped electroacoustic interdigital transducers (4) have their fan-shaped divergence direction facing one side of the U-shape of the bus ridge waveguide (6), and the other half of the total number of fan-shaped electroacoustic interdigital transducers (4) have their fan-shaped divergence direction facing the other side of the U-shape of the bus ridge waveguide (6); The fan-shaped electroacoustic interdigital transducer (4) is used to excite a sound field in the thin-film lithium niobate layer (3) under the drive of an external radio frequency signal. This enables different optical field modes transmitted unidirectionally in the bus ridge waveguide (6) to interact with the sound field. Ultimately, different optical field modes can be routed from different ports of the mode multiplexer and the mode demultiplexer, realizing a non-reciprocal optical isolator based on intermodal acousto-optic scattering for unidirectional optical transmission.

2. The thin-film lithium niobate non-reciprocal optical isolator based on acousto-optic scattering as described in claim 1, characterized in that, The total divergence angle of the fan-shaped electroacoustic interdigital transducer (4) is 0.1° to 10°.

3. The thin-film lithium niobate non-reciprocal optical isolator based on acousto-optic scattering as described in claim 1, characterized in that, The center emission direction of the excitation sound field of the fan-shaped electroacoustic interdigital transducer (4) forms an angle with the cross section of the bus ridge waveguide (6), and this angle is denoted as θ. The total divergence angle of the fan-shaped electroacoustic interdigital transducer (4) is denoted as 2α; Then, the angle between the emission angle of the fan-shaped electroacoustic interdigital transducer (4) and the cross-section of the bus ridge waveguide (6) is... θ' For the interval [ θ - α , θ + α ].

4. The thin-film lithium niobate non-reciprocal optical isolator based on acousto-optic scattering as described in claim 1, characterized in that, The total thickness of the thin-film lithium niobate layer (3) is 300 nanometers to 1000 nanometers, and the mode multiplexer, mode demultiplexer and bus ridge waveguide (6) disposed therein are formed by etching, with an etching depth of 150 nanometers to 500 nanometers. The bus ridge waveguide (6) has a trapezoidal cross-section and a waveguide sidewall tilt angle of 50 to 70 degrees; the bus ridge waveguide (6) is a multimode waveguide and the upper width of the trapezoidal cross-section is 1800 nanometers to 5000 nanometers.

5. The thin-film lithium niobate non-reciprocal optical isolator based on acousto-optic scattering as described in claim 1, characterized in that, The metal material selected for the fan-shaped electroacoustic interdigital transducer (4) is gold, copper or aluminum, with a thickness of 50 nanometers to 1000 nanometers. The output aperture of a single fan-shaped electroacoustic interdigital transducer (4) is more than 10 micrometers, the period length is equal to the ridge width of the bus waveguide (6), and the number of periods is more than 2.

6. The thin-film lithium niobate non-reciprocal optical isolator based on acousto-optic scattering as described in claim 1, characterized in that, When the thin film lithium niobate layer (3) is made of X-cut lithium niobate crystal, the emission direction of the sound field center of the fan-shaped electroacoustic interdigital transducer (4) is at a positive 30-degree angle with the Y-axis of the crystal. When the thin film lithium niobate layer (3) is made of Y-cut lithium niobate crystal, the emission direction of the sound field center of the fan-shaped electroacoustic interdigital transducer (4) is at an angle of ±45 degrees to the Z-axis of the crystal. When the thin-film lithium niobate layer (3) is made of Z-cut lithium niobate crystal, the emission direction of the sound field center of the fan-shaped electroacoustic interdigital transducer (4) is at an angle of 0 degrees or ±60 degrees to the Y-axis of the crystal.

7. The thin-film lithium niobate non-reciprocal optical isolator based on acousto-optic scattering as described in claim 1, characterized in that, The acoustic isolation layer is a buried oxygen layer (2), and the acoustic field mode excited by the fan-shaped electroacoustic interdigital transducer (4) in the thin film lithium niobate layer (3) is surface acoustic wave. Alternatively, the acoustic isolation layer may be an air layer, and the acoustic field mode excited by the fan-shaped electroacoustic interdigital transducer (4) in the thin-film lithium niobate layer (3) may be a Lamb wave.

8. The thin-film lithium niobate non-reciprocal optical isolator based on acousto-optic scattering as described in claim 1, characterized in that, The mode multiplexer has the same structure as the mode demultiplexer. The mode multiplexer has an upper port and a lower port, which are used to input the fundamental mode and higher-order modes, respectively. The optical coupling input to the upper port or the lower port is either vertical or horizontal.

9. The thin-film lithium niobate non-reciprocal optical isolator based on acousto-optic scattering as described in claim 1, characterized in that, The upper cladding of the thin-film lithium niobate optical isolator based on acousto-optic scattering is an air upper cladding. Alternatively, the thin-film lithium niobate photoisolator based on acousto-optic scattering may also have an upper cladding layer, the material of which is silicon oxide, silicon nitride, or HSQ type photoresist.

10. The method for fabricating a thin-film lithium niobate non-reciprocal optical isolator based on acousto-optic scattering as described in any one of claims 1-9, characterized in that, Includes the following steps: Step 1: Prepare a substrate consisting of a substrate (1), an acoustic isolation layer, and a thin film of lithium niobate from bottom to top. Use electron beam evaporation to grow metallic chromium with a thickness of 80 nanometers to 160 nanometers on the substrate. Step 2: Spin-coat electron beam adhesive onto a chromium-plated substrate. The thickness of the electron beam adhesive is 300 nanometers to 500 nanometers. After spin-coating, place the substrate on a heating plate at a preset heating temperature of 150 to 180 degrees Celsius and let it stand for at least 1 minute. Step 3: Expose the substrate using an electron beam exposure system, then develop and fix it, and use oxygen plasma to remove the residual adhesive from the developed substrate to obtain a waveguide pattern. Step 4: After etching the substrate using chlorine-based inductively coupled plasma etching, the remaining electron beam resist is removed to obtain a chromium layer pattern. Step 5: After etching the substrate with fluorine-based ICP, the remaining chromium metal is removed to obtain a thin film lithium niobate layer with a mode multiplexer, a mode demultiplexer and a bus ridge waveguide (6). Step six: Clean the substrate; Step 7: Perform thermal annealing on the substrate. Specifically, under normal pressure in an oxygen atmosphere, heat the substrate from room temperature to a preset holding temperature at a rate of 1 to 5 degrees Celsius per minute, and hold it at that temperature for more than 1 hour; then gradually cool it down from the holding temperature at a rate of 0.5 to 2 degrees Celsius per minute back to room temperature; wherein the preset holding temperature is between 500 degrees Celsius and 1100 degrees Celsius. Step 8: First, spin-coat the substrate with electron beam resist. The thickness of the electron beam resist is 300 nm to 500 nm. After spin-coating, place the substrate on a heating plate at a preset heating temperature of 100 to 200 degrees Celsius and let it stand for at least 1 minute. Then, expose the substrate with an electron beam exposure system, followed by development and fixing. Oxygen plasma is used to remove the residual resist from the development process to obtain a fan-shaped electroacoustic interdigital transducer pattern. Step nine: Place the above substrate in an electron beam evaporation system to deposit the metal of the fan-shaped electroacoustic interdigital transducer, and use a stripping process to remove the remaining electron beam adhesive and metal film, thereby obtaining a thin-film lithium niobate optical isolation device based on acousto-optic scattering.

11. The preparation method according to claim 10, characterized in that, In step two, the electron beam adhesive is ARP6200; In step three, the electron beam exposure system is a VISTEC EBPG 5000PES; In step six, the cleaning specifically involves placing the substrate in a mixed solution obtained by mixing ammonia solution, hydrogen peroxide solution, and deionized water in a volume ratio of 1:1:5 and allowing it to stand for 1 to 15 minutes at a temperature of 60 to 80 degrees Celsius. The concentration of the ammonia solution is 25 wt% to 28 wt%, and the concentration of the hydrogen peroxide solution is 30 wt%.

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