On-chip integrated positive and negative acousto-optic frequency shifter and preparation method thereof

Abstract

The invention relates to an on-chip integrated positive and negative acousto-optic frequency shifter, and belongs to the technical field of optoelectronics. According to the frequency shifter, positive and negative frequency movement of an optical signal is realized on a single chip by integrating an acousto-optic effect and a mode conversion technology. Specifically, a piezoelectric material layer and an optical waveguide structure are utilized, a first acousto-optic frequency shift unit and a second acousto-optic frequency shift unit are combined, fixed positive frequency shift and adjustable negative frequency shift are generated respectively, and continuous adjustment of optical signal frequency is achieved by adjusting the frequency of the negative frequency shift. In addition, a mode filter is arranged to filter residual useless signals generated due to non-ideal mode conversion. The device has the advantages of being compact in structure, stable in performance, flexible to adjust and the like, can be widely applied to the fields of optical communication, optical information processing, optical sensing and the like, and provides a new solution for frequency control of optical signals.

Description

Technical Field

[0001] The invention relates to an integrated acousto-optic frequency shifter, in particular to an on-chip integrated acousto-optic frequency shifter capable of realizing positive and negative frequency shifting and a preparation method thereof. Background Art

[0002] Realizing frequency shift of light is an important part of on-chip signal processing. Frequency shift based on the acousto-optic effect has the advantages of high frequency shift efficiency and simple structure, and is widely used. Traditional integrated acousto-optic frequency shifter technology is mainly based on the principle of Bragg diffraction of acousto-optic gratings. The light beam is incident at a certain angle, and after passing through the acousto-optic grating, 0th and ±1st order diffracted light is generated. The ±1st order diffracted light is emitted at a certain angle and generates positive or negative frequency shift respectively. This scheme can only achieve unilateral frequency shift, that is, the output frequency must be higher or lower than the input frequency, and it cannot take into account the needs of positive and negative frequency shift at the same time.

[0003] Invention patent CN102279479B proposes an integrated acousto-optic frequency shifter technology, which uses Ti diffusion technology to construct a lithium niobate waveguide, and couples with the grating formed by the surface acoustic wave to achieve frequency shifting, but this technology has weak light field binding ability, low frequency shifting efficiency, and can only produce unilateral frequency shifting. In order to improve the frequency shifting efficiency, invention patent CN107065235A proposes an acousto-optic frequency shifter based on tellurium oxide crystal material, but the acousto-optic frequency shifter based on bulk material can only produce a fixed frequency shift on one side, and the power consumption is higher. The document (Yama Nicholas S, et al. Advanced Materials 2024, 36.5: 2305434.) proposes that a large-area suspended structure can be manufactured to reduce acoustic wave loss to improve the frequency shifting efficiency, but the suspended structure is very fragile and easy to collapse, which will greatly reduce the robustness and stability of the device. In addition, there are also documents (Yu Zejie, et al. ACS Photonics 2021, 8.3: 798-803.) that use polymer materials with high elastic-optic coefficients as optical waveguides to improve the efficiency of acousto-optic coupling, but polymer processes are not CMOS compatible, which increases the complexity of the preparation process, and the long-term stability of polymer materials is poor, which cannot adapt to future large-scale on-chip system integration. Therefore, it is necessary to study an integrated acousto-optic frequency shifter that can achieve positive and negative frequency shifting. Summary of the invention

[0004] In view of the above-mentioned deficiencies in the prior art, the present invention provides an integrated positive and negative acousto-optic frequency shifter based on piezoelectric materials and a preparation method thereof, which utilizes two acousto-optic frequency shifts combined with mode selection to achieve the effect of positive and negative frequency shifts.

[0005] Working principle of the present invention:

[0006] In a device that integrates thin-film piezoelectric materials, two pairs of acousto-optic frequency shifters with opposite frequency shift directions are used to first convert mode 1 into mode 2, while generating a fixed positive frequency shift f1, and then convert mode 2 back to mode 1, while generating an adjustable negative frequency shift f2. The total frequency shift is then f1-f2, and by adjusting the frequency of f2 from less than f1 to greater than f1, positive and negative frequency shifts relative to the input light frequency can be achieved. Furthermore, due to the non-ideality of mode conversion, it is necessary to combine mode filtering to filter out residual useless signals to obtain the final high-performance integrated positive and negative frequency shifter.

[0007] The technical solution of the present invention is as follows:

[0008] On the one hand, the present invention provides an on-chip integrated positive and negative acousto-optic frequency shifter, which is characterized by comprising:

[0009] A substrate having an acousto-optic thin film piezoelectric material integrated thereon;

[0010] A first acousto-optic frequency shifter, disposed on the acousto-optic thin film piezoelectric material, is used to convert an input optical signal from mode 1 to mode 2 and generate a fixed positive shift frequency f1 during the conversion process;

[0011] A second acousto-optic frequency shifter is arranged after the first acousto-optic frequency shifter, and is used to convert the optical signal of mode 2 back to mode 1, and generate an adjustable negative frequency shift f2 during the conversion process, so that the total frequency shift amount is f1-f2, and by adjusting the frequency of f2, continuous adjustable frequency shift from positive frequency shift to negative frequency shift relative to the input optical frequency is achieved;

[0012] The mode filter is arranged between or after the first acousto-optic frequency shifter and the second acousto-optic frequency shifter, and is used to filter out residual useless signals generated by non-ideal mode conversion to ensure high performance of the output optical signal.

[0013] Furthermore, the first acousto-optic frequency shifter and the second acousto-optic frequency shifter both include an optical waveguide and an interdigital transducer (IDT), wherein the IDT is disposed on one side of the optical waveguide and is used to excite acoustic waves when a microwave modulation signal is applied, interact with the optical signal in the optical waveguide, and realize mode conversion and frequency shift.

[0014] Furthermore, the optical waveguide supports mode 1 and mode 2 at the same time and is at a certain angle to the IDT. The frequency of the microwave modulation signal applied to the IDT of the first acousto-optic frequency shifter is f1, and the frequency of the microwave modulation signal applied to the IDT of the second acousto-optic frequency shifter is f2, f2 is an adjustable frequency, and the electrode period of the IDT of the second acousto-optic frequency shifter changes in a chirp.

[0015] Furthermore, it also includes a driving module for generating a coherent microwave modulation signal and providing the coherent microwave modulation signal to the IDTs of the first acousto-optic frequency shifter and the second acousto-optic frequency shifter respectively, so as to drive them to perform mode conversion and frequency shift.

[0016] Furthermore, an input coupler is included, which is used to introduce the input optical signal into the chip.

[0017] Furthermore, an input coupler is included, and the thin film piezoelectric material is a lithium niobate thin film.

[0018] Furthermore, the acousto-optic material film has a piezoelectric effect, the crystal axis is the z-axis, the x-axis is along the normal direction of the film, the film plane is the yz plane, and the propagation direction of the optical signal in each of the above devices is the +y-axis;

[0019] Furthermore, the acoustic waves generated by the IDT in the first and second acousto-optic frequency shifters will cause a change in the refractive index in the waveguide, and the overlapping integral of the refractive index change on the waveguide cross section, the mode 1 mode field and the mode 2 mode field cannot be zero; the wave vector of the acoustic waves generated by the IDT in the first and second acousto-optic frequency shifters along the optical waveguide direction should meet the phase matching condition to improve the mode conversion efficiency; the waveguide length in the first and second acousto-optic frequency shifters should be moderate so that mode 1 (2) can be fully converted into mode 2 (1) but will not be converted in reverse;

[0020] Furthermore, in the first and second acousto-optic frequency shifters, the waveguide may be processed into a suspended waveguide, and / or an acoustic wave reflector may be prepared on the other side of the waveguide relative to the IDT to improve the mode conversion efficiency caused by the acousto-optic effect;

[0021] Furthermore, the first and second mode filters are based on the mode evolution principle or the adiabatic mode coupling principle;

[0022] Furthermore, among the microwave frequencies generated by the driving module, f1 is a fixed frequency, f2 is an adjustable frequency, and the tuning range of f2 is from less than f1 to greater than f1.

[0023] Preferably, the acousto-optic material film is an x-cut lithium niobate film with a thickness ranging from several hundred nanometers to several micrometers.

[0024] Preferably, the mode 1 of the optical signal is TE0, and the mode 2 is TE1. The input optical signal is mode 1.

[0025] Preferably, the frequency range of the microwave frequency f1 generated by the driving module can be selected from 1 GHz to 4 GHz, and the tuning range of the microwave frequency f2 is ±1% to 5% on both sides of f1.

[0026] Second, the present invention also provides a method for preparing the above-mentioned on-chip integrated positive and negative acousto-optic frequency shifter, which is characterized in that it comprises the steps of:

[0027] S1. Integrate acousto-optic thin film piezoelectric materials on a substrate;

[0028] S2. An optical waveguide portion of an input coupler, a first acousto-optic frequency shifter, a second acousto-optic frequency shifter and a mode filter is prepared by etching on the acousto-optic thin film piezoelectric material;

[0029] S3. IDT electrodes are prepared by deposition and stripping processes on one side of the light waves of the first acousto-optic frequency shifter and the second acousto-optic frequency shifter;

[0030] S4. Prepare an upper cladding layer by a deposition process to protect the device and the IDT electrode;

[0031] S5. Assembling a driving module to generate and distribute a microwave modulation signal to the IDT electrode of the first acousto-optic frequency shifter and the second acousto-optic frequency shifter;

[0032] S6. Perform packaging and testing.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] 1) Compared with the surface acoustic wave acousto-optic frequency shifter technology based on Ti diffused waveguide proposed in patent CN102279479B, the present invention has higher acousto-optic frequency shifting efficiency and lower acoustic and optical transmission losses.

[0035] 2) Compared with the tellurium oxide crystal-based acousto-optic frequency shifter technology proposed in patent CN107065235A, the present invention can achieve arbitrarily tunable positive and negative frequency shifts, as well as lower device power consumption.

[0036] 3) Compared with the large-area suspended structure acousto-optic frequency shifter technology proposed in the literature (Yama Nicholas S, et al. Advanced Materials 2024, 36.5: 2305434.), the device of the present invention has higher stability and robustness, and can be applied to a variety of complex usage environments.

[0037] 4) Compared with the polymer material acousto-optic frequency shifter technology proposed in the literature (Yu Zejie, et al. ACS Photonics 2021, 8.3:798-803.), the device of the present invention is CMOS compatible and simplifies the preparation process.

[0038] 5) All components of the present invention are integrated on the same chip, which greatly reduces the size and weight of the device. By adjusting the frequency of the microwave modulation signal, the positive and negative frequency shift of the optical signal can be easily achieved, and the frequency shift amount is continuously adjustable. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a schematic diagram of an embodiment of the integrated positive and negative acousto-optic frequency shifter of the present invention.

[0040] In the figure: 1-chip, 2-input coupler, 3-first acousto-optic frequency shifter, 4-first mode filter, 5-second acousto-optic frequency shifter, 6-second mode filter, 7-driving module.

[0041] Figure 2 Schematic diagram of the energy bands in which the first acousto-optic frequency shifter and the second acousto-optic frequency shifter produce two mode conversions and frequency shifts. The horizontal axis is the wave vector k and the vertical axis is the frequency. DETAILED DESCRIPTION

[0042] The present invention will be further described below in conjunction with the accompanying drawings and examples, but the protection scope of the present invention shall not be limited thereto. The embodiments of the present invention include but are not limited to the following examples.

[0043] Please refer to Figure 1 , Figure 1 Schematic diagram of an embodiment of the integrated positive and negative acousto-optic frequency shifter of the present invention. Figure 1 As shown, an integrated positive and negative acousto-optic frequency shifter comprises a chip 1, the upper layer of the chip has an acousto-optic material film with piezoelectric effect (such as an x-cut lithium niobate film), and the lower layer is a lower cladding layer.

[0044] On the acousto-optic material film, along the propagation direction of the optical signal, an input coupler 2, a first acousto-optic frequency shifter 3, a first mode filter 4, a second acousto-optic frequency shifter 5 and a second mode filter 6 are sequentially prepared.

[0045] Input coupler 2 contains a waveguide for optical signal mode 1, which is then adiabatically widened along the propagation direction of the optical signal so that it can support the propagation of both mode 1 and mode 2. The input optical signal (mode 1) is introduced into the chip and prepared for subsequent mode conversion by waveguide widening.

[0046] The first acousto-optic frequency shifter 3 is a two-port device, comprising an optical waveguide that supports both mode 1 and mode 2. An interdigital transducer (IDT) is prepared on one side of the optical waveguide, and the IDT is at a certain angle to the waveguide. A microwave modulation signal with a fixed frequency of f1 is applied to the IDT, and the microwave signal excites an acoustic wave on the acousto-optic material film through the IDT. The component of the acoustic wave along the waveguide direction propagates in the opposite direction to the optical signal in the waveguide. The acousto-optic effect is used to convert the optical signal from mode 1 to mode 2, and the frequency of the optical signal increases by f1.

[0047] The first mode filter 4 filters out the mode 1 component in the output optical signal of the first acousto-optic frequency shifter 3 and only retains the mode 2 component to ensure the accuracy of subsequent processing.

[0048] The second acousto-optic frequency shifter 5 is a two-port device, comprising an optical waveguide that supports both mode 1 and mode 2. An interdigital transducer (IDT) is prepared on one side of the optical waveguide, the IDT is at a certain angle to the waveguide, and the electrode period of the IDT changes in a chirp to support microwave modulation signals in a certain frequency range. A microwave modulation signal with a frequency of f2 is applied to the IDT, and the microwave signal excites an acoustic wave on the acousto-optic material film through the IDT. The component of the acoustic wave along the waveguide direction propagates in the opposite direction to the optical signal in the waveguide, so that the optical signal is converted from mode 2 to mode 1, and the frequency of the optical signal is reduced by f2;

[0049] The second mode filter 6 filters out the mode 2 component in the output optical signal of the second acousto-optic frequency shifter 5 and only retains the mode 1 component as the final output.

[0050] The driving module 7 generates two microwave signals with coherent frequencies f1 and f2 respectively, and drives the IDTs in the first acousto-optic frequency shifter and the second acousto-optic frequency shifter respectively to generate the required mode conversion.

[0051] The acousto-optic material film has a piezoelectric effect, the crystal axis is the z axis, the x axis is along the normal direction of the film, the film plane is the yz plane, and the propagation direction of the optical signal in the above-mentioned devices is the +y axis;

[0052] The acoustic waves generated by the IDT in the first and second acousto-optic frequency shifters will cause a change in the refractive index in the waveguide, and the overlapping integral of the refractive index change on the waveguide cross section, the mode 1 mode field and the mode 2 mode field cannot be zero; the wave vector of the acoustic waves generated by the IDT in the first and second acousto-optic frequency shifters along the optical waveguide direction should meet the phase matching condition to improve the mode conversion efficiency; the waveguide length in the first and second acousto-optic frequency shifters should be moderate so that mode 1 (2) can be fully converted into mode 2 (1), but will not be converted in the opposite direction;

[0053] In the first and second acousto-optic frequency shifters, the waveguide may be processed into a suspended waveguide, and / or an acoustic wave reflector may be prepared on the other side of the waveguide relative to the IDT to improve the mode conversion efficiency caused by the acousto-optic effect;

[0054] The first and second mode filters are based on the mode evolution principle or the adiabatic mode coupling principle;

[0055] Among the microwave frequencies generated by the driving module, f1 is a fixed frequency, f2 is an adjustable frequency, and the tuning range of f2 is from less than f1 to greater than f1.

[0056] The acousto-optic material film is an x-cut lithium niobate film with a thickness ranging from several hundred nanometers to several micrometers.

[0057] Mode 1 of the optical signal is TE0, and mode 2 is TE1. The input optical signal is mode 1.

[0058] The frequency range of the microwave frequency f1 generated by the driving module can be selected from 1 GHz to 4 GHz, and the tuning range of the microwave frequency f2 is ±1% to 5% on both sides of f1 as the center.

[0059] The method for preparing the integrated positive and negative acousto-optic frequency shifter comprises:

[0060] S1. Integrate acousto-optic thin film piezoelectric materials on a substrate;

[0061] S2. Preparing an input coupler, a first acousto-optic frequency shifter, a second acousto-optic frequency shifter and an optical waveguide portion of a mode filter by etching on the acousto-optic thin film piezoelectric material (such as a lithium niobate film);

[0062] S3. IDT electrodes are prepared by deposition and stripping processes on one side of the light waves of the first acousto-optic frequency shifter and the second acousto-optic frequency shifter;

[0063] S4. Prepare an upper cladding layer by a deposition process to protect the device and the IDT electrode;

[0064] S5. Assembling a driving module to generate and distribute a microwave modulation signal to the IDT electrode of the first acousto-optic frequency shifter and the second acousto-optic frequency shifter;

[0065] S6. Perform packaging and testing.

[0066] Figure 2 This is a schematic diagram of the energy band of mode conversion and frequency shift after the optical signal passes through the first acousto-optic frequency shifter and the second acousto-optic frequency shifter. Assume that the input optical mode is mode 1, the frequency is f0, and the direction of light propagation is opposite to the direction of the sound wave. Then after passing through the first acousto-optic frequency shifter, the optical mode becomes mode 2, and the frequency becomes f0+f1. After passing through the second acousto-optic frequency shifter, the optical mode becomes mode 1, and the frequency becomes f0+f1-f2. Therefore, the final total frequency shift amount is f1-f2. By controlling the size of f2 relative to f1, positive and negative frequency shift effects can be achieved.

[0067] This embodiment discloses an on-chip integrated positive and negative acousto-optic frequency shifter, which realizes the positive and negative frequency shift of optical signals on a single chip by integrating the acousto-optic effect and mode conversion technology. Specifically, by utilizing a piezoelectric material layer and an optical waveguide structure, in combination with a first acousto-optic frequency shift unit and a second acousto-optic frequency shift unit, a fixed positive frequency shift and an adjustable negative frequency shift are generated respectively, and by adjusting the frequency of the negative frequency shift, the continuous adjustable frequency of the optical signal is achieved. In addition, a mode filter is also provided to filter out residual useless signals generated by the non-ideality of mode conversion. The present invention has the advantages of compact structure, stable performance, and flexible adjustment. It can be widely used in optical communications, optical information processing, optical sensing and other fields, and provides a new solution for the frequency control of optical signals.

Claims

1. An on-chip integrated positive and negative acousto-optic frequency shifter, characterized in that: include: A substrate having an acousto-optic thin film piezoelectric material integrated thereon; A first acousto-optic frequency shifter, disposed on the acousto-optic thin film piezoelectric material, is used to convert an input optical signal from mode 1 to mode 2 and generate a fixed positive shift frequency f1 during the conversion process; A second acousto-optic frequency shifter is arranged after the first acousto-optic frequency shifter, and is used to convert the optical signal of mode 2 back to mode 1, and generate an adjustable negative frequency shift f2 during the conversion process, so that the total frequency shift amount is f1-f2, and by adjusting the frequency of f2, continuous adjustable frequency shift from positive frequency shift to negative frequency shift relative to the input optical frequency is achieved; The mode filter is arranged between or after the first acousto-optic frequency shifter and the second acousto-optic frequency shifter, and is used to filter out residual useless signals generated by non-ideal mode conversion to ensure high performance of the output optical signal.

2. The on-chip integrated positive and negative acousto-optic frequency shifter according to claim 1, characterized in that: The first acousto-optic frequency shifter and the second acousto-optic frequency shifter both include an optical waveguide and an interdigital transducer (IDT). The IDT is arranged on one side of the optical waveguide and is used to excite acoustic waves when a microwave modulation signal is applied, interact with the optical signal in the optical waveguide, and realize mode conversion and frequency shift.

3. The on-chip integrated positive and negative acousto-optic frequency shifter according to claim 2, characterized in that: The optical waveguide supports mode 1 and mode 2 at the same time and is at a certain angle to the IDT. The frequency of the microwave modulation signal applied to the IDT of the first acousto-optic frequency shifter is f1, and the frequency of the microwave modulation signal applied to the IDT of the second acousto-optic frequency shifter is f2, f2 is an adjustable frequency, and the electrode period of the IDT of the second acousto-optic frequency shifter changes in a chirp.

4. The on-chip integrated positive and negative acousto-optic frequency shifter according to any one of claims 1 to 3, characterized in that: It also includes a driving module for generating a coherent microwave modulation signal and providing the coherent microwave modulation signal to the IDTs of the first acousto-optic frequency shifter and the second acousto-optic frequency shifter respectively, so as to drive the IDTs to perform mode conversion and frequency shift.

5. The on-chip integrated positive and negative acousto-optic frequency shifter according to any one of claims 1 to 4, characterized in that: An input coupler is also included for introducing an input optical signal into the chip.

6. The on-chip integrated positive and negative acousto-optic frequency shifter according to any one of claims 1 to 4, characterized in that: It also includes an input coupler, and the thin film piezoelectric material is a lithium niobate thin film.

7. A method for preparing the on-chip integrated positive and negative acousto-optic frequency shifter according to any one of claims 1 to 5, characterized in that: Includes steps: S1. Integrate acousto-optic thin film piezoelectric materials on a substrate; S2. An optical waveguide portion of an input coupler, a first acousto-optic frequency shifter, a second acousto-optic frequency shifter and a mode filter is prepared by etching on the acousto-optic thin film piezoelectric material; S3. IDT is prepared by deposition and stripping processes on one side of the light wave of the first acousto-optic frequency shifter and the second acousto-optic frequency shifter, respectively; S4. As needed, a mode filter is prepared between or after the first acousto-optic frequency shifter and the second acousto-optic frequency shifter; S5. Assembling a driving module to generate and distribute a microwave modulated signal to the IDT of the first acousto-optic frequency shifter and the second acousto-optic frequency shifter; S6. Perform packaging and testing.

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