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

Abstract

The present application relates to a kind of integrated positive and negative acousto-optic frequency shifter on chip, belong to the technical field of optoelectronics.The frequency shifter is realized by integrating acousto-optic effect and mode conversion technology, and the positive and negative frequency movement of optical signal is realized on single chip.Specifically, the present application utilizes piezoelectric material layer and optical waveguide structure, combines first acousto-optic frequency shift unit and second acousto-optic frequency shift unit, respectively generates fixed positive frequency shift and adjustable negative frequency shift, and realizes the continuous adjustable frequency of optical signal by adjusting the frequency of negative frequency shift.In addition, mode filter is also provided to filter out residual unwanted signal due to mode conversion non-ideality.The present application has the advantages of compact structure, stable performance, flexible adjustment, etc., and can be widely applied in optical communication, optical information processing, optical sensing and other fields, and provides a new solution for frequency control of optical signal.

Description

TECHNICAL FIELD

[0001] The application 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 shifts and a preparation method thereof. BACKGROUND

[0002] The realization of optical frequency shift is an important part of on-chip signal processing, and the frequency shift based on the acousto-optic effect has the advantages of high frequency shift efficiency and simple structure and is widely used. The traditional integrated acousto-optic frequency shifter technology is mainly based on the principle of acousto-optic grating Bragg diffraction. The light beam is incident at a certain angle, and after passing through the acousto-optic grating, 0th-order and +1st-order and-1st-order diffraction lights are generated. The +1st-order and-1st-order diffraction lights are emitted at a certain angle and generate positive or negative frequency shifts, respectively. This scheme can only realize one-side frequency shift, that is, the output frequency must be higher or lower than the input frequency, and cannot simultaneously meet the demand of positive and negative frequency shifts.

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

[0004] In view of the deficiencies of the prior art, the application provides an integrated positive and negative acousto-optic frequency shifter based on piezoelectric material and a preparation method thereof. The positive and negative frequency shift effect is realized by using twice acousto-optic frequency shift and mode selection.

[0005] The working principle of the application is as follows:

[0006] On the device integrated with thin film piezoelectric material, two pairs of acousto-optic frequency shifters with opposite frequency shift directions are used to convert mode 1 into mode 2, and at the same time, a fixed positive frequency shift f1 is generated, and then mode 2 is converted back to mode 1, and at the same time, an adjustable negative frequency shift f2 is generated. Then the total frequency shift is f1-f2, and by adjusting the frequency of f2 from less than f1 to greater than f1, the positive and negative frequency shifts relative to the input light frequency can be realized. Further, due to the non-ideality of mode conversion, mode filtering is needed to filter out the residual unwanted signals to obtain the final high-performance integrated positive and negative frequency shifters.

[0007] The technical solutions of the present application are as follows:

[0008] In one aspect, the present application provides an on-chip integrated positive and negative acousto-optic frequency shifter, characterized in that it comprises:

[0009] a substrate integrated with acousto-optic thin film piezoelectric material;

[0010] a first acousto-optic frequency shifter arranged on the acousto-optic thin film piezoelectric material, for converting an input light signal from mode 1 to mode 2, and generating a fixed positive frequency shift f1 in the conversion process;

[0011] a second acousto-optic frequency shifter arranged after the first acousto-optic frequency shifter, for converting the mode 2 light signal back to mode 1, and generating an adjustable negative frequency shift f2 in the conversion process, so that the total frequency shift is f1-f2, and by adjusting the frequency of f2, the continuous adjustable positive and negative frequency shifts relative to the input light frequency are realized;

[0012] a mode filter arranged between or after the first and second acousto-optic frequency shifters, for filtering out residual unwanted signals generated due to the non-ideality of mode conversion, to ensure the high performance of the output light signal.

[0013] Further, the first and second acousto-optic frequency shifters each include an optical waveguide and an interdigital transducer (IDT), the IDT is arranged on one side of the optical waveguide, for exciting acoustic waves when a microwave modulation signal is applied, and interacting with the light signal in the optical waveguide to realize mode conversion and frequency shift.

[0014] Further, the optical waveguide simultaneously supports mode 1 and mode 2, and is at an angle with the IDT, the microwave modulation signal frequency applied to the IDT of the first acousto-optic frequency shifter is f1, the microwave modulation signal frequency 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 is chirped.

[0015] Further, the application further comprises a driving module for generating a phase-locked microwave modulation signal and providing the IDT of the first and second acousto-optic frequency shifters respectively to drive the mode conversion and frequency shift.

[0016] Further, the application further comprises an input coupler for introducing the input optical signal into the chip.

[0017] Further, the application further comprises an input coupler, and the thin film piezoelectric material is lithium niobate thin film.

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

[0019] Further, the acoustic waves generated by the IDT in the first and second acousto-optic frequency shifters cause the refractive index change in the waveguide, the refractive index change, mode 1 mode field and mode 2 mode field on the waveguide cross section 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 satisfy the phase matching condition to improve the mode conversion efficiency, and 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 not vice versa.

[0020] Further, the waveguide in the first and second acousto-optic frequency shifters can be processed into a suspended waveguide, and / or a sound wave mirror is 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] Further, the first and second mode filters are based on the mode evolution principle or the adiabatic mode coupling principle.

[0022] Further, in the microwave frequency 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 thin film is x-cut lithium niobate thin film, and the thickness is between several hundred nanometers and several microns.

[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 is selected from 1GHz to 4GHz, and the tuning range of the microwave frequency f2 is ±1% to 5% on both sides of f1 as the center.

[0026] Secondly, the application further provides a method for preparing the above-mentioned on-chip integrated positive and negative acousto-optic frequency shifters, which is characterized by comprising the following steps:

[0027] S1. Integrating an acousto-optic thin film piezoelectric material on a substrate;

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

[0029] S3. Preparing an IDT electrode on one side of the optical wave of the first acousto-optic frequency shifter and the second acousto-optic frequency shifter by a deposition and stripping process;

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

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

[0032] S6. Carrying out packaging and testing.

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

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

[0035] 2) Compared with the acousto-optic frequency shifter technology based on tellurium oxide crystal proposed in the CN107065235A patent, the present application can realize arbitrary tunable positive and negative frequency shifting, and has 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 application has higher stability and robustness, and can be applied to various complex use 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 application is CMOS compatible and simplifies the preparation process.

[0038] 5) All elements of the present application are integrated on the same chip, greatly reducing the volume and weight of the device. By adjusting the frequency of the microwave modulation signal, the positive and negative frequency shifts of the optical signal can be easily realized, and the frequency shift amount is continuously adjustable. BRIEF DESCRIPTION OF DRAWINGS

[0039] Figure 1 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 Figure 1 is a schematic diagram of the energy bands that produce two mode conversions and frequency shifts by the first and second acousto-optic frequency shifters. 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 with reference to the accompanying drawings and examples, but the scope of protection 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 includes 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 consists of a waveguide section for optical signal mode 1, which is then adiabatically widened along the direction of optical signal propagation to support both mode 1 and mode 2. The input optical signal (mode 1) is introduced into the chip and prepared for subsequent mode conversion through waveguide widening.

[0046] The first acousto-optic frequency shifter 3 is a two-port device consisting of an optical waveguide that supports both mode 1 and mode 2. An interdigital transducer (IDT) is fabricated on one side of the waveguide, positioned at a predetermined angle to the waveguide. A microwave modulated signal with a fixed frequency of f1 is applied to the IDT. This microwave signal excites acoustic waves in the acousto-optic material film. The acoustic wave component along the waveguide propagates in the opposite direction to the optical signal in the waveguide. The acousto-optic effect shifts the optical signal from mode 1 to mode 2, while simultaneously increasing its frequency 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, which contains an optical waveguide supporting both mode 1 and mode 2. On one side of the waveguide, an interdigital transducer (IDT) is prepared, which is at an angle with the waveguide. The electrode period of the IDT is chirped to support microwave modulation signals in a certain frequency range. A microwave modulation signal with a frequency of f2 is applied to the IDT, which excites acoustic waves on the acousto-optic material film. The component of the acoustic waves along the direction of the waveguide propagates in the opposite direction of 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 frequencies of f1 and f2 respectively, which are in phase, and drives the IDTs in the first and second acousto-optic frequency shifters 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 IDTs in the first and second acousto-optic frequency shifters cause changes in the refractive index in the waveguide. The overlap integral of the refractive index change, the mode 1 mode field and the mode 2 mode field on the cross section of the waveguide cannot be zero. The wave vector of the acoustic waves generated by the IDTs in the first and second acousto-optic frequency shifters along the direction of the optical waveguide should satisfy the phase matching condition to improve the mode conversion efficiency. The length of the waveguide in the first and second acousto-optic frequency shifters should be moderate, so that mode 1(2) can be fully converted to mode 2(1), but not vice versa.

[0053] In the first and second acousto-optic frequency shifters, the waveguide can also be processed into a suspended waveguide, and / or a sound wave reflector can 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 principle of mode evolution or adiabatic mode coupling.

[0055] In the microwave frequencies generated by the driving module, f1 is a fixed frequency, and f2 is an adjustable frequency. 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 of several hundred nanometers to several microns.

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

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

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

[0060] S1. Integrating an acousto-optic thin film piezoelectric material on a substrate;

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

[0062] S3. Preparing the IDT electrode on one side of the light wave of the first acousto-optic frequency shifter and the second acousto-optic frequency shifter by the deposition and peeling process;

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

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

[0065] S6. Packaging and testing.

[0066] Figure 2 is a band diagram of mode conversion and frequency shift of the optical signal after passing through the first acousto-optic frequency shifter and the second acousto-optic frequency shifter. Assuming that the input optical mode is mode 1 and the frequency is f0, the direction of light propagation is opposite to the direction of sound wave. 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 total frequency shift is f1-f2, and by controlling the size of f2 relative to f1, the positive and negative frequency shift effect can be achieved.

[0067] The embodiment discloses an on-chip integrated positive and negative acousto-optic frequency shifter, which realizes the positive and negative frequency shift of the optical signal on a single chip by integrating the acousto-optic effect and the mode conversion technology. Specifically, by using the piezoelectric material layer and the optical waveguide structure, in combination with the first acousto-optic frequency shifting unit and the second acousto-optic frequency shifting unit, fixed positive frequency shift and adjustable negative frequency shift are respectively generated, and by adjusting the frequency of the negative frequency shift, the continuous adjustable frequency of the optical signal is realized. In addition, a mode filter is also arranged to filter out the residual useless signal generated due to the non-ideality of mode conversion. The present application has the advantages of compact structure, stable performance, flexible adjustment, etc., and can be widely applied in the fields of optical communication, optical information processing, optical sensing, etc., 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 by, It comprises: a substrate, on which an acousto-optic thin film piezoelectric material is integrated; a first acousto-optic frequency shifter, which is arranged on the acousto-optic thin film piezoelectric material, and is used to convert an input optical signal from mode 1 to mode 2, and generate a fixed positive frequency shift f1 during the conversion; a second acousto-optic frequency shifter, which 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, so that the total frequency shift is f1-f2, and by adjusting the frequency of f2, continuous adjustable positive frequency shift to negative frequency shift relative to the input light frequency is realized; a mode filter, which 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 unwanted signals generated due to mode conversion non-ideality, and ensure high performance of the output optical signal.

2. The on-chip integrated positive and negative acousto-optic frequency shifter of claim 1, wherein, Both the first acousto-optic frequency shifter and the second acousto-optic frequency shifter comprise 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, and interact with the optical signal in the optical waveguide to realize mode conversion and frequency shift.

3. The on-chip integrated positive and negative acousto-optic frequency shifter of claim 2, wherein, The optical waveguide supports mode 1 and mode 2 at the same time, and is at a certain angle with the IDT, the frequency of the microwave modulation signal applied on the IDT of the first acousto-optic frequency shifter is f1, the frequency of the microwave modulation signal applied on 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 is chirped.

4. The on-chip integrated positive and negative acousto-optic frequency shifter according to any one of claims 1-3, characterized in that, It also comprises a driving module, which is used to generate in-phase microwave modulation signals, and provide them 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.

5. The on-chip integrated positive and negative acousto-optic frequency shifter according to any one of claims 1-4, characterized in that, It also comprises an input coupler, which is used to introduce the input optical signal into the chip.

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

7. A method of fabricating an on-chip integrated positive and negative acousto-optic frequency shifter as claimed in any one of claims 1-5, characterized in that, It comprises the following steps: S1. Integrating an acousto-optic thin film piezoelectric material on a substrate; S2. Preparing the optical waveguide parts of the input coupler, the first acousto-optic frequency shifter, the second acousto-optic frequency shifter and the mode filter on the acousto-optic thin film piezoelectric material by etching; S3. Preparing the IDTs on one side of the optical wave of the first acousto-optic frequency shifter and the second acousto-optic frequency shifter respectively by deposition and peeling processes; S4. Preparing the mode filter between or after the first acousto-optic frequency shifter and the second acousto-optic frequency shifter as needed; S5. Assembling a driving module to generate and distribute microwave modulation signals to the IDTs of the first acousto-optic frequency shifter and the second acousto-optic frequency shifter; S6. Packaging and testing.

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