Integrated polarization-maintaining electro-optical non-reciprocal device based on mode evolution coupling filtering and preparation method thereof

Abstract

The invention discloses an integrated polarization-maintaining electro-optical non-reciprocal device based on mode evolution coupling filtering and a preparation method thereof.The device structurally comprises a single-mode waveguide, a cascade mode converter, a TE1 / TE0 mode filter and an electro-optical mode coupler, residual light interference is restrained through the synergistic effect of mode evolution and filtering, the electro-optical mode coupler adopts an x-cut lithium niobate thin film waveguide, and the x-cut lithium niobate thin film waveguide is connected with the cascade mode converter. Applying a back propagation microwave signal in combination with a traveling wave electrode, and triggering non-reciprocal mode coupling of the TE0 and the TM0; the cascade multimode interference filter effectively filters residual modes; according to the preparation method, high-precision processing is realized through ion beam etching, electron beam photoetching and electrode integration processes. Under the condition of no magneto-optical material, the forward insertion loss is 10dB, the reverse isolation degree is 20dB, the C-band broadband work is supported, the output polarization extinction ratio is greater than 20dB, and the filter has the characteristics of miniaturization, high process compatibility and excellent anti-interference performance, and is suitable for optical communication and quantum integration systems.

Description

Technical Field

[0001] The present invention belongs to the technical field of integrated electro-optic non-reciprocal devices, and particularly relates to an integrated polarization-maintaining electro-optic non-reciprocal device based on mode evolution coupled filtering and a preparation method thereof. By combining mode conversion, coupling and filtering technologies, the device realizes efficient non-reciprocal transmission characteristics in electro-optic materials, and is particularly suitable for the design and application of on-chip optical isolators. Background Art

[0002] Integrated non-reciprocal devices have many uses. A typical application is to use non-reciprocity to realize an on-chip optical isolator to block the interference of echo signals to the transmitting end, which has important research value. Traditional non-reciprocal devices mainly rely on magneto-optical crystal materials or optical nonlinear effects. Magneto-optical crystal materials are difficult to integrate and miniaturize. Optical nonlinearity is sensitive to optical power.

[0003] In recent years, the method of realizing non-reciprocity through mode coupling has become a research hotspot. Non-reciprocity between mode conversions can be realized through acousto-optic modulation, but the frequency shift amount is small and narrowband filtering is required, so the practicality is poor. The work of realizing integrated non-reciprocal devices through electro-optic modulation has also been publicly reported, but narrowband filters are often required for cooperation.

[0004] The inventor team proposed an electro-optic mode conversion non-reciprocal device in previous applications (CN202311201716.3 and CN202411144766.7). Among them, CN202311201716.3 proposed the idea of realizing mode coupling non-reciprocity through electro-optic modulation, and CN202411144766.7 further introduced the crystal birefringence characteristic to realize broadband and efficient mode coupling and non-reciprocity. However, there are still key defects: when the mode conversion efficiency is low, the residual initial optical signal will seriously interfere with the operation of the device, and when the residual initial light is also reflected, the optical isolation degree brought by non-reciprocity will be greatly reduced.

[0005] Therefore, it is necessary to study a scheme that suppresses residual initial light and maintains consistent input and output polarization under non-ideal mode conversion. Summary of the Invention

[0006] Aiming at the deficiencies of the above-mentioned prior art, the present invention provides an integrated polarization-maintaining electro-optic non-reciprocal device based on mode evolution coupled filtering and a preparation method thereof. By using the evolution and coupling effects of modes in the waveguide, the sufficient suppression of unnecessary modes is realized, and on-chip polarization-maintaining electro-optic non-reciprocity and optical isolators are realized. The structure of the present invention is simple and has broad application prospects.

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

[0008] An integrated polarization-maintaining electro-optic non-reciprocal device based on pattern evolution coupled filtering, including a chip. The upper layer of the chip is an electro-optic material thin film with birefringence characteristics, and the lower layer is a lower cladding. Its characteristics are as follows:

[0009] Along the propagation direction of the input light, it successively passes through a single-mode waveguide, a first mode converter, a TE1 mode filter, a second mode converter, an electro-optic mode coupler, a TE0 mode filter, a third mode converter, and an output mode coupler;

[0010] The single-mode waveguide only supports two modes, TE0 and TM0;

[0011] The first mode converter is a two-port device, which is a tapered waveguide with a gradually increasing width. Its narrower input end is connected to the single-mode waveguide, and its wider output end is connected to the subsequent TE1 mode filter; the function of this device is to use the mode hybridization effect to convert the TM0 mode input from the first port into the TE1 mode, while the input TE0 mode passes through without being affected;

[0012] The TE1 mode filter is a group of cascaded 1x1 multimode interferometers, whose function is to generate high loss (loss not less than 30 dB) for the TE1 mode, while having no influence on the TE0 mode (loss less than 0.5 dB);

[0013] The second mode converter is a two-port device, which is a tapered waveguide with a gradually decreasing width. Its wider input end is connected to the TE1 mode filter, and its narrower output end is connected to the electro-optic mode coupler; the function of this device is the same as that of the first mode converter. For the forward signal, that is, the input signal from which TM0 has been filtered and only TE0 remains, it has no influence, while for the reflected TM0 signal, it will be converted into TE1 and then filtered by the TE1 mode filter;

[0014] The electro-optic mode coupler is a waveguide at a certain angle with the y-axis of the birefringent crystal. The waveguide only supports TE0 and TM0, and the refractive index difference between the two is less than 1e-3; there is a microwave electrode above the waveguide and a microwave modulation signal is applied; when the propagation direction of the microwave signal is opposite to the propagation direction of the optical signal in the waveguide, mode coupling occurs, and the TE0 (TM0) mode will be partially converted into TM0 (TE0); when the microwave signal and the optical signal propagate in the same direction, no mode coupling occurs, and only modulation sidebands within the mode are generated;

[0015] The described TE0 mode filter is a set of cascaded TE0-TE1 mode couplers. Each TE0-TE1 mode coupler is formed by coupling a single-mode waveguide with a wide waveguide. The TE0 mode in the single-mode waveguide matches the refractive index of the TE1 mode in the wide waveguide, resulting in coupling. Thus, high suppression of TE0 is achieved (loss not less than 30 dB), while there is no impact on TM0 (loss less than 0.5 dB).

[0016] The described third mode converter is similar to the first mode converter. It is a tapered waveguide with a gradually increasing width. The narrow-width end is the input end (the first port), and the wide-width end is the output end (the second port). It can convert the TM0 mode into the TE1 mode.

[0017] The described output mode coupler is formed by coupling a wide waveguide with a single-mode narrow waveguide at one end. The wide waveguide is connected to the output end of the third mode converter. The TE1 mode in the wide waveguide matches the refractive index of the TE0 mode in the single-mode waveguide, resulting in coupling to achieve the conversion from TE1 to TE0 and output, and at the same time, the original TE0 mode is filtered again.

[0018] There is also a microwave source for driving the electro-optic mode coupler to generate a single-frequency microwave signal. The wave vector generated by its frequency should exactly compensate for the wave vector difference between TE0 and TM0 in the waveguide, so that when the microwave signal and the optical signal propagate in opposite directions, the most efficient mode conversion occurs.

[0019] Furthermore, the crystal axis of the electro-optic material thin film with birefringence characteristics is the z-axis, the x-axis is along the normal direction of the thin film, the thin film plane is the yz plane, and the refractive index of the crystal axis is different from that of the non-crystal axis. When the applied electric field is parallel to the crystal axis, the refractive index of the electro-optic material along the crystal axis direction will change.

[0020] Furthermore, the electro-optic material is lithium niobate thin film, x-cut.

[0021] Furthermore, the electro-optic mode coupler is an electro-optic modulation device, which includes an optical waveguide and a traveling-wave electrode around the optical waveguide. The optical waveguide is fabricated on the lithium niobate thin film, located in the yz plane, and the angle with the y-axis is θ. The angle θ is selected such that the refractive index difference between TE0 and TM0 in the waveguide at the working wavelength is less than 1e-3. The microwave traveling-wave electrode is located above the waveguide and is parallel to the waveguide. The traveling-wave electrode includes a signal electrode located above the waveguide and two ground electrodes located on both sides of the signal electrode. The low-refractive-index upper cladding material, such as silicon oxide, is between the traveling-wave electrode and the waveguide. The electric field direction in the traveling-wave electrode is distributed along the cross-section of the optical waveguide and overlaps with the optical field in the waveguide.

[0022] Furthermore, the propagation direction of the optical signal in the first mode converter, the second mode converter, and the third mode converter is along the z-axis.

[0023] The working principle and process of the present invention are as follows:

[0024] First, the input signal is input from a single-mode waveguide, with the modes being TE0 and a small amount of TM0; after passing through the first mode converter, TE0 is not affected, and TM0 is converted to TE1; after passing through the TE1 mode filter, TE0 passes through without being affected, and TE1 is fully filtered out (loss is not less than 30dB), so that the TM0 mode in the input signal is suppressed;

[0025] Subsequently, the TE0 signal passes through the second mode converter and enters the electro-optical mode coupler without being affected. Since the microwave signal propagates in opposite directions to the optical signal, mode coupling occurs, and part of the TE0 is converted to TM0 (e.g., 10%) and leaves the electro-optical mode coupler together with the remaining TE0 (e.g., 90%).

[0026] Next, the optical signal enters the TE0 mode filter, TM0 is not affected, and TE0 is fully filtered out (loss is not less than 30dB), so that the residual TE0 after the non-ideal electro-optical mode coupling is suppressed;

[0027] Finally, the TM0 signal enters the third mode converter, is converted into the TE1 mode, and is converted into the TE0 mode output through the output mode coupler, thereby achieving a polarization-maintaining output that fully suppresses the residual original light.

[0028] When the output light returns due to external reflection, it is assumed that it contains both TE0 and TM0 modes. The input TE0 mode is converted to TM0 through the output mode coupler and the third mode converter in a low-loss manner, while the input TM0 mode will experience a loss (such as 10-15dB) at the output mode coupler and be converted to TM0 through the third mode converter with low loss. Therefore, only the TM0 mode enters the electro-optical mode coupler. At this time, since the microwave signal and the optical signal propagate in the same direction, no mode coupling occurs, but two sidebands of the TM0 mode are generated due to the non-ideality of the device. Then the TM0 mode enters the second mode converter and becomes TE1, and is finally suppressed by the TE1 mode filter (loss is not less than 30dB).

[0029] This shows that the present invention has the characteristics of non-reciprocal transmission and can be used as a polarization-maintaining optical isolator in TE0 mode. Among them, during forward propagation, the insertion loss of TE0 is about 10dB, which is caused by the -10dB conversion efficiency of the electro-optical mode coupler. During reverse propagation, the insertion loss of TE0 is about 30dB, which is generated by the TE1 mode filter, and the insertion loss of TM0 is about 45dB, including the 30dB insertion loss of the TE1 mode filter and the 15dB insertion loss of the output coupler. That is, the present invention has an isolation of about 20dB, which is comparable to the performance of existing commercial isolators based on magneto-optical crystal materials, and supports broadband operation at the same time.

[0030] The present invention also provides a method for fabricating a non-magnetic integrated polarization-maintaining electro-optic isolator based on mode evolution coupled filtering, which is characterized in that a mode converter is fabricated on an electro-optic material thin film, i.e., an x-cut lithium niobate thin film; the above device fabricates an optical waveguide by etching the lithium niobate thin film, fabricates an upper cladding by deposition, and fabricates a traveling-wave electrode by deposition and lift-off. The traveling-wave electrode includes a signal electrode and a ground electrode. The signal electrode is located above the optical waveguide and parallel to the waveguide, and the ground electrodes are located on both sides parallel to the signal electrode. The signal electrode and the ground electrode generate an electric field along the cross-sectional direction of the waveguide and passing through the waveguide.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0032] 1) By combining three technologies of mode evolution (tapered waveguide), electro-optic coupling (traveling-wave electrode), and cascaded filtering (MMI), the present invention can still ensure high isolation and pure output even when the electro-optic mode conversion efficiency is not high. Compared with acousto-optic devices and other electro-optic devices (such as Yu, et al., Nature Photonics 17, 666-671 (2023)), it can support broadband operation.

[0033] 2) Compared with the electro-optic mode coupling schemes proposed in the inventor's prior applications CN202311201716.3 and CN202411144766.7, the present invention fully considers the implementation method for the device to achieve high isolation and pure signal output under non-ideal mode coupling. The process of the present invention is insensitive and has the advantages of polarization-maintaining, broadband, and can suppress the reflected signals of two polarizations. Residual light is suppressed through multi-stage filtering design.

[0034] 3) Abandoning the dependence on magneto-optical crystals and realizing non-reciprocity by electro-optic modulation and mode coupling provides a new paradigm for silicon-based photon integration. Brief Description of the Drawings

[0035] Figure 1 It is a schematic diagram of the integrated polarization-maintaining electro-optic non-reciprocal device based on mode evolution coupled filtering of the present invention, and a typical cross-sectional design of the mode converter is also given (Illustration: AA' cross-section example). In the figure: 1-chip, 2-single-mode waveguide, 3-first mode converter, 4-TE1 mode filter, 5-second mode converter, 6-electro-optic mode coupler, 7-TE0 mode filter, 8-third mode converter, 9-output mode coupler, 10-microwave source.

[0036] Figure 2 It is a schematic diagram of the effect comparison, where (a) is the prior application and (b) is this embodiment. Detailed Embodiments

[0037] The present invention will be further described below in conjunction with the accompanying drawings and embodiments, but the protection scope of the present invention should not be limited thereby. The embodiments of the present invention include but are not limited to the following examples.

[0038] First, please refer to Figure 1 , Figure 1 Figure 1 is a schematic diagram of an integrated polarization-maintaining electro-optic non-reciprocal device based on mode evolution coupled filtering. As shown in the figure, the integrated polarization-maintaining electro-optic non-reciprocal device based on mode evolution coupled filtering includes a chip 1, with an electro-optic material thin film having birefringence characteristics on its upper layer and a lower cladding on its lower layer; a single-mode waveguide (2), a first mode converter (3), a TE1 mode filter (4), a second mode converter (5), an electro-optic mode coupler (6), a TE0 mode filter (7), a third mode converter (8), and an output mode coupler (9) connected in sequence along the input light propagation direction.

[0039] Single-mode waveguide 2: Supports TE0 and TM0 modes and is formed by etching a lithium niobate thin film.

[0040] First mode converter 3: Is a two-port device, a tapered waveguide with a gradually increasing width. Its narrower input end is connected to the single-mode waveguide, and its wider output end is connected to the subsequent TE1 mode filter; the function of this device is to use the mode hybridization effect to convert the TM0 mode input from the first port into the TE1 mode, while the input TE0 mode passes through unaffected.

[0041] TE1 mode filter 4: Consists of a group of cascaded 1x1 multimode interferometers, which generates high loss (loss not less than 30 dB) for the TE1 mode and has no effect on the TE0 mode (loss less than 0.5 dB);

[0042] Second mode converter 5: Is a two-port device, symmetrically designed with the first mode converter 3. Its wider input end is connected to the TE1 mode filter, and its narrower output end is connected to the electro-optic mode coupler; the function of this device is the same as that of the first mode converter, having no effect on the forward signal, that is, the input signal with TM0 filtered out and only TE0 remaining, while for the reflected TM0 signal, it will be converted into TE1 and then filtered out by the TE1 mode filter;

[0043] Electro-optic mode coupler 6 is a waveguide at a certain angle with the y-axis of the birefringent crystal. The waveguide only supports TE0 and TM0, and the refractive index difference between the two is less than 1e-3; there is a microwave electrode above the waveguide and a microwave modulation signal is applied; when the propagation direction of the microwave signal is opposite to the propagation direction of the optical signal in the waveguide, mode coupling occurs, and the TE0 (TM0) mode will be partially converted into TM0 (TE0); when the microwave signal and the optical signal propagate in the same direction, no mode coupling occurs, and only modulation sidebands within the mode are generated;

[0044] The TE0 mode filter 7 is composed of a set of cascaded TE0-TE1 mode couplers. Each TE0-TE1 mode coupler is formed by coupling a single-mode waveguide with a wide waveguide. The TE0 mode in the single-mode waveguide matches the refractive index of the TE1 mode in the wide waveguide, resulting in coupling. Thus, high suppression of TE0 (loss not less than 30 dB) is achieved, while there is no impact on TM0 (loss less than 0.5 dB).

[0045] The third mode converter 8 is similar to the first mode converter. It is a tapered waveguide with a gradually increasing width. The narrow-width end is the input end (the first port), and the wide-width end is the output end (the second port). It can convert the TM0 mode into the TE1 mode.

[0046] The output mode coupler 9 is formed by coupling a wide waveguide with one end of a single-mode narrow waveguide. The wide waveguide is connected to the output end of the third mode converter. The TE1 mode in the wide waveguide matches the refractive index of the TE0 mode in the single-mode waveguide, resulting in coupling, realizing the conversion from TE1 to TE0 and output, and filtering the original TE0 mode again.

[0047] The preparation process flow is as follows:

[0048] Substrate processing: Bond an x-cut lithium niobate thin film on the SiO / Si substrate.

[0049] Lithography and etching: Use electron beam lithography to define the geometries of the tapered waveguide, filter, and coupler. Complete the device etching through argon ion physical etching, and control the sidewall roughness ≤ 2 nm.

[0050] Cladding deposition: Deposit a 1.0 ± 0.1 μm thick silicon oxide upper cladding on the waveguide and device surface by plasma-enhanced chemical vapor deposition, and make the surface roughness ≤ 1 nm through chemical mechanical polishing.

[0051] Electrode preparation: Prepare gold electrodes above the waveguide through a lift-off process. The signal electrode is parallel to the waveguide, and the distance between the signal electrode and the ground electrodes on both sides is 6 - 30 μm.

[0052] Figure 2It is a schematic diagram of the effect comparison. Among them, (a) is the output spectrum without mode filtering and only with an electro-optic mode coupler, that is, the solutions in the prior applications CN202311201716.3 and CN202411144766.7. In the spectrum, the peak with the highest power in the middle is the input TE0 signal, the left sideband is the TM0 signal after mode coupling, and the right sideband is the sideband generated after non-ideal mode coupling, which contains both TE0 and TM0 components, and the TE0 component has a higher power. (b) is the output spectrum including mode filtering. It can be seen that the power of the useful TM0 signal on the left remains almost unchanged, while the power of the TE0 signal in the middle is fully suppressed, and the TE0 part in the right sideband where TE0 and TM0 are mixed is fully suppressed, leaving only the TM0 part, whose power is more than 15 dB lower than the power of the useful TM0 signal on the left.

Claims

1. An integrated polarization-maintaining electro-optical non-reciprocal device based on mode evolution coupled filtering, characterized in that: include: The chip (1) is composed of a lower cladding layer and an upper electro-optical material film, wherein the electro-optical material film has a birefringence characteristic, and its crystal axis direction is along the light propagation direction (z axis); The following functional modules are connected in sequence along the light propagation direction: Single-mode waveguide (2), supporting TE0 and TM0 modes; The first mode converter (3) is a tapered waveguide with gradually increasing width, the width of the input end matches the single-mode waveguide, and the width of the output end matches the TE1 mode filter, and is used to convert the TM0 mode into the TE1 mode; A TE1 mode filter (4), which is composed of a cascaded multi-mode interferometer, and has a loss of 30 dB or more for the TE1 mode and a loss of 0.5 dB or less for the TE0 mode; The second mode converter (5) is symmetrical with the first mode converter (3), and is a tapered waveguide with a gradually decreasing width, the width of the input end matches the TE1 mode filter (4), and the width of the output end is restored to the size of the single-mode waveguide (2); An electro-optical mode coupler (6) comprises a waveguide and a traveling wave electrode which are at an angle of 25° to 35° with the crystal axis y, and triggers the non-reciprocal mode coupling between TE0 and TM0 through a reversely propagating microwave signal; A TE0 mode filter (7), which is composed of cascaded TE0-TE1 mode couplers and has a loss of TE0 mode ≥ 30 dB; The third mode converter (8) is a tapered waveguide with gradually increasing width, the width of the input end matches the single-mode waveguide, and the width of the output end matches the output mode coupler (9), and is used to convert the TM0 mode into the TE1 mode and filter the original TE0 mode again; An output mode coupler (9) is formed by coupling a wide waveguide with a single-mode narrow waveguide, and achieves conversion output from TE1 to TE0 by matching the refractive index of the wide waveguide with the single-mode narrow waveguide; and The microwave source (10) is used to output a modulated signal that satisfies a phase matching condition when propagating in the reverse direction with the optical signal.

2. The integrated polarization-maintaining electro-optical non-reciprocal device based on mode evolution coupled filtering according to claim 1, characterized in that: The electro-optical material film is x-cut lithium niobate, with a thickness of not less than 600nm, a waveguide etching depth of 300-500nm, and an etching edge roughness of ≤2nm.

3. The integrated polarization-maintaining electro-optical non-reciprocal device based on mode evolution coupled filtering according to claim 1, characterized in that: The traveling wave electrode includes a signal electrode and ground electrodes on both sides. The signal electrode is located directly above the waveguide. The electrode material is gold and the thickness is not less than 600nm. The wave vector formed by the electrical signal propagating in the traveling wave electrode should just make up for the wave vector difference between the TE0 and TM0 modes, thereby achieving efficient mode conversion.

4. A method for preparing a device as claimed in any one of claims 1 to 3, characterized in that: The following steps are involved: Substrate processing: bonding x-cut lithium niobate thin films on SiO / Si substrates; Lithography and Etching: Use electron beam lithography to define the geometry of tapered waveguides, filters, and couplers, and use argon ion physical etching to complete device etching and control the sidewall roughness to ≤2nm; Cladding layer deposition: Plasma-enhanced chemical vapor deposition is used to deposit a 1.0±0.1μm thick silicon oxide cladding on the surface of the waveguide and device, and chemical mechanical polishing is used to make the surface roughness ≤1nm; Electrode preparation: A gold electrode is prepared on top of the waveguide by a lift-off process. The signal electrode is parallel to the waveguide, and the distance between the signal electrode and the ground electrodes on both sides is 6 to 30 μm.

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