Integrated electro-optical non-reciprocal mode converter based on birefringent crystal refractive index matching and preparation method thereof

Abstract

An integrated electro-optical non-reciprocal mode converter based on birefringent crystal refractive index matching comprises a chip, the upper layer of the chip is an electro-optical material film with birefringent characteristics, a mode converter is prepared on the electro-optical material film, and the mode converter is a two-port device; the first port is used for receiving an input optical signal of a mode 1 and allowing the optical signal to enter the mode converter; and the second port is used for outputting the converted optical signal in the mode 2. The refractive index difference of a birefringent material to a transverse electric mode (TE) and a transverse magnetic mode (TM) is utilized to realize the pre-coarse matching of the refractive indexes of TE and TM modes, and then the fine matching of the refractive indexes and wave vectors is realized in combination with electro-optical modulation, so that the integrated electro-optical mode converter insensitive to the process is realized. The device is simple in structure and has a wide application prospect.

Description

Technical Field

[0001] This invention relates to an integrated electro-optic non-reciprocal mode converter, and more particularly to a broadband integrated electro-optic non-reciprocal mode converter based on refractive index matching of a birefringent crystal and its fabrication method. Background Technology

[0002] Implementing mode conversion is a crucial part of mode manipulation. Non-reciprocal mode conversion can achieve unidirectional mode conversion, so it can be used not only for mode reuse but also for applications such as optical isolation, and has significant research value.

[0003] Traditional mode converters utilize refractive index matching between coupled waveguides, which cannot achieve non-dissimilarity.

[0004] Acousto-optic modulation can achieve non-reciprocity between mode transitions, but the frequency shift of acousto-optic modulation is small and usually requires the assistance of resonant devices, making it impractical.

[0005] Electro-optic modulation can also achieve mode coupling, but it is difficult to implement. The inventors proposed an electro-optic mode conversion device (CN117233986A) in their prior application, but this device is very sensitive to process errors, which limits its performance.

[0006] Therefore, there is a need to study a process-insensitive integrated electro-optic mode converter. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides an integrated electro-optic non-reciprocal mode converter based on refractive index matching of a birefringent crystal and its fabrication method. It utilizes the refractive index difference between the transverse electric mode (TE) and the transverse magnetic mode (TM) of the birefringent material to achieve pre-coarse matching of the refractive indices of the TE and TM modes. This is then combined with electro-optic modulation to achieve fine matching of the refractive index and wave vector, thereby realizing a process-insensitive integrated electro-optic mode converter. This invention has a simple structure and broad application prospects.

[0008] Working principle of the invention:

[0009] By utilizing the refractive index difference between TE and TM modes in a birefringent crystal material, and by selecting an appropriate waveguide orientation, a coarse pre-matching of the refractive indices of TE and TM is achieved. Then, electro-optic modulation is used to achieve a fine match between their refractive indices and wave vectors. Combined with optimized mode field overlap, a process-insensitive broadband electro-optic mode converter is realized. In this invention, it is assumed that the input optical signal is mode 1, and the waveguide orientation is in a specific direction relative to the crystal axis, making the refractive index difference between mode 1 and mode 2 in the waveguide extremely small (typically less than 1e-3). When the optical signal propagates from the input to the output, it is opposite to the propagation direction of the applied modulation electrical signal, resulting in inter-mode coupling and a conversion from mode 1 to mode 2. However, if the signal light is reflected at the output, the propagation direction of the reflected light in the waveguide is consistent with the propagation direction of the modulation electrical signal, resulting in intra-mode coupling and no mode conversion. The electrodes of the electrical signal should be optimized to ensure good overlap between the refractive index distribution generated by the electric field in the waveguide through the electro-optic effect and the two optical modes. This scheme utilizes the fact that when an optical signal propagates in a birefringent crystal, its refractive index is related to both the propagation direction and the polarization direction. The pre-coarse matching of the refractive indices between the two optical modes has a greater tolerance for process errors, thus enabling a process-insensitive electro-optic mode converter.

[0010] The technical solution of the present invention is as follows:

[0011] An integrated electro-optic non-reciprocal mode converter based on refractive index matching of a birefringent crystal includes a chip, wherein the upper layer of the chip is an electro-optic material thin film with birefringent properties, and the lower layer is a lower cladding layer, characterized in that:

[0012] A mode converter is fabricated on the electro-optic material thin film, and the mode converter is a two-port device.

[0013] The first port of the mode converter is used to receive the input optical signal of mode 1 and allow the optical signal to enter the mode converter.

[0014] The mode converter also includes an input path for a microwave signal opposite to the direction of optical signal propagation, used to receive microwave signals from an off-chip microwave signal source, and to convert the optical signal from mode 1 to mode 2 by utilizing electro-optic modulation effect and inter-mode coupling.

[0015] The second port of the mode converter is used to output the converted mode 2 optical signal;

[0016] When the output optical signal of mode 2 is reflected and re-enters the mode converter, since the optical signal and the microwave signal propagate in the same direction, intra-mode coupling occurs, keeping the optical signal mode unchanged and generating modulation sidebands, thereby realizing the mutuality of unidirectional mode conversion.

[0017] Furthermore, the crystal axis of the birefringent electro-optic material thin film is the z-axis, the x-axis is along the normal direction of the film, the film plane is the yz plane, and the refractive index of the crystal axis is different from that of the amorphous 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. This mechanism is usually achieved through the electro-optic coefficient r33.

[0018] Furthermore, the mode converter is an electro-optic modulation device comprising an optical waveguide and traveling-wave electrodes surrounding the waveguide. The optical waveguide is fabricated on an electro-optic material thin film, located in the yz plane, and makes an angle θ with the y-axis. The angle θ is chosen such that the refractive index difference between mode 1 and mode 2 at the operating wavelength in the waveguide is less than 1e-3. The microwave traveling-wave electrodes are located above the waveguide and parallel to it. The traveling-wave electrodes include a signal electrode located above the waveguide and two ground electrodes located on either side of the signal electrode. Between the traveling-wave electrodes and the waveguide is a low-refractive-index cladding material, such as silicon oxide. The electric field direction in the traveling-wave electrodes is distributed along the cross-section of the optical waveguide and is odd-symmetric, overlapping with the optical field in the waveguide. The propagation speed of the electrical signal in the traveling-wave electrodes should match the group velocity of the optical signal in the waveguide, with a deviation of less than 20%. The electrical signal propagates backward from the output waveguide side to the input waveguide side in the mode converter. When the optical signal and the electrical signal propagate in opposite directions, the optical signal will undergo intermode coupling, changing from mode 1 to mode 2, or from mode 2 to mode 1. When the optical signal and the electrical signal propagate in the same direction, the optical signal will undergo intramode coupling, without mode conversion, but instead generate two sidebands on both sides of the optical frequency. The interval between the sidebands and the optical frequency is equal to the frequency of the electrical signal.

[0019] The microwave signal source generates a single-frequency microwave signal. The microwave frequency should be such that when the optical signal in the mode converter propagates in the opposite direction to the microwave signal, its mode is converted most efficiently.

[0020] Optical signals exist in two modes in the aforementioned devices: Mode 1 and Mode 2. A mode refers to a transverse mode in the waveguide, such as TE0, TM0, or TE1. The input and output waveguides of all the aforementioned devices should simultaneously support both Mode 1 and Mode 2.

[0021] Preferably, the birefringent electro-optic material film is an x-cut lithium niobate film with a thickness between several hundred nanometers and several micrometers.

[0022] Preferably, the optical signal has mode 1 as TE0 and mode 2 as TM0. The input optical signal is mode 1.

[0023] Preferably, the impedance of the traveling wave electrode in the mode converter is 50 ohms, and the output impedance of the microwave signal source is also 50 ohms. The microwave signal source and the traveling wave electrode are connected by wire bonding, and the wires should be as short as possible to reduce microwave loss.

[0024] Preferably, the output optical signal of the present invention will undergo frequency shifting, and the frequency shifting frequency is equal to the modulation frequency of the microwave signal source. An inverse frequency shifter can be added after the output of the present invention to cancel the frequency shifting effect.

[0025] The core of this invention is to achieve coarse matching of the refractive indices of Mode 1 and Mode 2 (refractive index difference < 1e-3) using the polarization- and propagation-direction-dependent refractive indices of a birefringent electro-optic crystal material. Then, fine matching of the refractive index and wave vector is achieved using a microwave signal propagating in the opposite direction to the signal light, ultimately realizing broadband and non-reciprocal mode switching of the optical signal. First, since the optical waveguide is at an angle θ to the y-axis, for a specific operating wavelength, a specific angle θ can always be found such that the refractive indices of the TE and TM modes are equal. Furthermore, considering that the refractive index of a mode only changes slowly with wavelength, a small refractive index difference between the TE and TM modes can be achieved over a relatively wide wavelength range. Therefore, this invention possesses broadband mode coupling characteristics. Second, considering the electric field distribution of the traveling wave electrode on the waveguide cross-section, the horizontal component of the electric field exhibits a horizontally odd-symmetric distribution on the waveguide cross-section (equal amplitude, opposite direction). Therefore, the refractive index variation in the horizontal direction also exhibits odd symmetry. Considering that the electric fields of the TE and TM modes are even-symmetric and odd-symmetric respectively in the horizontal direction of the waveguide cross-section, the overlap integral of the three modes is not zero, enabling efficient mode coupling. Finally, during device fabrication, process errors mainly manifest as deviations in the dimensions of the waveguide cross-section, such as waveguide width, waveguide height, and sidewall tilt angle. However, high fabrication accuracy can be maintained for the angle between the waveguide and the y-axis. Therefore, this invention is not sensitive to process errors.

[0026] This invention also provides a method for fabricating a non-magnetic integrated electro-optic isolator based on mode coupling. The method is characterized by fabricating a mode converter on an electro-optic material thin film, specifically an x-cut lithium niobate thin film. The device is fabricated by etching the lithium niobate thin film to create an optical waveguide, depositing an upper cladding layer, and depositing and stripping traveling-wave electrodes. The traveling-wave electrodes include a signal electrode and a ground electrode. The signal electrode is located above and parallel to the optical waveguide, while the ground electrode is 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 it.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0028] 1) By utilizing the polarization- and propagation-direction-dependent refractive index characteristics of birefringent crystal materials, coarse matching of the refractive indices for TE and TM modes was achieved, providing a new technical approach for broadband mode conversion. Simultaneously, using a microwave signal propagating in the opposite direction to the optical signal to achieve fine matching of the refractive index and wave vector not only improves the efficiency of mode conversion but also endows the mode converter with non-reciprocity, solving the problem of reflected light interference in traditional mode converters. Optimizing the design of the traveling wave electrodes enabled efficient overlap of the electric and optical fields, thereby improving the efficiency of mode coupling. Furthermore, utilizing the symmetry of the electric field distributions in TE and TM modes further enhanced the mode coupling effect. Coarse refractive index matching was achieved by precisely controlling the angle between the waveguide and the y-axis, reducing the requirements for processing precision and improving the yield and reliability of the device.

[0029] 2) Compared with the electro-optic mode conversion scheme proposed in CN117233986A, the coarse matching of the mode refractive index in this invention is mainly achieved by adjusting the angle between the waveguide and the y-axis. It is process-insensitive and inherently possesses broadband characteristics. Mode conversion can occur continuously as the optical signal propagates within the device, offering advantages such as process insensitivity, large operating bandwidth (tens to hundreds of nm), and high conversion efficiency. Unlike CN117233986A, it does not require a gradual change in waveguide width to achieve broadband operation. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the integrated electro-optic non-reciprocal mode converter based on birefringent crystal refractive index matching of the present invention, which also shows two cross-sectional designs of the mode converter (Example 1 and Example 2 of AA' section in the figure).

[0031] Figure 2 This is a schematic diagram of the energy bands of mode coupling in a mode converter. The horizontal axis represents the wave vector k, where a wave vector greater than 0 indicates forward propagation (right quadrant in the diagram) and less than 0 indicates backward propagation (left quadrant in the diagram). The vertical axis represents the frequency.

[0032] Figure 3 This is a schematic diagram illustrating mode refractive index matching achieved through the angle with the y-axis;

[0033] In the diagram: 1-Chip; 2-Mode converter; 201-First port; 202-Second port; 3-Microwave signal source. Detailed Implementation

[0034] The present invention will be further described below with reference to the accompanying drawings and embodiments, but this should not be construed as limiting the scope of protection of the present invention. Embodiments of the present invention include, but are not limited to, the following embodiments.

[0035] Please refer to Figure 1 , Figure 1This is a schematic diagram of the integrated electro-optic non-reciprocal mode converter based on refractive index matching of birefringent crystal of the present invention. As shown in the figure, a broadband integrated electro-optic non-reciprocal mode converter based on refractive index matching of birefringent crystal includes a chip 1, the upper layer of the chip is an electro-optic material thin film with birefringent properties, and the lower layer of the chip is a lower cladding layer. The feature is that a mode converter 2 is fabricated on the electro-optic material thin film. The mode converter 2 is a two-port device. The input optical signal of mode 1 enters the mode converter 2 through the first port 201. An external microwave signal source 3 is coupled into the mode converter 2 and propagates in the mode converter in the opposite direction to the optical signal. By utilizing inter-mode coupling, the optical signal is converted from mode 1 to mode 2. The optical signal of mode 2 is output through the second port 202 of the mode converter 2. If the optical signal of mode 2 output by the chip is reflected, the reflected optical signal of mode 2 enters the mode converter 2 through the second port 202 of the mode converter 2. The optical signal of mode 2 and the electrical signal propagate in the same direction, resulting in intra-mode coupling, that is, generating two modulation sidebands without changing the mode. Therefore, the present invention has the effect of unidirectional mode conversion, that is, it has the dissimilarity of mode coupling.

[0036] Figure 1 Examples 1 and 2 of the AA' section illustrate two different arrangements of ground electrodes, both of which can produce an odd-symmetric electric field distribution in the horizontal direction of the waveguide cross section.

[0037] Figure 2 This is a schematic diagram of the energy bands for mode coupling in a mode converter. The microwave signal has a frequency of Ω and propagates in the reverse direction. When the optical signal of mode 1, with a frequency of ω1, propagates in the forward direction (right quadrant in the diagram), mode coupling occurs, converting it into an optical signal of mode 2, with a frequency of ω2 = ω1 + Ω. The other frequency conversion, ω1 - Ω, cannot occur because there is no corresponding mode. At this point, assuming some light is reflected at the output, the optical signal of mode 2, with a frequency of ω2 = ω1 + Ω, propagates in the reverse direction into the mode converter (left quadrant in the diagram), in the same direction as the microwave signal propagation, resulting in an intramode conversion and generating two sidebands, ω1 and ω3 = ω1 + 2Ω, both of which are mode 2.

[0038] Figure 3 This is a schematic diagram illustrating mode refractive index matching achieved through the angle between the waveguide and the y-axis. In x-cut lithium niobate, the refractive index changes of the TE0 and TM0 modes under different waveguide angles with the y-axis reveal an intersection point in the refractive index curves. This device is designed to operate at the y-axis angle represented by this intersection point.

[0039] Experiments show that this invention achieves coarse refractive index matching (refractive index difference less than 1e-3) between TE and TM modes at the operating wavelength by precisely designing the angle θ between the optical waveguide and the y-axis. Since the refractive index of the modes changes slowly with wavelength, this design allows the mode converter to maintain high mode conversion efficiency over a wide wavelength range, thus exhibiting excellent broadband characteristics. Unidirectional mode conversion of the optical signal is achieved using a microwave signal propagating in the opposite direction to the optical signal. When the reflected optical signal re-enters the mode converter, intra-mode coupling occurs because the optical signal and microwave signal propagate in the same direction, resulting in modulation sidebands instead of mode conversion. This characteristic effectively prevents interference from reflected light, improving system stability and reliability. During device fabrication, process errors mainly affect the cross-sectional dimensions of the waveguide, while the control precision of the angle between the waveguide and the y-axis is high. Since the mode converter mainly relies on the angle between the waveguide and the y-axis to achieve coarse refractive index matching, it has a high tolerance for process errors, reducing fabrication difficulty and cost. The design of the traveling-wave electrode results in a horizontally odd-symmetric electric field distribution across the waveguide cross-section, matching the even and odd-symmetric characteristics of the electric fields in TE and TM modes, thereby improving mode coupling efficiency. Furthermore, the group velocity matching design between the electrical and optical signals further enhances the mode conversion effect. This invention integrates the mode converter onto a chip, using an x-cut lithium niobate thin film as the electro-optic material. Combined with structures such as the cladding, optical waveguide, and traveling-wave electrode, a highly integrated electro-optic non-reciprocal mode converter is achieved, reducing the device's size and weight while improving system integration and scalability.

Claims

1. An integrated electro-optic non-reciprocal mode converter based on refractive index matching of a birefringent crystal, comprising a chip, wherein the upper layer of the chip is an electro-optic material thin film with birefringent properties, and the lower layer is a lower cladding layer, characterized in that: A mode converter is fabricated on the electro-optic material thin film, and the mode converter is a two-port device. The first port of the mode converter is used to receive the input optical signal of mode 1 and allow the optical signal to enter the mode converter. The mode converter also includes an input path for a microwave signal opposite to the direction of optical signal propagation, used to receive microwave signals from an off-chip microwave signal source, and to convert the optical signal from mode 1 to mode 2 by utilizing electro-optic modulation effect and inter-mode coupling. The second port of the mode converter is used to output the converted mode 2 optical signal; When the output optical signal of mode 2 is reflected and re-enters the mode converter, since the optical signal and the microwave signal propagate in the same direction, intra-mode coupling occurs, keeping the optical signal mode unchanged and generating modulation sidebands, thereby realizing the mutuality of unidirectional mode conversion.

2. The integrated electro-optic non-reciprocal mode converter according to claim 1, characterized in that: The crystal axis of the birefringent electro-optic material thin film is the z-axis, the x-axis is along the normal direction of the film, the film plane is the yz plane, and the refractive index of the crystal axis is different from that of the amorphous 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 is changed through the electro-optic coefficient r33.

3. The integrated electro-optic non-reciprocal mode converter according to claim 1, characterized in that: The mode converter is an electro-optic modulation device, comprising an optical waveguide and traveling-wave electrodes around the optical waveguide; the optical waveguide is located in the yz plane and makes an angle θ with the y-axis, the angle θ being selected such that the refractive index difference between mode 1 and mode 2 at the operating wavelength is less than 1e-3; the traveling-wave electrodes are located above the waveguide and parallel to the waveguide, and include a signal electrode located above the waveguide and two ground electrodes located on both sides of the signal electrode.

4. The integrated electro-optic non-reciprocal mode converter according to claim 3, characterized in that: The electric field direction in the traveling wave electrode is distributed along the cross-section of the optical waveguide and is odd symmetrical, overlapping with the optical field in the waveguide; the propagation speed of the electrical signal in the traveling wave electrode should match the group velocity of the optical signal in the waveguide, with a deviation of less than 20%.

5. The integrated electro-optic non-reciprocal mode converter according to claim 1, characterized in that: The microwave signal source generates a single-frequency microwave signal, the frequency of which should be such that the mode conversion efficiency of the optical signal is highest when the optical signal and the microwave signal propagate in opposite directions.

6. The integrated electro-optic non-reciprocal mode converter according to claim 1, characterized in that: The electro-optic material film with birefringence is an x-cut lithium niobate film with a thickness ranging from several hundred nanometers to several micrometers.

7. The integrated electro-optic non-reciprocal mode converter according to any one of claims 1-6, characterized in that: The optical signal has mode 1 as TE0, mode 2 as TM0, and the input optical signal is mode 1.

8. The integrated electro-optic non-reciprocal mode converter according to any one of claims 1-6, characterized in that: It also includes an inverting frequency shifter at the output end to counteract the frequency shifting effect of the optical signal caused by microwave signal modulation.

9. A method for fabricating an integrated electro-optic non-reciprocal mode converter, characterized in that, Includes the following steps: An electro-optic material thin film is provided as a substrate, and an optical waveguide is fabricated on the electro-optic material thin film by etching to guide the transmission of optical signals therein; An upper cladding is deposited above and around the optical waveguide to protect the waveguide and adjust its optical properties. On the upper cladding, a traveling wave electrode is fabricated by deposition and stripping processes. 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 extension direction of the optical waveguide to ensure that the electric field modulates the optical signal in the optical waveguide. The ground electrode is located on both sides parallel to the signal electrode and together with the signal electrode generates an electric field along the cross-sectional direction of the optical waveguide and at least partially through the optical waveguide. This electric field uses the electro-optic effect to induce mode coupling of the optical signal in the optical waveguide, thereby achieving the function of optical signal isolation.

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