Non-magnetic integrated electro-optical isolator based on mode coupling

Abstract

A non-magnetic integrated electro-optical isolator based on mode coupling comprises a chip with an electro-optical thin film on the upper layer and a lower cladding layer on the lower layer. A first mode combiner, a first mode absorber, a mode modulator, a second mode combiner, a second mode absorber, and a mode converter are fabricated on the electro-optical thin film. The present invention utilizes the non-reciprocal transmission of optical signals in the mode modulator, combined with mode filtering, to achieve a broadband electro-optical isolator.

Description

Technical Field

[0001] The present invention relates to a broadband integrated electro-optical isolator, in particular to a non-magnetic integrated electro-optical isolator based on mode coupling. Background Art

[0002] Compared with discrete optical systems, integrated photonics has the advantages of small size, low cost and high reliability, and has attracted widespread attention. The optical isolator in the general discrete optical system is realized by the magneto-optical effect, which requires a magnet to provide an external magnetic field and is relatively large in size. Therefore, an alternative solution needs to be found on the integrated photonic chip. The technical principles currently used include electro-optic effect, acousto-optic effect, optical nonlinearity, etc. Among them, optical nonlinearity has requirements on the power of the input light and has limited applicable scenarios. The existing on-chip integrated optical isolators based on electro-optic and acousto-optic effects are usually narrow-band devices and need to be combined with spectral filtering to truly realize the function of the isolator (such as the paper Yu, Nature Photonics 17, 666-671 (2023); Sohn, Nature Photonics 15, 822-827 (2021)), and cannot be applied to broadband optical signal systems, such as dense wavelength division multiplexing systems or broadband optical communication systems. Therefore, it is necessary to study an integrated optical isolator that is non-magnetic and supports the passage of broadband optical signals. Summary of the Invention

[0003] To address the shortcomings of the aforementioned prior art, the present invention provides a non-magnetic integrated electro-optical isolator based on mode coupling. This device achieves mode coupling related to propagation direction through electro-optical modulation, and combines this with mode filtering to realize an optical isolator. The present invention has a simple structure and broad application prospects.

[0004] Working principle of the present invention:

[0005] By utilizing the non-reciprocal transmission of optical signals in the mode modulator and combining it with mode filtering, a broadband electro-optical isolator is realized. In the mode modulator, assuming that the input optical signal is mode 1, when the optical signal propagates from the input end to the output end, it is opposite to the propagation direction of the electrical signal, and the optical signal generates ModeThe optical signal is coupled from mode 1 to mode 2, and then output to the second port through the second mode combiner, and then converted back to mode 1 as the final output through the mode converter. The residual optical signal that has not been converted to mode 2 (that is, it is still mode 1) is output to the first port through the second mode combiner and absorbed by the second mode absorber. Assuming that the final output is reflected, the reflected light (mode 1) passes through the mode converter and the second mode coupler, and enters the mode modulator in the reverse direction as mode 2. At this time, the optical signal and the electrical signal have the same propagation direction, and intra-mode coupling occurs, that is, two modulation sidebands are generated without changing the mode. After leaving the input port of the mode modulator, it enters the combining port of the first mode coupler, is output from the second port of the first mode coupler, and is absorbed by the first mode absorber, thereby isolating the input optical signal of mode 1. It can be seen that the present invention can realize the coupling conversion of mode 1 and mode 2 within a wider bandwidth by optimizing the design of the mode modulator, and the first and second mode combiners and mode converters used are broadband devices, so the entire system operates in broadband.

[0006] The technical solutions of the present invention are as follows:

[0007] A non-magnetic integrated electro-optical isolator based on mode coupling, comprising a chip, an upper layer of the chip being an electro-optical material film, and a lower layer of the chip being a lower cladding layer, characterized in that a first mode combiner, a first mode absorber, a mode modulator, a second mode combiner, a second mode absorber, and a mode converter are fabricated on the electro-optical material film;

[0008] The first mode combiner and the second mode combiner are both three-port devices having a first port, a second port and a combining port;

[0009] The first mode absorber and the second mode absorber are both one-port devices, and the mode modulator and the mode converter are both two-port devices;

[0010] The optical signal of mode 1 enters the mode modulator through the first port of the first mode combiner, the combining port of the first mode combiner, and the first port of the mode modulator; an off-chip microwave signal source is coupled into the mode modulator and propagates in the mode modulator in the opposite direction to the optical signal, and the optical signal undergoes inter-mode coupling, and most of it is converted from mode 1 to mode 2. The optical signal of mode 2 enters the mode converter through the second port of the mode modulator, the combining port of the second mode combiner, and the second port of the second mode combiner in sequence, and is converted back to mode 1 as the final output through the mode converter; a small part of the optical signal of mode 1 that has not been converted is absorbed by the second mode absorber through the second port of the mode modulator, the combining port of the second mode combiner, and the first port of the second mode combiner in sequence.

[0011] If the final output mode 1 optical signal is reflected, the reflected light in mode 1 is converted to mode 2 by the mode converter, and then passes through the second port of the second mode coupler and the beam combining end of the second mode coupler in sequence, and then enters the mode modulator through the second port of the mode modulator as a mode 2 optical signal. The mode 2 optical signal has the same propagation direction as the electrical signal, and intra-mode coupling occurs, that is, two modulation sidebands are generated without changing the mode. The reflected light is output through the first port of the mode modulator, the beam combining end of the first mode coupler, and the second port of the first mode coupler, and is absorbed by the first mode absorber, thereby isolating the mode 1 optical signal.

[0012] Furthermore, the electro-optic material should have the following characteristics: when the applied electric field is perpendicular to the crystal axis, the refractive index ellipse of the electro-optic material will rotate, and this mechanism is usually achieved through the electro-optic coefficient r42 or r51.

[0013] The electro-optical material film is an x-cut y-conductive lithium niobate film with a thickness ranging from several hundred nanometers to several micrometers.

[0014] The microwave signal source generates a single-frequency microwave signal, the microwave frequency of which is precisely designed to ensure that the optical signal in the mode modulator undergoes the most efficient mode conversion when propagating in the opposite direction to the microwave signal. The electric field is directed perpendicularly through the waveguide along the x-direction. To ensure efficient mode conversion, the speed at which the electrical signal propagates in the traveling-wave electrode should match the group velocity of the optical signal in the waveguide, with a deviation of less than 20%, and the effective refractive index difference between Mode 1 and Mode 2 in the waveguide should be less than 1e-3 in the operating wavelength band. When the specific cross-sectional design of the waveguide only allows the effective refractive index difference between Mode 1 and Mode 2 to be less than 1e-3 at a specific wavelength, a waveguide width gradient design can be introduced along the direction of optical signal propagation, so that different wavelengths can achieve a smaller effective refractive index difference within the corresponding waveguide width, thereby enabling the electro-optical isolator to support broadband operation.

[0015] Furthermore, optical signals in the aforementioned devices exist in two modes: Mode 1 and Mode 2. Mode refers to the transverse mode in the waveguide, such as TE0, TM0, or TE1. The input and output waveguides of all the aforementioned devices should support both Mode 1 and Mode 2.

[0016] Furthermore, the functions of the first and second mode combiners are: when an optical signal enters the first port in mode 1, it will be output from the combining port, and vice versa; when an optical signal enters the second port in mode 2, it will be output from the combining port, and vice versa.

[0017] Furthermore, the mode modulator is an electro-optical modulation device, which includes an optical waveguide and a traveling wave electrode around the optical waveguide. The direction of the electric field in the traveling wave electrode is perpendicular to the crystal axis of the electro-optical material film and overlaps with the optical field in the waveguide. The speed at which the electrical signal propagates in the traveling wave electrode should match the group velocity of the optical signal in the waveguide, with a deviation of less than 20%. The effective refractive index difference between mode 1 and mode 2 in the waveguide at the working wavelength should be less than 1e-3 to ensure that the mode coupling can be carried out efficiently. The electrical signal propagates in the mode modulator in the opposite direction from the output waveguide to the input waveguide. When the propagation direction of the optical signal is opposite to that of the electrical signal, the optical signal will Mode Coupling, changing from mode 1 to mode 2, or from mode 2 to mode 1. When the propagation direction of the optical signal is the same as that of the electrical signal, the optical signal will be coupled. In mode Coupling produces two sidebands on both sides of the optical frequency, and the interval between the sidebands and the optical frequency is equal to the frequency of the electrical signal.

[0018] Furthermore, the mode converter converts the input mode 2 optical signal into a mode 1 optical signal.

[0019] Furthermore, the first and second mode absorbers absorb the input optical signals and eliminate the reflection of the optical signals in the waveguide.

[0020] Furthermore, the microwave signal source generates a single-frequency microwave signal, and the microwave frequency should just make the optical signal in the mode modulator undergo the most efficient mode conversion when propagating in the opposite direction to the microwave signal.

[0021] Preferably, the electro-optical material film is an x-cut lithium niobate film with a thickness ranging from several hundred nanometers to several microns.

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

[0023] Preferably, the first and second mode combiners are based on the mode coupling principle or the mode evolution principle.

[0024] Preferably, the optical waveguide in the mode modulator should be designed so that the group velocities of mode 1 and mode 2 within the operating band are as equal and constant as possible, so as to improve the conversion efficiency between the two modes.

[0025] Preferably, the impedance of the traveling wave electrode in the mode modulator is 50 ohms, the output impedance of the microwave signal source is also 50 ohms, and 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.

[0026] Preferably, the output optical signal of the present invention will undergo frequency shift, and the frequency of the frequency shift is equal to the modulation frequency of the microwave signal source. A reverse frequency shifter can be added after the output of the present invention to offset the frequency shift effect.

[0027] The core of the present invention is to realize a broadband electro-optical isolator by utilizing the non-reciprocal transmission of optical signals in a mode modulator in combination with mode filtering.

[0028] The present invention also provides a method for preparing a non-magnetic integrated electro-optical isolator based on mode coupling, which is characterized in that an optical device is prepared along the y-axis propagation direction on an electro-optical material film, namely an x-cut lithium niobate film, including a first mode combiner, a first mode absorber, a mode modulator, a second mode combiner, a second mode absorber and a mode converter, wherein the first mode combiner and the second mode combiner are designed based on mode evolution and can realize broadband mode combining and splitting; the first mode absorber and the second mode absorber are realized by a coiled waveguide with a small bending radius and a metasurface design; the mode converter is designed based on mode evolution; the above-mentioned device can be prepared by etching the lithium niobate film; the mode modulator is a waveguide with an upper cladding layer of silicon oxide above the waveguide, and a coplanar electrode above the upper cladding and in a direction parallel to the waveguide, the signal electrode of the coplanar electrode is located directly above the waveguide, and the ground electrodes are located on both sides parallel to the signal electrode, and the signal electrode and the ground electrode generate an electric field along the x-direction and perpendicular to the waveguide.

[0029] Compared with the prior art, the present invention has the following advantages:

[0030] 1) The present invention does not require spectral filtering and can therefore support broadband optical isolation.

[0031] 2) The present invention does not require an external magnetic field, and thus can realize an integrated micro optical isolator. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 Schematic diagram of a broadband integrated electro-optical isolator of the present invention, wherein the illustration in the dotted box is a working schematic diagram of the first mode combiner.

[0033] Figure 2 This is a schematic diagram of the energy band of mode coupling in the mode modulator, where the horizontal axis is the wave vector k. A wave vector greater than 0 indicates forward propagation (the right quadrant in the figure), and less than 0 indicates reverse propagation (the left quadrant in the figure). The vertical axis represents the frequency.

[0034] In the figure: 1-chip; 2-first mode combiner, 201-first port of the first mode coupler, 202-second port of the first mode coupler, 203-combining port of the first mode coupler; 3-first mode absorber; 4-mode modulator, 401-first port of the mode modulator, 402-second port of the mode modulator; 5-microwave signal source, 6-second mode combiner, 601-first port of the second mode coupler, 602-second port of the second mode coupler, 603-combining port of the second mode coupler; 7-second mode absorber, 8-mode converter, 801-output port of the mode converter. DETAILED DESCRIPTION

[0035] 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.

[0036] Please refer to Figure 1 As shown in the figure, a broadband integrated electro-optical isolator includes a chip 1, the upper layer of the chip is an electro-optical material film, and the lower layer of the chip 1 is a lower cladding. A first mode combiner 2, a first mode absorber 3, a mode modulator 4, a second mode combiner 6, a second mode absorber 7, and a mode converter 8 are prepared on the electro-optical material film. The first and second mode combiners are three-port devices, having a first port, a second port, and a combining port. The first and second mode absorbers are one-port devices. The mode modulator is a two-port device, having an input end and an output end. The mode converter is a two-port device, having an input end and an output end. The optical signal in the present invention includes two modes: mode 1 and mode 2. Along the propagation direction of the optical signal, the optical signal passes through the first port of the first mode combiner, the combining port of the first mode combiner, the input port and output port of the mode modulator, the combining port of the second mode combiner, the second port of the second mode combiner, the input port and output port of the mode converter in sequence. The second port of the first mode combiner is connected to the first mode absorber, and the first port of the second mode combiner is connected to the second mode absorber. An off-chip microwave signal source is coupled into the mode modulator via wire bonding from the side of the mode modulator near the output port, propagating in the opposite direction of the optical signal to the input port of the mode modulator.

[0037] Furthermore, optical signals in the aforementioned devices exist in two modes: Mode 1 and Mode 2. Mode refers to the transverse mode in the waveguide, such as TE0, TM0, or TE1. The input and output waveguides of all the aforementioned devices should support both Mode 1 and Mode 2.

[0038] Furthermore, the functions of the first and second mode combiners are: when an optical signal enters the first port in mode 1, it will be output from the combining port, and vice versa; when an optical signal enters the second port in mode 2, it will be output from the combining port, and vice versa.

[0039] Furthermore, the mode modulator is an electro-optical modulation device comprising an optical waveguide and a traveling wave electrode surrounding the optical waveguide. The direction of the electric field in the traveling wave electrode is perpendicular to the crystal axis of the electro-optical material film and overlaps with the optical field in the waveguide. The speed at which the electrical signal propagates in the traveling wave electrode should match the group velocity of the optical signal in the waveguide, with a deviation of less than 20%. In the mode modulator, the electrical signal propagates in the opposite direction from the output waveguide to the input waveguide. When the propagation directions of the optical signal and the electrical signal are opposite, the optical signal will undergo inter-mode coupling, transforming from mode 1 to mode 2, or from mode 2 to mode 1. When the propagation directions of the optical signal and the electrical signal are the same, the optical signal will undergo intra-mode coupling, generating two sidebands on both sides of the optical frequency, with the interval between the sidebands and the optical frequency being equal to the frequency of the electrical signal.

[0040] Furthermore, the mode converter converts the input mode 2 optical signal into a mode 1 optical signal.

[0041] Furthermore, the first and second mode absorbers absorb the input optical signals and eliminate the reflection of the optical signals in the waveguide.

[0042] Furthermore, the microwave signal source generates a single-frequency microwave signal, and the microwave frequency should just make the optical signal in the mode modulator undergo the most efficient mode conversion when propagating in the opposite direction to the microwave signal.

[0043] The electro-optical material film is an x-cut lithium niobate film with a thickness between several hundred nanometers and several microns. Mode 1 of the optical signal is TE0, and mode 2 is TM0. The input optical signal is mode 1. The first and second mode combiners are based on the mode coupling principle or the mode evolution principle. The optical waveguide in the mode modulator should be designed so that the group velocities of mode 1 and mode 2 in the working band are as equal and unchanged as possible to improve the conversion efficiency between the two modes. The impedance of the traveling wave electrode in the mode modulator 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 wire should be as short as possible to reduce microwave loss. The output optical signal of the present invention will undergo frequency shift, and the frequency of the frequency shift is equal to the modulation frequency of the microwave signal source. A reverse frequency shifter can be added after the output of the present invention to offset the frequency shift effect.

[0044] In a mode modulator, assuming the input optical signal is mode 1, as the optical signal propagates from the input port to the output port, it undergoes inter-mode coupling, converting from mode 1 to mode 2. It then passes through the second mode combiner and is output to the second port. It then passes through the mode converter and is converted back to mode 1 as the final output. The remaining optical signal that has not been converted to mode 2 (i.e., still mode 1) passes through the second mode combiner and is output to the first port, where it is absorbed by the second mode absorber. Assuming the final output is reflected, the reflected light (mode 1) passes through the mode converter and the second mode coupler and enters the mode modulator in the opposite direction as mode 2. At this point, the optical signal propagates in the same direction as the electrical signal, resulting in intra-mode coupling, generating two modulation sidebands without changing mode. After exiting the input port of the mode modulator, it enters the combining port of the first mode coupler, is output from the second port of the first mode coupler, and is absorbed by the first mode absorber, thereby isolating the input optical signal in mode 1. It can be seen that the present invention can realize the coupling conversion of mode 1 and mode 2 within a wider bandwidth by optimizing the design of the mode modulator, and the first and second mode combiners and mode converters used are both broadband devices, so the entire system operates in broadband.

[0045] Figure 2 This is a schematic diagram of the energy bands for mode coupling in a mode modulator. The microwave signal has a frequency of Ω and propagates in the reverse direction. When the optical signal in mode 1, with a frequency of ω1, propagates in the forward direction (right quadrant of the diagram), mode coupling occurs, converting it into an optical signal in 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 in mode 2, with a frequency of ω2 = ω1 + Ω, propagates in the reverse direction into the mode modulator (left quadrant of the diagram), in the same direction as the microwave signal. Intra-mode conversion occurs, generating two sidebands, ω1 and ω3 = ω1 + 2Ω, both in mode 2. By adding a mode combiner at the input to filter out the reflected mode 2, optical isolation of the input optical signal (mode 1) can be achieved.

Claims

1. A non-magnetic integrated electro-optical isolator based on mode coupling, comprising a chip, an electro-optical material thin film on the upper layer of the chip, and a lower cladding layer on the lower layer of the chip, characterized in that: A first mode combiner, a first mode absorber, a mode modulator, a second mode combiner, a second mode absorber and a mode converter are prepared on the electro-optical material film; The first mode combiner and the second mode combiner are both three-port devices having a first port, a second port and a combining port; The first mode absorber and the second mode absorber are both one-port devices, and the mode modulator and the mode converter are both two-port devices; The optical signal of mode 1 enters the mode modulator through the first port of the first mode combiner, the combining port of the first mode combiner, and the first port of the mode modulator; An off-chip microwave signal source is coupled into the mode modulator and propagates in the mode modulator in the opposite direction to the optical signal. The optical signal undergoes inter-mode coupling, and most of the optical signal is converted from mode 1 to mode 2. The optical signal of mode 2 enters the mode converter through the second port of the mode modulator, the beam combining port of the second mode combiner, and the second port of the second mode combiner in sequence, and is converted back to mode 1 through the mode converter as the final output. A small portion of the unconverted mode 1 optical signal is absorbed by the second mode absorber through the second port of the mode modulator, the combining port of the second mode combiner, and the first port of the second mode combiner; If the final output mode 1 optical signal is reflected, the reflected light in mode 1 is converted to mode 2 by the mode converter, passes through the second port of the second mode coupler and the beam combining end of the second mode coupler, and then enters the mode modulator through the second port of the mode modulator as a mode 2 optical signal. The mode 2 optical signal propagates in the same direction as the electrical signal, and intra-mode coupling occurs, that is, two modulation sidebands are generated without changing the mode. The optical signal is outputted through the first port of the mode modulator, the beam combining port of the first mode coupler, and the second port of the first mode coupler, and is absorbed by the first mode absorber, thereby isolating the optical signal of mode 1.

2. The non-magnetic integrated electro-optical isolator based on mode coupling according to claim 1, characterized in that: The electro-optical material film is an x-cut y-conductive lithium niobate film.

3. The non-magnetic integrated electro-optical isolator based on mode coupling according to claim 1, characterized in that: The microwave signal source generates a single-frequency microwave signal, and the microwave frequency should just make the optical signal in the mode modulator undergo the most efficient mode conversion when propagating in the opposite direction to the microwave signal.

4. The non-magnetic integrated electro-optical isolator based on mode coupling according to claim 3, characterized in that: The direction of the electric field is perpendicular to the crystal axis of the electro-optical material film. The speed of the electrical signal propagating in the traveling wave electrode should match the group velocity of the optical signal in the waveguide, with a deviation of less than 20%, and the effective refractive index difference between mode 1 and mode 2 in the waveguide in the operating band should be less than 1e-3. When the specific cross-sectional design of the waveguide can only make the effective refractive index difference between mode 1 and mode 2 at a specific wavelength less than 1e-3, a waveguide width gradient design can be introduced along the propagation direction of the optical signal, so that different wavelengths can achieve a smaller effective refractive index difference at the corresponding waveguide width, thereby enabling the electro-optical isolator to support broadband operation.

5. The non-magnetic integrated electro-optical isolator based on mode coupling according to claim 1, characterized in that: Mode 1 of the optical signal is TE0, and mode 2 is TM0.

6. A method for preparing a non-magnetic integrated electro-optical isolator based on mode coupling according to any one of claims 1 to 5, characterized in that: An optical device is prepared on an electro-optical material film, i.e., an x-cut lithium niobate film, along the y-axis propagation direction, including a first mode combiner, a first mode absorber, a mode modulator, a second mode combiner, a second mode absorber and a mode converter, wherein the first mode combiner and the second mode combiner are designed based on mode evolution and can realize broadband mode combining and splitting; the first mode absorber and the second mode absorber are realized by a coiled waveguide with a small bending radius and a metasurface design; the mode converter is designed based on mode evolution; the above-mentioned device can be prepared by etching the lithium niobate film; the mode modulator is a waveguide with an upper cladding layer of silicon oxide above the waveguide, a coplanar electrode above the upper cladding and parallel to the waveguide, the signal electrode of the coplanar electrode is located directly above the waveguide, and the ground electrodes are located on both sides parallel to the signal electrode, and the signal electrode and the ground electrode generate an electric field along the x-direction and perpendicular to the waveguide.

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