Electro-optic modulator and method based on thin film lithium niobate and metasurface

Abstract

An electro-optic modulator and method based on thin-film lithium niobate and metasurfaces, by monolithically integrating a thin-film lithium niobate electro-optic modulator with a polarization-conversion metasurface, can complete the intensity modulation of input light and the conversion from linear polarization to circular polarization on a single chip. The method is characterized by including a thin-film lithium niobate platform on an insulator, with a grating coupler disposed on the inner side of the left-center portion of the platform. The left end of the grating coupler has a linearly polarized light input interface, and the right end of the grating coupler is connected to a Mach-Zehnder intensity modulator via a ridge-type lithium niobate main waveguide. The Mach-Zehnder intensity modulator is connected to a planar lithium niobate waveguide via the ridge-type lithium niobate waveguide. The planar lithium niobate waveguide is formed by thermally widening the right end of the ridge-type lithium niobate waveguide to the right. A polarization-modulating metasurface is integrated on the surface of the planar lithium niobate waveguide. The polarization-modulating metasurface is used to realize the conversion of linearly polarized light to a specified circularly polarized state, thereby obtaining output light with a predetermined intensity modulation depth and a predetermined polarization state.

Description

Technical Field

[0001] This invention relates to the field of integrated optics and micro / nano photonic devices, and in particular to an electro-optic modulator and method based on thin-film lithium niobate and metasurfaces, which is applicable to the joint control of light intensity and polarization state in the visible to near-infrared bands. Background Technology

[0002] Optical modulation devices are core components in some quantum precision measurement instruments, especially optically pumped atomic magnetometers, and their performance directly affects the system's bandwidth, power consumption, and integration. Traditional acousto-optic or electro-optic modulators are mostly based on bulk crystal materials, often resulting in large sizes. Furthermore, traditional polarization control relies on discrete optical components such as waveplates and polarizers, further increasing system complexity and size. These characteristics make it difficult for optical modulation devices based on traditional components to achieve multidimensional control of light intensity and polarization state on a single chip.

[0003] In recent years, lithium niobate-on-insulator platforms have become ideal platforms for high-performance, miniaturized electro-optic modulators due to their excellent electro-optic properties, low optical loss, and strong optical field confinement capabilities. On the other hand, optical metasurfaces, as two-dimensional planar devices composed of subwavelength-scale artificial structures, can flexibly and efficiently control the amplitude, phase, and polarization state of light waves.

[0004] Currently, Bell-Bloom magnetometers are high-performance, miniaturized, all-optical atomic magnetometers suitable for use in integrated systems. However, traditional Bell-Bloom magnetometers often employ acousto-optic modulators and discrete optics for pump light modulation, limiting system miniaturization due to device size constraints. Theoretically, monolithically integrating a thin-film lithium niobate electro-optic modulator with a polarization-conversion metasurface on a chip can simultaneously achieve dynamic intensity modulation and static polarization state conversion of light at the sub-millimeter scale, greatly simplifying the system architecture and meeting the demands of highly integrated applications. Summary of the Invention

[0005] The present invention aims to overcome the shortcomings of the prior art and provide an electro-optic modulator and method based on thin-film lithium niobate and metasurface. By integrating the thin-film lithium niobate electro-optic modulator with a polarization-conversion metasurface monolithically, the intensity modulation of the input light and the conversion from linear polarization to circular polarization can be completed on a single chip.

[0006] The technical solution of the present invention is as follows:

[0007] An electro-optic modulator based on thin-film lithium niobate and metasurfaces is characterized by comprising a thin-film lithium niobate platform on an insulator, a grating coupler disposed on the inner side of the left middle portion of the thin-film lithium niobate platform, the left end of the grating coupler having a linearly polarized light input interface, the right end of the grating coupler being connected to the input end of a Mach-Zehnder intensity modulator via a ridge-type lithium niobate main waveguide, the output end of the Mach-Zehnder intensity modulator being connected to a planar lithium niobate waveguide via a ridge-type lithium niobate waveguide, the planar lithium niobate waveguide being formed by thermally widening the right end of the ridge-type lithium niobate waveguide to the right, and a polarization-modulating metasurface integrated on the surface of the planar lithium niobate waveguide, the polarization-modulating metasurface being used to realize the conversion of linearly polarized light to light of a specified circularly polarized state, thereby obtaining output light with a predetermined intensity modulation depth and a predetermined polarization state.

[0008] The input terminal of the Mach-Zehnder intensity modulator is the left end of the first multimode interference coupler, and the output terminal of the Mach-Zehnder intensity modulator is the right end of the second multimode interference coupler. The upper side of the first multimode interference coupler is connected to the bottom left end of the first modulation arm waveguide, which is convex at the top. The upper side of the second multimode interference coupler is connected to the bottom right end of the first modulation arm waveguide. The lower side of the first multimode interference coupler is connected to the top left end of the second modulation arm waveguide, which is concave at the bottom. The lower side of the second multimode interference coupler is connected to the top right end of the second modulation arm waveguide. A first gold electrode is disposed above the first modulation arm waveguide. A second gold electrode is disposed between the first and second modulation arm waveguides. A third gold electrode is disposed below the second modulation arm waveguide. The second gold electrode has a voltage source interface. Both the first and third gold electrodes are grounded.

[0009] The polarization-tunable metasurface is composed of a subwavelength-scale dielectric nanostructure array, which consists of silicon nanopillars of a predetermined size and arranged according to a predetermined period. The coupling of guided mode light to free space is achieved by changing the geometric parameters and spatial arrangement of the nanounits.

[0010] The multimode interference coupler is a 3dB multimode interference coupler.

[0011] The linearly polarized light input interface is connected to the laser via a polarization-maintaining fiber, and the voltage source interface is connected to a voltage source.

[0012] The Mach-Zehnder intensity modulator includes the following expression:

[0013] ,

[0014] ,

[0015] In the formula It represents the phase change of a single arm of the modulator under the signal V(t), where V(t) is the modulating signal. Where λ is the length of the modulation region, and λ is the wavelength of the incident light. In optical waveguides The change in the effective refractive index of the mode, TE0 is the lowest order transverse electric mode, n e For effective refractive index, It is the electro-optic coefficient of lithium niobate along the z-axis. For external electric field and optical waveguide Normalized overlap integral of the pattern, The distance between the two electrodes. It is half-wave voltage, which is the voltage required to change the output optical power from maximum to minimum.

[0016] The polarization-tunable metasurface comprises the following formula:

[0017] ,

[0018] ,

[0019] In the formula Let be the incident light wavenumber, θ be the angle between the outgoing light and the normal, and β be the propagation constant of the waveguide TE0 mode. The geometric phase gradient is caused by the rotational angular gradient of the nanopillar, where m is the diffraction order. The period of the nanopillar is in the direction of light propagation.

[0020] The electro-optic modulation method based on thin-film lithium niobate and metasurface is characterized by employing the aforementioned electro-optic modulator based on thin-film lithium niobate and metasurface.

[0021] Includes the following steps:

[0022] Step 1: Guide the 795nm linearly polarized light generated by the laser into the grating coupler through the polarization-maintaining fiber to excite the transverse electric TE fundamental mode light.

[0023] Step 2: Apply a modulation voltage signal to the electrodes in the Mach-Zehnder intensity modulator using a signal driving system to generate an electric field along the optical axis of the crystal between the push-pull structure electrodes. Change the effective refractive index of the light in the waveguide through the Pockels electro-optic effect of lithium niobate material to generate a phase difference in the two arms of the Mach-Zehnder interference structure, thereby achieving intensity modulation of the output light.

[0024] Step 3: The intensity-modulated light is transmitted along the waveguide propagation direction to the polarization-modulated metasurface region integrated at the top of the waveguide to obtain output light with a predetermined intensity modulation depth and a predetermined polarization state;

[0025] Step 4: Receive the output light signal through the photoelectric detection system, measure the modulation curve of the output light intensity as a function of the driving voltage, and calculate the extinction ratio and half-wave voltage of the electro-optic modulator; measure the Stokes parameter or ellipticity of the output light through the polarization analysis system to obtain the polarization conversion efficiency of the output light.

[0026] The technical advantages of this invention are as follows: This invention is based on an electro-optic modulator and method using thin-film lithium niobate and a metasurface. It includes an integrated input / output fiber coupling structure, a thin-film lithium niobate waveguide electro-optic modulator, a waveguide-top polarization-conversion metasurface, an electrode driving system, and an optical detection system. By adjusting the phase difference of the guided mode light through a driving voltage, dynamic modulation of the output light intensity is achieved. Furthermore, the polarization state of the output light is controlled through the waveguide metasurface, enabling simultaneous dynamic control of both the output light intensity and polarization state. This invention has a compact overall structure and can achieve dynamic electro-optic modulation of the guided mode light intensity and polarization state. It can be applied to on-chip integrated optical systems and pump light modulation systems for optically pumped atomic magnetometers, providing an effective technical solution for miniaturization and integration of optical systems. Attached Figure Description

[0027] Figure 1 is a schematic diagram of the electro-optic modulator structure based on thin-film lithium niobate and metasurface for implementing the present invention.

[0028] The reference numerals in the attached figures are explained as follows: 1-Lithium niobate platform on insulator; 2-Polarization-maintaining fiber (left end connected to the laser, right end connected to the grating coupler); 3-Grate coupler; 4-Ridged lithium niobate main waveguide; 5-Mach-Zehnder intensity modulator (including a top-positioned first modulation arm waveguide and a bottom-positioned second modulation arm waveguide, a left-positioned first multimode interference coupler and a right-positioned second multimode interference coupler, the first multimode interference coupler being connected to the right end of the ridged lithium niobate main waveguide and the left ends of the two modulation arm waveguides respectively, the second multimode interference coupler being connected to the left end of the ridged lithium niobate waveguide and the right ends of the two modulation arm waveguides respectively, the first modulation arm... A first gold electrode is disposed above the waveguide, a second gold electrode is disposed between the first and second modulation arm waveguides, and a third gold electrode is disposed below the second modulation arm waveguide. The second gold electrode is connected to a voltage source, and both the first and third gold electrodes are grounded. The right end of the ridge-shaped lithium niobate waveguide is thermally extended to the right to form a planar lithium niobate waveguide; 6-Planar lithium niobate waveguide (obtained by thermally extending the ridge-shaped lithium niobate waveguide after the Mach-Zehnder modulator); 7-Polarization-modulated metasurface (composed of silicon nanopillars of a preset size and arranged according to a preset rule); XYZ-Cartesian coordinate system (i.e., X, Y, Z axes). Detailed Implementation

[0029] The following is in conjunction with the attached diagram ( Figure 1 The present invention will be described in conjunction with the examples.

[0030] Figure 1 is a schematic diagram of the electro-optic modulator structure based on thin-film lithium niobate and metasurface, implementing the present invention. (Reference) Figure 1 As shown, an electro-optic modulator based on thin-film lithium niobate and metasurfaces includes a thin-film lithium niobate platform 1 on an insulator. A grating coupler 3 is disposed on the inner side of the left middle part of the thin-film lithium niobate platform 1. The left end of the grating coupler 3 has a linearly polarized light input interface. The right end of the grating coupler 3 is connected to the input end of a Mach-Zehnder intensity modulator 5 through a ridge-type lithium niobate main waveguide 4. The output end of the Mach-Zehnder intensity modulator 5 is connected to a planar lithium niobate waveguide 6 through a ridge-type lithium niobate waveguide. The planar lithium niobate waveguide 6 is formed by thermally widening the right end of the ridge-type lithium niobate waveguide to the right. A polarization-modulating metasurface 7 is integrated on the surface of the planar lithium niobate waveguide 6. The polarization-modulating metasurface 7 is used to realize the conversion of linearly polarized light to light in a specified circularly polarized state, thereby obtaining output light with a predetermined intensity modulation depth and a predetermined polarization state.

[0031] The input terminal of the Mach-Zehnder intensity modulator 5 is the left end of the first multimode interference coupler, and the output terminal of the Mach-Zehnder intensity modulator 5 is the right end of the second multimode interference coupler. The upper side of the first multimode interference coupler is connected to the bottom left end of the first modulation arm waveguide, which is convex at the top. The upper side of the second multimode interference coupler is connected to the bottom right end of the first modulation arm waveguide. The lower side of the first multimode interference coupler is connected to the top left end of the second modulation arm waveguide, which is concave at the bottom. The lower side of the second multimode interference coupler is connected to the top right end of the second modulation arm waveguide. A first gold electrode is disposed above the first modulation arm waveguide. A second gold electrode is disposed between the first and second modulation arm waveguides. A third gold electrode is disposed below the second modulation arm waveguide. The second gold electrode has a voltage source interface. Both the first and third gold electrodes are grounded.

[0032] The polarization-tunable metasurface 7 is composed of a subwavelength-scale dielectric nanostructure array, which consists of silicon nanopillars of a preset size and arranged according to a predetermined period. The coupling of the guided mode light to free space is achieved by changing the geometric parameters and spatial arrangement of the nanounits.

[0033] The multimode interference coupler is a 3dB multimode interference coupler. The linearly polarized light input interface is connected to the laser via polarization-maintaining fiber 2, and the voltage source interface is connected to a voltage source.

[0034] The Mach-Zehnder intensity modulator includes the following expression:

[0035] ,

[0036] ,

[0037] In the formula It represents the phase change of a single arm of the modulator under the signal V(t), where V(t) is the modulating signal. Where λ is the length of the modulation region, and λ is the wavelength of the incident light. In optical waveguides The change in the effective refractive index of the mode, TE0 is the lowest order transverse electric mode, n e For effective refractive index, It is the electro-optic coefficient of lithium niobate along the z-axis. For external electric field and optical waveguide Normalized overlap integral of the pattern, The distance between the two electrodes. It is half-wave voltage, which is the voltage required to change the output optical power from maximum to minimum.

[0038] The polarization-tunable metasurface comprises the following formula:

[0039] ,

[0040] ,

[0041] In the formula Let be the incident light wavenumber, θ be the angle between the outgoing light and the normal, and β be the propagation constant of the waveguide TE0 mode. The geometric phase gradient is caused by the rotational angular gradient of the nanopillar, where m is the diffraction order. The period of the nanopillar is in the direction of light propagation.

[0042] The electro-optic modulation method based on thin-film lithium niobate and metasurface employs the aforementioned electro-optic modulator based on thin-film lithium niobate and metasurface.

[0043] The process includes the following steps: Step 1, guiding 795nm linearly polarized light generated by the laser into a grating coupler via a polarization-maintaining fiber to excite the transverse electric TE fundamental mode light; Step 2, applying a modulation voltage signal to the electrodes in the Mach-Zehnder intensity modulator using a signal driving system, generating an electric field along the optical axis of the crystal between the push-pull structure electrodes, changing the effective refractive index of the light in the waveguide through the Pockels electro-optic effect of lithium niobate material, causing a phase difference in the two arms of the Mach-Zehnder interference structure, and achieving intensity modulation of the output light; Step 3, transmitting the intensity-modulated light along the waveguide propagation direction to the polarization-controlled metasurface region integrated at the top of the waveguide to obtain output light with a predetermined intensity modulation depth and a predetermined polarization state; Step 4, receiving the output light signal through a photoelectric detection system, measuring the modulation curve of the output light intensity as a function of the driving voltage, and calculating the extinction ratio and half-wave voltage of the electro-optic modulator; measuring the Stokes parameter or ellipticity of the output light through a polarization analysis system to obtain the polarization conversion efficiency of the output light.

[0044] This invention discloses an electro-optic modulator based on a thin-film lithium niobate waveguide integrated metasurface and its applications. The modulator includes an optical input structure, a ridge-shaped lithium niobate waveguide, an electro-optic modulation unit, a planar lithium niobate waveguide, and a metasurface structure disposed above the planar lithium niobate waveguide. The optical input structure is used to couple external input light into the lithium niobate waveguide to form guided mode light. The electro-optic modulation unit is based on a Mach-Zehnder interference structure and achieves intensity modulation of the guided mode light through the electro-optic effect of lithium niobate material. The metasurface structure is composed of a subwavelength-scale dielectric nanostructure array, used to couple the modulated guided mode light into free space and control the polarization state of the outgoing light, realizing the conversion of linearly polarized light to circularly polarized light and the control of the outgoing direction, thereby achieving integrated output of intensity modulation and polarization control. This invention has a compact structure and high integration, is suitable for optical field modulation with a working wavelength of 795nm, can be used for pump optical path modulation in atomic magnetometers, and can be applied to related fields such as atomic clocks and quantum precision measurement.

[0045] An electro-optic modulator based on a thin-film lithium niobate platform includes an optical input structure, a ridged lithium niobate waveguide, an electro-optic modulation unit, a planar lithium niobate waveguide, and an output modulation unit disposed above the planar lithium niobate waveguide, all disposed on the thin-film lithium niobate platform on an insulator. The optical input structure couples external input light into the ridged lithium niobate waveguide to form a guided mode light. The electro-optic modulation unit, disposed on the ridged lithium niobate waveguide, includes a beam splitting structure, two modulation arm waveguides, a beam combining structure, and an electrode structure. It is used to change the effective refractive index in the modulation arm waveguides through the electro-optic effect of lithium niobate material, causing the guided mode light to generate a phase difference in the two modulation arms and form intensity-modulated light at the output end. The output end of the electro-optic modulation unit is connected to the planar lithium niobate waveguide through the waveguide structure. The output modulation unit includes a dielectric nanostructure array disposed on the planar lithium niobate waveguide, used to couple the intensity-modulated guided mode light into free space and modulate the polarization state of the output light, thereby achieving integrated output of intensity modulation and polarization modulation.

[0046] The electro-optic modulation unit is a Mach-Zehnder interference structure, and both the beam splitting structure and the beam combining structure are multimode interference couplers, specifically 3dB multimode interference couplers.

[0047] The electrode structure includes electrodes disposed on both sides of the two modulation arm waveguides and an electrode disposed between the two modulation arms, for applying a modulation voltage signal.

[0048] The electrodes are metal electrodes, with the electrodes on both sides grounded and the electrode between the two modulation arms used to input the drive voltage signal.

[0049] The planar lithium niobate waveguide is formed by the ridge-shaped lithium niobate waveguide through an adiabatic broadening structure.

[0050] The dielectric nanostructure array is composed of subwavelength-scale nanounits arranged according to a predetermined period, and the coupling of guided mode light to free space is achieved by changing the geometric parameters and spatial arrangement of the nanounits.

[0051] The nanounit is a silicon nanopillar structure, and the Pancharatnam-Berry geometric phase is introduced by changing the rotation angle of the nanopillar, thereby realizing the conversion of linearly polarized light to circularly polarized light.

[0052] The electro-optic modulator operates at a wavelength of 795 nm and is used for pump light modulation in an atomic magnetometer system.

[0053] An electro-optic modulation method, employing the aforementioned electro-optic modulator, includes the following steps: coupling input light into a thin-film lithium niobate waveguide to form guided mode light; applying an electric field to the guided mode light during its propagation via an electro-optic modulation unit to generate a phase difference between the guided mode light and two modulation arms, thereby forming intensity-modulated light at the output end; transmitting the modulated guided mode light to an output control unit; coupling the guided mode light into free space via a dielectric nanostructure array; and controlling the polarization state and propagation direction of the output light.

[0054] The modulation depth and modulation frequency of the output light are controlled by adjusting the amplitude and frequency of the voltage signal applied to the electro-optic modulation unit.

[0055] The emitted light is circularly polarized and radiates in the near-normal direction.

[0056] The method is used in a Bell-Bloom type rubidium atomic magnetometer, which drives the atomic spin to generate a magneto-optical resonance signal by adjusting the modulation frequency to be close to the atomic Larmor precession frequency.

[0057] This invention aims to overcome the shortcomings of existing technologies and provide a compact and functionally integrated electro-optic modulation device based on a lithium niobate and metasurface cascade. This device integrates a thin-film lithium niobate electro-optic modulator with a polarization-conversion metasurface monolithically, enabling intensity modulation of input light and conversion from linear to circular polarization on a single chip. To achieve the above objectives, this invention adopts the following technical solution:

[0058] A dynamic electro-optic modulator based on a thin-film lithium niobate integrated metasurface is fabricated on a silicon dioxide substrate. The thin film is an x-section lithium niobate film, with the x-axis perpendicular to the film plane and pointing upwards. A waveguide coupler is disposed on the film. A ridge-shaped lithium niobate waveguide is etched along the y-axis of the lithium niobate, starting from the waveguide coupler. A Mach-Zehnder interferometer is disposed along the ridge-shaped lithium niobate waveguide. The interferometer includes, in sequence according to the beam propagation direction, a first multimode interference coupler, two parallel and identical modulation arms, and a second multimode interference coupler. A gold electrode structure is covered above the modulation arm region. The electrode structure is divided into three electrodes along the z-axis of the lithium niobate. The electrodes are sequentially grounded, connected to an RF signal source, and grounded again. The two arms of the interferometer are arranged in the interval between the electrodes, and the electrodes provide the modulation voltage. The waveguide at the rear end of the interferometer is thermally broadened to obtain a planar lithium niobate waveguide. A silicon nanopillar array is heterogeneously integrated on the top of the planar waveguide. The Mach-Zehnder interferometer controls the refractive index of the lithium niobate waveguide by controlling the electrode voltage, and then controls the light intensity through interference. The silicon nanopillar array controls the exit angle of the guided mode light coupled into space by a precisely designed period and rotation angle gradient, and controls the exit light to a specific polarization state by a precisely designed nanopillar geometry, finally obtaining a specific circularly polarized light with adjustable intensity in the near-normal direction.

[0059] The intensity modulator module includes the following expressions:

[0060] ,

[0061] ,

[0062] In the formula This represents the phase change of a single arm of the modulator under a signal V(t), where V(t) is the modulation signal and λ is the incident light wavelength, corresponding to the rubidium atom D1 line at 795 nm in the application environment of a rubidium atomic magnetometer. In optical waveguides The change in the effective refractive index of the mode; TE0 is the lowest-order transverse electric mode. It is the electro-optic coefficient of lithium niobate along the z-axis. n is the length of the modulation region. e For effective refractive index, The distance between the two electrodes. For external electric field and optical waveguide Normalized overlap integral of the pattern, It is half-wave voltage, which is the voltage required to change the output optical power from maximum to minimum.

[0063] The metasurface module contains the following formula:

[0064] ,

[0065] In the formula, , where is the incident light wavenumber, θ is the angle between the outgoing light and the normal, and β is the propagation constant of the TE00 mode of the waveguide. The geometric phase gradient is caused by the rotational angular gradient of the nanopillar, where m is the diffraction order. The period of the nanopillar is in the direction of light propagation.

[0066] The electro-optic modulator couples fiber optic light into a waveguide via a grating coupler, propagating in the TE fundamental mode (TE, Transverse Electric). By applying a radio frequency signal to the common electrode between the two arms of the modulator, the two arms are subjected to equal and opposite time-varying electric fields, with the electric field direction along the z-axis of the lithium niobate material. This causes the refractive index in the waveguide to change with voltage, generating opposite phase differences in the two arms. When the phase difference between the two arms is 0, i.e., no voltage is applied, constructive interference occurs in the multimode interference coupler at the end of the modulator, resulting in maximum light intensity. When the phase difference between the two arms is π, i.e., the voltage reaches... At this time, destructive interference occurs in the multimode interference coupler, and the light intensity is at its minimum. By changing the frequency of the radio frequency signal, the intensity of the output light can be controlled to change at the same frequency.

[0067] The metasurface, through a carefully designed rotation angle gradient and period, enables left-handed circularly polarized light that has undergone geometric phase modulation to achieve phase matching in the near-normal direction, thereby outputting left-handed circularly polarized light in the near-normal direction.

[0068] The dynamic electro-optic modulator based on thin-film lithium niobate and metasurface employs the aforementioned dynamic intensity modulation and polarization conversion system based on thin-film lithium niobate and metasurface.

[0069] Includes the following steps:

[0070] Step 1: Guide the 795nm linearly polarized laser generated by the laser into the grating coupler through the polarization-maintaining fiber to excite the TE fundamental mode light;

[0071] Step 2: Apply a modulation voltage signal to the electrodes of the thin-film lithium niobate electro-optic modulator using a signal driving system. This generates an electric field along the optical axis of the crystal between the push-pull structure electrodes. By changing the effective refractive index of the light in the waveguide through the Pockels electro-optic effect of the lithium niobate material, a phase difference is generated in the two arms of the Mach-Zehnder interference structure, thereby achieving intensity modulation of the output light.

[0072] Step 3: The intensity-modulated light is transmitted along the waveguide propagation direction to the polarization-tunable metasurface region integrated at the top of the waveguide. The waveguide metasurface is composed of a subwavelength nanostructure array. The guided mode light is decoupled to free space through the coupling between the waveguide and the metasurface, and the linearly polarized light is converted to a specified circularly polarized state light through the geometric phase modulation of the nanostructure, thereby obtaining output light with a predetermined intensity modulation depth and polarization state.

[0073] Step 4: Receive the output light signal through the photoelectric detection system, measure the modulation curve of the output light intensity as a function of the driving voltage, and calculate the extinction ratio and half-wave voltage of the electro-optic modulator; measure the Stokes parameter or ellipticity of the output light through the polarization analysis system to obtain the polarization conversion efficiency of the output light.

[0074] refer to Figure 1As shown, an electro-optic modulator based on a lithium niobate waveguide integrated metasurface includes a polarization-maintaining fiber 2, a grating coupler 3, a ridged lithium niobate main waveguide 4, a Mach-Zehnder intensity modulator 5, and a planar lithium niobate waveguide 6 disposed on the surface of a thin-film lithium niobate platform 1 on an insulator. A metasurface (i.e., a polarization-modulated metasurface 7) is disposed on top of the planar lithium niobate waveguide 6. The input end of the polarization-maintaining fiber 2 is connected to a 795nm fiber laser, and the output end is aligned with the input end of the grating coupler 3. The output end of the grating coupler 3 is connected to the ridged lithium niobate main waveguide 4, which is connected to the input end of the Mach-Zehnder intensity modulator 5. The Mach-Zehnder intensity modulator 5 has an axisymmetric structure along the extension of the lithium niobate main waveguide 4 and includes two modulation arm waveguides, two multimode interference couplers, and three gold electrodes. The two multimode interference couplers are interconnected with the ridged lithium niobate waveguide 4. The main waveguide 4 and two modulation arm waveguides are provided. Three gold electrodes are located on both sides and in the middle of the two modulation arm waveguides, respectively. The electrodes on both sides are grounded. The electrodes located on the inner side of the two arms of the modulator are controlled by a voltage source. The output end of the Mach-Zehnder intensity modulator 5 is connected to the planar lithium niobate waveguide 6 through adiabatic broadening. The metasurface (i.e., polarization-modulated metasurface 7) is integrated on the top of the planar lithium niobate waveguide 6. The metasurface (i.e., polarization-modulated metasurface 7) is composed of silicon nanopillars of specific size and arranged in a specific pattern. The aspect ratio and height of the silicon quadrangular prisms are precisely designed to be equivalent to a quarter waveplate at a wavelength of 795nm. By precisely designing the period and rotation angle of the nanopillars, the nanopillar array is given a specific geometric phase value for the outgoing light to control the far-field emission angle. The output end of the metasurface (i.e., polarization-modulated metasurface 7) faces free space, and the theoretical output angle is 0°.

[0075] A voltage source is disposed on the outer side of the thin-film lithium niobate platform 1 on the insulator. The voltage source is connected to the inner electrode of the Mach-Zehnder intensity modulator 5. The voltage source generates an electric field along the optical axis of the crystal between the push-pull structure electrodes by controlling the modulation voltage signal applied to the electrodes. The effective refractive index of the light in the waveguide is changed by the first-order electro-optic effect of the lithium niobate material, so that a phase difference is generated in the two arms of the Mach-Zehnder interference structure, thereby realizing intensity modulation of the output light. The intensity-modulated light is transmitted along the waveguide propagation direction to the polarization-modulated metasurface 7 region integrated above the planar lithium niobate waveguide 6. The guided mode light is decoupled to free space through the coupling effect of the waveguide and the metasurface, and the linearly polarized light is converted to a specified circularly polarized light through the geometric phase modulation of the nanostructure, thereby obtaining the output light with a predetermined intensity modulation depth and polarization state.

[0076] Including the following expressions:

[0077] ,

[0078] ,

[0079] in It is the half-wave voltage of the electro-optic modulator, the voltage required for the output light intensity to change from maximum to minimum, where λ is the incident light wavelength, d is the distance between the two electrodes, and n is the voltage required for the output light intensity to change from maximum to minimum. e For the effective refractive index, r 33 Γ represents the electro-optic coefficient along the z-axis, the operating direction of lithium niobate; L is the length of the modulation region; Γ is the normalized overlap integral of the applied electric field and the TEO mode of the optical waveguide; k0 is the incident light wavenumber; θ is the angle between the emitted light and the metasurface normal; and β is the propagation constant of light in the lithium niobate planar waveguide. denoted as the geometric phase step of the metasurface nanopillar array, where m is an integer and p is the period of the nanopillar along the direction of light propagation.

[0080] In step 1, a 795 nm linearly polarized laser generated by the laser is guided into a grating coupler via a polarization-maintaining fiber. Under the diffraction coupling effect of the grating structure, the linearly polarized light propagating in free space is coupled into the thin-film lithium niobate waveguide, exciting the propagation of the TE fundamental mode in the waveguide. The polarization-maintaining fiber is used to keep the polarization direction of the incident light stable, ensuring that the polarization direction of the incident light is consistent with the designed coupling polarization direction of the grating coupler. This improves the grating coupling efficiency and ensures that the guided mode light entering the waveguide maintains a stable linear polarization state, enabling the optical signal to propagate stably along the propagation direction in the thin-film lithium niobate waveguide.

[0081] In step 2, a modulation voltage signal is applied to the push-pull structure electrodes of the thin-film lithium niobate electro-optic modulator using a signal driving system, generating an electric field distribution along the optical axis of the lithium niobate crystal between the electrodes. This electric field alters the effective refractive index of light in the waveguide through the Pockels electro-optic effect of the lithium niobate material, causing a difference in the propagation constants of the two arms of the Mach-Zehnder interference structure, thereby creating a controllable phase difference between the two arms. This phase difference is converted into a change in light intensity at the interference output end, causing the output light intensity to be periodically modulated with the driving voltage, thus achieving intensity modulation of the guided mode light.

[0082] In step 3, the guided-mode light, after intensity modulation in step 2, continues to propagate along the waveguide propagation direction to the polarization-modulated metasurface region integrated at the top of the waveguide. The waveguide metasurface is composed of a subwavelength-scale array of nanostructures, each arranged at a preset rotation angle to form a spatial geometric phase gradient. During waveguide propagation, the guided-mode light gradually decouples from the waveguide mode and radiates into free space through scattering coupling with the nanostructures. Simultaneously, the geometric phase modulation generated by the nanostructures modulates the polarization state of the outgoing light, converting the originally linearly polarized guided-mode light into circularly polarized light with a specified chirality, thus obtaining output light with both a predetermined intensity modulation depth and a predetermined polarization state.

[0083] In step 4, the output optical signal radiated from the waveguide metasurface into free space is received by the photoelectric detection system and converted into an electrical signal. By changing the amplitude of the driving voltage applied to the electrodes of the electro-optic modulator by the signal driving system, the modulation curve of the output light intensity changing with the driving voltage is measured, and the extinction ratio and half-wave voltage of the electro-optic modulator are calculated based on the modulation curve. At the same time, the polarization state of the output light is measured by the polarization analysis system, and the circular polarization degree and polarization conversion efficiency of the output light are calculated by measuring the Stokes parameter or ellipticity parameter of the output light, thereby characterizing the modulation performance and polarization control performance of the thin-film lithium niobate electro-optic modulator and the waveguide metasurface integrated device.

[0084] The above method enables the coordinated control of the intensity and polarization state of the guided mode light on a thin-film lithium niobate platform, so that the output light has both modulated intensity and controllable circular polarization state.

[0085] This invention discloses an integrated electro-optic modulator and its modulation method based on a thin-film lithium niobate platform for intensity and polarization synergistic modulation. The device includes a grating coupler, a thin-film lithium niobate ridge waveguide, an electro-optic modulation electrode structure, a Mach-Zehnder interferometry modulation unit, and a polarization-tunable metasurface structure integrated on the top of the waveguide. 795 nm linearly polarized light generated by a laser is coupled into the thin-film lithium niobate waveguide via the grating coupler. In the Mach-Zehnder interferometry modulation unit, the incident light is split into two beams by a 3 dB multimode interferometry coupler and propagates in both arms of the waveguide. An electric field along the optical axis of the lithium niobate crystal is generated in the waveguide by the electro-optic modulation electrode. The effective refractive index of the waveguide is changed using the Pockels electro-optic effect of the lithium niobate material, creating a controllable phase difference between the two arms, which is converted into a change in light intensity at the beam combining end, thereby achieving intensity modulation of the guided mode light. The intensity-modulated guided-mode light propagates along the waveguide to the metasurface region integrated at the top of the waveguide. Decoupled radiation of the guided-mode light into free space is achieved through coupling between the guided-mode light and the subwavelength nanostructure array. Furthermore, Pancharatnam-Berry geometric phase modulation generated by the nanostructures enables the conversion of linearly polarized light to a specified circularly polarized state, resulting in output light radiating near the normal direction. Applying the technical solution of this invention, on-chip integration of intensity modulation and polarization state control can be achieved on a thin-film lithium niobate photonic platform, significantly reducing the volume and complexity of the modulation optical path in an atomic magnetometer system. This provides a key modulation light source device for the miniaturization and integration of Bell-Bloom type rubidium atomic magnetometers.

[0086] An electro-optic modulator based on a thin-film lithium niobate waveguide integrated metasurface includes an optical input unit, an on-chip optical coupling unit, a waveguide transmission unit, an electro-optic intensity modulation unit, and a polarization control unit.

[0087] The optical input unit includes a polarization-maintaining fiber (2), the input end of which is connected to a 795nm fiber laser, used to stably transmit the linearly polarized light generated by the laser to the on-chip optical structure and maintain the polarization state of the input light.

[0088] The on-chip optical coupling unit includes a grating coupler (3) disposed on the surface of a thin-film lithium niobate platform (1) on an insulator. The output end of the polarization-maintaining fiber (2) is aligned with the input end of the grating coupler (3). The grating coupler (3) is used to couple the linearly polarized light output from the polarization-maintaining fiber into the lithium niobate waveguide and excite transverse electric (TE) guided mode propagation.

[0089] The waveguide transmission unit includes a ridged lithium niobate main waveguide (4), and the output end of the grating coupler (3) is connected to the ridged lithium niobate main waveguide (4) for guided mode transmission of optical signals on the thin-film lithium niobate platform.

[0090] The electro-optic intensity modulation unit is a Mach-Zehnder intensity modulator (5). The Mach-Zehnder intensity modulator (5) is set on the ridge-type lithium niobate main waveguide (4) and includes two modulation arm waveguides, two multimode interference couplers and three gold electrodes. The two multimode interference couplers are respectively connected between the ridge-type lithium niobate main waveguide (4) and the two modulation arm waveguides to split the input light and re-combine it after modulation. The three gold electrodes are respectively located on both sides and in the middle of the two modulation arm waveguides. The electrodes on both sides are grounded and the electrodes located on the inner side of the two modulation arms are connected to an external voltage source. By applying a modulation voltage between the electrodes, an electric field along the optical axis of the crystal is generated in the lithium niobate waveguide. The Pockels electro-optic effect of the lithium niobate material is used to change the effective refractive index of the waveguide, so that the light in the two modulation arms generates a phase difference and is converted into a change in light intensity at the output end, thereby realizing the intensity modulation of the guided mode light.

[0091] The output of the Mach-Zehnder intensity modulator (5) is connected to the planar lithium niobate waveguide (6) through an adiabatic broadening structure. The planar lithium niobate waveguide (6) is used to transmit the modulated guided mode light to the metasurface region and provide a uniform guided mode light field distribution for the metasurface structure.

[0092] The polarization control unit includes a metasurface (i.e., polarization control type metasurface 7) disposed on top of the planar lithium niobate waveguide (6). The metasurface (i.e., polarization control type metasurface 7) is composed of an array of silicon nanopillars of specific size arranged according to a preset rule. The aspect ratio and height of the silicon nanopillars are optimized and designed to be equivalent to a quarter-wave plate structure at a working wavelength of 795nm. By precisely designing the period and rotation angle of the nanopillars, the nanopillar array is given a specific Pancharatnam-Berry geometric phase of the outgoing light, thereby realizing the control of the propagation direction and polarization state of the outgoing light.

[0093] After being modulated by the metasurface (i.e., polarization-modulated metasurface 7), the guided mode light is decoupled from the planar lithium niobate waveguide (6) and radiated into free space. Its theoretical exit direction is near the normal direction, that is, the exit angle is 0°. At the same time, the polarization state of the output light can be converted into circularly polarized light with a specified chirality.

[0094] By integrating a Mach-Zehnder electro-optic intensity modulation structure and a polarization-controlled metasurface structure on a thin-film lithium niobate waveguide, this invention achieves integrated output of guided mode light intensity modulation and polarization state control. Intensity modulation and circular polarization output can be obtained simultaneously in a single on-chip device, providing a miniaturized and integrated modulation light source device for Bell-Bloom type rubidium atomic magnetometer systems.

[0095] An electro-optic modulator system based on a thin-film lithium niobate waveguide integrated metasurface includes: a thin-film lithium niobate platform on an insulator, an optical input unit, an electro-optic modulation unit, and a polarization control unit.

[0096] The thin-film lithium niobate platform on the insulator is used to form the basic structure of on-chip photonic devices. Its surface is processed to form grating couplers, ridge-type lithium niobate waveguides, Mach-Zehnder intensity modulators, and planar lithium niobate waveguide structures to realize the coupling, transmission, and modulation of optical signals.

[0097] The optical input unit includes a polarization-maintaining fiber and a 795nm fiber laser. The polarization-maintaining fiber is used to stably transmit the linearly polarized light output from the fiber laser to the on-chip grating coupler and keep the polarization direction of the input light stable, so as to improve the grating coupling efficiency.

[0098] The grating coupler is disposed on the surface of the thin-film lithium niobate platform on the insulator and is used to couple the linearly polarized light output from the polarization-maintaining fiber into the lithium niobate waveguide and excite transverse electric (TE) guided mode propagation.

[0099] The electro-optic modulation unit is a Mach-Zehnder intensity modulator, which includes two modulation arm waveguides, two multimode interference couplers, and three gold electrodes. The two multimode interference couplers are used to split and combine the guided mode light, respectively. The three gold electrodes are located on both sides and in the middle of the two modulation arm waveguides, respectively. The electrodes on both sides are grounded, and the electrodes located on the inner side of the two modulation arms are connected to an external voltage source. By applying a modulation voltage to the electrodes, an electric field along the optical axis of the crystal is generated in the lithium niobate waveguide. This utilizes the Pockels electro-optic effect of the lithium niobate material to change the effective refractive index of the waveguide, causing a phase difference in the light in the two modulation arms, which is then converted into a change in light intensity at the output end, thereby achieving intensity modulation of the guided mode light.

[0100] The output of the Mach-Zehnder intensity modulator is connected to a planar lithium niobate waveguide via an adiabatic broadening structure, which is used to transmit the modulated guided mode light to the metasurface region and provide a uniform guided mode light field distribution.

[0101] The polarization control unit includes a metasurface structure disposed on top of a planar lithium niobate waveguide. The metasurface is composed of an array of silicon nanopillars of specific size arranged according to a preset pattern. The aspect ratio and height of the silicon nanopillars are optimized and designed to be equivalent to a quarter-wave plate structure at a working wavelength of 795 nm. By precisely designing the period and rotation angle of the nanopillars, the nanopillar array imparts a specific Pancharatnam-Berry geometric phase to the outgoing light, thereby achieving control over the propagation direction and polarization state of the outgoing light.

[0102] The guided mode light, after being modulated by the metasurface, is decoupled from the planar lithium niobate waveguide and radiates into free space. Its theoretical exit direction is near the normal direction, that is, the exit angle is 0°. At the same time, the polarization state of the output light can be converted into circularly polarized light with a specified chirality.

[0103] The electrodes of the electro-optic modulation unit are connected to an external signal driving system to apply a modulation voltage signal to the modulation electrodes, thereby adjusting the intensity modulation depth and modulation frequency of the output light.

[0104] By integrating a Mach-Zehnder electro-optic intensity modulation structure and a polarization-controlled metasurface structure on a thin-film lithium niobate waveguide, this invention achieves integrated output of guided mode light intensity modulation and polarization control. Intensity modulation and circular polarization output can be obtained simultaneously in a single on-chip device, providing a miniaturized and integrated modulation light source device for Bell-Bloom type rubidium atomic magnetometer systems.

[0105] The specific principle of this invention is as follows:

[0106] Lithium niobate, due to its wide transparency window and high electro-optic coefficient, has become an ideal substrate for constructing electro-optic modulators. The refractive index *n* of the material as a function of the electric field can be expressed by the following formula.

[0107] (1)

[0108] In the formula, Let represent the refractive index without an applied electric field, E be the electric field strength, and a, b, and c be coefficients. When the refractive index is linearly related to the applied electric field (a≠0, b=c=0), it is called the first-order electro-optic effect, also known as the Pockels effect.

[0109] Lithium niobate is an anisotropic negative uniaxial crystal, and its Pockels effect under an applied electric field can be described in tensor form.

[0110] (3-10)

[0111] In the formula Let i be a second-order tensor representing the refractive index change, where i and j are both positive integers. Let E be the component of the electric field E in the k direction (representing one of x, y, z). Let be the electro-optic coefficient tensor element. The electro-optic coefficient tensor matrix of lithium niobate material can be simplified from the refractive index ellipsoid formula and Voigt notation:

[0112] (2)

[0113] In the design of electro-optic modulators, it is generally considered to apply an external electric field along the optical axis to utilize the maximum electro-optic coefficient of lithium niobate. pm stands for picometer, V stands for volt, and r stands for r. 13 r 22 r 51 Both are electro-optic coefficients, thus achieving the maximum refractive index change. When an electric field is applied along the z-axis... When, the refractive index changes in the three directions as follows:

[0114] (3)

[0115] (4)

[0116] in It is the change in refractive index in the xy-plane direction. It is the refractive index in the xy-plane direction. It is the change in refractive index in the z-direction. This refers to the refractive index in the z-direction. The most common lithium niobate EOM (electro-optical matrix) is the Mach-Zehnder interferometer (MZI) modulator, which operates by fabricating a waveguide on a lithium niobate wafer. This type of modulator commonly uses x-cut wafers, meaning wafers with their planes perpendicular to the crystal's x-axis. When designing the modulator, it's crucial to ensure that the electric field is applied along the crystal axis to utilize the maximum electro-optic coefficient of lithium niobate. To achieve optimal modulation, the electric field of the transmission mode in the waveguide should also have a polarization direction consistent with the optical axis. To meet these requirements, the modulator fabricated from x-cut lithium niobate needs to employ a horizontal electrode structure, and the transmission mode in the waveguide must be a transverse electric (TE) mode.

[0117] In a Mach-Zehnder modulator (MZM), the optical signal is input into the waveguide. It can be represented as

[0118] (5)

[0119] in Here, e is the amplitude, j is the natural constant, and j is the imaginary unit. Here, 't' is the frequency, and 't' is time. The input light is split into two equal paths by a 3dB beam splitter, entering the upper and lower arms of the modulator. The optical signal at this point can be represented as...

[0120] (6)

[0121] in It is an input light signal from the upper arm. The lower arm receives the input optical signal, and the two arms of the modulator respectively form two phase modulators. For a single phase modulator, the modulation signal... The resulting phase change is

[0122] (7)

[0123] In the formula It represents the phase change of a single arm of the modulator under the signal V(t), where V(t) is the modulating signal. In optical waveguides The change in the effective refractive index of the mode, n e For effective refractive index, For wavelength, The length of the modulation region. The distance between the two electrodes. For external electric field and optical waveguide The normalized overlap integral of the mode, TE0 is the lowest order transverse electric mode. Equation (7) can be simplified to the following equation:

[0124] (8)

[0125] in Let be the half-wave voltage of the modulator's single arm, which is the voltage required for the optical phase change π. According to equations (7) and (8), the half-wave voltage can be written as:

[0126] (9)

[0127] MZM typically employs a push-pull modulation structure, where the central electrode is connected to the signal source, and the two side electrodes are grounded. In this configuration, the electric fields on both arms of the modulator are equal in magnitude but opposite in direction.

[0128] As can be seen from equation (7), the change in the optical phase caused by the modulator is proportional to the applied electric field. Therefore, the changes in the optical phase of the two arms of the modulator should also be equal in magnitude and opposite in sign, i.e.

[0129] (10)

[0130] (11)

[0131] in It is the upper arm that outputs the optical signal. This is the output optical signal from the lower arm. After passing through the 3dB combiner, the output light can be represented as...

[0132]

[0133]

[0134] (12)

[0135] in It is the output optical signal. The output optical power is proportional to the square of the output optical signal amplitude, therefore...

[0136] (13)

[0137] in This refers to the output optical power. The static response of a Mach-Zehnder modulator (MZM) It can be defined as

[0138] (14)

[0139] in Also known as the modulator's transfer function. It is the input optical power. And because... Equation (3-23) can also be written as

[0140] (15)

[0141] The voltage required to change the output optical power from maximum to minimum is defined as the half-wave voltage of the MZM. From equation (15), it can be seen that the half-wave voltage of the push-pull MZM is half of the half-wave voltage of the single-arm phase modulator, that is... .

[0142] The metasurface on the waveguide is designed to be periodic, and the outgoing process follows the discrete transfer of the parallel wave vector given by scalar diffraction theory: (16)

[0143] in , These are all intermediate quantities under waveguide propagation conditions. And in free space, Therefore, the phase-matching formula for the leakage channel (the output channel of light with the same chirality as the incident light) is obtained:

[0144] (17)

[0145] According to the generalized Snell's law, the rotational gradient of the nanopillar introduces a phase gradient into the outgoing light. Therefore, the phase-matching equation for the cross-outgoing channel (the output channel of light with the opposite chirality to the incident light) is:

[0146] (18)

[0147] To design near-normal exit (θ = 0°), take =30° / period =300nm, which allows m= -1 order light to exit near the normal. When the nanopillar size is designed to be 220nm×100nm×380nm, high-purity left-handed linearly polarized light with circular polarization degree DoCP < -0.99 is obtained in the far field. period is the period (representing the distance between two adjacent nanopillars).

[0148] This invention also discloses a method for achieving intensity modulation and polarization control using the above-mentioned thin-film lithium niobate waveguide integrated metasurface electro-optic modulator, specifically including the following steps:

[0149] Step 1: Linearly polarized light output from a 795nm fiber laser is input to an on-chip grating coupler via a polarization-maintaining fiber. Under the action of the grating coupler, the light is coupled into a thin-film lithium niobate waveguide and excited to propagate a transverse conductive mode. A modulation voltage is applied to the Mach-Zehnder intensity modulator electrode by an external signal driving system, generating an electric field along the optical axis of the lithium niobate crystal in the modulation arm waveguide. The effective refractive index of the waveguide is changed by utilizing the Pockels electro-optic effect, causing a phase difference in the light in the two modulation arms, which is converted into a change in light intensity at the beam combining end, thereby obtaining intensity-modulated guided mode light.

[0150] Step 2: The intensity-modulated guided mode light is transmitted along the planar lithium niobate waveguide to the metasurface region integrated on the top of the waveguide. The guided mode light is decoupled from the radiation of the guided mode light into free space through the coupling between the guided mode light and the subwavelength silicon nanopillar array. The linearly polarized light is converted into a specified chiral circularly polarized light through the Pancharatnam-Berry geometric phase modulation generated by the nanopillar array. At the same time, the output light is controlled to radiate in the near-normal direction, thereby obtaining the output light with predetermined intensity modulation characteristics and polarization state.

[0151] Step 3: By adjusting the amplitude and frequency of the driving voltage input to the Mach-Zehnder modulator electrodes, measure the modulation response curve of the output light intensity as a function of the driving signal, and calculate the extinction ratio and half-wave voltage of the electro-optic modulator; at the same time, measure the polarization state of the output light through a polarization analysis system to obtain the Stokes parameter or ellipticity parameter of the output light, thereby obtaining the polarization conversion efficiency.

[0152] Step 4: By continuously adjusting the driving voltage signal parameters and the input optical power, the intensity modulation characteristics and polarization control characteristics of the output light are jointly optimized to obtain modulated light output that meets the specific application requirements.

[0153] An electro-optic modulator system based on a thin-film lithium niobate waveguide integrated metasurface, see details. Figure 1 The structure includes a thin-film lithium niobate platform on an insulator (1), a polarization-maintaining fiber (2), a grating coupler (3), a ridge-type lithium niobate main waveguide (4), a Mach-Zehnder intensity modulator (5), a planar lithium niobate waveguide (6), and a metasurface (7).

[0154] The polarization-maintaining fiber is used to input the linearly polarized light output from the fiber laser to the grating coupler; the grating coupler is used to couple the incident light into the lithium niobate waveguide to form guided mode light; the ridge-type lithium niobate waveguide is used for guided mode transmission of optical signals.

[0155] The Mach-Zehnder intensity modulator is used to achieve intensity modulation of the guided mode light. It splits and combines the light beams through a multimode interference coupler and changes the refractive index of the waveguide by applying an electric field through electrodes, thereby generating a phase difference and achieving light intensity modulation.

[0156] The planar lithium niobate waveguide is used to transmit the modulated guided mode light to the metasurface region and form a uniform light field distribution.

[0157] The metasurface is composed of an array of silicon nanopillars. Geometric phase modulation enables the radiative coupling of the guided mode light into free space and modulates the propagation direction and polarization state of the outgoing light, so that the output light is emitted in the near-normal direction and forms circularly polarized light.

[0158] The signal driving system is connected to the Mach-Zehnder modulator electrodes and is used to control the input voltage signal, thereby adjusting the modulation depth and modulation frequency of the output light.

[0159] Through the above methods and systems, this invention achieves coordinated control of guided mode light intensity modulation and polarization state regulation, enabling simultaneous intensity modulation and circular polarization output in a single on-chip device. This provides a miniaturized and integrated modulation light source for Bell-Bloom type rubidium atomic magnetometer systems and is beneficial for on-chip integration and performance improvement of atomic magnetometer systems.

[0160] The dynamic electro-optic modulator based on the thin-film lithium niobate waveguide integrated metasurface of this embodiment is used for pump optical path modulation in a Bell-Bloom type rubidium atomic magnetometer. The modulation method includes the following steps:

[0161] Step 1: Linearly polarized light with a center wavelength of 795nm is output from a fiber laser and input to an on-chip grating coupler through a polarization-maintaining fiber. Under the action of the grating coupler, the incident light is coupled into a thin-film lithium niobate waveguide and excited transverse conductive mode propagation. A modulation voltage is applied to the electrodes of the Mach-Zehnder intensity modulator through a signal driving system, generating an electric field along the optical axis of the lithium niobate crystal in the two modulation arm waveguides. The Pockels electro-optic effect of the lithium niobate material is used to change the effective refractive index of the waveguide, causing a phase difference in the light in the two modulation arms. The light is then combined at the multimode interference coupler and converted into a change in light intensity, thereby obtaining a modulated guided mode light whose intensity varies with time.

[0162] Step 2: The intensity-modulated guided mode light is transmitted along the thermally adiabatic broadened planar lithium niobate waveguide to the metasurface region integrated on the top of the waveguide. Through the scattering coupling between the guided mode light and the subwavelength silicon nanopillar array, the guided mode light is decoupled from the waveguide and radiated into free space. At the same time, the Pancharatnam-Berry geometric phase modulation generated by the silicon nanopillar array is used to realize the conversion of linearly polarized light to a specified chiral circularly polarized light, and the outgoing light is radiated in the near-normal direction, thereby obtaining circularly polarized modulated pump light that satisfies the atomic spin polarization condition.

[0163] Step 3: The circularly polarized modulated pump light is coupled into the rubidium atom gas cell to optically pump the alkali metal atoms. Under the action of an external magnetic field, the atomic spins generate Larmor precession driven by the modulated light. By adjusting the amplitude and frequency of the driving voltage of the Mach-Zehnder intensity modulator, the modulation frequency of the output light intensity is made close to or matched with the Larmor precession frequency, thereby enhancing the resonance response of the atomic spin system.

[0164] Step 4: By continuously adjusting the modulation voltage signal parameters and input optical power, the intensity modulation characteristics and polarization control characteristics of the output pump light are optimized to meet the requirements of the Bell-Bloom type rubidium atomic magnetometer for pump light modulation conditions, thereby improving the amplitude and signal-to-noise ratio of the magneto-optical resonance signal.

[0165] By integrating an electro-optic intensity modulation structure and a polarization-controlled metasurface structure onto a thin-film lithium niobate waveguide, the technical solution of this embodiment achieves integrated pump light intensity modulation and circular polarization output, enabling modulation and polarization conversion functions to be completed on a single on-chip device. Compared with traditional solutions using discrete electro-optic modulators and waveplate combinations, this invention significantly reduces the number of optical components, simplifies the optical path structure, and improves system stability and integration. This modulator is particularly suitable for pump light path modulation in Bell-Bloom type rubidium atomic magnetometers, facilitating miniaturization, on-chip integration, and performance improvement of magnetometer systems, and can be widely applied in related fields such as atomic clocks and quantum precision measurement.

[0166] Contents not described in detail in this specification are existing technologies known to those skilled in the art. It is hereby indicated that the above description is intended to help those skilled in the art understand the present invention, but does not limit the scope of protection of the present invention. Any equivalent substitutions, modifications, improvements, and / or simplifications of the above descriptions that do not depart from the essence of the present invention fall within the scope of protection of the present invention.

Claims

1. An electro-optic modulator based on thin-film lithium niobate and metasurface, characterized in that, The device includes a thin-film lithium niobate platform on an insulator. A grating coupler is disposed on the inner side of the left-center portion of the thin-film lithium niobate platform. The left end of the grating coupler has a linearly polarized light input interface. The right end of the grating coupler is connected to the input end of a Mach-Zehnder intensity modulator via a ridge-type lithium niobate main waveguide. The output end of the Mach-Zehnder intensity modulator is connected to a planar lithium niobate waveguide via the ridge-type lithium niobate waveguide. The planar lithium niobate waveguide is formed by thermally widening the right end of the ridge-type lithium niobate waveguide to the right. A polarization-modulating metasurface is integrated on the surface of the planar lithium niobate waveguide. The polarization-modulating metasurface is used to realize the conversion of linearly polarized light to a specified circularly polarized state, thereby obtaining output light with a predetermined intensity modulation depth and a predetermined polarization state.

2. The electro-optic modulator based on thin-film lithium niobate and metasurface according to claim 1, characterized in that, The input terminal of the Mach-Zehnder intensity modulator is the left end of the first multimode interference coupler, and the output terminal of the Mach-Zehnder intensity modulator is the right end of the second multimode interference coupler. The upper side of the first multimode interference coupler is connected to the bottom left end of the first modulation arm waveguide, which is convex at the top. The upper side of the second multimode interference coupler is connected to the bottom right end of the first modulation arm waveguide. The lower side of the first multimode interference coupler is connected to the top left end of the second modulation arm waveguide, which is concave at the bottom. The lower side of the second multimode interference coupler is connected to the top right end of the second modulation arm waveguide. A first gold electrode is disposed above the first modulation arm waveguide. A second gold electrode is disposed between the first and second modulation arm waveguides. A third gold electrode is disposed below the second modulation arm waveguide. The second gold electrode has a voltage source interface. Both the first and third gold electrodes are grounded.

3. The electro-optic modulator based on thin-film lithium niobate and metasurface according to claim 1, characterized in that, The polarization-tunable metasurface is composed of a subwavelength-scale dielectric nanostructure array, which consists of silicon nanopillars of a predetermined size and arranged according to a predetermined period. The coupling of guided mode light to free space is achieved by changing the geometric parameters and spatial arrangement of the nanounits.

4. The electro-optic modulator based on thin-film lithium niobate and metasurface according to claim 2, characterized in that, The multimode interference coupler is a 3dB multimode interference coupler.

5. The electro-optic modulator based on thin-film lithium niobate and metasurface according to claim 1, characterized in that, The linearly polarized light input interface is connected to the laser via a polarization-maintaining fiber, and the voltage source interface is connected to a voltage source.

6. The electro-optic modulator based on thin-film lithium niobate and metasurface according to claim 1, characterized in that, The Mach-Zehnder intensity modulator includes the following expression: ， ， In the formula It represents the phase change of a single arm of the modulator under the signal V(t), where V(t) is the modulating signal. Where λ is the length of the modulation region, and λ is the wavelength of the incident light. In optical waveguides The change in the effective refractive index of the mode, TE0 is the lowest order transverse electric mode, n e For effective refractive index, It is the electro-optic coefficient of lithium niobate along the z-axis. For external electric field and optical waveguide Normalized overlap integral of the pattern, The distance between the two electrodes. It is half-wave voltage, which is the voltage required to change the output optical power from maximum to minimum.

7. The electro-optic modulator based on thin-film lithium niobate and metasurface according to claim 1, characterized in that, The polarization-tunable metasurface comprises the following formula: ， ， In the formula Let be the incident light wavenumber, θ be the angle between the outgoing light and the normal, and β be the propagation constant of the waveguide TE0 mode. The geometric phase gradient is caused by the rotational angular gradient of the nanopillar, where m is the diffraction order. The period of the nanopillar is in the direction of light propagation.

8. An electro-optic modulation method based on thin-film lithium niobate and metasurfaces, characterized in that, An electro-optic modulator based on thin-film lithium niobate and metasurface as described in any one of claims 1-7 is used.

9. The electro-optic modulation method based on thin-film lithium niobate and metasurface according to claim 8, characterized in that, Includes the following steps: Step 1: Guide the 795nm linearly polarized light generated by the laser into the grating coupler through the polarization-maintaining fiber to excite the transverse electric TE fundamental mode light. Step 2: Apply a modulation voltage signal to the electrodes in the Mach-Zehnder intensity modulator using a signal driving system to generate an electric field along the optical axis of the crystal between the push-pull structure electrodes. Change the effective refractive index of the light in the waveguide through the Pockels electro-optic effect of lithium niobate material to generate a phase difference in the two arms of the Mach-Zehnder interference structure, thereby achieving intensity modulation of the output light. Step 3: The intensity-modulated light is transmitted along the waveguide propagation direction to the polarization-modulated metasurface region integrated at the top of the waveguide to obtain output light with a predetermined intensity modulation depth and a predetermined polarization state; Step 4: Receive the output light signal through the photoelectric detection system, measure the modulation curve of the output light intensity as a function of the driving voltage, and calculate the extinction ratio and half-wave voltage of the electro-optic modulator; measure the Stokes parameter or ellipticity of the output light through the polarization analysis system to obtain the polarization conversion efficiency of the output light.

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