Single-electrode integrated multimode optical phased array based on thin film lithium niobate platform

Abstract

The invention provides a single-electrode integrated multimode optical phased array based on a thin-film lithium niobate platform, and relates to the technical field of photoelectronics, and the optical phased array comprises a mode conversion module, a phase modulation module and a grating antenna. The mode conversion module, the phase modulation module and the grating antenna are connected in sequence; the mode conversion module is used for converting the input fundamental mode light into a high-order mode; the phase modulation module is used for modulating the phase difference between the high-order mode spots through an external electric field; and the grating antenna is used for radiating the modulated multimode light to a free space for interference so as to realize beam steering. The technical scheme of the invention has the main advantages that a traditional multichannel phased array which needs a large number of light splitting arrays is replaced by multimode light in one waveguide, a phase control unit is simplified, power consumption and loss are reduced, the size is reduced, and process errors do not need to be considered.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optoelectronics, and in particular to a single-electrode integrated multi-mode optical phased array based on a thin-film lithium niobate platform. BACKGROUND

[0002] Optical phased arrays have emerged as a simple, cost-effective, and lightweight solution for laser beam steering, showing great potential in applications such as light detection and ranging systems, 3D imaging, space laser communication, and photonic switching.

[0003] Integrated photonic-based optical phased arrays have been implemented on a variety of material platforms, including silicon, silicon nitride, and indium phosphide. In particular, silicon platforms have achieved significant success due to their mature processing technology and compatibility with complementary metal-oxide-semiconductor (CMOS) processes. However, silicon-based integrated optical phased arrays have limitations such as two-photon absorption, high third-order nonlinearity, and high power consumption, which limit their scalability and output power. Silicon nitride has emerged as an alternative platform for integrated optical phased arrays, offering low nonlinear coefficients and low propagation losses, which can overcome the limitations of silicon-based optical phased arrays. Despite these advantages, most silicon and silicon nitride integrated operational amplifiers achieve phase modulation per channel through the thermo-optic effect, which limits the modulation speed and power consumption, especially in systems with hundreds of thousands of phase modulators.

[0004] Lithium niobate has a wide transparent window (0.35-5 μm), low absorption loss, piezoelectric effect, high second-order nonlinear optical coefficient, and relatively high Pockels electro-optic coefficient. In particular, the electro-optic properties of lithium niobate are well suited for phase modulation in optical phased arrays, while providing high modulation efficiency and low optical loss. However, traditional bulk lithium niobate waveguides are mostly fabricated through metal diffusion, ion implantation, and proton exchange, which result in a large bending radius, hindering their large-scale photonic circuits. With the breakthroughs in manufacturing technology of thin-film lithium niobate on insulator (LNOI) over the past few years, LNOI has emerged as a high-performance integrated photonic platform that combines the superior optical properties and electro-optic effects of lithium niobate materials with the large refractive index contrast of LNOI, making it possible for future large-scale integrated optical phased arrays.

[0005] In traditional optical phased array systems, each channel requires a separate electronic unit for control. Therefore, as the number of channels increases, not only will the complexity of the control electronics increase, but so will the power consumption. Researchers have achieved beam control in a single lithium niobate waveguide structure by combining microstructure sawtooth electrodes and gradient microstructure electrodes. While this approach simplifies the control structure, it is limited by the constrained scan angle. SUMMARY

[0006] The application aims at solving the problems of complex electric control equipment, high power consumption and loss, and phase calibration of devices due to process errors in conventional multi-channel optical phased arrays, and using thin film lithium niobate material because of its superior electro-optic effect to achieve fast beam steering.

[0007] The above-mentioned purpose of the application is realized by the following technical scheme: The optical phased array comprises a mode conversion module, a phase modulation module and a grating antenna; the mode conversion module, the phase modulation module and the grating antenna are sequentially connected in order; The mode conversion module is used for converting input fundamental mode light into high-order mode; The phase modulation module is used for modulating the phase difference between high-order mode spots by an external electric field; The grating antenna is used for radiating the modulated multi-mode light to free space for interference to realize beam steering.

[0008] Further, the optical phased array is designed based on a standard CMOS process LNOI wafer platform of a Z-cut thin film lithium niobate layer and an oxide layer.

[0009] Further, the mode conversion module, the phase modulation module and the grating antenna are sequentially connected and all located in a flat plate multi-mode waveguide.

[0010] Further, the lithium niobate waveguide mode converter comprises an input port, an output port and a coupling region for completing energy coupling of mode conversion.

[0011] Further, the mode converter in the mode conversion module converts the fundamental mode in the waveguide into high-order mode, and the order of the mode should correspond to the number of array elements in the optical phased array.

[0012] Further, a single phase modulation module is used to replace the conventional phase shift network, and the fixed phase characteristics of each mode spot in the high-order mode are used to realize the equal phase difference condition required for beam forming and scanning of the optical phased array by using a single phase modulation unit.

[0013] Optionally, the phase modulated by the phase modulation module is in a range greater than 0 degrees and less than 360 degrees.

[0014] Optionally, a strip waveguide grating is arranged behind the phase modulation module and used for radiating the modulated multi-mode light.

[0015] The technical scheme provided by the application has the following beneficial effects: The application aims at the problem that the demand for large-scale integrated optical phased array of thin film lithium niobate platform is increasing, and proposes an integrated single lithium niobate waveguide optical phased array, which provides a highly integrated solution for beam control applications. Through the integration of mode converters, single electrode modulation zones and grating antennas, the device realizes two-dimensional beam scanning capability. The single electrode integrated multimode optical phased array uses the unique characteristics of the ferroelectric electro-optic crystal lithium niobate to realize beam control, which greatly simplifies the electronic control unit compared with the traditional on-chip optical phased array. The single electrode integrated multimode optical phased array can realize an inter-element spacing of less than half a wavelength, thereby realizing 180° beam scanning. This capability opens up the possibility for applications that require precise and fast beam control within a wide field of view. The single electrode integrated multimode optical phased array proposed in the application has good performance in integration, control simplicity, efficiency and beam scanning capability. BRIEF DESCRIPTION OF DRAWINGS

[0016] The application will be further described below with reference to the drawings and examples, wherein: Figure 1 is a single electrode integrated multimode optical phased array structure in the embodiment of the application; Figure 2 is a traditional multi-channel optical phased array structure diagram; Figure 3 is a phase relationship diagram between mode spots before and after the phase modulation front area in the embodiment of the application; Figure 4 is a single electrode integrated multimode optical phased array far field beam pointing angle diagram when different voltages are applied in the embodiment of the application. DETAILED DESCRIPTION

[0017] In order to have a clearer understanding of the technical features, purposes and effects of the application, the specific embodiments of the application will be described in detail with reference to the drawings.

[0018] The embodiment of the application provides a single electrode integrated multimode optical phased array based on a thin film lithium niobate platform.

[0019] Please refer to Figure 1 , Figure 1 is a structure diagram of a single electrode integrated multimode optical phased array based on a thin film lithium niobate platform in the embodiment of the application, which mainly includes: a mode conversion module, a phase modulation module and a grating antenna; the mode conversion module, the phase modulation module and the grating antenna are sequentially connected in order; The mode conversion module is used to convert the input fundamental mode light into a high-order mode; The phase modulation module is used to modulate the phase difference between the high-order mode spots through an external electric field; The grating antenna is used for interference of the modulated multi-mode light radiation to free space to realize beam steering.

[0020] The application uses the advantages of the LNOI platform to propose a new optical phased array scheme: all the array elements are controlled by a single electronic unit regardless of the number of array elements.

[0021] The application provides an embodiment as follows: a single-electrode integrated multi-mode optical phased array based on a thin film lithium niobate includes three parts: a mode conversion area, a phase modulation area, and a grating antenna.

[0022] As an embodiment, the application uses multi-mode light input to integrate the entire optical phased array in a single multi-mode waveguide.

[0023] The application provides an embodiment as follows: the grating antenna is a periodic structure, and the optical antenna is a core device of the optical phased array.

[0024] The optical phased array is designed based on a Z-Cut thin film lithium niobate layer and an oxide layer of a standard CMOS process LNOI wafer platform.

[0025] As an embodiment, the corresponding refractive indexes of the Z-Cut thin film lithium niobate and the silicon dioxide are , The device is designed for TM mode and works in C-band for regular communication.

[0026] As an embodiment, common thin film lithium niobate wafers are divided into three types: x-cut, y-cut and z-cut, the defined types are determined according to the axis perpendicular to the wafer surface, the z-axis of x-cut and y-cut is in the plane, and the z-axis of z-cut is perpendicular to the wafer plane. In the x-cut lithium niobate wafer, in order to utilize the maximum electro-optic effect, the waveguide and the electrode need to be placed parallel to the y-axis, the electric field direction of the electrode is parallel to the polarization direction of the TE mode in the waveguide structure, at this time the maximum electro-optic effect of lithium niobate can be utilized, but this limits that the electrode must be on both sides of the waveguide. The z-cut lithium niobate wafer, because the z-axis is perpendicular to the wafer surface, when a vertical upper and lower electrode structure design is adopted, the maximum electro-optic effect of lithium niobate can be utilized, and the phase between the mode spots can be accurately controlled, the z-cut wafer plane is isotropic, and there is no mode hybridization problem, therefore, the z-cut wafer is adopted in this example.

[0027] As an embodiment, the circuit is designed and manufactured based on LNOI wafer, electron beam lithography (EBL) and inductively coupled plasma (ICP) etching are used to form the waveguide structure, and 200 nm deep waveguide patterns are transferred into lithium niobate by optimized argon plasma and CHF3. 2μm thick The cladding layer covers the entire device and forms a buffer layer between the electrode and the waveguide. The metal aluminum electrode is sputtered on a separate layer, and the connection with the external power supply is realized through a DC probe. In order to support multi-mode transmission, a multi-mode slab waveguide design is adopted as a whole.

[0028] The mode conversion module, the phase modulation module and the grating antenna are connected in sequence, and all are located in a flat plate multi-mode waveguide.

[0029] The lithium niobate waveguide mode converter in the mode conversion module includes: an input port, an output port and a coupling region for completing energy coupling of mode conversion.

[0030] The mode converter used in the mode conversion module converts the fundamental mode in the waveguide into a high-order mode, and the order of the mode should correspond to the number of array elements in the optical phased array.

[0031] The lithium niobate waveguide mode converter in the mode conversion module includes: an input port, an output port and a coupling region for completing energy coupling of mode conversion.

[0032] The single phase modulation module is used to replace the traditional phase shift network, and the phase of each mode spot in the high-order mode is fixed, so that the equal phase difference condition required by the optical phased array beam forming and scanning is realized by using a single phase modulation unit.

[0033] The phase modulated by the phase modulation module is in the range of greater than 0 degrees and less than 360 degrees.

[0034] As an embodiment, in order to realize the efficient conversion between the input fundamental mode light and the high-order mode, the mode converter needs to meet the phase matching requirement along the propagation direction and the refractive index distribution requirement in the transverse direction of the short coupling length at the same time.

[0035] As an embodiment, according to the coupling mode theory, when the light propagates in the medium, the energy of the light field can be coupled from one mode to another mode by designing a proper medium structure. In this process, the perturbation of the medium structure not only meets the phase matching requirement of the two conversion modes along the z-axis propagation direction, but also has a proper refractive index distribution in the transverse direction to obtain a proper coupling coefficient and realize a shorter coupling length.

[0036] The phase modulation module comprises a lithium niobate electro-optic modulator; and the electrode of the lithium niobate electro-optic modulator is made of a metal material.

[0037] As an embodiment, in order to realize the regulation of the phase between the high-order mode spots, the electrode of the modulation region is designed based on the Pockels effect of the thin film lithium niobate material. The refractive index of the lithium niobate material will change under the action of the applied electric field. By applying an electric field, the phase difference between the mode spots is linearly changed, so as to regulate the deflection of the far-field light beam.

[0038] As an embodiment, the lithium niobate electro-optic modulator utilizes the first-order electro-optic effect in the Pockels effect of the lithium niobate crystal to realize. The working principle is that the refractive index perturbation quantity is proportional to the applied radio frequency electric field, and the complex electro-optic modulation is realized by inputting the radio frequency signal to the electrode. According to the electro-optic tensor of lithium niobate, when the applied electric field and the polarization direction of the light mode are parallel to the z-axis, the highest electro-optic modulation can be realized.

[0039] A strip waveguide grating is arranged behind the phase modulation module and is used to radiate the modulated multi-mode light.

[0040] Figure 2is a traditional multi-channel optical phased array structure diagram, which is composed of fiber-chip coupler, cascaded power beam splitter, phase shifter and grating antenna array. The working principle is as follows: the light from the tunable laser is coupled into the chip through the fiber-chip coupler, and is divided into multiple channels by the cascaded 1x2 multimode interferometer (MMI) beam splitting network. Then it goes into the silicon waveguide of the phase shift array, and the phase of each channel can be independently controlled by adjusting the electrical power on the thermal-optic phase shifter. After the phase adjustment, the light of all channels is finally emitted into free space through the optical antenna array. By adjusting the relative phase between the array elements, the deflection of the light beam in one dimension (X direction) can be controlled, and by wavelength tuning, the deflection of the light beam in the other dimension (y direction) can be controlled. It is a great challenge to design such an optical phased array that can generate complex far-field patterns through controlled electronic devices, especially when dealing with a large number of array elements.

[0041] The present application provides a single-electrode integrated multi-mode optical phased array based on a thin film lithium niobate platform as shown in Figure 1 The device is composed of lithium niobate waveguide mode converter, phase modulator and grating antenna. After the fundamental mode light input of the light source, it first enters the mode converter, which converts the fundamental mode to high-order mode, so that the phased array has multiple channels; then it enters the phase modulation area: by applying an electric field, the phase between the high-order mode spots is controlled, so that a gradient phase difference between the mode spots is generated; finally, through the grating antenna, the modulated multi-mode light is radiated to free space for interference, thereby realizing beam steering. Due to the different applied voltages, the phase difference between the mode spots can be flexibly controlled, thereby realizing beam deflection without angle in the transverse direction. Due to the dispersion effect of the grating, by changing the wavelength of the input light, the longitudinal beam deflection can be realized.

[0042] Figure 3 The phase relationship between the mode spots after the multi-mode light passes through the phase modulation area is shown. Due to the linear electro-optic effect (Pockels effect) of lithium niobate material, the change of applied voltage and refractive index is linearly related. By controlling the direction and size of the electric field, the increase or decrease of the refractive index of lithium niobate material can be effectively controlled. For lithium niobate material, thanks to the high electro-optic coefficient of , electro-optic modulators, optical switches and optical isolators have been demonstrated on thin film lithium niobate for the modulation and processing of optical signals. As shown in Figure 3 (a), the phase difference between the mode spots before modulation is π, and the appropriate voltage is applied to the modulation electrode. Due to the triangular design of the electrode, the modulation length received by different mode spots is different, and the phase difference between the mode spots after modulation can be changed between π and 0. Here, taking 1V voltage as an example, as shown in Figure 3 (b), after modulation, the phase difference between the mode spots is 0.9π.

[0043] Figure 4 The X-direction beam steering angle of the single-electrode integrated multimode control is shown under different electric fields. If the modulation electrode is not applied with voltage, the multimode light is directly radiated to the far field by the grating antenna to interfere. Since there is no longer an eigenmode in free space, interference will occur between the mode spots, thereby realizing beam steering. The far-field beam deflection angle is shown in Figure 4 (a). The beam deflection angle is 26.7°. If the modulation electrode is applied with voltage, the phase between the multimode light spots will change, and is no longer π. Finally, the light is emitted to free space through the grating antenna. The far-field beam deflection angle is shown in Figure 4 (b). The beam deflection angle is 18.2°. Compared with the traditional optical phased array of other material platforms, the single-electrode integrated multimode optical phased array has a fast response time (about 100 fs or less), thereby realizing the potential of high-speed communication.

[0044] The single-electrode integrated multimode optical phased array based on the thin film lithium niobate platform has good beam control ability. Only the applied voltage of the phase modulation region needs to be changed to realize the regulation of the phase between different mode spots, thereby realizing the deflection of the far-field beam. By changing the wavelength of the input light, the deflection of the beam in another direction can be controlled.

[0045] The above are only exemplary embodiments of the present disclosure, and cannot limit the scope of the present disclosure. That is, any equivalent changes and modifications made in accordance with the teachings of the present disclosure are still within the scope of the present disclosure.

[0046] The present application is intended to cover any variations, uses, or adaptive changes of the present disclosure that follow the general principles of the present disclosure and include common knowledge or conventional technical means in the technical field of the present disclosure not described in the present disclosure. The specification and examples are only considered as exemplary, and the scope and spirit of the present disclosure are defined by the claims.

Claims

1. A single-electrode integrated multimode optical phased array based on thin-film lithium niobate, characterized in that: The optical phased array comprises: A mode conversion module, a phase modulation module and a grating antenna; the mode conversion module, the phase modulation module and the grating antenna are connected in sequence; The mode conversion module is used to convert the input fundamental mode light into a higher-order mode; The phase modulation module is used to modulate the phase difference between high-order mode spots by applying an external electric field; The grating antenna is used to radiate modulated multi-mode light into free space for interference, thereby achieving beam steering.

2. The single-electrode integrated multi-mode optical phased array based on a thin-film lithium niobate platform according to claim 1, characterized in that: The optical phased array is designed based on a standard CMOS process LNOI wafer platform of Z-Cut thin film lithium niobate layer and oxide layer.

3. The single-electrode integrated multi-mode optical phased array based on a thin-film lithium niobate platform according to claim 1, characterized in that: The mode conversion module, the phase modulation module and the grating antenna are connected in sequence and are all located in a planar multimode waveguide.

4. The single-electrode integrated multi-mode optical phased array based on a thin-film lithium niobate platform according to claim 1, characterized in that: The mode converter used in the mode conversion module converts the fundamental mode in the waveguide into a higher-order mode, and the order of the mode should correspond to the number of array elements in the optical phased array.

5. The single-electrode integrated multi-mode optical phased array based on a thin-film lithium niobate platform according to claim 1, characterized in that: The lithium niobate waveguide mode converter in the mode conversion module includes an input port, an output port, and a coupling region for energy coupling to complete mode conversion.

6. The single-electrode integrated multi-mode optical phased array based on a thin film mode lithium niobate platform according to claim 1, characterized in that: A single phase modulation module is used to replace the traditional phase shift network. By utilizing the fixed phase characteristics of each mode spot in the high-order mode, a single phase modulation unit is used to achieve the equal phase difference conditions required for optical phased array beam forming and scanning.

7. The single-electrode integrated multi-mode optical phased array based on a thin-film lithium niobate platform according to claim 6, characterized in that: The phase adjusted by the phase modulation module is within a range greater than 0 degrees and less than 360 degrees.

8. The single-electrode integrated multi-mode optical phased array based on a thin-film lithium niobate platform according to claim 1, characterized in that: A strip waveguide grating is arranged behind the phase modulation module for radiating modulated multi-mode light.

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