Back-emitting lidar chip based on nanobeam switch array

Abstract

A back-emitting lidar transmitter chip based on a nanobeam switch array includes a nanobeam switch routing network, a metal reflective layer, and a metasurface lens serving as a Fourier transform lens. The nanobeam switch routing network is fabricated on the front side of the chip. After the input optical signal enters the nanobeam switch branch of the network, it is routed by specific nanobeam switch units to the corresponding nanobeam emission branch, and then emitted into free space by specific nanobeam emission units. The front metal reflective layer reflects the upward-radiated light waves downwards, and finally, all downward-emitted light waves are collimated and emitted in a designated direction by the back metasurface lens. This invention enables directional transmission, simultaneously achieving switch selection and vertical emission functions. The silicon-based metasurface lens can serve as a Fourier transform lens to achieve high-precision, wide-range beam scanning, providing a superior technical path for the large-scale manufacturing of silicon-based integrated lidar transmitter chips.

Description

Technical Field

[0001] This invention belongs to the field of optoelectronic integrated chips, specifically relating to a back-emitting lidar emitting chip based on a nanobeam switch array, which can realize directional laser beam emission with compact structure, high integration, low power consumption, and large beam scanning range. Background Technology

[0002] LiDAR technology is an optical remote sensing technology that uses lasers to measure the position, velocity, and other information of targets with high precision, ultimately giving the sensor the ability to model the real world. In recent years, with the development of LiDAR technology, high-precision LiDAR has been widely used in robotics, autonomous driving, optical tomography (OCT), and intelligent manufacturing, and is expected to have a market potential of hundreds of billions of dollars in the future. Taking Velodyne, a long-established LiDAR company, as an example, the products currently used on a large scale are mainly mechanical LiDARs. These LiDARs beam the laser beam through beamforming and are then emitted into free space by rotating mirrors, ultimately achieving one-dimensional beam scanning. However, limited by moving parts and the number of laser channels, mechanical LiDARs generally suffer from high cost, large size, low detection accuracy, slow field-of-view refresh rate, complex packaging and testing, and deterioration in stability during use. These are all problems that urgently need to be overcome before large-scale application of LiDAR.

[0003] To overcome the limitations of mechanical lidar, high-precision, high-refresh-rate, low-manufacturing-cost, and miniaturized solid-state lidar has become the development direction of lidar technology. Solid-state lidar mainly follows four technological paths: MEMS (Micro-Electro-Mechanical Systems), Flash (floodlight), OPA (Optical Phased Array), and focal plane array optical switch. Focal plane array optical switch lidar arranges a transmitting array on the front focal plane of a lens. A switch selects transmitters at different positions at different times, resulting in output beams in different directions after collimation by the lens. This type of switch array lidar has advantages such as low control complexity and low system power consumption. This is the core reason why companies such as Argo AI, LedderTech, TetraVue, Wanji, and Xuchuang are developing along this technological path and integrating optical transmitter arrays and photodetector arrays to advance the development of this type of solid-state lidar. This beam deflection method selects a single transmitter at a time, requiring transmitters with high emission efficiency and large power tolerance to achieve long-distance detection. This necessitates maximizing the utilization of the light power diffracted into free space by the transmitting unit. Commonly used micro-ring switch arrays are limited by the size of the micro-rings, making large-scale expansion difficult. Therefore, there is a need for switching unit devices with small unit size, simple structure, and strong scalability. When conventional four-port nanobeam resonators are used for optical switches, due to the characteristics of their standing wave cavities, the input optical power is evenly distributed across the four ports during switch selection, resulting in low usable transmit power. Focal plane optical switch array schemes require focusing lenses to participate in beam scanning, but using discrete focusing lenses would significantly increase packaging costs. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention aims to propose a back-emitting lidar transmitting chip based on a nanobeam switch array. Through a nanobeam switch routing network, simply activating the corresponding nanobeam units in the nanobeam switch branch and the nanobeam transmitting branch enables the selection of any unit in the routing network, thereby illuminating the corresponding nanobeam transmitting unit and emitting light into free space. The optical switch units in the nanobeam switch routing network utilize mode conversion to achieve unidirectional transmission, effectively reducing channel crosstalk and increasing transmitted optical power without increasing the size of the switch units. The nanobeam transmitting units in the nanobeam transmitting branch employ asymmetrical nanobeam resonant cavities, allowing the semi-reflective, semi-diffractive Bragg grating on the right side of the nanobeam to simultaneously function as a beam-emitting device. This multiplexing design reduces the unit size, making the entire chip more compact. When a specific nanobeam transmitting unit is activated, a portion of the light within the resonant cavity radiates into free space. By optimizing the coupling coefficient of the nanobeam transmitting units, they operate in a critical coupling state, achieving maximum optical power output. The metal interconnect layer on the chip surface serves both as a metal interconnect to control the nanobeams and as a reflective layer for front-facing light emission, reflecting the upward-radiated light from the emitting units to the back side, thereby increasing the back-side emission power. A metasurface lens fabricated on the back side of the chip can act as a Fourier lens to collimate the light emitted by the grating, ensuring a one-to-one correspondence between the selected nanobeam emitting units and the beam deflection angle. This enables a lidar emission system with high output power, high integration, and a large deflection range. This invention effectively reduces system power consumption, improves system integration, and enhances output power and signal-to-noise ratio.

[0005] The technical solution of the present invention is as follows:

[0006] A back-emitting lidar transmitting chip based on a nanobeam switch array is characterized by including a nanobeam switch routing network, a metal reflective layer, and a metasurface lens as a Fourier transform lens.

[0007] The nanobeam switch routing network is fabricated on the front side of the chip, including a nanobeam switch branch, multiple nanobeam emission branches placed orthogonally to the nanobeam switch branch, and corresponding nanobeam switch branches. Each nanobeam emission branch is provided with multiple nanobeam emission units. When an input light wave enters the nanobeam switch branch, it is routed by a designated nanobeam switch unit to the corresponding nanobeam emission branch, and then emitted into free space by a selected nanobeam emission unit in the nanobeam emission branch.

[0008] The metal reflective layer is fabricated on the upper cladding surface of the chip and is composed of nanobeam metal interconnects. It is used to control the nanobeam switching unit and reflect the light waves radiated upward by the nanobeam emitting unit downward.

[0009] The metasurface lens is etched on the back of the chip, and its focal length is equal to the chip thickness. That is, the focal length of the metasurface lens is equal to the distance from the metal reflective layer to the back of the chip, ensuring that all light waves radiated downward by the nanobeam emitting unit are deflected at a certain angle and collimated through the focal point. This ensures that the position of the nanobeam emitting unit corresponds one-to-one with the light wave deflection angle, thereby achieving light wave deflection at different angles.

[0010] When light waves enter the nanobeam switch routing network, they are first routed by designated switching units in the nanobeam switch branches to the corresponding nanobeam emission branches. Then, designated nanobeam emission units in the emission branches emit light into free space. The metal reflective layer on the chip surface reflects some of the upward-radiated light waves from the nanobeam emission units, while all downward-radiated light waves are collimated at corresponding deflection angles by the metasurface lens on the back of the chip. The metasurface lens on the back of the chip can collimate the light emitted by the nanobeam emission units, thus corresponding different emission unit positions to different beam deflection angles.

[0011] The described nanobeam switch routing network can transmit light waves from a specific location by activating designated nanobeam switch units and nanobeam emitting units, requiring only the control of two devices at a time. This topology significantly reduces transmission loss, control power consumption, and control complexity in the routing network. The operating wavelength of the nanobeam resonant cavity and the passband filtering bandwidth at the download end can be designed according to actual requirements.

[0012] The nanobeam switching unit transmits the input light unidirectionally to the through end and the download end. The width of the nanobeam, the spacing with the coupled waveguide, and other parameters need to be optimized for mode matching conditions. This can realize a nanobeam resonant cavity with traveling wave characteristics, so that the resonant wavelength is output only from the download end.

[0013] The nanobeam emitting unit couples a portion of the light transmitted in the nanobeam waveguide into free space. Its parameters, such as period, aperture size, and number of apertures, need to be optimized to improve optical power emission efficiency.

[0014] The metal reflective layer can reflect the upward radiation light from the emitting unit and coherently enhance it together with the downward radiation light. Then, it is collimated and emitted through a metasurface lens fabricated on the silicon substrate on the back of the chip, further improving the emitted light power.

[0015] The metasurface can be viewed as a Fourier transform lens, with the lens focal length equal to the distance between the lens and the nanobeam emitting unit. By adjusting the deflection angle of the light emitted from the back of the chip, the requirements of solid-state lidar beam scanning can be met. The microstructure units constituting the metasurface include, but are not limited to, cylindrical structures, ensuring that the effective refractive index of the central portion is higher than that of the edge portion, thereby achieving the light wave focusing function.

[0016] Compared with the prior art, the beneficial effects of this invention are mainly reflected in the following aspects:

[0017] This invention is based on a nanobeam switch routing network, a metal reflective layer, and a metasurface. For a specified transmitting unit, simply activating the corresponding nanobeam unit in the nanobeam switch branch and the nanobeam transmitting branch enables the selection of any unit in the routing network, thereby illuminating the corresponding nanobeam transmitting unit and transmitting into free space. Compared to phased array schemes, this routing network significantly reduces system power consumption and control complexity while also offering flexibility and scalability. The switch unit employs a nanobeam resonant cavity design, resulting in an extremely small mode volume. This reduces device size and integration density while also lowering control power consumption. The nanobeam transmitting unit operates in a critical coupling state, maximizing the transmission of input optical power into free space and increasing transmitted optical power. The light wave diffracts from a Bragg grating on one side of the nanobeam transmitting unit into free space. The upward diffracted portion is reflected by the metal interconnect reflective layer on the front side of the chip and ultimately exits from the back side of the chip, effectively improving the optical power of the lidar detection beam. The metaplane lens is located on the back of the chip. Compared with discrete spherical lenses, it allows for more freedom in designing parameters such as focal length and aberrations, while also having extremely high surface flatness. The metaplane lens is processed directly on the back of the chip through chip fabrication technology, which can better align with the nanobeam emission unit and save assembly costs. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of an embodiment of the back-emitting lidar transmitting chip based on a nanobeam switch array of the present invention, wherein a is a schematic diagram of the overall structure and b is a partial enlarged view.

[0019] Figure 2 This is a schematic diagram of the beam scanning principle of the present invention.

[0020] Figure 3 This is a schematic diagram of an embodiment of the nanobeam switch routing network of the present invention.

[0021] Figure 4 This is a schematic diagram of an embodiment of the nanobeam switching unit of the present invention, wherein a is a schematic diagram of the cross-section of the nanobeam switch, and b is the mode switching principle.

[0022] Figure 5 This is a schematic diagram of an embodiment of the nanobeam emission unit of the present invention, wherein a is an enlarged view and b is a wave vector diagram.

[0023] Figure 6 This is a schematic diagram of an embodiment of the metasurface lens of the present invention. Detailed Implementation

[0024] To further clarify the purpose, technical solution, and core advantages of this invention, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. The specific embodiments described below are for illustrative purposes only and are not intended to limit the scope of the invention.

[0025] Please see Figure 1 , Figure 1 This is a schematic diagram of an embodiment of the back-emitting lidar transmitting chip based on a nanobeam switch array according to the present invention. As shown in the figure, it includes a nanobeam switch routing network 101, a metal reflective layer 102, and a metasurface lens 103 serving as a Fourier transform lens. Light waves enter the nanobeam switch branch 104 in the nanobeam switch routing network 101, are routed by designated nanobeam switch units 106 to the corresponding nanobeam transmitting branch 105, and then emitted into free space by designated nanobeam transmitting units 107 in the nanobeam transmitting branch 105. The metal reflective layer 102 on the chip surface reflects some of the upward-radiated light waves from the nanobeam transmitting units 107, and all downward-radiated light waves are collimated and emitted at the corresponding deflection angle by the metasurface lens 103 on the back of the chip. The focal plane of the metasurface lens 103 on the back of the chip falls on the plane of the metal reflective layer; therefore, all radiated light waves are emitted through the focal plane of the metasurface lens 103, thereby causing the transmitting units at different positions to correspond to different beam deflection angles.

[0026] like Figure 2 As shown, a schematic diagram of the beam scanning principle of the present invention includes a nanobeam emitting unit 107, a metal reflective layer 102 on the chip surface, and a metasurface lens 103 on the back of the chip. The metasurface lens 103 can be regarded as a Fourier transform lens, and the focal length of the lens is equal to the distance between the lens and the nanobeam emitting unit. By adjusting the deflection angle of the light emitted from the back of the chip, the beam scanning requirements of the solid-state lidar are met. Part of the upward-radiated light wave in the nanobeam emitting unit 107 is reflected by the metal reflective layer 102, and after coherent enhancement with the upward-radiated light wave, it is collimated and emitted by the metasurface lens 103 through the lens focal point. That is, nanobeam emitting unit 1 corresponds to the back-emitted light 1, and nanobeam emitting unit 2 corresponds to the back-emitted light 2, ultimately achieving a one-to-one correspondence between the position of the nanobeam emitting unit and the beam emission deflection angle. This design effectively utilizes the longitudinal dimension of the chip and reduces the system complexity of the solid-state lidar.

[0027] like Figure 3As shown, the nanobeam switch routing network 101 includes a nanobeam switch branch 104 and a set of nanobeam transmitting branches 105 placed orthogonally to it. The nanobeam transmitting branches 105 and the nanobeam switch branch 104 are connected through nanobeam switch units 106. After the light wave enters the nanobeam switch branch 104, it is first routed by the designated nanobeam switch unit 106 to the corresponding nanobeam transmitting branch 105, and then transmitted into free space by the designated nanobeam transmitting unit 107 in the nanobeam transmitting branch 105. This topology can greatly reduce transmission loss, control power consumption, and control complexity in the routing network. In addition, this routing network structure can directly increase the number of nanobeams in the row and column selection paths to expand the routing network scale and realize the design of larger-scale lidar transmitting chips.

[0028] like Figure 4 As shown, the present invention discloses a nanobeam resonant cavity structure, which is essentially a one-dimensional photonic crystal, achieved by etching a set of specially designed small holes on a silicon waveguide. The entire resonant cavity consists of three parts: a central section with a series of gradually changing air holes, called the gradient section; two ends of the gradient section each have a row of periodically arranged small holes, which act as mirrors for the resonant cavity. These three parts constitute a Fabry-Perot resonant cavity (FP resonant cavity). The central gradient section is used to reduce scattering loss and achieve phase matching between the photonic crystal and the straight waveguide mode. The mirror sections on both sides reflect as much light as possible back to the central position to achieve strong confinement of the light field, thereby forming a resonant mode. Based on this design, the nanobeam resonant cavity can confine the light field to a size of a few micrometers, greatly reducing the device size and control power consumption. The resonant frequency of the nanobeam resonant cavity can be controlled by a thermo-optical or PIN electro-optical phase shifter.

[0029] like Figure 4As shown, the present invention discloses the structure of the nanobeam switching unit and the mode distribution of optical wave transmission. The nanobeam switching unit consists of upper and lower bus waveguides 201 and 203 and a nanobeam resonant cavity 202. An optical signal enters the optical switching unit from the input end. When the wavelength of the optical signal matches the resonant wavelength of the nanobeam resonant cavity, the optical signal is coupled into the nanobeam resonant cavity and oscillates, outputting from the input end. This state corresponds to the switching unit being turned on (gated). Conversely, when the wavelength of the input light mismatches with the resonant wavelength of the nanobeam, the optical wave in the input end will continue to propagate through the direct end. This state corresponds to the switching unit being turned off. The modes involved in the entire process are mode a (TE0) in the bus waveguide, mode b (TE2) propagating forward in the resonant cavity, and mode c (TE1) propagating backward in the resonant cavity. The design ensures that mode a in the bus waveguide and mode b propagating forward in the nanobeam resonant cavity satisfy the phase matching condition. Mode a in the bus waveguide passes through an S-shaped bend waveguide, couples to mode b in the nanobeam resonant cavity, and propagates forward. Mode b propagating forward is then reflected and converted into mode c, propagating backward. Since the backward propagating modes c and a do not satisfy the phase matching condition in the coupling region, they cannot couple into the bus waveguide. That is, only mode b can couple into the bus waveguide throughout the entire process; therefore, the resonant wavelength is only output unidirectionally from the download segment. Thus, by controlling the resonant wavelength of the nanobeam, the nanobeam switch can be switched on and off. This type of switch has advantages such as small unit size, low transmission loss, low power consumption, low inter-channel crosstalk, and fast switching response speed. Furthermore, to improve the switching response speed, the nanobeam switch unit of this invention can employ a PIN-phase-shifted nanobeam resonant cavity. This approach reduces the switch response time, ultimately improving the scanning refresh rate of the lidar.

[0030] like Figure 5 As shown, the working principle of the nanobeam emitting unit is illustrated. The nanobeam emitting unit consists of a coupled waveguide and a nanobeam resonant cavity. The nanobeam resonant cavity employs an asymmetrical design, with the aperture spacing on the left side slightly smaller than that on the right. According to the Bragg diffraction condition, when the aperture spacing Λ < π / x... x (where Λ is the grating spacing, β) x When the beam's propagation constant component in the x-direction is 0, the grating does not satisfy the phase-matching condition, and there is no diffracted beam. Therefore, the left side is equivalent to a mirror of the resonant cavity. The right side of the nanobeam emitting unit is designed as a semi-reflective, semi-diffractive Bragg grating. According to the wave vector diagram, the first-order diffracted beam is emitted into space, satisfying condition β. x +2π / Λ2=k 1,x The second-order diffracted beam propagates in the opposite direction to the input beam, satisfying condition β. x +4π / Λ2=k 2,xTherefore, the first-order diffracted beam is emitted into space, while the second-order diffracted beam propagates in the opposite direction within the waveguide. This nanobeam emitter unit design not only enables switching and selection but also allows for emission at specific wavelengths, and reduces the size of the emitter unit, making the entire chip more compact.

[0031] like Figure 5 As shown, the working principle of a nanobeam emitting unit of the present invention is as follows: the nanobeam emitting unit operates in a critical coupling state, with zero power at the waveguide's direct end, and all light is coupled into the resonant cavity, at which point the light energy within the resonant cavity is maximized. Therefore, when the nanobeam resonant cavity operates in a critical coupling state, the light energy radiated from the semi-reflective, semi-diffractive Bragg grating on the right side of the nanobeam resonant cavity into free space is maximized. This design can achieve maximum light power output, improving the detection range of the lidar.

[0032] like Figure 6 As shown, this invention discloses a metasurface lens structure. The metasurface lens 103 is a diffraction device composed of two-dimensional periodic arrangement of microstructure units, which performs beam scanning control on the coupled outgoing light field. The microstructure units constituting the metasurface include, but are not limited to, cylindrical structures, ensuring that the effective refractive index of the central portion is higher than that of the edge portion, thereby achieving light wave focusing. The unit structure parameters can be designed according to the phase distribution corresponding to the required optical parameters such as focal length and field of view of the lens. In this embodiment, the focal length of the metasurface lens is equal to the distance from the metal reflective layer to the back of the chip. Furthermore, it is necessary to reduce lens aberrations to ensure the imaging effect of the emitting units at the chip edge. After obtaining the relationship between geometric parameters and the phase delay, transmittance, and other characteristics of the microstructure units through simulation, the qualified microstructure units are placed in the corresponding positions to generate the final device.

[0033] Metasurface lenses 103 are typically fabricated on the back side of a chip. They can be generated by directly etching microstructures onto the silicon substrate surface, or by etching a certain depth into the back side of the silicon substrate at the matching location of the grating coupler before etching the microstructures. Compared to discrete spherical lenses, they offer greater freedom in designing parameters such as focal length and aberrations, while also possessing extremely high surface flatness. Metasurface lenses are fabricated directly on the back side of the chip using chip fabrication processes, allowing for better alignment with nanobeam emission units and eliminating assembly costs. Furthermore, due to the modulation principle of metasurfaces, the impact of errors during manufacturing can be reduced. Therefore, the metasurface strategy is convenient for fabrication, simplifying the manufacturing process and reducing costs while improving modulation accuracy.

[0034] This invention is not limited to the embodiments described above. Those skilled in the art can make various improvements and modifications without departing from the principles of this invention, and these improvements and modifications are also considered within the scope of protection of this invention. Contents not described in detail in this specification are prior art known to those skilled in the art.

Claims

1. A back-emitting lidar emitting chip based on a nanobeam switch array, characterized in that: It includes a nanobeam switch routing network (101), a metal reflective layer (102), and a metasurface lens (103) that acts as a Fourier transform lens. The nanobeam switch routing network (101) is fabricated on the front side of the chip, including a nanobeam switch branch (104), multiple nanobeam emission branches (105) orthogonally placed to the nanobeam switch branch (104), and corresponding nanobeam switch branches (104). Each nanobeam emission branch (105) is provided with multiple nanobeam emission units (107). When an input light wave enters the nanobeam switch branch (104), it is routed by a designated nanobeam switch unit (106) to the corresponding nanobeam emission branch (105), and then emitted into free space by the selected nanobeam emission unit (107) in the nanobeam emission branch (105). The metal reflective layer (102) is fabricated on the upper cladding surface of the chip and is composed of nanobeam metal interconnects. It is used to regulate the nanobeam switching unit (106) and reflect the light waves radiated upward by the nanobeam emitting unit (107) downward. The metasurface lens (103) is etched on the back of the chip, and its focal length is equal to the chip thickness. That is, the focal length of the metasurface lens (103) is equal to the distance from the metal reflective layer (102) to the back of the chip, ensuring that all light waves radiated downward by the nanobeam emitting unit (107) are deflected at a certain angle and collimated through the focal point. Finally, the position of the nanobeam emitting unit (107) corresponds one-to-one with the light wave deflection angle, realizing the deflection of light waves at different angles.

2. The back-emitting lidar transmitting chip based on a nanobeam switch array according to claim 1, characterized in that: The nanobeam switching unit (106) consists of an upper bus waveguide (201), a lower bus waveguide (203), and a nanobeam resonant cavity (202); the upper bus waveguide (201) includes an input end and a through end of a straight waveguide, and the lower bus waveguide (203) includes a download end of a straight waveguide; the nanobeam resonant cavity (202) is a one-dimensional photonic crystal, realized by etching small holes with different spacings on a silicon waveguide; The optical signal enters through the input terminal of the upper bus waveguide (201). When the wavelength of the optical signal matches the resonant wavelength of the nanobeam resonant cavity, the optical signal is coupled into the nanobeam resonant cavity and oscillates before being output through the lower bus waveguide (203), i.e., the nanobeam switching unit is turned on. When the wavelength of the input light does not match the resonant wavelength of the nanobeam resonant cavity, the optical signal is output through the through terminal of the upper bus waveguide (201), i.e., the nanobeam switching unit is turned off. By controlling the resonant wavelength of the nanobeam resonant cavity, the nanobeam switching unit (106) can be switched on and off, and emitted at a specific wavelength.

3. The back-emitting lidar chip based on a nanobeam switch array according to claim 2, characterized in that: The nanobeam resonant cavity (202) adopts an asymmetrical structure, that is, small holes with different spacings are distributed on the left and right sides. The side with smaller hole spacing serves as the reflector of the nanobeam resonant cavity (202), and the side with larger hole spacing serves as the Bragg grating of the nanobeam resonant cavity (202) for half reflection and half diffraction, which satisfies the Bragg diffraction condition, that is, part of the light diffracts into free space, and the rest is reflected in the resonant cavity.

4. The back-emitting lidar chip based on a nanobeam switch array according to claim 1, characterized in that: When the nanobeam switching unit is selected, some of the light waves radiated upward through the nanobeam emitting unit are reflected downward by the metal reflective layer and finally emitted from the back. The design of the metal reflective layer can effectively improve the back-emitted light power while independently controlling the nanobeam.

5. The back-emitting lidar transmitting chip based on a nanobeam switch array according to claim 1, characterized in that: The metasurface lens (103) is a set of two-dimensional periodically arranged microstructure units processed on the back of the chip, which can be obtained by directly etching microstructures on the back. The microstructure unit is a cylindrical structure, and the geometric parameters of the unit structure and the phase delay and transmittance characteristics of the microstructure unit can be designed according to the focal length and field optical parameters of the lens.

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