Since there are no macroscopic or microscopic moving parts inside the all-solid-state LiDAR, the advantages of durability and reliability are self-evident, and it conforms to the trend of solid-state, miniaturization and cost reduction of LiDAR for autonomous driving, so it has become a vehicle for vehicle use. Trends in lidar. The following is a brief introduction to different solid-state lidar technologies. The first thing to introduce is the optical phased array LiDAR.
1. Optical Phased Array LiDAR
The evolution trend of lidar from mechanical rotation to beamforming is exactly the same as that of radar: phased array radars widely used in the military generally have thousands of transmitting antenna units. By adjusting the beamforming method, the direction of radar scanning can be changed without It requires mechanical components to operate, is highly flexible, and is suitable for dealing with highly maneuverable targets. It can also emit narrow beams as electronic warfare antennas.
For lidar, in order to completely cancel the mechanical structure, it is considered to change the outgoing angle of laser light by adjusting the phase difference of each transmitting unit in the transmitting array, and adopt the principle of phased array to realize solid-state lidar.
So what is the principle of phased array? The most common example of interference in life is water waves. The water waves generated by two vibrations are superimposed on each other. In some directions, the two waves strengthen each other, and some directions just cancel each other. This principle is amplified, and multiple light sources are used to form an array. The time difference of emission can synthesize the main beam with flexible angle and precise controllability, which is the principle of phased array.
In Optical Phased Array (OPA, Optical Phased Array) LiDAR, the phased array transmitter consists of a number of transmitting and receiving units to form an array. By changing the voltage loaded on different units, the light wave characteristics (such as light intensity, phase, etc.) emitted by different units are changed. ), to achieve independent control of the light waves of each unit, by adjusting the phase relationship between the light waves radiated from each phased unit, to generate mutually reinforcing interference in the set direction to achieve high-intensity beams, while other directions The light waves emanating from the individual cells cancel each other out, so the radiation intensity is close to zero. Under the control of the program, each phased unit that constitutes the phased array can make one or more high-intensity beams point to realize random airspace scanning according to the designed program.
How does the optical phased array change the exit angle of the laser by controlling the phase difference of each emitting unit in the emitting array?
We can understand how an optical phased array works through a simple analogy (as shown in the figure below):
Suppose there are 10 people walking forward side by side in a row on the left, take their connection line as the array plane of their overall movement, and the direction perpendicular to the connection line to the right is the forward direction.
• If 10 people walk at the same speed, the array surface will move forward in parallel, and its forward direction will not change, as shown in the figure (a) below;
• If the person at the top walks the slowest, the speed of the others gradually increases from top to bottom, and the person at the bottom walks the fastest, then the array surface is no longer moving in parallel. The person at the top walks the farthest, and the person at the top walks the shortest, and the forward direction of the array surface will change significantly upward, as shown in Figure (b) below;
• If the person at the top walks the fastest, the speed of other people gradually decreases from top to bottom, and the person at the bottom walks the slowest, after a period of time, the advancing direction of the array surface will have an obvious angle downward change, as shown in Figure (c) below.
Optical phased arrays work in a similar way to the analogy above, with each element being able to control the speed of the light (people) passing through it. When a beam of light is divided into many small units (persons), the beams (persons) of each small unit pass through an optical phased array unit, and its speed is strictly controlled by the phased array unit. When the light beam of each small unit passes through the optical phased array at the same time, its speed returns to the speed before entering the optical phased array, but because the optical path (distance) traveled by the light beam of each small unit is different , the wavefront synthesized by the optical phased array (the array surface in the above analogy) will change significantly, so that the direction of the beam will be deflected, which is the basic working principle of the optical phased array.
The above is an example of one-dimensional scanning. If we make the optical phased array into a two-dimensional array (such as the Quanergy scheme described below), we can achieve two-dimensional scanning. Optical phased arrays generally use electrical signals to strictly control their phase to achieve beam pointing scanning, so it can also be called electronic scanning technology.
At the CES 2016 in the United States, the "solid-state" SolidState lidar exhibited by Quanergy is an optical phased array lidar, which meets the general trend of lidar miniaturization, and the overall size is only 90mm x 60mm x 60mm. The core technologies used are Optical Phased Array, Optical Integrated Circuit Photonic IC, Far Field Radiation Pattern. This product has no mechanical firmware at all, and can be called a pure solid-state lidar. The following figure is a schematic diagram of the working principle of the Solid State LiDAR S3 optical phased array scanning radar disclosed by Quanergy. It can be seen that the S3 uses optical phased array technology to achieve laser scanning. The principle is the same as that of phased array radar. The output angle of the laser is changed by adjusting the phase difference of each transmitting unit in the transmitting array.
Considering the technical background of Quanergy's CEO and co-founder Louay Eldada and the working principle of S3 released by Quanergy, as shown in the figure above, Quanergy should use the mature planar optical waveguide technology in optical communication to make optical phased array scanning devices . In order to obtain a good coherent synthesis effect, the size of the waveguide structure is required to be very small, only in the order of hundreds of nanometers, and the laser power that can be passed through is limited. This is because the smaller the water pipe, the smaller the water flow that can be accommodated. If the pulse ranging system is used, the SNR will be insufficient and the detection distance will be limited. Other means must be used to make up for it, such as multi-pulse, pulse coding or continuous wave modulation, etc. to improve the SNR.
In addition, we often say that the anti-interference ability of lidar is strong, that is because the traditional mechanical scanning lidar has a very small receiving field of view, and it is difficult for the directly irradiated interference signal from the outside to align and enter the receiving field of view of the lidar. Moreover, the background light noise power that the lidar can receive is proportional to the receiving field of view. The larger the field of view, the higher the background light noise power. Quanergy's optical phased array scanning can only control the direction of the emitted laser beam, and cannot achieve synchronous scanning of the receiving optical path. This requires that the S3 lidar must use a receiving optical system with a large field of view to receive the echo signal of the laser. If the scanning angle range is ±60º, the angle of the receiving field of view must also be ±60º, which will cause a decrease in the signal-to-noise ratio and be susceptible to interference from laser signals emitted by other similar systems and direct sunlight.
To sum up, compared with traditional mechanical scanning technology, optical phased array scanning technology has three major advantages:
• Fast scanning speed: The scanning speed of an optical phased array depends on the electronic properties of the materials used and the structure of the device, and can generally reach the order of MHz or more.
•High scanning accuracy or pointing accuracy: The scanning accuracy of the optical phased array depends on the accuracy of the control electrical signal (generally a voltage signal), which can be above the order of μrad (one thousandth of a degree).
•Good controllability: The beam pointing of the optical phased array is completely controlled by electrical signals, and can be pointed arbitrarily within the allowable angle range, high-density scanning can be performed in the target area of ​​interest, and sparse scanning can be performed in other areas , which is very useful for autonomous driving environment perception.
But optical phased array scanning technology also has its disadvantages:
• It is easy to form side lobes, which affects the beam action distance and angular resolution: the beam synthesis after the beam passes through the optical phased array device is actually formed by the mutual interference of light waves. The interference effect is easy to form side lobes as shown in the figure below, which makes the laser energy be dispersed.
• The processing difficulty is high, and the manufacturing process is difficult. The optical phased array requires that the size of the array unit must be no greater than half a wavelength. Generally, the working wavelength of the current lidar is about 1 micron, which means that the size of the array unit must be no greater than 500 nanometers. Moreover, the more the number of arrays, the smaller the size of the array unit, and the energy is concentrated in the main lobe, which requires higher processing accuracy.
• In addition, the research and selection of materials is also a very critical factor. So far, lithium niobate crystals, PLZT piezoelectric ceramics, liquid crystals and AlGaAs-based waveguide optical phase control have been developed. In the future, in terms of devices, the size of the phased array unit will be further reduced, the diffraction efficiency will be improved, and the size of the device will be reduced; in terms of more fundamental material research, the development of liquid crystal materials in the mid-wave infrared, long-wave infrared, and ultraviolet bands will continue to be pursued. High-performance electro-optical materials with large birefringence, fast response speed, high thermal stability, and strong laser resistance, while developing electro-optical materials with good transmittance for medium and long wavelengths and ultraviolet bands to expand the application of optical phased array devices field.
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