Shulou(Shulou.com)06/03 Report--

Lidar is considered to be the key sensing technology in various industries, acting as a promoter in the fields of robots, self-driving, smart cities and so on. In recent years, solid-state lidar, which has been highly expected, has become the focus of attention in the industry.

What is solid-state lidar?

Theoretically, solid-state lidar is a radar with no moving parts at all. Optical phased array (Optical Phased Array) and Flash are typical technical routes, and they are also considered to be pure solid-state lidar schemes.

But in recent years, some incompletely rotating lidars are also collectively called "solid-state lidars". They have many performance characteristics of solid-state lidars, such as high resolution, limited level FOV (forward rather than 360 °), etc., but these technical solutions will have some tiny moving parts, which can not be regarded as pure solid-state lidars in a strict sense.

Working principle of solid-state lidar

Solid-state lidar mainly depends on the reflection or reception of waves to detect the characteristics of targets, mostly from the study of three-dimensional image sensors, and actually from infrared focal plane imagers. The focal plane of the focal plane detector is lined with an array of photosensitive elements. Infrared light emitted from an infinite distance passes through the optical system and is imaged on these photosensitive elements of the focal plane of the system. The detector converts the received optical signal into an electrical signal and carries out integral amplification, sampling and holding, and finally sends it to the monitoring system to form an image through the output buffer and multiplexing system.

Three technical routes for the formation of solid-state lidar

After years of development, the basic framework of solid-state lidar has been relatively clear, the following are the three mainstream schemes.

1.MEMS (Micro-Electro-Mechanical System) MEMS

MEMS refers to the miniaturized and electronic design of the mechanical mechanism, which integrates the originally large mechanical structure on the silicon-based chip through the microelectronic process for large-scale production. The technology is mature and can be produced in large quantities. It is mainly through the MEMS micromirror to achieve vertical one-dimensional scanning, the whole machine is rotated horizontally to complete the horizontal scanning, and its light source is a fiber laser, which is mainly due to the low repetition frequency of the 905nm tube, and the high average power will be too large, which will affect the life of the laser tube.

Strictly speaking, MEMS is not a pure solid-state lidar, this is because the mechanical is not completely eliminated in the MEMS scheme, but the mechanical is miniaturized, and the scanning unit is turned into a MEMS micromirror.

2.OPA (optical phased array) Optical phased Array Technology

Compared with other technical schemes, the OPA scheme describes the bright prospect of a lidar chip-level solution, which mainly uses multiple light sources to form an array to synthesize the main beam with a specific direction by controlling the time difference of each light source. Then the main beam can be scanned in different directions by controlling it. The radar accuracy can reach the millimeter level, and conforms to the trend of solid state, miniaturization and low cost of lidar in the future, but the difficulty lies in how to improve the point cloud data measured in unit time and the huge input cost.

3.Flash

The principle of Flash lidar is also flash. Unlike the scheme of MEMS or OPA, it directly emits a large area of laser covering the detection area in a short time, and then uses a highly sensitive receiver to complete the drawing of the image around the environment.

Advantages and disadvantages of solid-state lidar

Solid-state lidar using optical phased array scanning technology does have many advantages, such as:

① has the advantages of simple structure, small size, no need to rotate parts, and can be greatly compressed in structure and size, increasing service life and reducing its cost.

The scanning accuracy of ② is high, and the scanning accuracy of optical phased array depends on the accuracy of the control electrical signal, which can reach more than 1/1000 degrees.

③ is controllable and can be pointed arbitrarily within the allowable angle range, and high-density scanning can be carried out in key areas.

The scanning speed of ④ is fast, and the scanning speed of optical phased array depends on the electronic characteristics of the materials used, which can generally reach the order of MHz.

Of course, solid-state lidar also has some disadvantages, such as:

The ① scanning angle is limited, and the solid state means that the lidar cannot rotate 360 degrees and can only detect the front. Therefore, in order to realize omni-directional scanning, it is necessary to arrange multiple (at least two front and rear) solid-state lidars in different directions.

② sidelobe problem, grating diffraction in addition to the central stripe will also form other bright lines, this problem will make the laser form sidelobe in the direction of the maximum power and disperse the energy of the laser.

It is difficult to process ③. Optical phased array requires that the size of the array element must be no more than half a wavelength. Generally, the working wavelength of the lidar is about 1 micron, so the size of the array unit must not be larger than 500nm. And the higher the array density is, the more concentrated the energy is, which improves the requirements of machining accuracy and requires a certain technological breakthrough.

④ has large receiving surface and poor signal-to-noise ratio: traditional mechanical radar only needs a small receiving window, but solid-state lidar needs a whole receiving surface, so it will introduce more ambient light noise and increase the difficulty of scanning and analysis.

Generally speaking, at present, in terms of the characteristics that solid-state lidar should have (high reliability, low cost and long range), it is difficult for the existing radar products on the market to meet at the same time, which also determines that solid-state lidar is difficult to be manufactured in a short time. At the same time, it has also led to the continuous extension of the delivery dates of all solid-state radar companies.

Although many people in the industry predict that solid state, miniaturization and low cost will be the development trend of lidar in the future, mechanical lidar is still the mainstream at present.

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