Principle and Application of LiDAR Structure

1. Basic concepts

The name "Laser" means the amplification of stimulated radiation, which is a process of light amplification. Laser belongs to a type of electromagnetic wave, which is a form of motion in the electromagnetic field. Laser emits a highly directional beam of light, forming a wave that propagates along a straight line without spreading. The light waves inside the laser beam are all of the same color, which is called monochromaticity. The light waves emitted by ordinary light sources will diffuse in various directions, and ordinary light is generally a mixture of several colors of light that appears white. Lidar is a radar system that emits detection signals (shock beams) to the target being measured, and then measures the arrival time, intensity, and other parameters of the reflected or scattered signals to determine the distance, orientation, motion state, and surface optical characteristics of the target. Lidar has advantages such as high angular resolution and range resolution, strong anti-interference ability, and the ability to obtain various image information of targets.


2. Classification

(1) Classified by function

1) Laser ranging radar. Laser ranging radar is a device that emits laser beams from the object being measured and receives reflected waves. By recording this time difference, it determines the distance between the object being measured and the testing point.

2) Laser velocimetry radar. Laser velocimetry radar is a measurement of the speed of a moving object, which measures the distance of the object by emitting two laser pulse signals to obtain its movement speed.

3) Laser imaging radar. Laser imaging radar is a comprehensive product of laser technology, radar technology, optical scanning and control technology, high sensitivity detection technology, and high-speed computer processing technology. It has high angle resolution and distance resolution, which can form high-resolution three-dimensional images.

4) Atmospheric detection lidar. Atmospheric detection lidar is used to detect the density, temperature, wind speed, wind direction, and water vapor concentration of molecules and smoke in the atmosphere, monitor the atmospheric environment, and predict catastrophic weather such as storms and sandstorms

As shown in Figure 1.


5) Tracking radar. Tracking radar can continuously track a target and measure its coordinates, providing the target's motion trajectory.

(2) Classification by working medium

1) Solid state LiDAR. Solid state LiDAR refers to a LiDAR without moving components, also known as solid-state LiDAR, as shown in Figure 2. It has the advantages of simple structure, small size, high lifespan, and low cost.
Figure 1 Atmospheric Detection Lidar

Figure 2 Solid state LiDAR


2) Gas LiDAR. Gas LiDAR is represented by CO2 LiDAR. Laser pulses travel through the atmosphere, being dissipated by aerosols and absorbed by atmospheric substances. The information extracted by gas LiDAR is reflected in the absorption of laser pulse energy by CO2 gas. In the laser radar absorption detection system, both the echo formed by aerosol heat dissipation and the information of CO2 obtained through gas absorption are utilized. The strength of the absorption signal reflects the concentration of CO2.

Gas LiDAR operates in the infrared band, with low atmospheric transmission attenuation and long detection distance, and has played a significant role in atmospheric wind field and environmental monitoring.


3) Semiconductor LiDAR. Semiconductor laser, also known as laser diode, is a laser that uses semiconductor materials as working materials. Common working substances include gallium arsenide, cadmium sulfide, indium phosphide, etc. Semiconductor LiDAR can operate continuously at high repetition rates, with the advantages of long lifespan, small size, low cost, and minimal harm to the human eye.

(3) Classification by number of lines

1) Single line LiDAR. The basic components of LiDAR are the transmitter and receiver. A single line LiDAR has only one laser transmitter and one laser receiver, which are projected onto a straight line through the rotation of the motor, as shown in Figure 3. The advantages of single line LiDAR are small data volume, high efficiency, good stability, and mature technology. However, it can only be used for planar tracing and cannot measure the height of objects, which has certain limitations. Mainly used in sweeping robots, hotel service robots, etc.

Figure 3 Single line LiDAR


Figure 4 Multi line LiDAR

2) Multi line LiDAR. Multi line LiDAR is mainly used in radar imaging systems. Compared to single line LiDAR, it has made qualitative changes in dimension enhancement and scene restoration, and can recognize the height information of objects. Multi line LiDAR can achieve 3D imaging and high-precision modeling of the driving environment, as shown in Figure 4. At present, the multi line LiDARs launched in the market mainly include 4-line, 8-line, 16 line, 32 line, 64 line, and 128 line.

(4) Classification by presence or absence of rotating parts

1) Mechanical LiDAR. Mechanical LiDAR refers to a transmitting and receiving system that continuously rotates the transmitting head to transform the emitted laser from a line to a surface, and arranges multiple beams of laser vertically to form multiple surfaces, thereby achieving the goal of dynamic 3D scanning and continuous reception of information. Mechanical LiDAR, as the first LiDAR product applied in autonomous vehicles, has advantages such as fast scanning speed, large receiving field of view, and the ability to withstand high laser power; But it also has disadvantages such as bulky structure, large weight and volume, complex installation and adjustment work, and high price.

2) All solid state LiDAR. There are no moving parts inside the solid-state LiDAR. Currently, the main solid-state LiDAR products on the market include optical phased array LiDAR, frequency modulated continuous wave LiDAR, nano antenna array LiDAR, and floodlight array LiDAR. All solid-state LiDAR has the best durability and reliability, meeting the needs of autonomous driving for miniaturization and low-cost solid-state radar


3. Structural principle

(1) Basic structure

Lidar mainly consists of four parts: laser emission system, laser receiving system, scanning system, and information processing system.

The basic structure of the LiDAR is shown in Figure 5

Figure 5 Basic structure of LiDAR

1) Laser emission system. The excitation source of the laser emission system periodically drives the laser, emits laser pulses, uses a laser modulator to control the direction and number of lines emitted by the beam controller, and finally emits the laser to the target object through the emission optical system.

2) Laser receiving system. The laser receiving system receives the laser reflected back from the target object through the receiving optical system photodetector, generating a receiving signal

3) Information processing system. The information processing system amplifies and analogizes the received signal, which is then calculated by the information processing module to obtain the surface morphology, physical properties, and other features of the target, ultimately establishing an object model.

4) Scan the system. The scanning system rotates at a stable speed to scan the plane and generate real-time planar information.

(2) Working principle

The working principle of LiDAR is very similar to that of ultrasonic radar. It uses laser as the signal source, and the pulse laser emitted by the laser hits trees, roads, bridges, and buildings on the ground, causing scattering. Some of the light waves will be reflected onto the receiver of LiDAR. According to the principle of laser ranging, the distance from LiDAR to the target point can be calculated. By continuously scanning the target object with pulsed laser, data on all target points on the target object can be obtained. After imaging processing with this data, accurate three-dimensional images can be obtained.



There is an optical transmission and reception system at the front end of the LiDAR, with N sets of transmission modules at the back end of the transmission system and N sets of reception modules corresponding to the transmission modules at the back end of the reception system. When the LiDAR starts working, the N group transmitting module and N group receiving module take turns working in a certain time sequence under the precise control of the system circuit, transmitting and receiving laser beams.

Encoder is a sensor used for motion control, which utilizes principles such as optoelectronics, electromagnetism, and inductance to detect the mechanical position and changes of objects, and converts this information into electrical signals as feedback for motion control, which are transmitted to various motion control devices. Optical rotary encoder is a special type of encoder that can convert mechanical quantities such as angular displacement and angular velocity of the output shaft into corresponding electrical pulses for digital output through photoelectric conversion. It can accurately test the motor angular displacement and rotation position. The rotating motor drives the scanning mirror to rotate in a certain order and speed, emitting the laser beam emitted by the laser. The reflected laser beam is then processed and calculated through an optical receiving system, forming an optical scan, as shown in Figure 6.

Figure 6 Working principle of LiDAR

(3) Product parameters

1) Transmission power. Transmission power is an important indicator that needs to be predetermined in the design of LiDAR schemes. By analyzing the energy conversion process during laser transmission, a LiDAR equation is established, and the transmission power is determined based on the relationship between the detection probability, false alarm probability, and system signal-to-noise ratio. The required transmission power is obtained through simulation technology to meet the requirements of system device parameters and application background. The maximum transmission power of a LiDAR determines whether safety protection is required.

2) Field of view angle. Lidar field of view angle is divided into horizontal field of view angle and vertical field of view angle. The horizontal field of view angle is in the horizontal direction
The angle range that can be observed is 360 ° for a rotating LiDAR, so the horizontal field of view angle is 360 °. The vertical field of view angle is an angle that can be observed in the vertical direction, usually 40 °. It is not symmetrically and uniformly distributed because it mainly needs to scan obstacles on the road surface, rather than shooting the laser towards the sky. In order to make good use of the laser, the laser beam will be biased downwards by a certain angle as much as possible. Moreover, in order to detect obstacles and concentrate the laser beam on the key areas of focus in the middle, in order to better detect vehicles, the laser radar beam is not vertically evenly distributed, but dense in the middle and sparse on both sides. Taking the beam of a 64 line LiDAR as an example, as shown in Figure 7, it can be seen that the LiDAR beam has a certain bias, with an upward angle of 15 ° and a downward angle of 25 °, and the laser beam is dense in the middle and sparse on both sides.
Figure 7 Lidar 64 line vertical field of view angle

3) Light source wavelength. Laser wavelength refers to the output wavelength of the laser and is an important parameter for the laser beam output. The corresponding output frequency is called laser frequency. The wavelength unit of lasers is usually measured in nanometers (nm), and lasers can be divided into two categories: visible lasers and invisible lasers. In general, the visible light wavelength that the human eye can clearly distinguish is between 400 and 700m. The shorter the wavelength of the laser, the bluer and more purple its color becomes, until ultraviolet rays are invisible to the human eye. The longer the wavelength of the laser, the more its color leans towards red, until infrared rays are invisible to the human eye. The laser beam emitted by LiDAR belongs to invisible light, with a typical wavelength of around 905m.

4) Measure distance. When the speed and point frequency of the LiDAR are constant, the farther the distance is measured, the thinner the point density, and the accuracy decreases accordingly; To ensure a long testing distance, the point frequency setting should be adjusted accordingly to be lower, resulting in lower point density and lower accuracy. For car grade LiDAR, the measurement distance usually needs to reach 150 meters or more.

5) Distance measurement accuracy. In the measurement process, the precision of any measurement method can only be relative and cannot achieve absolute accuracy. There will always be errors caused by various reasons. To ensure accurate and reliable measurement results, errors should be minimized as much as possible to improve measurement accuracy. The current vehicle mounted LiDAR ranging accuracy is usually controlled at the centimeter level.