I. Classification by Scanning Method
1. MEMS-type lidar
MEMS-type lidar can dynamically adjust its scanning mode to focus on specific objects, capturing and identifying detailed information about smaller, more distant objects—something traditional mechanical lidar cannot do. The entire MEMS system requires only a small mirror to guide a fixed laser beam in different directions. Because the mirror is small, its inertial torque is low, allowing it to move quickly—fast enough to track a 2D scanning pattern in less than a second.
2. Flash-type lidar
Flash-type LiDAR can quickly record an entire scene, avoiding the various problems caused by target or LiDAR movement during scanning. It operates somewhat like a camera. The laser beam diffuses directly in all directions, so a single flash is enough to illuminate the entire scene. The system then uses a miniature sensor array to collect the laser beams reflected back from different directions. Flash LiDAR has its advantages, but it also has certain drawbacks. The larger the pixel count, the more signals need to be processed. Packing a massive number of pixels into a photodetector inevitably introduces various interferences, resulting in a decrease in accuracy.
3. Phased array lidar
A phased array lidar uses a row of transmitters to change the direction of the laser beam by adjusting the relative phase of the signals. Currently, most phased array lidars are still in the laboratory, while we are still in the era of rotating or MEMS lidar.
4. Mechanically Rotating LiDAR
Mechanically rotating lidar is one of the earliest types of lidar technology developed and is currently quite mature. However, its system structure is very complex, and the prices of its core components are quite high. These components mainly include lasers, scanners, optical components, photodetectors, receiver ICs, and positioning and navigation devices. Due to the high hardware costs, mass production is difficult, and stability needs improvement. Therefore, solid-state lidar is becoming the development direction for many companies.
II. Classification by Detection Method
1. Direct detection lidar
The basic structure of a direct-detection lidar is quite similar to that of a laser rangefinder. During operation, a signal is transmitted by the transmitting system, reflected by the target, and collected by the receiving system. The distance to the target is determined by measuring the round-trip time of the laser signal. The radial velocity of the target can be determined by the Doppler frequency shift of the reflected light, or by measuring two or more distances and calculating their rate of change to obtain the velocity.
2. Coherent detection lidar
Coherent detection lidar is divided into monostable and bistable systems. In a monostable system, the transmitting and receiving signals share a single optical aperture and are isolated by a transmit-receive switch. A bistable system, on the other hand, includes two optical apertures, one for transmitting and one for receiving signals. The transmit-receive switch is then unnecessary, and the rest of the system is the same as a monostable system.
III. Classification by Laser Emission Waveform
1. Continuous LiDAR
From the perspective of laser principles, continuous-wave lidar continuously emits light, much like turning on a flashlight switch; the light stays on continuously (except in special circumstances). Continuous-wave lidar relies on maintaining continuous light at the target altitude to collect data at that specific height. Due to the characteristics of continuous-wave lidar, only data from one point can be collected at any given moment. Given the uncertain nature of wind data, using a single point to represent the wind conditions at a particular altitude is clearly incomplete. Therefore, some manufacturers use a compromise approach: rotating the device 360 degrees and collecting data from multiple points along the edge of this circle for averaging. This is essentially a concept of multi-point statistical data on a virtual plane.
2. Pulse-type lidar
Pulsed lasers output discontinuous, flickering light. The principle behind pulsed lasers is emitting tens of thousands of laser particles. Based on the internationally accepted Doppler principle, the reflections of these tens of thousands of laser particles are used to comprehensively evaluate wind conditions at a certain altitude. This is a three-dimensional concept, hence the theory of detection range. Considering the characteristics of lasers, pulsed lasers can measure dozens of times more points than continuous lasers, thus providing a more accurate reflection of wind conditions at a specific altitude.
