I. How to classify lidar according to its working medium
1. Solid-state lidar
Solid-state lidar boasts high peak power, an output wavelength range compatible with existing optical components and devices (such as modulators, isolators, and detectors), and atmospheric transmission characteristics. Furthermore, it readily implements a master oscillator-power amplifier (MOPA) structure. Combined with its high efficiency, small size, light weight, high reliability, and good stability, solid-state lidar is prioritized for application in airborne and space-based systems. In recent years, the focus of lidar development has been on diode-pumped solid-state lidar.
2. Gas lidar
Gas lidar, represented by CO2 lidar, operates in the infrared band, with low atmospheric transmission attenuation and long detection range. It has played a significant role in atmospheric wind field and environmental monitoring. However, its large size and the requirement that the mid-infrared HgCdTe detector used in its operation must be at a temperature of 77K limit the development of gas lidar.
3. Semiconductor LiDAR
Semiconductor lidar can operate continuously at high repetition rates, offering advantages such as long lifespan, small size, low cost, and minimal eye strain. It is widely used in Mie scattering measurements, where backscattering signals are strong, such as detecting cloud base height. Potential applications of semiconductor lidar include measuring visibility, obtaining aerosol extinction profiles in the atmospheric boundary layer, and identifying rain and snow. It is also easily manufactured into airborne equipment. Currently, the VP300 lidar developed by [Company Name] is a typical example of its application in AGVs (Automated Guided Vehicles), autonomous mobile robots, and commercial service robots, with a detection range of up to 50 meters.
II. What are the methods for measuring distance using lidar?
"The Eye in Laser Ranging"—Pulse Laser Ranging Method
Pulsed lasers are characterized by their high peak power, enabling them to travel long distances in space. Therefore, pulsed laser ranging can measure very distant targets. How far is "very far"? The farthest distance ever measured by laser in human history is the distance between the Earth and the Moon, which was achieved using pulsed laser ranging. Since June 2019, my country's Tianqin Project team has successfully measured the Earth-Moon distance multiple times. Through precise timing of the pulse flight time, they have determined that the Earth-Moon distance fluctuates between 351,000 km and 406,000 km (elliptical orbit).
Pulsed laser ranging is a well-developed ranging system that can be used not only for detecting distant targets but also for measuring distances of several kilometers or even tens of meters. It is also widely used in current autonomous driving systems, offering ranging accuracy down to the centimeter level, which is sufficient for most applications.
"Exquisite Observation" Phase Laser Ranging
For applications requiring high ranging accuracy, such as the final approach segment in "space rendezvous and docking," centimeter-level ranging accuracy is no longer sufficient. In such cases, phase laser ranging is required.
Phase-array laser ranging emits a modulated continuous laser signal and determines the target distance by measuring the phase difference between the emitted and echoed signals. Compared to pulsed laser ranging, phase-array laser ranging offers higher accuracy, reaching millimeter-level precision. However, because the laser emitted in phase-array ranging is a continuous wave, its average power is much lower than the peak power of a pulsed laser, making it unsuitable for detecting distant targets. Most handheld laser rangefinders commonly used in our daily lives employ phase-array laser ranging.
Frequency-modulated continuous wave laser ranging method that "tracks you"
If the target is moving, how do we determine its velocity in addition to its distance? With the development of aerospace technology, ensuring a safe soft landing for spacecraft has become a crucial issue. Single-pulse laser ranging and phase ranging methods can only obtain the target's distance information. To obtain its velocity, at least two measurements are needed, combined with the time interval between the two measurements. However, the accuracy of the average velocity calculated in this way is far lower than that of laser Doppler velocimetry radar.
Frequency-modulated continuous wave laser ranging can solve this problem. It can measure not only distance but also velocity, and therefore can be applied to ranging targets with relatively high speeds. NASA's Autonomous Landing and Obstacle Avoidance (ALHAT) program, proposed in 2006 for returning to the Moon and exploring Mars, uses this ranging method. The radar underwent flight tests in 2008 and 2010, achieving good results.
