Automotive LiDARs are gaining increasing interest, gradually covering a wide range of applications. However, Automotive LiDAR has two mainstream laser bands. We will discuss the advantages and disadvantages of these two bands without considering the imaging technology solution. In a LiDAR system, laser generates laser pulses; Laser modulator controls the direction and channels of the light through the beam controller. Finally, the light is emitted to the targets through the emission optical system.
Wavelength is the most critical indicator of lasers, and four factors are generally considered: human eye safety, interaction with the atmosphere, optional lasers, and photodetectors. Laser is a single-color, single-wavelength light. Different lasers could be generated by different generator for different application, including ultraviolet (10-400nm), visible light (390-780nm) and infrared light(760nm-1mm). In the 400-1400nm wave band, the laser could pass through the vitreous and focus on the retina. Photoreceptor cells will be damaged if the temperature in retinal rises more than 10°C. The Lasers below 400nm or above 1400nm could be absorbed by the lens and cornea, so high-power lasers in such bands can cause cataracts or burn crystalline. Nearinfrared lasers are most likely to cause damage to the human eyes, as the photoreceptor cells are not sensitive to such lasers. Permanent wear to the eyes may has occurred before the eyes could feel it.
In order to avoid the damage to the human eyes, two wavelength ranges could be selected by LiDARs. One is within 1000nm, and the typical value is 905nm. In this case silicon could be used as the receivers, which are mature products with low cost. Another one is between 1000 and 2000nm, the typical value is 1550nm, this wavelength cannot be detected by silicon, and Ge/InGaAs detectors are required. Compared with InGaAs photodetectors, Si detectors are indeed more mature. However, with the rapid development of 1550nm LiDAR in recent years, the price gap between the Si and InGaAs detectors (both refer to APD) is quickly narrowed. With the same power, the eye safety performance of 1550nm laser could be 40 times of 905nm, allowing the LiDAR with higher power so as to achieve a longer detection range. Therefore, we can draw the first conclusion: 1550nm laser has better eye safety and longer detection range than 905nm laser.
The second advantage of 1550nm over 905nm is that the former has strong atmospheric penetration and higher detection accuracy. The 1550nm laser has strong anti-interference ability, better beam collimation and brightness. These advantages allow a LiDAR owns more efficient laser emission and reception, so as to achieve more refined object recognition. Moreover, the 1550nm laser has better beam divergence, and the spot diameter size is 1/4 of that of 905nm laser at a distance of 100 meters.
One of the disadvantages for 1550nm laser over 905nm is the penetration ability in rainy and snowy weather. Secondly, 1550 nm LiDAR generally adopts fiber laser as the light source, a more complex technology than the 905nm light source. There are obvious deficiencies in the cost of light source and detector, the size of the 1550nm LiDAR, and the maturity of the supply chain. Due to the high-power consumption of 1550nm laser, the heat dissipation is also a challenge.
The respective advantages and disadvantages of 905nm and 1550nm are very obvious, so we believe the two wavelengths of lasers will co-exist on automotive LiDARs. 2
