Laser radar is mainly composed of four system elements: laser emission, laser manipulation (scanning system), laser reception and information processing. The four systems complement each other, and then obtain a large amount of location point information in a short time, and realize 3D modeling based on these information.
Laser emission: EEL to VCSEL, 905nm to 1550nm
The laser is generated from the laser transmitter. The excitation source periodically drives the laser and emits laser pulses. The laser modulator controls the direction and number of lines of laser emission through the beam controller. Finally, the laser is emitted to the target object through the emission optical system. The transmitter develops from EEL to VCSEL. At present, most vehicle mounted lidars use semiconductor lasers, which are specifically divided into edge emitting laser (EEL) and vertical cavity surface emitting laser (VCSEL) whose laser is perpendicular to the top surface.
The light-emitting surface of the EEL laser is located on the side of the semiconductor wafer, which has the advantages of high optical output power, good heat dissipation, etc., but often the production cost is high and the consistency is difficult to guarantee; The light-emitting surface of VCSEL laser is parallel to the semiconductor wafer, which makes it easier to bond with the planar circuit chip, and improves the efficiency of light modulation. Compared with EEL, VCSEL has the advantages of low cost, high efficiency and long life. The traditional VCSEL laser has the defects of low luminous density power and ranging distance less than 50m. In recent years, many domestic and foreign VCSEL laser companies have developed multi-layer junction VCSEL lasers, which has increased its luminous power density by 5~10 times, which makes it possible to develop long-distance laser radar by using VCSEL.
At present, the mainstream laser radar mainly emits light at 905nm and 1550nm. Among them, the 905nm laser receiver can directly use silicon materials with lower price, which makes the cost more controllable and the size of the final product relatively small, so it has become the wavelength selected by the most mainstream laser radar at present; However, in order to avoid damage to the human eye, its transmission power and detection distance will be limited (lasers in 400-1400nm wave band can pass through the vitreous body of the human eye and focus on the retina, while the temperature of the human retina rises by 10 ℃, causing damage to photoreceptors). The 1550nm laser will not damage the retina, so it can emit more power and have a longer detection distance; At the same time, the light at 1550nm is far away from the visible spectrum and is not easily interfered by sunlight. However, it is necessary to use high valence indium gallium arsenic (InGaAs) as the substrate material of the detector, and the production cost is relatively high. At present, some manufacturers, such as Luminar, Huawei, Innovusion, Benewake, and Tudatong, have chosen 1550nm laser, which is expected to further reduce costs with the increase of mass production in the future.
The emission optical system is composed of a diffuser, a collimating mirror and a beam splitter, which plays an important role. The original laser emitted by the laser itself is uneven point light. Its "hot spot" will burn the irradiated devices and objects. At the same time, it has the shortcomings of irregular spot shape (such as oval or strip shape), different divergence angles, and cannot be directly emitted. The emission optical system can convert the original laser into a uniform beam through the mutual cooperation of the diffuser, collimator, and beam splitter, The effect should not be underestimated.
Source: Lemon Feelings
