Interview conducted byVignesh MukundJul 30 2018
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Image Credits Shutterstock/temp-64GTX
The self-driving car has several sensors that detect obstacles and map its environment. Amongst these sensors, the LiDAR is most like the human eye in its function. It’s a detection and ranging system much like the radar but instead of radio waves, the LiDAR operates on light waves.
LiDAR systems illuminate the target area with a pulsed laser signal and calculate the time it takes for the reflected signal to be returned to the receiver. These systems comprise of a laser source, a photodetector, data processing electronics, and motion-control equipment.
自动驾驶汽车中使用了两种类型的激光盆。这些是根据它们如何扫描周围环境进行分类的。3D Flash LiDAR由广角源和广角光学元件组成，可将一个暴露于检测器的所有反射光聚焦。然后，该信息根据飞行时间计算来对周围环境进行建模。扫描激光雷达是一个系统，该系统沿各个方向依次发出光，并逐一检测回声以绘制其周围环境。
The metrics that are used to gauge the performance of a LiDAR system are:
- 横向分辨率
- 纵向分辨率
- 远程检测
- scan rate
在激光雷达的不同组件中，激光源对性能最重要。激光束的质量及其差异对于高横向分辨率至关重要。短的激光脉冲持续时间和较低的正时抖动可确保良好的纵向精度。脉冲能量是达到远距离检测的关键，而高脉冲重复速率可以更快地扫描导致更高的数据吞吐量。
Wavelengths between 600 nm and 1000 nm are usually used for non-scientific applications. The human eye is sensitive to these wavelengths and can easily absorb them. The power at which the laser has to operate to map obstacles at long ranges is beyond what is considered eye-safe. LiDAR systems that operate at 905 nm are, hence, limited in their range. Infrared lasers of 1.5 um are a great alternative for detecting solid bodies and obstacles at long ranges, and since they are not absorbed by the human eye, they are suitable for high-performance applications. In addition, the atmosphere is largely transparent at this wavelength, and highly efficient detectors are available.
激光源对良好性能至关重要的另一个方面是脉冲持续时间。几秒钟的短脉冲的短脉冲非常需要达到几毫米的纵向分辨率。但是，如此短的脉冲会导致激光光谱的扩大，从而降低信噪比（SNR）。尽管脉冲超过1纳秒消除了噪声，但它们也降低了系统的分辨率。为了平衡高态精度和良好的SNR，当前系统的脉冲持续时间为几百个皮秒。
激光二极管和纤维激光器都是激光雷达系统中常用的来源。激光 - 二极管源通常垂直堆叠，这意味着堆栈中的层之间的不连贯性通常会导致激光功率超过1类安全性激光的激光功率。通过用垂直腔表面发射激光替换垂直堆栈，可以降低总输出功率，但因此降低了这些系统可起作用的最大范围。另一方面，纤维激光器在功率水平为10 W至250 kHz的功率水平下的脉冲重复速率高为5 kHz。但是，与脉冲二二极管系统相比，这些系统昂贵。
理想的LIDAR系统必须在具有足够功率的眼睛安全波长下包含一个激光源，以在10 cm精度的距离处检测到黑暗障碍物，并在-40 c至85 C之间的温度下运行。
Sources:
- Lasers for Lidar: Application parameters dictate laser source selection in lidar systems.https://www.laserfocusworld.com/yabo214articles/print/volume-53/issue-03/features/lasers-for-lidar-application-parameters-dictate-dictate-laser-source-source-selection-selection-inclection-inclection-in-lidar-systems.html
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