## 2 FMCW LiDAR
### FMCW LiDAR System Architecture  
The FMCW LiDAR operates as an advanced radar system, emitting light waves of constant, significant power that vary in temporal frequency or phase during cyclic changes. This system uses coherent detection technology to perform measurements by analyzing differences between echo and transmitted signals, demonstrating a high degree of precision. In FMCW LiDAR systems, the core components include the trigger system module, the signal acquisition and processing system module, the receiver-end beam scanning system, the frequency-modulated laser, and the photodetector, as depicted in Figure 1. Upon receiving a command to start measurement, the trigger module initiates and simultaneously activates the frequency-modulated laser module, the receiver-end beam scanning system, and the signal acquisition and processing module. In the single-point measurement mode, the receiver-end beam scanning system may remain stationary, typically used for precise measurements of fixed targets, hence focusing on a fixed point without the need for dynamic scanning. However, in dynamic measurement modes, scanning across multiple targets or entire target areas is required; thus, the receiver-end beam scanning system operates synchronously, dynamically adjusting the direction of received beams to continuously scan points in space, relaying data to the computer for processing in real-time. The frequency-modulated laser module, under the control of the trigger signal, produces a laser frequency modulated by triangular or sawtooth waves. Ninety-nine percent of the laser is transmitted through an optical fiber path into free space for measurement, while one percent is directly transmitted to the photodetector for interference with the reflected measurement light. The signal acquisition and processing module starts synchronously, processing the interference signals from the photodetector. The trigger module is composed of both software and hardware triggering elements. In continuous measurement mode, this module is tasked with delivering software-generated instructions to the laser. As the laser emits a linearly frequency-modulated beam, it concurrently sends synchronization signals to the acquisition card and the receiver-end beam scanning system. Upon receiving the beat frequency signals and data from the receiver's beam scanning device, the system processes this information collectively to estimate the target's characteristics. The synchronization of the triggering is vital for the accuracy of measurements. Imprecise synchronization may lead to inconsistent data, thereby impacting the final measurement results. The receiver-end beam scanning devices of FMCW LiDAR systems can take various forms in different designs, each with its specific advantages and application scenarios. The  
Figure 1: Structure of FMCW LiDAR system. main types include one-dimensional and two-dimensional steering mirrors, which can be precisely controlled through mechanical or motor mechanisms to adjust the beam's direction, suitable for applications requiring rapid and precise angular adjustments; micro-electromechanical systems (MEMS) scanning mirrors utilize miniature motors to drive small mirrors for fast and accurate beam scanning, ideal for portable or miniaturized LiDAR systems where space and weight are critical considerations; optical phased array (OPA) chips are key hardware components for implementing OPA technology, using electronically controlled optical phase modulators to change the beam's propagation direction without mechanical movement, offering extremely high scanning speed and flexibility; reflective mirror systems use directional mirrors to optimize optical path design, ensuring laser transmission to the target area; and three-dimensional rotation platforms typically include two motors, controlling rotation in horizontal and vertical directions, capable of dynamic scanning in multiple directions, thus providing a comprehensive view and high-precision angular resolution.