## 2 FMCW LiDAR
### FMCW LiDAR Key Parameters
#### 2.3.5 FM Linearity
The use of a laser with a narrow linewidth as the emitting source enables the modulated output signal to have a large modulation bandwidth and a very narrow linewidth, thereby achieving high resolution and high accuracy detection of the target. This method provides a solution to effectively reduce nonlinear interference while maintaining the advantage of modulation bandwidth [24]. Chirped-pulse lasers are produced using various techniques, including time-domain stretching, Fourier domain mode-locked lasers, and frequency-shifted feedback lasers [25; 26; 27]. Although these signals are not continuous light beams, their low-duty cycle allows chirped pulse laser signals to be regarded as FMCW LiDAR signals. Within each pulse period, the frequency of the signal linearly varies with time. Therefore, FMCW LiDAR systems utilizing chirped pulse laser sources are widely considered an effective realization of FMCW LiDAR technology. This approach enhances the performance of FMCW LiDAR by leveraging the characteristics of chirped pulse lasers, enabling operation over a broader frequency range and thus providing higher measurement accuracy and update rates. This technology extends the application range of FMCW LiDAR across various fields, offering significant support for high-precision measurements and dynamic target tracking. In the modulation method, the internal modulation method is relatively easy to achieve a large tuning range by directly changing the resonant cavity parameters; however, the existence of the laser buildup time causes the instantaneous linewidth of the output FM light to be relatively wide, resulting in a reduction in the coherence length of the light source, or the tuning rate must be limited in order to establish a stable light field. External modulation can rapidly change the instantaneous frequency of the light field while maintaining the excellent properties of the seed light through tuning mechanisms such as electro-optic effects and sound and light effects, but the limited bandwidth of the electro-optic effect limits the increase in the tuning range of the light source (i.e., it limits the maximum resolving power that can be achieved with the system) [28]. For the selection of the light source in FMCW LiDAR, the specific application scenarios must be considered (e.g., the current widely concerned about the automated driving of the vehicle LiDAR). In vehicular applications, FMCW LiDAR must meet specific performance criteria. These include a minimum detection range of 300 m, centimeter-level distance resolution of at least 0.1\({}^{\circ}\) angular resolution, and an omnidirectional data update rate exceeding 10 Hz. To satisfy these specifications, the coherence length of the LiDAR light source must exceed 600 m, implying a linewidth of less than 150 kHz and a repetition period greater than 2 \(\upmu\)s. To achieve the 1 cm distance resolution criterion, the frequency tuning range must exceed 15 GHz. Additionally, for a 5 Hz data update rate, the scanning rate of the light source should surpass 300 GHz/ms [7]. For the vehicle unmanned LiDAR, such as the long detection distance, fast refresh, and high resolution requirements of the application scenario, we believe that the external modulation-based FMCW LiDAR technology is worth looking forward to. The program can maintain the characteristics of the seed light long coherence to achieve a wide enough frequency tuning range and fast enough tuning rate. Although the main reason that limits the practical application of this solution is the cost of electro-optic modulators, with the development of silicon optical integration technology, we believe that the cost of electro-optic modulators will be greatly reduced and put into practical production.