## 2 FMCW LiDAR
### FMCW LiDAR Key Parameters
#### 2.3.1 Detection Distance and Accuracy
OPA technology represents a significant research avenue within this domain, offering substantial benefits for improving the integration of radar systems. OPA technology, which precisely controls the phase of light beams to create a main beam with specified directivity for accurate scanning [11]. The optical beams emitted by each waveguide or antenna can be considered as slits propagating through the air and interfering with each other. According to the multi-slit Fraunhofer diffraction model, these interfering beams will be reinforced in certain directions while being attenuated in others, thus enabling beam steering. By controlling the phase differences between each beam, it is possible to direct the enhanced interference beams and achieve beam scanning. Optical phased arrays can be classified into one-dimensional and two-dimensional scanning systems based on the dimension of the scanned output beam. One-dimensional scanning in optical phased arrays can be directly realized by modulating the phase difference of the beams. Two-dimensional scanning, on the other hand, is typically achieved using one of two methods: one involves etching a grating on the waveguide and utilizing the diffraction characteristics of the grating, where altering the wavelength of the laser can control the direction in the second dimension; the other method uses a two-dimensional emitter array to achieve two-dimensional beam scanning. The primary materials used in the fabrication of optical phased arrays (OPA) include lithium niobate, liquid crystals, ceramics, and silicon-based materials. Currently, with the rapid advancement of silicon photonics technology, it has become feasible to fabricate optical phased array chips using silicon-on-insulator (SOI) technology. Compared to traditional mechanical radar systems, OPA chips achieve fully solid-state beam scanning and control using electrical signals, eliminating the need for mechanical movement and thereby significantly enhancing radar reliability and lifespan. Additionally, OPA chips manufactured using complementary metal oxide semiconductor (CMOS) technology are compact, cost-effective, and have low drive power consumption. These chips can also be integrated with laser and detector chips, making them an optimal choice for developing solid-state, low-cost, and automotive-grade LiDAR systems. Furthermore, advancements in OPA technology open new possibilities for the integration of FMCW LiDAR systems, including combined integration with laser emitters and detector chips. This represents a new trend in the development of low-cost, high-performance FMCW LiDAR technology. Due to their substantial benefits in size reduction and cost efficiency, OPA technology is considered one of the most promising directions in LiDAR technology development. The array scale of OPA has continually expanded, advancing from 128 channels to as many as 9216 channels. Despite the high complexity in driving and controlling large-scale OPA arrays, there are now dense electronic driving circuits and sophisticated algorithms available to coordinate the elements within these arrays. Additionally, the scanning field of view of OPA is progressively broadening, and the level of sidelobe suppression is also being enhanced. Some entities have developed complete chip packaging, high-speed driving circuits, and advanced high-throughput signal processing techniques, successfully achieving high-quality FMCW point cloud outputs [12]. In 2023, the WATTS MR team at the Massachusetts Institute of Technology proposed an on-chip FMCW LiDAR system based on a 9216-channel OPA chip. The inter-element spacing within the OPA chip is 1.7 \(\upmu\)m, with an aperture size of 94 mm\({}^{2}\), and a scanning field of view of \(50^{\circ}\times 11^{\circ}\). The system achieved coherent detection of targets beyond 50 m, with an average power of 125 \(\upmu\)W per driving unit, a resolution time of less than 3.8 \(\upmu\)s, and a capability of producing 10,000 point clouds per second [13].