High-linearity terahertz FMCW generation using two-chirped optical tone photomixing in a self-delayed fiber-optic system.

Terahertz frequency-modulated continuous-wave (THz-FMCW) radar has attracted significant attention due to its unique advantages in high-precision sensing and imaging, offering broad prospects for practical applications. However, the nonlinearity of FMCW chirp signals can introduce multiple frequency components, thereby degrading the detection capability of the system. In this work, we propose a two-chirped optical tone photomixing method to address this issue. Without introducing any additional hardware-based linearization components or software-based pre-compensation algorithms, the proposed approach combines a self-delayed fiber-optic system constructed from standard single-mode fiber with a single free-running laser source. Despite the free-running laser exhibiting an intrinsic chirp linearity of Rlin2 below 0.99, the proposed method enables the generation of high-linearity THz-FMCW chirp signals at a sweep rate of 20.53 GHz/μs, with the chirp linearity markedly enhanced to Rlin2=0.9994. Furthermore, the generated high-linearity THz-FMCW chirp signals are applied to a Michelson interferometer ranging system, achieving a ranging error below 0.60 cm and a signal-to-noise ratio exceeding 30 dB. The proposed method significantly enhances the chirp linearity, making this approach a highly attractive solution for future high-precision THz-FMCW radar systems.