FMCW lidar combined frequency modulation and frequency shift

The frequency-modulated continuous-wave (FMCW) lidar is widely applied due to its high sensitivity and strong antiinterference capabilities. However, when detecting high-speed moving targets, FMCW lidar encounters range-velocity coupling issue. This paper proposes a method to decouple the target distance and velocity by combining linear frequency modulation with a constant frequency shift. In this approach, the frequency of a single-frequency seed laser is modulated periodically. Each cycle of the frequency modulation signal consists of a sawtooth wave and a DC part. The chirped signal is split into two parts. one part serves as the local oscillator (LO), while the other part passes through a continuously operating frequency shifter and an amplifier, and finally emits to the target. The returned signal from the target beats with the LO to produce a radio frequency signal containing both target range and velocity information. The beat signal frequency of the chirped part comes from both the range induced frequency delay and the Doppler shift, while the target velocity only induces the Doppler frequency shift within the constant frequency duration. Therefore, using the known velocity the target range can be calculated from the beat chirped signal. Thereby, the velocity and the range of the target is decoupled. The simulation and experimental results validate the effectiveness of this method, demonstrating the high measurement resolution and low setup complexity.