FMCW Lidar Using Phase-Diversity Coherent Detection to Avoid Signal Aliasing

A frequency-modulated continuous-wave (FMCW) lidar is proposed to avoid signal aliasing in the measurement of distance and velocity. In the transmitter, a lightwave is amplitude-modulated in a Mach-Zehnder modulator by a linear-frequency modulated (LFM) waveform. The modulated light is used as a probe signal. The reflected optical signal beats with a local optical (LO) signal in a phase-diversity coherent optical receiver. The in-phase and quadrature components of the output signal are simultaneously recorded. The distance is extracted from the sum of the squares of the two components, while the velocity is calculated from the ratio of them. An experiment is carried out, in which the real-time distance and velocity of a spinning disk are measured. The proposed method can be used to solve the problems of signal aliasing which may lead to the ambiguity of distance and velocity measurements in traditional FMCW lidars. Moreover, the impacts of optical phase fluctuations are removed from the distance measurement in the proposed method.