Optical parametric-assist frequency modulation for FMCW Lidar enabling double-range velocity measurement

Frequency-modulated continuous waves (FMCW) Lidar functioned as simultaneous measurement of velocity and distance has shown promising prospects in many applications. Direct modulation on semiconductor lasers is a common approach to achieve linear optical frequency scanning in FMCW Lidar for the simple configuration and easy implementation. However, the directly-modulated laser used to suffer from the limited scanning bandwidth, resulting in poor range resolution and limited velocity measurement range of FMCW Lidar. In particular, the issue becomes critical in the case of high velocity measurement over short distance. Here, we propose an optical parametric-assist frequency modulation (OPAFM) method to improve the velocity and distance measurement of FMCW Lidar simultaneously. Firstly, a chirp rate doubled idler light is generated via the four-wave mixing of a frequency-swept pump with a continuous-wave signal. The idler light then serves as a linear frequency swept laser source in the FMCW Lidar to double the range of velocity measurement and improve ranging resolution. We experimentally demonstrated the chirp rate improvement of frequency-swept light from 80.75 MHz/μs to 163.75 MHz/μs. To apply the idler light to the FMCW Lidar, the maximum measurable velocity can be doubled from 2.13 m/s to 4.41 m/s for a target at 5.2 m. Moreover, ranging measurement with improved resolution of 117 mm is also achieved, compared to the 234 mm ranging resolution by using the traditional FMCW Lidar solely. The maximum range can reach 41.54 m. The precision of velocity and distance measurement are 0.1 m/s and 3.5 cm, respectively. The method provides a promising solution to enhance the velocity and distance measurement of FMCW Lidar, enabling diverse applications in sensing and mapping.