QCL-Based FMCW Ranging and Free-Space Optical Communication in the Mid-Infrared

Frequency-modulated continuous-wave light detection and ranging (FMCW LiDAR) is a technique for fast and precise measurements of distances and speeds of hard and diffuse targets [1]. Moreover, it has been shown that the use of an optical intensity modulator could allow simultaneous communication capabilities [2]. In this paper we show the first experimental demonstration of a quantum cascade laser (QCL)-based FMCW LiDAR in the mid-infrared. The original and compact optical setup that has enabled our work is illustrated in fig. 1. The output of a $9 \mu m$ QCL is split in two and one part (blue path in fig. 1) is sent to a compact Mach-Zehnder interferometer, allowing real-time monitoring and correction of the frequency modulation. More specifically, the use of a predistortion algorithm allows the injection current of the laser to be adequately modulated in order to achieve linear optical frequency modulation (LFM), which increases the signal-to-noise ratio of the FMCW signal. High-speed and precise LFM up to 8 GHz in $1\ \mu s$ , with less than 1% error to linearity and with a very low optical power withdrawal for monitoring (< 1mW) has been achieved. To our knowledge, this is the first demonstration of such results whose scope of application extend to other fields such as spectroscopy.