Free-Space Communications Enabled by Quantum Cascade Lasers

Future generations of wireless communication systems are expected to support orders of magnitude faster data transfer with much lower latency than the currently deployed solutions. Development of wireless transceivers of higher bandwidth, low energy consumption, and small footprint becomes challenging with radio frequency (RF) electronic technologies. Photonics-assisted technologies show many advantages in generating signals of ultrabroad bandwidth at high carrier frequencies in the millimeter-wave, terahertz, and IR bands. Among these frequency options, the mid-IR band has recently attracted great interest for future wireless communication due to its intrinsic merits of low propagation loss and high tolerance of atmospheric perturbations. A promising source for mid-IR free-space communications is the semiconductor quantum cascade laser (QCL), which can be directly modulated at a high speed and facilitates monolithic integration for compact transceivers. Herein, the research and development of QCL-based free-space communications are reviewed and a recent experimental study of multi-gigabit transmission with a directly modulated mid-IR QCL and a commercial off-the-shelf IR photodetector is reported on. Up to 4 Gb s(-1)transmission of two advanced modulation formats, namely, four-level pulse amplitude modulation (PAM-4) and discrete multitone (DMT) modulation, is demonstrated.