Recent advances in high-speed data communications using mid infrared quantum cascade lasers

Mid-Wave Infrared (MIR) free-space optical communication offers multiple advantages, such as improved transmission capacity through the atmosphere and immunity to electromagnetic interference. In addition, MIR transmission between 8-12 microns provides stealth for the communication signal thanks to the random thermal blackbody radiation having a strong background at these wavelengths, hence greatly reducing the probability of adversaries intercepting a MIR laser signal. Quantum cascade lasers (QCL) are optical sources of choice to target this wavelength domain. They are unipolar semiconductor lasers from which stimulated emission is obtained via electronic transitions between discrete energy states inside the conduction band. This work reports on a full unipolar quantum optoelectronics communication system based on a 9-micron QCL and on a Stark-effect modulator. Two different receivers are considered for high-speed detection, namely an uncooled quantum cascade detector (QCD) and a nitrogen-cooled quantum well infrared photodetector (QWIP). We evaluate the maximum data rate of our link in a back-to-back (B2B) configuration before adding a multi-pass Herriott cell so as to increase the transmission length of the light path up to 31 meters. By using pulse shaping, pre- and post-processing, we reach a record bitrate both 2-level (OOK) and 4-level (PAM-4) modulation scheme for a 31-meter propagation link and a bit error rate (BER) compatible with standard error-correction codes. Overall, we believe that our unipolar quantum system is of paramount importance for the development of cost-effective, reliable and versatile free-space optics data links.