Towards a turnkey private communication system using a quantum cascade laser emitting at 4 microns

Quantum cascade lasers (QCLs) are optical sources exploiting radiative intersubband transitions within the conduction band of semiconductor heterostructures.1 Mid-infrared QCLs have been thoroughly considered for applications such as spectroscopy,2 free-space communications3 and countermeasure systems.4 Under self-optical feedback, QCLs have been proven to operate in several non-linear dynamic regimes,5 including low-frequency fluctuations and deterministic chaos, which are suitable for private communications taking advantage of both chaos masking and background stealth. However, the previous experiments focused on distributed feedback (DFB) quantum cascade lasers emitting at 5.7 μm, which is not an optimized wavelength for free-space applications. Indeed the atmosphere is characterized by two transparency windows between 3-4 μm and 8-12 μm, which are called bandpass L and bandpass N, respectively.6 Furthermore, the 5.7 μm lasers were studied at the chip level, which means that end users must own the dedicated mounts, connectors and mid-infrared optics in order to take advantage of these quantum cascade sources. This work extends our knowledge by exploring the non-linear dynamics of a packaged Fabry-Perot (FP) QCL emitting at 4 μm. The advantage of the FP configuration is an increased output-power compared to DFB sources, though the FP configuration is not well-known yet.7 Moreover, this laser comes in a handy environment with embedded focusing optics and high-heat load (HHL) packaging for plug-and-play operation. Consequently, the current findings pave the way for off-the-shelf private s at mid-infrared wavelength where high-power and compact turnkey systems are required.