Free-space chaos-based private communication with quantum cascade lasers emitting in the mid-infrared second thermal atmospheric window

This work describes the first proof-of-concept of mid-infrared (MIR) free-space cryptosystem, based on chaos synchronization between two distributed-feedback (DFB) quantum-cascade lasers (QCL), operating continuous wave at room temperature. Based on a superlattice heterostructure where the light emission is due to intersubband transitions, QCLs are now mature sources emitting in a wide range of wavelengths, from the MIR up to the long-wave infrared (LWIR, 2.6-20 μm) domains, extending also to the Terahertz region (60-150 μm). This feature, together with high output power and modulation bandwidth beyond tens of GHz, elects QCLs among the leading devices that find application in countless fields: countermeasure systems, high-resolution spectroscopy, and telecommunication. MIR and LWIR windows yield several advantages over the near-infrared (NIR) wavelengths in free-space optical (FSO) communication systems: among the others, high atmospheric transparency, associated to a strong resistance to degraded atmospheric condition and stealth of the beam, conferred by the background emission. This makes classical communication systems based on QCLs realistic and very desirable. Besides, the observation of chaotic outcomes in the nonlinear dynamics of QCLs drew attention for private communication: based on message enciphering within the chaos produced by a transmitter source and subsequent synchronization by a coupled matched receiver, it is cost-effective and easy-to-implement. Discovered in 1999 in semiconductor lasers, chaos synchronization served free-space and fiber-based secure communication, reaching dozens of Gbps over hundreds of km. Because of the absence of relaxation oscillations, contrarily to other semiconductor lasers, the chaos bandwidth of QCLs is estimated to be around 130 GHz, and high-speed dynamics over hundreds of GHz might be revealed. Up to now, chaos bandwidths of only dozens of MHz have been observed under external optical feedback (EOF), mainly for the complex structure of QCLs. In this work, we implement a cryptosystem of one-meter-distant unilaterally coupled QCLs. The master QCL is driven chaotic through EOF. The message is hidden within the chaos, directly modulating the master bias through a non-return-to-zero (NRZ) pseudorandom-binary-sequence (PRBS-7). The matched slave QCL is optically injected with the transmitted beam, allowing for chaos synchronization or anti-synchronization due to the injection-locking effect. For the chaos pass filter effect, the slave reproduces only the master chaos: the initial message is retrieved by subtracting the transmitted signal to the slave signal, obtaining a new “difference” timetrace. Ideally, it is identical to the enciphered message; practically, channel impairments and imperfect synchronization cause additional noise. Filtering and equalization improve the quality and reliability of transmission. Considering the implementation of a forward-error-correcting (FEC) decoder, the experimental bit-error rate (BER) and eye diagrams for data rates from 1 Mbps to 8 Mbps demonstrate an error-free transmission, with increasing performance as the data rate decreases. Security is also granted: an eavesdropper with the best matching filter obtains a BER higher than the lower limit commonly accepted for a non-decipherable transmission. This work represents a pillar of building a real-field free-space cryptosystem based on chaos synchronization in a wavelength range that is of utter interest, yet still unknown to many.