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This book provides an integrated theoretical and experimental perspective on nonlinear dynamics in quantum cascade lasers. This PhD thesis investigates quantum cascade lasers (QCLs) as a platform for advanced free-space optical systems, with the dual objective of deepening the understanding of their nonlinear dynamics and exploiting these properties for secure and resilient photonic applications in the mid- and long-wave infrared spectral regions. Motivated by the growing demand for high-speed, covert, and robust wireless communication and sensing technologies, this book explores the advantages of LWIR free-space links, including favorable atmospheric transmission windows, reduced scattering, and intrinsic covertness. Unlike conventional interband diode lasers, QCLs are intersubband light sources characterized by ultrafast gain recovery, strong nonlinearities, and fundamentally different dynamical behavior. These properties place QCLs in a distinct dynamical class and enable access to unconventional regimes that are not observed in standard semiconductor lasers. A central focus of the thesis is the study of deterministic chaos in QCLs. Using a rigorous modeling framework based on the Effective Semiconductor Maxwell-Bloch Equations, the intrinsic and feedback-induced dynamics of distributed-feedback QCLs are analyzed, revealing complex multimode and chaotic regimes arising from the interaction between internal and external cavity modes, enhanced by spatial hole burning and a non-zero linewidth enhancement factor. A detailed dynamical analysis demonstrates the emergence of hyperchaotic behavior dominated by ultrafast, picosecond-scale processes, highlighting a fundamental departure from chaos mechanisms in interband lasers. Building on this physical understanding, the thesis demonstrates the exploitation of QCL-based chaos for free-space optical applications. A chaos-based LiDAR system operating in the LWIR band is experimentally implemented, achieving meter-scale range resolution with sub-centimeter ranging precision. In parallel, a physical random number generator based on QCL chaos is demonstrated, delivering data rates up to 2.5 Gbps. The impact of atmospheric turbulence on chaotic free-space propagation is also investigated, and chaos recovery is demonstrated using real-time wavefront correction.