8 - Frequency Noise and Frequency Stabilization of QCLs

Summary

Quantum cascade lasers (QCL) can be powerful testing grounds of the fundamental physical parameters determined by their quantum nature. In this chapter we describe a set of experimental techniques to explore the linewidth, frequency and phase stability of far-infrared QCLs. By performing noise measurements with unprecedented sensitivity levels, we highlight the key role of gain medium engineering and demonstrate that properly designed semiconductor-heterostructure lasers can unveil the mechanisms underlying the laser-intrinsic phase noise, revealing the link between device properties and the quantum-limited linewidth. We discuss phase-locking of THz QCL to a free-space comb generated in a LiNbO3 waveguide, and present phase and frequency control of miniaturized QCL frequency combs. This work paves the way to novel metrological-grade THz applications, including high-resolution spectroscopy, manipulation of cold molecules, astronomy and quantum technologies. The physical processes and dynamics presented here open groundbreaking perspectives for the development of quantum sensors, quantum imaging devices and q-bits made by entangled teeth for photonic-based quantum computation.