Quantum cascade lasers for high-resolution spectroscopy

Despite the growing interest that quantum cascade lasers (QCLs) are gaining, they still present a few unclear aspects of their fundamental properties, such as spectral purity, that need to be deeply investigated when aiming to make these innovative laser sources suitable for high-resolution spectroscopy and metrology. This paper is a review of our efforts towards QCL-based high-resolution spectroscopy and of our experimental investigation of QCLs' frequency noise, aimed to discover the ultimate performances attainable by QCLs and to develop the experimental techniques required to achieve them. Our results, confirmed by several independent measurements, show that QCLs have a very small intrinsic linewidth buried under a large frequency-noise background. The development of appropriate frequency stabilization techniques will make QCLs well suited for high-resolution spectroscopy and metrology in the mid and far IR.