  Quantum cascade lasers (QCLs) are becoming a key tool for plenty of
applications, from the mid-infrared (mid-IR) to the THz range. Progress in
related areas, such as the development of ultra-low-loss crystalline
microresonators, optical frequency standards, and optical fiber networks for
time and frequency dissemination, is paving the way for unprecedented
applications in many fields. For most demanding applications, a thorough
control of QCLs emission must be achieved. In the last few years, QCLs unique
spectral features have been unveiled, while multifrequency QCLs have been
demonstrated. Ultra-narrow frequency linewidths are necessary for metrological
applications, ranging from cold molecules interaction and ultra-high
sensitivity spectroscopy to infrared-THz metrology. A review of the present
status of research in this field is presented, with a view of perspectives and
future applications.
