High-frequency modulation spectroscopy with a THz quantum-cascade laser

A terahertz absorption spectrometer with a quantum-cascade laser (QCL) for high-resolution molecular spectroscopy is realized. The spectrometer is based on high-frequency (up to 50 MHz) modulation of the QCL frequency. This allows for the determination of the absorption coefficient and dispersion of the absorbing medium along with a very precise measurement of the line shape of the absorption feature. The design and performance of the spectrometer are presented, and its sensitivity and frequency calibration are discussed