Heterodyne Spectroscopy of Frequency Instabilities in Terahertz Quantum-Cascade Lasers Induced by Optical Feedback

We study the influence of optical feedback on the frequency stability of terahertz (THz) quantum-cascade lasers (QCLs) by high-resolution heterodyne spectroscopy. The emission from two pairs of QCLs operating either at 3.4 or at 4.7 THz is mixed in a Schottky diode and the resulting difference frequency in the gigahertz range is measured. The high spectral resolution of less than 1 MHz and a time resolution of 1 ms enable us to investigate feedback-induced frequency instabilities on scales that are relevant for spectroscopy. Frequency changes of up to 70 MHz are observed with an optical feedback of only 3 × 10
^-5
of the output power of the QCL.