Heterodyne detection is a well known coherent detection technique, widely exploited from telecommunications to basic research. Its application in the Terahertz (THz) range remains however rather challenging owing to the scarcity of local oscillator sources, especially above 2THz . As shown in 2007, rectified mode-locked fs-lasers can become a powerful tool to generate broadband THz frequency rulers for the measurement of sources with an emission frequency up to several THz [2,3]. These rulers exploit the harmonics of a mode-locked laser repetition rate as local oscillators for heterodyne mixing.
In this work it will be shown how the 104 harmonic of the repetition rate of a mode-locked fs-fiber laser can be mixed with the radiation emitted by THz Quantum Cascade Lasers (QCLs) and used for their heterodyne detection . This opens the door to several applications that will be reviewed in the talk, starting from the frequency and phase stabilisation of THz QCLs, the measurement of their intrinsic line-widt and frequency, to coherent imaging [4-6]. In the final part of the presentation it will be shown how fs-laser combs can also be applied to multi-mode THz QCLs, in particular for the demonstration of active mode-locking .
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