The combined beam is used to illuminate a photo-mixer that generates THz radiation with a wavelength proportional to the frequency difference of the two laser diodes. Using this method, the THz frequency can be swept by precisely changing the temperature of the laser diode sources; see for example US Patents 9,134,054 and 9,806,491.
The frequency sweeping enables numerous absorption spectroscopy applications. Over the frequency range of interest, mode-hop free laser sources with optical isolators are required to enable stable operation.