Narrow linewidth tunable THz signal radiated by 1.55 µm photomixing
Résumé
THz has become a wide field of investigation opening new opportunities in a growing number of domains of physics, chemistry, and biology. Among the different techniques existing today to generate THz fields, heterodyning two optical frequencies is a useful approach when tunability is required. Moreover, to address high-resolution spectroscopy or metrology applications, a key point is the achievement of a narrow linewidth source. To this aim, two-propagation-axis dual-frequency lasers have been already shown to provide narrow linewidth tunable beat notes up to 2 THz. We report in this paper the demonstration of a narrow linewidth THz radiation source based upon this laser. Indeed the beat note provided by the laser is sent into a unitravelling carrier photodiode (UTC-PD), and radiated by a transverseelectromagnetic- horn antenna (TEM-HA). All components operate at room temperature. The emitted THz signal is detected by a subharmonic mixer coupled to an electrical spectrum analyzer. The THz signal is observed and analyzed thanks to a heterodyne detection. The measured dynamic range is 75 dB at 282 GHz, 50 dB at 500 GHz, 35 dB at 700 GHz and decreases to 20 dB at 1 THz. The decrease is due to the UTC-PD efficiency and conversion losses in the sub-harmonic mixer. The measured linewidth is better than 30 kHz at any frequency from DC to 1 THz.