Frequency Noise and Phase-Locking of a Quantum Cascade Laser-Pumped, 1.073 Terahertz Molecular Laser using a 1560nm Frequency Comb

Summary form only given. Recently, a novel type of molecular THz laser with ~mW-level output power was demonstrated [1,2]. This laser is optically pumped by a mid-IR Quantum Cascade Laser (QCL), therefore resulting in a much more compact, frequency agile and deployable source compared to standard, CO
₂
-laser pumped, far-infrared gas lasers. This achievement has revived the interest for molecular THz lasers, in particular for their exploitation as low-phase noise THz local oscillators (LOs) operating at room temperature. Compared to CO
₂
-laser pumping, a QCL pump offers an unrivalled frequency agility that allows accessing an unprecedented number of molecular transitions. On the other hand, its lower frequency stability can potentially impact the frequency noise of the THz laser source [3-5]. Here we report the first measurement of the frequency noise power spectral density (PSD) of a 1.073THz ammonia laser pumped by a 10.3μm QCL. This is achieved by using a commercial harmonic mixer to generate a beating between the ammonia laser frequency and the 1080
^th
harmonic of the 1GHz repetition rate of a custom-made, 1560nm frequency comb (FC). We then exploit a voltage-controlled oscillator to track the beat-note signal and realize the frequency to amplitude conversion [6]. The measured frequency noise PSD is shown in Fig.1. In the range ~30-100kHz we find a frequency noise <; 10Hz
²
/Hz (-95dBc), limited by the noise of the QCL current driver. We will show that the value of the frequency noise PSD depends on the relative position of the pump frequency with respect to center of the Doppler-broadened pump transition line. To further elucidate the effect of the stability of the pump laser we also measured the frequency noise PSD of a CO
₂
-pumped, 2.5THz methanol laser, reaching 0.1Hz
²
/Hz (-110dBc) at 40kHz from the carrier, limited by the frequency noise of the FC harmonic. Finally, we will show that it is possible to actively phase-lock the QCL-pumped ammonia laser to the FC repetition rate harmonic by controlling the QCL drive current, demonstrating a sub-Hz linewidth. These results pave the way to the development of QCL-pumped molecular lasers as compact, low-phase noise THz sources operating a room temperature.