Abstract-Quantum cascade laser-pumped terahertz molecular lasers: frequency noise and phase-locking using a 1560 nm frequency comb

Jean-Francois Lampin, Antoine Pagies, Giorgio Santarelli, Jeffrey Hesler, Wolfgang Hansel, Ronald Holzwarth,  Stefano Barbieri,

(a) Energy diagram of the levels relevant for laser operation. (b) Schematic of the QCL-pumped ML. The laser cavity is closed at one end (left) by an output coupler, obtained by depositing a metallic grid on a Silicon substrate, and on the other end (right) by a plane metallic mirror, with a 1.2 mm hole drilled in its center. An isolator (∼30 dB isolation, and ∼60% transmission) is used to reduce the optical feedback on the DFB QCL. (c) THz ouput power vs MIR pump power, measured without the optical isolator displayed in panel (b). Inset: intensity plot of the optical beam collected with a microbolometer camera positioned at about 4 cm from the laser output coupler.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-28-2-2091

We report the measurement of the frequency noise power spectral density (PSD) of a Terahertz (THz) molecular laser (ML) pumped by a mid-infrared (MIR) quantum cascade laser (QCL), and emitting 1 mW at 1.1THz in continuous wave. This is achieved by beating the ML frequency with the 1080th harmonic of the repetition rate of a 1560 nm frequency comb (FC). We find a frequency noise PSD < 10Hz2/Hz (-95dBc/Hz) at 100kHz from the carrier. To demonstrate the effect of the stability of the pump laser on the spectral purity of the THz emission we also measure the frequency noise PSD of a CO2-laser-pumped 2.5THz ML, reaching 0.1Hz2/Hz (-105dBc/Hz) at 40kHz from the carrier, limited by the frequency noise of the FC harmonic. Finally, we show that it is possible to actively phase-lock the QCL-pumped molecular laser to the FC repetition rate harmonic by controlling the QCL current, demonstrating a sub-Hz linewidth.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Quantum cascade laser-pumped terahertz molecular lasers: frequency noise and phase-locking using a 1560nm frequency comb

Jean-Francois Lampin, Antoine Pagies, Giorgio Santarelli, Jeffrey Hesler, Wolfgang Hänsel, Ronald Holzwarth, Stefano Barbieri

https://arxiv.org/abs/1911.06568

The recent demonstration of a terahertz (THz) molecular gas laser pumped by a mid-infrared quantum cascade laser (QCL) has opened up new perspectives for this family of sources, traditionally relying on CO2-laser pumping. A so far open question concerning QCL-pumped THz molecular lasers (MLs) is related to their spectral purity. Indeed, assessing their frequency/phase noise is crucial for a number of applications potentially exploiting these sources as local oscillators. Here this question is addressed by reporting the measurement of the frequency noise power spectral density (PSD) of a THz ML pumped by a 10.3{\mu}m-wavelength QCL, and emitting 1mW at 1.1THz in continuous wave. This is achieved by beating the ML frequency with the 1080th harmonic of the repetition rate of a 1560nm frequency comb. We find a frequency noise PSD < 10Hz2/Hz (-95dBc/Hz) at 100kHz from the carrier. To demonstrate the effect of the stability of the pump laser on the spectral purity of the THz emission we also measure the frequency noise PSD of a CO2-laser-pumped 2.5THz ML, reaching 0.1Hz2/Hz (-105dBc/Hz) at 40kHz from the carrier, limited by the frequency noise of the frequency comb harmonic. Finally, we show that it is possible to actively phase-lock the QCL-pumped molecular laser to the frequency comb repetition rate harmonic by controlling the QCL current, demonstrating a sub-Hz linewidth.