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.

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