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

doi:10.1364/OE.379960

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

Opt Express. 2020 Jan 20;28(2):2091-2106. doi: 10.1364/OE.379960.

Authors

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

PMID: 32121907
DOI: 10.1364/OE.379960

Abstract

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 1080^th 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.