Absolute frequency spectroscopy at 4.3 µm by direct referencing of a Quantum-Cascade-Laser to an Er: Fiber laser-based frequency-comb

The authors report the first direct referencing of a QCL (Alpes Lasers) at 4.3 μm to a near-IR comb emitted by a two-branch Er: fiber oscillator (100 MHz, Toptica FFS), exploiting a very simple scheme: an up-converted 1.14μm pulsed beam is generated by focusing together in a periodically poled lithium niobate non-linear crystal (PPLN) both the 1.55 μm comb beam and the 4.3 μm continuous wave (cw) beam from the QCL. The paper shows the beat signal used for QCL stabilization as obtained by overlapping onto an InGaAs fast detector the 1.14 μm up-converted beam with the spectrally overlapping super-continuum portion generated at the second branch of the comb source.