Fig. 1: Nonlinear laser source and detection scheme.
From: Optical frequency metrology in the bending modes region
a Experimental lay-out of the comb-referenced nonlinear laser source for long-wavelength mid-infrared. QCL quantum cascade laser; BC beam combiner; PM parabolic mirror; OP-GaAs orientation-patterned Gallium Arsenide nonlinear crystal; F low-pass filter; PD liquid nitrogen cooled photodetector; SFG: sum frequency generation. b Pictorial view of the difference frequency generation (DFG) process between the comb-locked CO2 laser and the quantum cascade laser (QCL). The optical frequency of the QCL beam (pump, orange line) is swept while keeping the CO2 beam (signal, red line) locked to the comb. The frequency of the DFG beam (idler, brown line) is correspondingly swept over a range that contains the long-wavelength mid-infrared (LWIR) absorption signatures (blue line). The acquisition of the QCL-comb radiofrequency (RF) beat note (fbeat, black line) permits the absolute calibration of the DFG frequency during the scan, synchronously with the acquisition of the idler transmission through the molecular sample (IPD). Pump and signal are shown to beat directly with a MIR comb, yet this happens through the interposition of a sum frequency generation process (see “Methods” for details).
