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Schematic of the experimental setup. There are three main parts: the beat-note detection between QCL and DFG for the phase-lock, the high-finesse cavity for FNPSD analysis, and the saturation spectroscopy signal detection for the absolute frequency measurement of the CO2 transition. See Refs. 9 and 23 for more details.
Beat-note signal between the DFG radiation and the phase-locked QCL. The inset shows the same beat-note with a narrower span and resolution bandwidth. In both cases, the width of the peak (FWHM) is limited by the RBW of the spectrum analyzer.
QCL FNPSDs in free-running (trace a) and phase-locked (trace b) conditions, acquired by using a CO2 line and the high-finesse cavity as frequency-to-amplitude converters, respectively. The cavity has been used also to measure the FNPSD of the DFG radiation (trace c) and that of the QCL, when it is frequency-locked to a molecular line as in our previous work 22 (trace d).
Dependence of the center of the Lamb dip of the CO2 ( ) P(29) transition on pressure, with the corresponding linear fit. For clarity, the constant value of 69 297 478 MHz has been subtracted from the absolute frequency values. Inset: example of a single Lamb-dip acquisition (black squares). Experimental conditions: lock-in amplifier time constant 10 ms, chopper frequency 616 Hz, frequency scan with 60-MHz span, and 50 kHz steps. The fit with a Voigt function (green line) and the residuals are also plotted.
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