Frequency-comb-calibrated Doppler broadening thermometry

We describe an implementation of Doppler broadening thermometry based upon a room-temperature continuous-wave quantum cascade laser at 9.07 $\mu$m, coherently phase locked to a thulium optical frequency comb centered at 2 $\mu$m. The thermodynamic temperature is retrieved from the analysis of multiple-line absorption spectra of the NH${}_3$ ${\nu }_2$ band, adopting semiclassical line-shape models and taking into full account the speed dependence of collisional broadening. We demonstrate that a precision of approximately five parts over 10${}^5$ can be reached as a result of the analysis of only 90 absorption spectra, as acquired as a function of the gas pressure.

Figure 1

Experimental setup used for comb-assisted absorption spectroscopy of ammonia at 9.07 $\mu$m. QCL, quantum cascade laser; PBS, polarizing beam splitter; SC, supercontinuum fiber; HW, half waveplate; OF, optical filter; FC, fiber coupler; PD, photodetector; BC, beam combiner; LO, local oscillator at radio frequency; MCT, mercury-cadmium-telluride detector; OC, optical chopper.

Figure 2

Beat-note signal between the Tm:fiber frequency comb and the QCL in free-running and phase-locking (in the inset) conditions.

Figure 3

Experimental ammonia spectrum near $\lambda$ $=$ 9.07 $\mu$m, at a pressure of about 0.1 Torr and a temperature of 295.18 K. Absolute residuals do not show any particular trend, thus demonstrating that the hypothesis of a linear variation for the laser power was very well satisfied. Their root-mean-square value (of about 2.4 mV) is consistent with a signal-to-noise ratio of about 650, for an equivalent noise detection bandwidth of 2.5 Hz.

Figure 4

Comparison between the spectroscopic determinations of the gas temperature, as retrieved by means of the MVP and SDVP models, and the values measured by means of the pt100 thermometer. Best-fit lines (in red) are also shown.

Figure 5

Slope of the best-fit lines obtained for different temperature-ratio data resulting from the application of different line-shape models.