## 3 Results and Discussion
An Allan-variance analysis of our data (not shown) reveals that drift effects become more dominant than white or 1/f noise for time constants higher than about 10 s. We found that this system drift is predominantly caused by two phenomena: variations in pulse characteristics due to thermal or electronic drifts of the quantum cascade laser (pulse drift noise) and interference due to internal reflections of the laser beam at optical components. The former case generally leads to both, instabilities in intensity and shifts in the frequency scale of the absorption spectra, while the latter case leads to unstable periodic fringes. Pulse drift noise is often encountered in QCL spectroscopy and may be widely avoided when a dual-beam setup with permanent sample and reference measurement is used. The interference from internal reflections most presumably originate from the collimating ZnSe lens and/or the BaF\({}_{2}\) detector window and may be reduced by more sophisticated anti-reflection coating design. Two of the key properties of QCL based devices is exceptional short response time and high time resolution. For our setup, these benefits can be demonstrated for measurements where the process under study occurs on a time-scale shorter than the time constant of system drift. As an example, we discuss \"dynamic\" measurements on simulated breath flow. Figure 5 shows representative results obtained on measurements with 5% CO\({}_{2}\) sample gas. The QCLAS and Pt1000 temperatures show a distinctively different behavior which can be consistently explained by their different capabilities to resolve short-time processes. Evidently, the breath-circle related temperature variations can be clearly resolved by our chirped QCL technique. On the contrary, the Pt1000 temperature data are affected by a significantly slower response-time as a result of inherent thermal inertia. Notably, the small temperature hop seen in the \(T_{\text{QCL}}\) data in periods of decreasing temperature at about 304 K  
is caused by an inherent mechanism in the breath simulator device and can also not be resolved by the Pt1000 sensor. Figure 5 also shows the CO\({}_{2}\) concentration obtained by the presented QCL technique. A mean concentration of 2.15 \(\pm\) 0.01 mol/m\({}^{-3}\) is obtained, which corresponds to a volume fraction of \(\sim\)4.8% if a behavior according to the ideal gas law is assumed. Importantly, the CO\({}_{2}\) concentration determined is independent of the actual gas temperature although the CO\({}_{2}\) transitions probed by our setup are highly temperature-sensitive. This result demonstrates that the method can also effectively be used as a temperature-insensitive probe of the CO\({}_{2}\) concentration. In the presented measurements, the sampling rate of our QCL technique was deliberately set to about 6 Hz in order to allow for simultaneous data transfer, storage and analysis. The fundamental upper limit for the sampling rate, however, is given by the maximum repetition rate of the pulsed QCL. The results of preliminary single-pulse measurements (no averaging) with similar experimental conditions to the ones from Figures 3 and 4 indicate that the temperature noise is dominantly affected by electronic noise of the detector system. For these single-pulse measurements, we obtain a statistical uncertainty in \(T_{\text{QCL}}\) of about 1 K.  
The strength of the QCL method presented is in situ sensing on systems with highly dynamic temporal temperature variations on time scales of about milliseconds to seconds that are too fast to be resolved by conventional temperature sensors and/or lead to strongly inhomogeneous temperature fields. Previous works towards the application of mid-IR QCLs as temperature sensors have focused on gases at elevated temperatures in plasma and combustion processes. These studies include, for instance, the intrapulse measurements on CH\({}_{4}\) and NO-admixed air plasma performed by [PERSON] et al. [19] and [PERSON] [20], respectively. [PERSON] et al.