## 3 Results and Discussion  
### QCL Characterization  
The laser emission spectrum in a wavelength range between 1400 cm\({}^{-1}\) and 1200 cm\({}^{-1}\) is shown in Figure 3a. A seven-point FFT filter was used to remove noise and to highlight the pure signal. The spectrum is plotted with the power normalized to the highest peak equal to one. The peak maximum is located at 1287 cm\({}^{-1}\) with a peak width of approximately 15 cm\({}^{-1}\). Two further peaks appear at 1307 cm\({}^{-1}\) and 1335 cm\({}^{-1}\), whereby the peak at 1307 cm\({}^{-1}\) overlaps with the main peak and broadens the main peak. Therefore, the laser does not strictly show single wavelength emission, which can be also explained by the Fabry-Perot cavity. As the main goal of this study is to show the feasibility of combining  
Figure 2: Schematics of the used analytical setups. (**a**) The QCL is coupled via a hollow-core fiber to the RobustATR assembly with the detector directly coupled to the distal end; (**b**) the QCL is also directly coupled to the RobustATR assembly. It should be noted that the iHWG in- and outcoupling structures integrated into the RobustATR assembly are not shown, for clarity, and that the laser beam path is therefore only schematically indicated. This light propagation does not show the real path. (Elements in the Figure not to scale). QCLs with the developed sensor assembly, no further laser beam characterization has been conducted. The laser was operated just above the recommended threshold current. However, if needed, a higher signal output can be achieved by adjusting the duty circle, voltage, or pulse length. Since a hollow-core fiber was placed in between the QCL and the sensor assembly, the initial beam shape is altered; hence, no further studies on the beam shape were performed. It should be noted that no further beam-shaping optics were used. As a measure of efficiency, the dependence between the laser signal and distance to the detector has been recorded by manual adjustment. The anticipated exponential decay of the emitted radiation intensity has been recorded and is shown in Figure 3b. Error bars were obtained via the standard deviation of a 40 s measurement period, and an assumed reading error was obtained by measuring the distance between laser and detector using a ruler. Figure 3: Characteristics of the Fabry–Pérot laser. (**a**) The emission spectrum of the laser treated with a seven-point FFT filter; (**b**) Signal decay as a function of distance between laser and detector. (**c**) Exemplary tryptophan spectra (top) and Allan variance of the laser (bottom). The colors are in accordance with Figure 4b, indicating the different measurements (yellow is the third measurement in Figure 4b and blue is the second measurement in Figure 4b). To evaluate the sensor stability, an Allan variance study was used. Originally, the Allan variance is used as a time domain measurement of frequency stability. The Allan variance is an increasingly adopted measure for characterizing noise, whereby the log-log plot of Allan deviation vs. cluster time \(\tau\) is used, providing direct access to characterize different sources of noise. White noise, for example, has a slope of \(-1/2\)[49; 50; 51; 52]. As the detection limit and the confidence interval of measured data are dependent on the variance of the measured data, averaging of the data is relevant. In white noise-dominated systems, the Allan variance is equivalent to the variance of the mean and, therefore, may be used to predict the detection limit. In turn, the minimum of the Allan variance plot provides the optimum integration time for the recording sample and background spectra [51; 53]. The Allan variance obtained using the developed sensor assembly is shown in Figure 3c. Interestingly, although the tryptophan measurements illustrated in Figure 4b show different noise levels when evaluating raw data, the Allan variance does not vary significantly between the two measurements, showing a small frequency fluctuation.