## 3 Remote Sensing with CM-CLaDS
### System Characterization  
Characterization of the CM-CLaDS instrument was conducted in the laboratory using two setups: a remote sensor arrangement with a retroreflector positioned \(\sim\)17 m away, or a setup with a 10 cm long gas cell filled with 151 ppmv N\({}_{2}\)O in N\({}_{2}\) at 300 Torr pressure for calibration purposes. A significant improvement in CLaDS system performance was achieved through suppression of the parasitic optical fringes originating from the AOM. Because the length of the AOM frequency shifter is \(>\)5 cm (which for germanium corresponds to an optical length of \(>\)20 cm), the free spectral range of the observed parasitic fringes is much smaller than the line width of the target spectral feature (\(>\)0.1 cm\({}^{-1}\)). Hence, additional non-synchronized sinusoidal modulation was very effective at fringe suppression. The effect of the additional modulation of the laser current at frequency f\({}_{2}\) is clearly visible in Figure 4, which compares N\({}_{2}\)O spectra acquired with and without the fringe suppression. Figure 4: An example of the 2f CM-CLaDS spectrum recorded in the remote configuration with and without additional small-amplitude wavelength modulation at f\({}_{2}\) (the sample is approximately 35 m of lab air at atmospheric conditions). The narrow fringes (red) are effectively suppressed with the modulation turned on (black). The CLaDS signal is strongly immune to changes in the received optical power [9, 19]. This is especially important in open-path sensing over long distances when mechanical drifts in the system, turbulence in the atmosphere, or environmental conditions (dust, fog) can significantly affect the amount of power that is received by the photodetector. This property was discussed in detail for direct CLaDS in [9, 18] and has been experimentally investigated in this work for CM-CLaDS by measuring the amplitude of 2f signal while the received power was varied. The laser wavelength was locked to the peak of the transition for continuous N\({}_{2}\)O detection, and the beat-note (carrier) level was varied through partial obstruction of the primary mirror. The concentration measurements and the variations of the RF signal power are shown in Figure 5. In FM-demodulation there is a slight dependence of the output noise on the carrier-to-noise ratio [28], which affects the SNR of the system. This is also observed in our instrument. Nevertheless, it is clear that despite power changes by more than 20 dB (100 times), the mean value of the CLaDS signal was not affected. To test the stability of the CM-CLaDS system, an arrangement with a gas cell containing a calibrated gas mixture instead of a remote sensing setup was used. A gas mixture of 151 ppmv N\({}_{2}\)O in N\({}_{2}\) was flowed through the cell with a fixed pressure of 300 Torr. Due to the limited data acquisition capabilities of the spectrum analyzer, an acquisition of a continuous data stream could not be performed. Hence, the signal was recorded for only 50 ms with an interval of 1 s between the consecutive acquisitions (5% duty cycle; the remaining 950 ms in the cycle were used for data processing). This has resulted in a 20-fold reduction in the effective integration time, but at the same time it enabled long-term stability analysis with a quasi-continuous signal acquisition. Both the  
Figure 5: A continuous measurement of N\({}_{2}\)O concentration (upper plot) performed in the laboratory using an open-path configuration with a retroreflector placed \(\sim\)17 m away from the instrument. The carrier (beat-note) power (lower plot) is varied through changes of received optical power through partial obstruction of the telescope. CM-CLaDS signal as well as the RF beat-note power were simultaneously recorded and are shown in Figure 6. Allan variance plots were calculated for both data series.