## 3 Remote Sensing with CM-CLaDS  
A preliminary test of remote open-path sensing with CM-CLaDS was performed using nitrous oxide as the target molecule. N\({}_{2}\)O is an important greenhouse gas and atmospheric pollutant [23]. Agricultural emissions are the primary source of a gradual increase of N\({}_{2}\)O concentration in the atmosphere at almost 1 ppbv/year rate [24, 25]. Unfortunately, the global budget of N\({}_{2}\)O is still poorly characterized. In order to analyze changes in atmospheric N\({}_{2}\)O concentration, an instrument that provides precision and accuracy at a single ppbv level is required. Such performance can be achieved with mid-IR QCLs that give access to the strongest N\({}_{2}\)O transitions around 2,200 cm\({}^{-1}\) (Figure 2(A)) [26, 27]. The N\({}_{2}\)O P18e transition located at 2,207.62 cm\({}^{-1}\) exhibits very small interference from other atmospheric constituents (such as H\({}_{2}\)O or CO) which makes it ideal for sensing in ambient conditions (Figure 2(B)). The experimental arrangement of the prototype CM-CLaDS sensor for remote N\({}_{2}\)O detection is shown schematically in Figure 3. It was configured as a transportable system with optics placed on a 24\({}^{\prime\prime}\times 36^{\prime\prime}\) breadboard and installed with all the necessary electronics on a cart. The total power consumption of this transportable platform is \(\sim\)500 W (primarily limited by an off-the-shelf RF spectrum analyzer consuming \(\sim\)300 W). A thermoelectrically cooled (TEC) DFB QCL (provided by  
Figure 2: **(A)** A HITRAN simulation showing that the N\({}_{2}\)O absorption band near 2,200 cm\({}^{-1}\) is significantly stronger than absorption bands at other wavelengths (inset shows magnification of the near-IR transitions); **(B)** A simulated absorption spectra for four atmospheric molecules shows that the transition at 2,207.6 cm\({}^{-1}\) is optimal for N\({}_{2}\)O detection with minimum interference from water vapor, carbon monoxide, and carbon dioxide. Corning Inc., Corning, NY, USA) generated up to 5 mW of optical power at 4.53 \(\upmu\)m. By changing the laser temperature and bias current multiple N\({}_{2}\)O transitions between 2,207.6 and 2,211.4 cm\({}^{-1}\) could be accessed. A CaF\({}_{2}\) wedge plate was used to out-couple a small fraction of the laser power for active laser frequency locking. The main laser beam was divided with 50/50 beam splitter and the two beams were directed into the AOM. Both incident angles on the AOM were set to provide two Bragg reflected beams with one of them being up-shifted and the second down-shifted in frequency by \(\Omega=50\) MHz. Both beams were re-combined into one beam using the Mach-Zehnder arrangement. The laser power available for measurement of N\({}_{2}\)O at 2,207.62 cm\({}^{-1}\) was only 250 \(\upmu\)W due to losses on multiple optical components placed after the laser (AOM, beam splitters, several mirrors), but this was still sufficient to achieve a satisfactory SNR. This power was directed towards a distant retroreflector (diameter of 65 mm). Due to the divergence of the laser beam, a 6 inch concave mirror was used to collect the returning radiation and to focus it onto high-speed TEC MCT photodetector (PVI-3 TE-10.6 from Vigo Systems S.A., Warsaw, Poland). The optical configuration of the collection optics can also be used as a standard Newtonian telescope to collect the scattered optical radiation from objects/hard-targets rather than a specular reflection from a retroreflector. The laser was frequency-chirped using a sinusoidal waveform at a modulation frequency of f\({}_{1}\) = 101 kHz and with an optimized optical frequency modulation depth [19]. Due to imperfections of the antireflection coatings on the AOM facets the signal contained narrowband optical fringes created by an etalon formed between the AOM facets. In order to suppress these etalon fringes an additional small-amplitude  
Figure 3: Optical setup of CM-CLaDS sensor for remote sensing (PD-photodetector).