## 2 Measurement Method and Experimental Setup Design
### Optical Measurements System  
The ICOS cell in the M-DLS carries two main functions: firstly, it is the main measurement instrument for spectroscopic research of Martian atmosphere; secondly, it is the reference channel for laser stabilization system to reach high spectral resolution and sensitivity. The sectional drawing of the ICOS cell is shown in Figure 4. The section is crossed along the main optical axis of the cavity throughout all mechanical, optical, and electronic components of the cuvette. As it could be seen in Figure 4, the ICOS cell consists of following main components: a couple of DFB diode lasers (DL); complex of input and output optics, and the photodetector (PD) module. The ICOS cell represents a two-channel measurement system, as it contains two DFB lasers emitting on wavelengths of 2.656 \(\upmu\)m and 2.808 \(\upmu\)m with the optical power of 6-7 mW. The construction of the ICOS cell implies to employ one photodetector, therefore lasers operate in a sequential regime. All mechanical components of the cuvette are made of aluminum with an oxide-fluoride coating for corrosion prevention, whereas mechanical parts which could interact with laser radiation have anodic oxidation coating, turning an aluminum surface into the porous surface with anti-corrosion and antireflection properties. In more detail about laser modules, Nanoplas DLs (nanoplus Nanosystems and Technologies GmbH, Gerbrunn, Germany) in TO8 case with built-in thermoelectric cooler (TEC) and thermistors are exploited in the M-DLS. The DLs emit in quasi-continuous operation mode with the repetition frequency of spectral sweep about 70 Hz. The laser temperature tuning is used to set the DL to the targeted spectral range, while the current tuning provides continuous sweeping of the DL frequency in the range of 1 cm\({}^{-1}\). The DL is powered by pumping current pulse of the special form, which is shown in Figure 5. The first part of the current pulse is intended for laser crystal heating within 1.8 ms, where the current is higher than the threshold level to compensate for a nonlinear transient process of laser switching on. The second part is the frequency sweeping interval where the spectroscopic measurements are carried out. It is a sawtooth current pulse of 11.4 ms which consists of 1000 stepped points. The final part of the DL pulse is a time period with the duration of 1 ms when DL is turned off, this state of DL is used to register a dark current of PD and background radiation. Both DLs operate in the described mode, where each DL has its own unique set of pumping current parameters. Figure 4: The sectional drawing of the ICOS cell, implemented in the M-DLS. Light blue parts show all optical components, including two collimating lenses, two optical wedges, two mirrors and the integrating lens. DLs (diode lasers) are brown, green elements are electronic boards, LM (laser module) constructions are yellow, optical wedge mounts are red and the LM flange is navy-blue. From the left cell side: PD (photodiode) is black, the integrating lens mount is orange and the PD flange is magenta. The ICOS cavity body and PD set are gray. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article). The manufacturer of DLs specified that the position of the crystal is not aligned with the center of TO8 case, thus it is mounted to the construction called laser module (LM) with the main function to form plane-parallel laser beam. LM consist of two elements: DL mounted on an aluminum plate with an electronic board for connection with atmospheric measurements unit (AMU); an aluminum mount with plano-convex collimating GeSbSe lens C036 TME-D (Thorlabs Inc., Newton, NJ, USA) with diameter of 9.24 mm and working distance of 2.67 mm. LM is highlighted in yellow in Figure 4. The distance between the collimating lens in its mount to DL could be regulated by threaded connection with LM.