## 1 Introduction  
Martian multichannel diode laser spectrometer (M-DLS) is a laser spectrometer developed by Space Research Institute of Russian Academy of Sciences (IKI RAS) in cooperation with Moscow Institute of Physics and Technology (MIPT) in the framework of the joint Russian-European ExoMars mission. M-DLS is a part of the science payload of the landing platform \"Kazachok\" to be launched to Mars along with \"Rosalind Franklin\" rover as a second stage of the ExoMars space mission. The main purpose of M-DLS is comprehensive study of the Martian atmosphere by continuous measurements of its chemical and isotopic compositions at a fixed location at the planet's surface within on least one Martian year. Comprehensive study of composition of the Martian atmosphere has been a priority of nearly all mission to the red planet since the very beginning of its spacecraft exploration. In addition to numerous experiments targeting atmospheric composition by remote sensing from the orbiter, such as Atmospheric Chemistry Suite (ACS) onboard ExoMars/ Trace Gas Orbiter (TGO) spacecraft [1], in situ measurements play an important role as well. In particular, atmospheric composition measurements onboard Mars rovers provide data to study complex chemical interactions at the atmosphere-subsurface interface [2], boundary layer meteorology [3], and even prebiotic synthesis and/or hypothetical biological activity on Mars [4]. All experiments listed above have been housed onboard mobile platforms and delivered information about atmosphere-surface interactions at different locations in the course of a rover's lifetime. Since the Viking landers activity (1976-1984) [5], no stationary platform has performed permanent long-term operation of both carbon dioxide and water vapor isotopologues. However, isotopic ratios in the atmosphere may substantially vary due to condensation/sublimation and adsorption desorption processes [6]. Thus, implications of laser spectroscopy technique for continuous monitoring isotopic ratios of main Martian volatiles remains an encouraging opportunity of Mars exploration. Following this task, the targeted molecules of the experiment are water vapor (H\({}_{2}\)O), carbon dioxide (CO\({}_{2}\)), and their isotopologues, in particular HD\({}^{16}\)O, H\({}_{2}\)\({}^{18}\)O, \({}^{13}\)C\({}^{16}\)O\({}_{2}\), \({}^{16}\)O\({}^{12}\)C\({}^{17}\)O and \({}^{16}\)O\({}^{12}\)C\({}^{18}\)O. The analysis of their spectral properties reveals that optimal intervals for spectroscopic measurements of all the molecules listed above are located in the vicinity of two wavelengths: 2.656 \(\upmu\)m (H\({}_{2}\)O, HD\({}^{16}\)O and H\({}_{2}\)\({}^{18}\)O) and 2.808 \(\upmu\)m (CO\({}_{2}\), \({}^{13}\)C\({}^{16}\)O\({}_{2}\), \({}^{16}\)O\({}^{12}\)C\({}^{17}\)O and \({}^{16}\)O\({}^{12}\)C\({}^{18}\)O). Synthetic transmission spectra of a near-surface layer of Martian atmosphere are shown in Figure 1. These spectra were calculated based on data published in [7; 8; 9; 10], where the seasonal distribution of near-surface Martian atmosphere chemical composition is described. Figure 1 shows that absorption lines of all listed above molecules are available for measurement by one frequency sweep of the laser. However, main isotopic features, especially those of HD\({}^{16}\)O, are weaker than one per cent for assumed measurement path length of 110 m. Therefore, relative noise level and systematic errors at the level of \(10^{-5}\) or less are required for sensible measurements of isotopic ratios, which is a serious challenge to the instrument's design and data treatment techniques. In this paper, we describe the concept of the M-DLS instrument and present preliminary pre-flight calibration data in the context of future operation on the Mars' surface. The instrument's design is aimed at robust, easy-to-interpret and reliable in situ measurements, yet characterized with sensitivity fair enough for long-term isotopic monitoring of the volatiles.