## 2 Measurement Method and Experimental Setup Design
### Optical Measurements System
The aluminum plate with installed DL allows aligning the optical axis of the collimating lens and the optical axis of DL because they are not centered by default. After all tunings, we get a plane-parallel laser beam formed by LM, which goes to an optical quartz wedge with filtering dichroic coating. It is installed to a cylindrical aluminum mount (highlighted out by in red in Figure 4). The wedge has two plane sides: the input side is perpendicular to incident laser beam, while on the output side the plane is angled at 6.25\({}^{\circ}\) for directing the laser beam to the certain point on a mirror of the resonator. Particularly, by rotation of the wedge a signal pattern shape on resonator mirrors could be controlled. The signal pattern is a picture on the mirror formed by laser radiation, which consists of closely set spots where laser beam incidents and reflects, making up circle or ellipse. The quality of the signal passed through the ICOS cell depends on the signal pattern shape; a detailed analysis of it will be described and discussed elsewhere. Two DLs together with a couple of wedges in their mounts are installed to the construction called LM flange (marked by navy-blue in Figure 4), which fixes an optical wedges' position and hermetically presses the input mirror to the body of the ICOS cell. Moving on to the main part of the system, the cavity of the ICOS cell is a pair of spherical flat-concave quartz mirrors with a curvature radius of 500 mm located at a distance of 220 mm from each other. This distance is a little shorter than mirror's focal length of 250 mm. It is a typical configuration for the OA-ICOS, because this mirrors spacing, together with off-axis laser input to the cavity, allows to simultaneously pump many transverse modes to produce a dense cavity mode spectrum. Each mirror has an anti-reflection coating on the flat side with a reflection coefficient R\({}^{*}\) = 0.2%, and high reflection coating on the concave side with datasheet coefficients of R\({}_{2.6\ \upmu\text{m}}\) = 99.8 \(\pm\) 0.1% and R\({}_{2.8\ \upmu\text{m}}\) = 99.6 \(\pm\) 0.1% for 2.656 \(\upmu\text{m}\) and 2.808 \(\upmu\text{m}\), respectively. The signal passed through the output mirror is collected and focused on the PD by a plano-convex quartz  
Figure 5: The schematic structure of the pumping current pulse, which consists of three stages: laser heating, frequency sweep, and background radiation measurement. integrating lens with a diameter of 28 mm and working distance of 21.8 mm. The integrating lens is installed into the aluminum mount (marked by orange in Figure 4), to set the distance to PD. The HgCdTe photodiode PVI-3 TE-3 (VIGO System S.A., Ozarow Mazowiecki, Poland) in TO8 case with built-in three-stage TEC, thermistor and input lens is exploited in the M-DLS for signal detection. The PD is mounted on the aluminum plate for heat dissipation and attached to two electronic boards for signal amplification and connection to the AMU. PD set with the integrating lens is installed into the construction called PD flange (marked by magenta in Figure 4), which fixes the lens position and hermetically presses the output mirror to the body of the resonator. All the optical components except the collimating lens were manufactured by OptoSigma Corporation, Santa Ana, CA, USA  
Because of high reflectivity the input mirror transmits weak laser signal to the cavity, with a power about several thousandth from incident laser beam power. Passed signal, which was injected to the resonator by the optical wedge with a small angle to the optical axis of the cuvette, repeatedly reflects between mirrors providing a long effective optical path and forms a signal pattern on mirrors. The output mirror also transmits weak laser signal from the resonator, with power about several thousandths from incident signal power, thus the power of the cavity output signal is a million times weaker than the DL power.