## 3 Results and Discussion
Figure 6: Averaged over 10\({}^{4}\) realizations and over time (\(\approx\)100 \(\upmu\)s) waveforms of the time-varying component of the probe radiation power U(t) in experiments with argon and ambient air: (**a**) Waveforms obtained when the heating and probe beams were co-propagated in argon (color), and an waveform (amplitude reduced 40 times) due to ambient air absorption (black). Different color curves correspond to five independent experiments; (**b**) Arithmetic averages from five waveforms similar to (**a**), which were obtained in two independent experiments conducted on different days (color) and a waveform due to ambient air absorption (black). The amplitudes of the black curves are reduced 40 times. The linearity of the form of the obtained curves during heating and the time of decay of their amplitudes during cooling after heating (\(\approx\)2 ms at the half-height level) indicates that the optical element of the scheme, which determines such a temporal shape of the signal, is quartz glass [15]. The only contenders for this role are optical elements M\({}_{2}\) and M\({}_{3}\) having quartz substrates with absorption sufficiently large relative to those measured in this paper. The observed negative amplitude of the time-varying component can be explained by the location of the projection plane of the aperture installed in front of the photodiode. This projection was 2 cm away from the sample exit, and the spectral splitting mirror M\({}_{3}\), which played the role of a nonlinear thermal positive lens formed during heating, was placed after the aperture projection. The polarizer M\({}_{2}\), located behind the projection of the aperture, also represented a positive thermal lens during heating. But due to its different location relative to the projection of the aperture plane, this resulted in positive deviations of the amplitude of the time-varying component. Thus, the observed negative deviation of the signal was the result of opposite influences of the polarizer M\({}_{2}\) and the spectral splitting mirror M\({}_{3}\). When the tested sample is mounted, due to its refractive index, the optical distance between the polarizer M\({}_{2}\) and the aperture increases and the influence of its thermal lens also increases accordingly. At the same time, the amplitude of the parasitic signal after the sample is installed will generally differ from that observed above to a lesser extent. In other words, this amplitude value obtained in the absence of the sample can be used only to estimate the magnitude of the correction that takes into account the influence of parasitic absorption of these elements on the measured values of the absorption coefficients: \(\leq\)7.8 \(\times\) 10\({}^{-8}\) cm\({}^{-1}\). Then, we investigated the operation of the scheme with a sample of hydrothermal crystalline quartz (Figure 4). During the study it was possible to move the sample perpendicularly to the axes of the beams, i.e., to scan inside the sample in different areas defined by the heating beam waist. The sample was placed on a stand that was moved by a micrometer screw. Figure 7a presents with black color the waveforms obtained at the same sample position when the cells were filled with ambient air (lower waveform in the negative range) and when they were filled with argon at a flow rate below the threshold of turbulence (upper waveform in the positive range). The observed ratio of signal amplitudes demonstrates the influence of ambient air when measuring the absorption coefficient of the sample at the level of 5.6 \(\times\) 10\({}^{-7}\) cm\({}^{-1}\). In all waveforms, the signal component, due to the influence of refractive index nonlinearity in the heating beam field [11], is clearly visible in the form of a spike of amplitude 140 \(\upmu\)V at the beginning and end of the heating pulse of rectangular shape. Figure 7a shows a rectangular function with a height of 140 \(\upmu\)V modeling the spike. This function was used to subtract this component from the observed waveforms.