We describe the functional capabilities of the time-resolved photothermal common-pass interferometry method. For two thermo-optical effect-based methods, photothermal common-pass interferometry and time-resolved photothermal common-pass interferometry, the achieved detection limit for absorption measurements in ultrapure quartz glasses (5 × 10⁻⁷ cm⁻¹ and 2 × 10⁻⁹ cm⁻¹, respectively) is given. The problem of calculating the variation of the refractive index tensor under local heating trigonal-symmetric crystals of class 32 by a focused laser beam is considered. For these crystals, it is shown that the problem of the influence of deformations on the measured signal is reduced to determine a thermo-optical parameter (an analog of dn/dT). The calculation of this parameter does not require a complete solution to the strain problem when the crystals are heated locally by laser radiation. The formulas for calculating the thermo-optical parameter P and its numerical values are presented. They are required to calibrate absorption measurements in crystalline quartz using the time-resolved photothermal common-pass interferometry scheme. For a full understanding, formulas of the theory of equilibrium deformations used in this study are presented. An analysis of the relevance of improving the thermo-optical method sensitivity for concentration measurements of pollutant inclusions in crystals, ultrapure quartz glasses, and ambient air is presented.

The problem of creating optical parametric amplifiers of femtosecond pulses up to the multiterawatt level was fiist discussed in [l, 21. In these papers, as well as in subsequent studies [3-91, only nonlinear elements made from KDP crystals, whose aperture can be as great as 30cm, were considered as nonlinear elements for final cascades of optical parametric amplifiers (OPAs). The most promising pump source is the second harmonic of a Nd:glass laser with a wavelength of 3L3=527nm. At this pumping, the KDP crystal has two serious disadvantages: it absorbs greatly at wavelengths higher than 1200nm and has maximal gain bandwidth (1000-1200~m~[l~]) at the wavelength of signal 1050-1 1 OOnm, i.e., at degenerated (or nearly degenerated) phasematching. In this wavelength range, however, it is difficult to create light sources with pulse duration of less than 50fs.

We present a study of the problem of measuring ultra-low absorption in quartz materials using the time-resolved photothermal common-path interferometry (TPCI) method, which we proposed and elaborated, with allowance for developed heat diffusion in the samples. This task is related to the implementation of one of the ways of increasing the sensitivity of measurements, namely, increasing the energy of the heating pulse by increasing its duration. Analytical formulas for the time dependence of the power of the time-varying component of the probe radiation are obtained in the Gaussian approximation for the laser beams. A correction factor that takes into account the heat diffusion effect is calculated theoretically and used during calibration. The rate at which the power of the time-varying component decreases when the sample is cooled after the end of the heating pulse is calculated. When measuring in crystalline quartz, quartz glass, and also in air, the calculated rate coincided with the experimental one, which is additional evidence for the reliability of the calculations of the correction factor for calibrating the measurements. When the duration of the heating pulse is increased to 5 ms, the calculated sensitivity of the scheme for measuring absorption in quartz glasses is 2 × 10⁻⁹ cm⁻¹.

We demonstrate measurements of the absorption coefficient α ≈ 2.5 × 10⁻⁷ cm⁻¹ in synthetic crystalline quartz at a wavelength of 1071 nm with a signal-to-noise ratio of 10/1 using the Time-resolved photothermal common-path interferometry (TPCI) scheme. It utilized cells filled with flowing argon and eliminated the influence of ambient air absorption. The scheme elements limiting the sensitivity of measurements at the level of ≈7.8 × 10⁻⁸ cm⁻¹ were revealed. When these elements are replaced by better ones in terms of their thermal influence, the sensitivity of absorption coefficient measurements in crystalline quartz is ~10⁻⁸ cm⁻¹. The calculation of the correction due to these optical elements of the values of the measured absorption coefficients is also described, which makes it possible to achieve the same sensitivity without replacing the elements. The improved scheme confirms the presence of the spatial inhomogeneity of absorption with a minimum coefficient of 2.5 × 10⁻⁷ cm⁻¹ in synthetic crystalline quartz. The discrepancy of the absorption coefficient values in different regions of the crystal in the presented series of experiments was 2.5 × 10⁻⁷ cm⁻¹ to 4 × 10⁻⁶ cm⁻¹. Taking into account the ratio of thermo-optical parameters and the heat diffusion effect, the calculation shows that for quartz glasses the corresponding sensitivity of the absorption coefficient measurements equals ≈1.5 × 10⁻⁹ cm⁻¹.