High-sensitive absorption measurement in ultrapure quartz glasses and crystals using time-resolved photothermal common-pass interferometry and its possible prospects

Abstract

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.

Problem of Measuring Absorption Using Time-Resolved Photothermal Common-Path Interferometry under Conditions of Developed Heat Diffusion

“…Figure 1 shows the optical part of the TPCI scheme, which we modified in comparison with its first version [17] to study the effect of ambient air absorption on measurements [22]. The essence of the modification was to employ a system of lenses L 1 , L 2 , and L 3 and lenses L 4 and L 5 to eliminate technically caused radiation distortions in the waist area of the heating and probe laser beams, respectively (these distortions appeared when the radiation was focused into the center of the sample with a single lens).…”

“…As before [17,[20][21][22], a single-mode fiber ytterbium laser (λ = 1071 nm, M 2 = 1.08) operating in a pulse-periodic regime was used as a heating laser, and a continuous-wave single-mode He-Ne laser (λ = 633 nm) with an output power of 2 mW was employed as a probe laser. Figure 2 shows a block diagram of the electronic part of the setup.…”

“…Oscillograms U(t) of the time-varying component of the probe radiation power, which were obtained for SCQ (red) and Suprasil 311 quartz glass (black), as well as in the absence of samples, i.e., when the entire propagation path of the heating and probe beams passed through the ambient air (blue). The red and blue oscillograms were corrected with allowance for the ambient air effect [22]. The maximum (the red and black curves) and minimum (the blue curve) of the oscillograms correspond to the end of the heating pulse.…”

“…The TPCI scheme is a modified version of the thermal lens scheme [18,19] and the photothermal common-path interferometry scheme [1][2][3][4][5][6][7][8]. Differences between these schemes are discussed in [20,21]. We have developed a theory that relates the values of the measured signals with the absorption coefficients of isotropic [17] and anisotropic solid dielectrics of some classes of symmetries [20].…”

“…Differences between these schemes are discussed in [20,21]. We have developed a theory that relates the values of the measured signals with the absorption coefficients of isotropic [17] and anisotropic solid dielectrics of some classes of symmetries [20]. In [22], a modified optical part of the TPCI scheme, which provided a Gaussian-like shape of laser beams in the volume of the tested samples, was presented, bringing the experimental conditions as close as possible to the theoretical description.…”