High-sensitive absorption measurement in transparent isotropic dielectrics with time-resolved photothermal common-path interferometry

Abstract

We propose a time-resolved photothermal common-path interferometry scheme at fast heating in the absence of heat diffusion and employ it to measure absorption in Suprasil 311 silica glass (2.8·10 cm) at a wavelength of 1071 nm and continual absorption in laboratory air (2.9·10 cm) for a signal-to-noise ratio of 100/1. The absorption was measured at a thermally induced phase incursion of less than 0.1 rad in a heating beam, which guaranteed correct calibration. To calibrate this scheme, we developed a theory of diffraction on deformations taking into account the stresses arising in an inhomogeneous temperature field. This allowed us to use a standard glass K8 for calibration. The low level of noise and time resolution of pulsed signals allowed the distinguishing of the contributions of Kerr and striction nonlinearities to absorption measurements in Suprasil 311 silica glass and enabled the observance of the time evolution of strictional deformations. Additionally, an anomalous temporal development of the absorption of broadband laser radiation in atmospheric air at 2.9·10 cm has been revealed.

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.…”

“…In [17], we presented a new method for measuring ultra-low absorption (with a sensitivity of ~10 −6 cm −1 achieved at the moment with a signal-to-noise ratio ≈100/1 for quartz glass), called time-resolved photothermal common-path interferometry (TPCI). 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]. 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.…”

22 kW near-diffraction-limited Yb:YAG slab laser amplifier without adaptive optics correction

“…To reduce the thermal effects of the AR and HR films, the absorptivity and structural defects of the films are effectively reduced. The photothermal common-path interferometer (PCI) [24] is used to measure the absorption rate of the films. By optimizing the coating materials, evaporating ion source energy, and rate, the absorption rate of the films is reduced from 300 ppm to less than 20 ppm, as shown in Figs.…”