Absorptance behavior of optical coatings for high-average-power laser applications

Abstract

A modified photothermal deformation technique is used to measure the absorptance behaviors of optical multilayered dielectric coatings for a high-power laser system. The surface thermal-lensing modification uses an enlarged probe beam to facilitate alignment of the laser beam and data acquisition. The coatings, both reflective and transmissive types, are made by a physical vapor-deposition process. Coating absorptances are observed to depend on the laser's exposure time and power density. Time-dependent absorptance defect models are proposed. Also, micrometer-sized sites of high absorptance and an area with physical damage can be found during the spatial scans. It is proposed that absorptance values reported for coatings in high-repetition-rate or cw-laser systems include time- and power-dependent behaviors in addition to other relevant irradiation parameters.

Development of a "turn-key" system for weak absorption measurement and analysis

“…The photothermal signal is also dependent on the time period of laser irradiation [13]. Figure 3 shows the time dependence of the measured absorption properties of two different samples under same laser irradiation.…”

“…For this reason, the photothermal technique is widely used for characterizing optics for high power / high energy laser systems, including industrial applications and important research programs such as the laser fusion programs, optics for space applications, and the laser interference gravitational observatory [1][2][3][4][5][6][7][8][9][10][11][12][13][14].…”

Sensitivity enhancement of surface thermal lens technique with a short-wavelength probe beam: Experiment

“…5,6 Surface thermal lens technique, first reported by Kuo and Munidasa 7 and Satio et al, 8 has also been developed to be a highly sensitive photothermal technique and was widely used in absorptance and surface displacement measurements of optical components. [9][10][11][12][13] Due to the development of the mode-mismatched thermal lens configuration, 14,15 the sensitivity of surface thermal lens technique has been considerably improved. Moreover, the use of top-hat beam excitation, first proposed by Li et al 9 and late adopted by Astrath et al, 16,17 has further improved the sensitivity of surface thermal lens technique, with a maximum sensitivity enhancement factor of 2.…”

“…Currently, He-Ne laser operated at 633 nm is commonly used as the probe beam in almost all surface thermal lens or in general thermal lens experiments or instruments. [9][10][11][12][13]16,17,20 On the other hand, with the rapid development of highpower laser technique, 21 the absorption loss of optical laser components used in high-power laser systems becomes a crucial parameter due to the high sensitivity of the laserinduced damage threshold and thermal distortion to the absorption loss. The absorption of laser power by optical laser components causes temperature rise inside the laser components and results in thermal expansion (thermal distortion) and thermal stress or melting of the laser components.…”

In situ measurement of absorption in high-power interferometers by using beam diameter measurements

“…7 The absorption of laser light in optical coatings and substrates has been extensively studied by using photothermal deformation. 8 Indeed, the coatings and the substrates of Initial LIGO optics were studied prior to installation. 9 Contamination of the LIGO core optics during the assembly and in-vacuum installation is a possibility.…”