| Abstract [eng] |
Optical elements of laser systems exposed to high intensity radiation have a certain resistance limit – laser induced damage threshold (LIDT), beyond which the element is irreversibly destroyed. The damaged optical element can no longer be used in the system, causing the entire optical circuit to fail. LIDT is not constant - it depends on various, meanwhile, exposure parameters: dose, wavelength, beam diameter, pulse duration, and so on. There are very little studies of the dependence of LIDT on sequential metal coatings on pulse duration or they only cover a narrow range. Therefore, the aim of this work is to experimentally evaluate the dependence of the optical resistance of metal coatings on the pulse duration of a laser pulse in a very wide range of laser pulse durations - from 200 fs to 10 s. 10 types of samples (gold, silver, aluminum, zinc, titanium, chromium, tungsten, tantalum and germanium and silicon mirrors) were measured using the „1on1“ and „Son1“ test procedures described in the ISO standard. The obtained results show that the PLSS characteristic of these coatings in the range of short laser pulses (200 fs - 8 ns) varied up to 10 times, and in the range of long pulses (1 ms - 10 s) over 10,000 times. This means that the damage mechanisms in the areas of long and short laser pulses are different – in the area of short pulses the damage is determined by the average power, in the area of long pulses – by the absorbed energy dose. In the regions of long and short laser pulses, the dependence of PLSS on the pulse duration can be described by the power law y=axb – only the values of the coefficients differ – the boundary of these regions can be determined by finding the point of intersection of approximations. This parameter correlates to a materials‘ thermal conductivity: a higher thermal conductivity of the metal corresponds to a higher LIDT value. The dependence of the LIDT of the studied metals on the pulse duration differed from each other, and the differences (especially in the field of long pulses) are due to the absorption of the material - higher absorption metal will have lower PLSS. In the field of short laser pulses (200 fs - 8 ns), the fatigue effect and PLSS “S(1000)on1” were evaluated by comparing the PLSS values of “S(1000)on1” and “1on1” Was up to 80% lower (silver) and differed among the studied starvations, where chromium had the lowest drop in LIDT by 30%. Comparing the properties of these metals - chromium has 2 times higher melting point, which means that this parameter can also be important in assessing the aging process of optics. |
