Work is ongoing at TSL to better understand and characterize the LIDT (laser-induced damage threshold) of our coated viewports and optics. We currently specify a CW (continuous wave) threshold of 500 W cm-2 at 530 nm. For each individual customer requirement, this threshold is scaled according to the wavelength. We specify pulsed lasers at 2 J cm-2 at 530 nm with a 10 ns pulse width. This threshold scales with both the wavelength and pulse width, and for higher duty cycles the CW threshold may also be applicable. For ps and fs pulses and shorter, damage may occur as the electric field of the pulse can break the bonds of the viewport materials. This type of damage is not captured by the scaling of the threshold and is not predictable. For short pulses such as these, testing to ISO LIDT standards is required.
This system has worked well and is still in widespread use in a number of industries, but knowledge of LIDT moves on, and research indicates that CW thresholds scale with beam diameter. The reason is that small beam spots dissipate heat more efficiently, having a higher edge to area ratio. ISO standards use a damage threshold with ‘linear power’ units of W cm‑1 rather than the areal unit W cm-2.
A recent series of tests, commissioned to customer specifications and performed to ISO standards, showed that TSL coated fused silica viewports have a damage threshold of at least 3.96 kW cm-1 at 1064 nm and with a 45° beam angle of incidence. For the tests, the fluence was over 50 kW cm-2 – this is around fifty times higher than our standard LIDT at this wavelength.
As we accrue test data with time, we will consider changing the way we specify LIDT in line with current knowledge and ISO standards. The thresholds are likely to increase, giving confidence to customers with requirements that exceed our current conservative specifications.
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We would like to thank Belford Research Ltd. for the use of their photographs.