Why Ultrafast Lasers are so Ultrafast in Damaging Optics?
With great [laser] power comes great responsibility [for coaters] – this slightly interpreted phrase of Uncle Ben (the uncle of Spiderman) is the main motivational slogan of the OPTOMAN team. It does not only prove that we are geeks at OPTOMAN but it is also the driving force in making optical components able to resist the ongoing increase of laser power and decrease in pulse duration. It makes OPTOMAN take innovation very seriously.
So why ultrafast lasers are so ultrafast in damaging optics?
While the high laser-induced damage threshold (LIDT) is a buzzword when talking about femtosecond and picosecond optics, it is not (only) the nominal LIDT value that matters. Laser damage is a complex phenomenon and, while the result is the same – the optical component is ruined and not suitable for further use, there are different laser damage types and mechanisms. The main ones speaking about ultrafast optics are catastrophic damage and color change. Catastrophic damage causes light scattering, while color change affects the laser pulse shape and temporal characteristics.
The measurements shown in graphs a) and b) above indicate that both types of damage are more likely to occur with an increasing number of pulses and that color-change damage occurs at lower fluence values compared to catastrophic damage. It can also be seen that color change is the dominant damage mechanism after prolonged radiation (>103 pulses), especially for high-power mirrors. At number of pulses as high as 105, fluence drops down to 0.4 J/CM2 for market-standart mirrors and even to 0.3 J/CM2 for high-power market-standart mirrors. This can be explained by the optical fatigue effect which causes degradation of coatings.
What separates standard mirrors from high-power ones is the choice of coating materials, as it is possible to increase the catastrophic damage threshold by choosing higher bandgap coating materials. However, the color-change effect still remains there working with ultrashort pulses of 10 ps and less. Therefore, it is clear that the color-change effect is an arch-enemy and a LIDT-limiting factor for ultrafast applications, and has to be eliminated in order to increase the lifetime of optics and reduce the total cost of ownership.
Beating Dr. Absorption
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