Figure 1 different root causes of damage create

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FIGURE 1. Different root causes of damage create various morphologies of damage, and identifying these morphologies is important for coating and process development; understanding these phenomena is imperative, as overspecifying will drive cost and the resultant damage will degrade laser system performance. Overspecifying optics for LIDT can unnecessarily increase costs; understanding the statistics behind LIDT will help avoid this problem. L A S E R O P T I C S
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continued July 2018 Laser Focus World 38 Damage mechanisms Damage from continuous-wave (CW) lasers is typically a result of thermal ef- fects caused by absorption in the op- tic’s coating or substrate. Damage from nanosecond pulses from pulsed lasers, on the other hand, is typically initiated by different processes before thermal buildup can occur. Nanosecond pulses are also too long for nonlinear effects to be the primary damage mechanism, but they can lead to dielectric break- down, where current flows through an electrical insulator because the voltage applied across it exceeds its breakdown voltage. This process is driven by the electric-field strength and is the most de- terministic of the damage mechanisms, because if the electric field is above a given threshold there will be dielectric breakdown. This can lead to delamina- tion and further damage. Dielectric breakdown is the typically the dominant mechanism of damage for small beams on the order of tens of microns or less or for femtosecond pulses. For large beams on the order of hundreds of microns and up, the dominant damage mechanism typically becomes the thermally driven ex- plosion of particulate contaminants. Damage mechanisms that are triggered by the interaction of light with a defect of some type are not so straightforward. For example, the grinding and polishing pro- cess can produce microfractures in the op- tic that can be the initiating sites of laser damage. Ideally, each successive stage in the polishing process is finer and will elim- inate any fractures from the previous steps, but no process is truly ideal and some level of subsurface damage will remain. Another potential defect source is mi- croscopic particles of polishing abrasive that can remain on the surface through successive stages, including coating. The coating process itself can lead to anoth- er potential defect source. Typical met- al oxide dielectric coatings are created by vacuum deposition of material from a sputtered target, and clusters of metal- lic elements can form during the coating process. Defect-driven LIDT can be rep- resented using a stochastic model because of the random location of these defects relative to the center of the laser beam. Each of these distinct defect sources ex- hibits equally distinct absorption charac- teristics, as the nature and size of any giv- en defect determines the laser fluence it can withstand without causing damage to the optic. The next step of designing with LIDT in mind is to determine how these defects impact the performance of the optics in a given system. This is what allows laser-optics integrators to extrap- olate LIDT specifications into an accept- able level of risk for their laser system.
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  • Fall '19
  • Photonics, LIDT

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