Minimizing Optical Component Surface Defects
Light scatter due to surface defects on laser resonator optics produces losses which lower system efficiency and output power. Defects can also degrade beam quality and become increasingly problematic as resonator energies increase, since they act as centers for laser damage. REO employs several strategies in both fabrication and metrology to reduce or eliminate defects.
One important fabrication tool for reducing the microroughness of a laser optic surface is “superpolishing,” which can deliver surface roughness values below 1 Ǻ, and, in conjunction with high quality IBS coatings, can reduce scattered light below the 1 ppm level. Another critical step in minimizing defects is contamination control. REO maintains a series of protocols that extend from incoming material inspection, through fabrication and coating that largely eliminates contaminants from entering the production process.
REO also employs a suite of specialized metrology tools to identify contaminants and quantify surface quality. The traditional methodology for surface quality inspection involves visual comparison of a component to scratch and dig (SAD) standards under controlled lighting and viewing conditions. Unfortunately, this process is subjective and operator dependent. Also, there is no clear correlation between inspection results and the actual performance impact of the optic in a laser resonator. As a result, laser manufacturers often overspecify surface quality in order to ensure that optics will not degrade laser performance due to scatter. This can drive up component costs and lengthen lead times.
REO has developed its own proprietary methods for surface quality measurement which are more quantitative and less subjective than these traditional means. These involve both automated and visual inspection of optical surfaces using a microscope under darkfield illumination conditions. As a result, we can routinely intensity illumination with darkfield illumination with microscopes eliminate surface defects larger than 20 – 30 µm, which corresponds to a surface quality level that surpasses 10-5.
Scatter losses can also be caused by surface defects, which can also degrade beam quality, and which become increasingly problematic as resonator energies increase, since they act as initiators for laser damage. High quality polishing is again a critical factor here, as is the choice of coating process. The image on the left shows a laser mirror polished conventionally and coated (by e-beam evaporation) with a high reflector. The image on the right shows a similar mirror produced using superpolishing and IBS coating. The inspection conditions are identical for these images.
Notice both the sharply decreased level of background light (scatter) and the much lower density of point defects present in the superpolished, IBS coated mirror. The standard quality, e-beam coated mirror has approximately 500 ppm of total loss, while the high performance IBS mirror has approximately 10 ppm of total loss. This lower roughness and reduced defect level translates directly into higher laser operating efficiency, better beam quality, and increased damage threshold.