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Producing Low Absorption Coatings
Coating absorption can affect the performance of even low power lasers
Absorption in optical thin films is a well-known issue in high power lasers, where it can directly lead to component damage and limit laser lifetime. But, even in lower power lasers, absorption can be a problem. For example, on radiused components, such as rear cavity mirrors and output couplers, any absorption in the thin film causes an increase in temperature which can result in a shift in component focal length. This can have a substantial effect on laser output power, mode quality or beam waist location.
The use of ion beam sputtering (IBS) technology is particularly advantageous in this connection.
Specifically, traditional e beam deposited coatings typically exhibit absorption in the hundreds of parts per million (ppm), while REO can routinely produce IBS coated optics with absorption levels below 10 ppm.
One reason that IBS tends to yield low absorption coatings is that the process is simpler than evaporative methods. Specifically, less hardware is required inside the coating chamber for sputtering than evaporation, which reduces the sources for contamination. Additionally, IBS is a largely automated method, so once the process is refined and established, it’s highly reproducible. In contrast, evaporative methods require frequent changes in the chamber environment, such as source material replenishment, making it more difficult achieve consistency.
At REO, another important factor in consistently achieving low coating absorption is the ability to measure it directly, in this case using a photothermal interferometer which can measure absorption to accuracies of below 10 ppm. With precise, accurate measurements of absorption we’re able to improve our process, as well as ensure its reliability on a production basis.