Standard laser optics typically require at least l/10 form error and 10-5 scratch-dig (DIN 5/4 x 0.025, Ø 1”) surfaces, which would be difficult to produce without absolute cleanliness during fabrication. Standard polishing machines can produce at best a 1nm to 0.5nm surface roughness figure (srf).
Applications using laser optics often require greater attention to the surface roughness than to individual scratches because the former can cause unacceptable scatter losses. Surface imperfections can become magnified once coated with dielectric thin films such as AR coatings. This is because the dielectric layers increase the peaks, thereby causing an increase in surface roughness. The result is that light scatters internally as well as on the surface. Especially complex substrate shapes (like the parabolic mirrors used for pumping thin disc lasers) pose a challenge to manufacturers.
Fortunately, today’s polishing techniques of aspheric blanks are so well advanced that the standard micro roughness figures achievable on flats and spherical surfaces are now available on aspheric surfaces as well. Dr. Giesen of the IFWS department at the University of Stuttgart, Germany, has measured Laser Components’ parabolic mirrors and found them to have srf = 0.5 nm. Together with Laser Components’ high performance coatings, the total improvement using these mirrors is typically 10 – 20% higher (depending upon the manner of operation) when compared to coated parabolic copper mirrors. Glass parabolic mirrors have better reflection and have more consistent parabolic profile (to within 1 lambda) and cleanliness of 4x0.1 (per dia 2”) up to dia 200 mm mirrors. Even large Yb:YAG disc lasers in the kW range benefit from this latest development.
As the future of such lasers is expected to require multiple kW amplification stages the use of high performance glass parabolic mirrors is key to Laser Components’ customers.
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