Surface Quality

Standard laser optics typically require at least L/10 form error and 10-5 scratch-dig (DIN 5/4x0.025, Ø 1”) surfaces, which would be difficult to produce without absolute cleanliness during fabrication. Standard polishing machines can produce at best a 1-nm to 0.5-nm surface roughness figure (srf).

Applications using laser optics often require greater attention to the surface roughness than 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 due to the fact that the dielectric layers increase the peaks, rather than reduce the ‘peaks and troughs,’ thereby causing an increase in surface roughness. Light therefore scatters internally as well as on the surface.

Compared to the original blank substrate a thirty-layer dielectric Rmax mirror coating can increase the scatter losses many times over. Although the physical scale of this roughness is still very small – it can perhaps be designated the smallest roughness category – the optic can still cause a problem. This is especially so for laser resonators, in which the laser gain and thus the output power are reduced.

For special applications like the parabolic mirror, which is used for pumping thin disk lasers, measurable losses can still be detected even though this system does not use a multiple cavity design. The pump laser (808 nm or 940 nm) is reflected again and again in the focusing mirror, resulting in as many as 32 reflections. Together with transmission losses, the total loss can be extremely high thus reducing the pump efficiency.

Fortunately, today’s polishing techniques of aspheric blanks are so far 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 has measured Laser Components’ parabolic mirrors and found them to have an 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) than coated parabolic copper mirrors. Glass parabolic mirrors have a better reflection, a more consistent parabolic profile (to within 1 lambda), and a cleanliness of 4x0.1 (per dia2”) up to dia 200-mm mirrors. Even large Yb:YAG disk 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.

We thank the IFSW and all partners who took part in providing the technical data for this project.