Bonding Developments

Polymer-free Joining of Optical Glasses and Components

N-SF6 and N-BK7 joined curved substrates.
© Fraunhofer IOF

N-SF6 and N-BK7 joined curved substrates.

Glass joining is used for various applications, such as light weight components, with blank covered gratings as well as for manufacturing grating-prism-systems (GRISM). Using existing bonding methods with and without interlayer for optical components were developed and investigated within the scope of the funded project “Optibond“.

During direct plasma activated bonding, covalent Si-O-Si bonds are created between the joining partners. A highly stable joining with optical transparency even in the UV-range is formed. The investigation of a large variety of optical glasses, e.g. N-BK7 and N-SF6, has shown that even lead-containing glasses, e.g. SF11, can be joined with high bonding strength.

Direct bonding of coated components requires their final polishing to reduce the surface roughness. This procedure yields a higher bonding strength of 40 MPa rather than 10 MPa, which is approximately half of uncoated joined components.

Fused silica and SF11 beam splitter cubes have been fabricated as demonstrator elements. The joining partners are aligned passively in high precision equipment. Applications are high power resistant beam splitters (laser induced damage threshold 0.25 J/cm² @ 500 fs) as well as broad band polarizers with high contrast.

As a second method, silicate bonding using a mineral solution was investigated. A polymer-free, solid and stable, visual transparent joining between the glass components is formed. Due to alkaline ions in the silicate solution, the transparency in the UV range is limited. Thus, joined substrates show bonding strengths of 80 % of the bulk material. The method compensates for flatness errors of a few micrometers and therefore allows the joining of curved surfaces and the application in optics and system technology. The method was demonstrated for the manufacturing of achromatic optics composed of N-SF6 and N-BK7 as well as fused silica cylindrical lenses.

 

Authors: Felix Dreisow, Carolin Rothhardt, Kevin Grabowski, Karina Jorke, Stefan Risse