For Notch Filters, Rugate designs with a small index contrast and apodisation are well known in the literature. The
required deposition of gradient index layers or so called flip flop structures is very complicated and difficult to
manufacture. Higher order H/L stacks of coating materials with high index contrast result in very thick layer stacks. In
our approach we replace the second refractive index by equivalent layers consisting of H/L materials with high index
contrast. This leads to a combination of thick (>100nm) and very thin layers. Stable coating processes with dense layers
are strict requirements. Another challenge is the accurate thickness control of very thin layers in the nanometer range. Single notch filters were produced with PIAD and broad-band optical monitoring. The most challenging filters were demonstrated with magnetron sputtering and monochromatic optical monitoring. Some outstanding results of single and multiple notch filter coatings will be presented.
Future production of high quality laser components asks for both, the precise preparation of low loss multilayer stacks and a clean room compatible innovative deposition process. In addition a cost-effective thin film filter production is required in order to transfer new developments like ultra fast pulse laser technique from research to economical products and applications. One of the most promising candidates is magnetron sputtering due to the potential of excellent film properties, a fully automatic clean room compatible manufacturing process and a high productivity. Similar film qualities as with ion beam sputtering are realised but with a more than 10 times higher productivity. The reproducible production of multilayer broad band mirrors with controlled group delay dispersion and low losses is still a big challenge. Promising results were achieved on the basis of silica and niobia and will be presented.