## 1 Introduction  
Joining-material-free processes for the direct contacting or connection of optically active surface components are of increasing importance due to the growing need and use of UV- and high power laser optics as well as vacuum applications. Here, the classical optical cements cannot be used due to their low laser-induced damage threshold and high UV-absorbance. The common method to generate direct contacted optical components without such cement is wringing, also known as optical contact bonding [1]. This method originates from the manufacturing process of optics with high demands on surface form deviation or angularity. In the optical manufacturing process, wringing is usually applied for the temporary attachment of optical components to manufacturing tools made of glass. Physically, the effect of wringing can be based on intermolecular phenomena [2] such as dipole-dipole or van-der-Waals forces [3] on the one hand and a thin water film and capillary forces on the other hand [4]. Due to the different expansion coefficients of different glasses, such an attachment can be easily released by heating. In addition to temperature fluctuations, environmental influences in the form of vibrations can also lead to loosening of the connection. Hence, such wringing is basically not suitable to generate permanently contacted optics. To realise the permanent attachment of uncoated optical components all the same, processes based on the methods used in the semiconductor industry for direct wafer bonding are modified to optics made of glass. The direct bonding processes consistsof the three process steps surface activation, merging and curing. Surface activation during direct bonding of wafers is either performed wet-chemically--for example by using ammonium hydroxide, hydrogen peroxide and water--or physically by plasma treatment at low pressure [5; 6]. After surface activation, the wafers are brought into contact and an annealing process is performed to generate covalent bonds. For annealing of silica wafers, temperatures of approx. 1100 \({}^{\circ}\)C are reached [7] since, at such temperatures, the silanol groups that perform the actual covalent bond between bonded components reaches the highest stability [8]. At lower annealing temperatures, wet chemical activation and plasma activation can also be combined to obtain high bond stability [9]. For direct bonding of optics, this bonding process can also be performed after slight modifications mainly concerning the annealing temperature. For surface activation of uncoated optics, there are already different wet-chemical procedures established in optics manufacturing [10]. Similar to the bonding of wafers, a dilution of ammonium hydroxide, hydrogen peroxide and water [11] or a dilution of potassium hydroxide and water can be applied [12]. Furthermore activation can be achieved by plasma treatment [13]. Fused silica and borosilicate glasses were already bonded successfully after surface activation via a dielectric barrier discharge (DBD) plasma treatment at atmospheric pressure and a subsequent annealing process [14]. The transfer of bonding technologies used for wafers and uncoated optics or glass surfaces to optical components with functional coatings is of great interest. Such direct contacting of coated elements would represent an added value for optics manufacturing. Due to the temperature sensitivity of functional coatings the annealing temperatures are lowered to a range of 100 \({}^{\circ}\)C up to 250 \({}^{\circ}\)C [11; 14]. The wet-chemical processes were already performed successfully on optical components with dielectric functional coatings, whereas the use of the plasma process has turned out to be problematic due to the fact that the used DBD penetrates into the coating material [15; 16]. For optical components coated with functional layers such as non-polarising beam splitters, which usually contain metal, wet-chemical bonding procedures are not applicable because the functional layers are destroyed.