Manufacturing of high performance VIS-NIR beam splitters by plasma assisted thin film deposition technologies

Abstract

Beam splitters separating visible (VIS) and near-infrared (NIR) light are an important component of many optical systems such as spectrometers or telescopes. Here, one part of the spectrum is transmitted while the other is reflected. Typical goal requirements are broadband high transmittance and high reflectance without local minima combined with a steep transition zone. These requirements drive the complexity of the coating design. Beside the coating design also the deposition technology has an impact on the feasibility of the coating. In this contribution, we address manufacturing challenges for manufacturing of an ideal beam splitter and compare Ion Assisted Deposition (IAD) and Plasma Enhanced Magnetron Sputtering (PARMS) technology by presenting examples of VIS-NIR beam splitters manufactured at Optics Balzers Jena GmbH (OBJ). These examples reach from beam splitters manufactured by IAD with a total coating thickness of about 3.5 μm to the beam splitter of Sentinel 2 multi-spectral instrument with more than 100 layers a and a coating thickness of about 13 μm. An example which overcomes the limitations of the IAD process is the dichroic plate of ESAs Euclid telescope manufactured by OBJ by means of PARMS process. This dichroic plate shows a reflectance of over 99% in the VIS spectral range and a transmittance of more than 98% in the NIR spectral range. Both mentioned manufacturing technologies are discussed in terms of process stability, coating homogeneity, and straylight limitations.