The project “High‑performance optical coatings for freeform optics” was carried out under the German Federal Ministry of Education and Research (BMBF) program “Innovative Regional Growth Kernels” as part of the Technology Platform VIS Freeform Optics. The consortium consisted of Optics Balzers Jena GmbH, the company Materion (project lead Dr. Stefan Jakobs), and the optics specialist asphericon. The work spanned from 2021 to 2023 and was financed with a total budget of 831 843 EUR, of which 23 558 EUR were spent on raw materials and 42 140 EUR on external services. The project’s outcomes are now being integrated into customer‑specific production of broadband high‑reflective and spectrally selective components, and the company’s annual turnover rose from about 8 million to 13 million euros during the period, with staff increasing from 68 to 95 employees.

Technically, the study focused on the design, deposition, and characterization of both dielectric‑only and metal‑dielectric multilayer coatings for freeform surfaces. A plasma‑assisted reactive magnetron sputtering (PARMS) process was adopted for the demonstrator mirrors. The coatings were evaluated for spectral performance, micro‑roughness, surface defects, and laser damage threshold (LIDT). In the ultraviolet range (230–350 nm) the absorption coefficient of hafnium oxide and tantalum oxide was measured spectrophotometrically, revealing a clear dependence on post‑deposition annealing. Using the measured data, a broadband high‑reflective layer was designed that achieved reflectivity above 99 % across 250–350 nm and 99.9 % at the central wavelength of 532 nm under oblique incidence. The LIDT tests, performed at 60° incidence for both s‑ and p‑polarized light, showed values between 2 and 4 J cm⁻² for s‑polarization in both dielectric and metal‑dielectric stacks; p‑polarized tests yielded lower thresholds, with the aluminum layer in the metal‑dielectric stack limiting the damage threshold due to its absorption. All investigated stack designs met the operational requirements of the freeform demonstrators.

A key development was a dedicated measurement system and analysis algorithm for surface defect characterization. The system can quantify four defect metrics and classify defects according to DIN ISO 10110‑7, enabling a person‑independent quality assessment. This tool is now part of the serial production quality control for laser mirrors, reducing inspection effort and improving reliability. Additionally, a method for precisely determining reflectivity at 99.9 % under oblique incidence was established, providing a robust benchmark for future coating designs.

The collaboration facilitated technology transfer between the partners. Optics Balzers supplied the freeform substrates and provided expertise in substrate‑coating interactions, while asphericon contributed to the design of broadband reflective stacks for the 532 nm beam‑expander demonstrator. Materion led the deposition process development, surface defect analysis, and laser damage testing. The joint effort not only produced a set of validated coating recipes and characterization protocols but also strengthened the partners’ capabilities in high‑performance UV coatings, positioning them to meet evolving customer demands in freeform optical systems.