Future optical systems in the sub-mm range require low loss, low reflectance and broadband optics. Presented here is the development process for a hot press technique for making broadband multi-layer anti-reflection coatings for plastic lenses and optical components. The elevated temperatures used in this method induce a change in index and mechanical deformation in the substrate material due to polymer chain rearrangements. To avoid this, these components are annealed before they are machined to their intended form and prior to the application of any coating. Study of precise dimensional and refractive index changes between 115-130°C across repeated annealing cycles shows that ultra-high molecular weight polyethylene (UHMWPE) requires 3-4 annealing cycles above 125°C before stabilisation. We then present 3 and 5 layer test coating recipes for 90-225GHz with comparison between experimental measurements and theory. Preliminary results show good agreement. In this study, the methods presented are kept generic for any sub-mm band focusing broadly on 60-600GHz; however future work will apply what is achieved for future CMB experiments.
This paper presents recent results of ongoing European Space Agency funded program of work aimed at developing large dielectric lenses suitable for future satellite missions, with a particular focus on requirements for CMB polarimetry. Two lens solutions are being investigated: (i) polymer lenses with broadband multi-layer antireflection coatings; (ii) silicon lenses with surface-structured anti-reflection coating represented by directly machined pyramidal features. For each solution, base materials with and without coatings have been optically characterized over a range of temperatures down to ~10 K. Full lens solutions are under manufacture and will be tested in a bespoke large cryo-optical facility.
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