Enabling Precise Multi-Layer Optical Coatings on Highly Curved Lenses with Atomic Layer Deposition

To improve satellite-based lightning detection for small, short duration flashes that could be key for monitoring severe weather, NASA’s Marshall Space Flight Center (MSFC) has begun development of a new lightning imager named CubeSat Lightning Imaging & Detection Experiment (CLIDE), however it requires small form factor lenses with a wide field of view (FOV). It has been proposed that an affordable lens solution could be a single concentric sphere if a very narrow spectral bandpass filter can be applied conformally onto the lens. Traditionally, such narrow bandpass filters are applied as multilayer coatings deposited by physical vapor deposition (PVD) and are thus limited to flat or only slightly curved surfaces. RMD proposes to address this challenge by demonstrating both the technical and manufacturing feasibility for producing such bandpass filters using a special technique atomic layer deposition (ALD). ALD is a conformal coating technique ensuring that the bandpass filter will be identical across highly curved lenses enabling a wide FOV for detection across large areas for accurate lightning mapping. In Phase I, RMD will demonstrate the ability of ALD to deliver a uniform multilayer coating on a curved surface. Based on initial simulations, RMD plans to develop and demonstrate scalable ALD processes for low refractive index magnesium fluoride (MgF2), and high refractive index zirconium dioxide (ZrO2) as well as utilized an already established process for titanium dioxide (TiO2). Concurrently RMD will also begin detailed modelling to design MgF2 and TiO2 bandpass filters for 777.4 nm and MgF2 and ZrO2 bandpass filters for 337 nm with narrow (< 3 nm) bandwidths. Such bandpass filters will be deposited onto flat and curved substrates and evaluated by reflectometry to verify uniform bandpass characteristics across the curved substrate. The end result of this program will be bandpass filters that can be integrated into CLIDE and other imaging systems.