Sun blocking filter for satellite based laser communication terminals

Abstract

Satellite based laser communication terminals (LCT’s) rely on the optomechanical stability and imaging performance of their beam expander and receiver telescope lens assemblies under extreme environmental conditions with respect to temperature and solar irradiation. Direct or partial sun exposure of the entrance aperture of any telescope optics will result besides a possible stray light background in a gradual transient heat load and thus thermal deformation of the optical elements. This can result into temporary degradation of the optical performance, making a datalink impossible within this time. We present a design and functional verification of a high reflectivity (HR) sun blocking filter (SBF), that is able to protect LCT telescopes against solar or earth albedo based electromagnetic radiation over a very broad spectral range from 450 nm up to > 5 μm while simultaneously providing a high transmissivity (HT) of > 95% for the LCT laser radiation at 1064.55 nm. The SBF is based on a 3.25” diameter sapphire substrate in high performance optical quality. Both sides of the filter have a multilayer coating with distributed functionalities accounting for broadband HR, low loss anti reflection (AR) coating and HT at the laser wavelength of 1064.55 nm. The HT coating design at 1064.55 nm is based on an induced transmittance of a thin ( < 25 nm) Ag layer, that is stacked within an interference coating, tuning the HT to the desired laser wavelength of the used LCT. Design verification is presented with respect to spectral transmission/reflection, transmitted wave front error and birefringence under all relevant operational conditions.