Performance verification of sun blocking filter for satellite based laser communication terminals

Extended experimental verification of the recently introduced high performance sun blocking filter (SBF) design is reported. The SBF is based on a high optical quality sapphire substrate coated on both sides with multilayers including an induced transparency metal layer, protecting satellite based laser communication terminals (LCTs) telescopes against solar irradiation while simultaneously providing high transmission on the used laser wavelength at 1064.55 nm. Low earth orbit (LEO) satellite based long range LCT offer a potential solution for the highest possible data link rates and availability. Laser transmitter (TX) and receiver (RX) units require a special beam expander telescope optics. The performance of the optics can be deteriorated by the stray light and extreme thermal load conditions caused by direct sun illumination in space. Transmitted wavefront performance is a key requirement for the SBF since the total wavefront budget determines the LCTs link budget. The transmitted wavefront error (TWE) of the SBF is investigated with respect to the coating uniformity. Besides the TWE the contribution of birefringent SBF substrate to the polarization extinction budget has been experimentally verified. Polarization extinction enables polarization coding between TX and RX radiation inside the LCT receiver unit. The interferometric measurements and setups of the transmitted wavefront of the SBF in dependence on substrate temperature are presented.