A Broad-Band Dual-Polarization All-Metal Dichroic Filter for Cryogenic Applications in Sub-THz Range

In this work, we report on the design and characterization of an all-metal, wideband single-layer dichroic filter operating at non-normal beam incidence. The dichroic filter consists of a perforated metal plate with an angular offset of the perforated channels equal to the beam incidence angle onto the dichroic surface. The fabricated filter is characterized using an especially designed quasi-optical test system. The filter demonstrates 96% transmission of the incoming electromagnetic radiation averaged in the signal band about 37-50 GHz for both polarizations while simultaneously achieving a rejection better than 20 dB for frequencies lower than 26 GHz at the designed beam incidence of 13 degrees. The cross-polarization level for each polarization is better than 30 dB in the passband. The experimental results of the transmission measurements are in very good agreement with electromagnetic simulations confirming the feasibility and benefits of our proposed design concept even at THz frequencies. The simulations of the dichroic scaled version demonstrate that, for instance, it can be employed in the Event Horizon Telescope project, where the 230 GHz and the 345 GHz receiver channels could be operating simultaneously.