Dichroic filters development for NOEMA receivers

The NOEMA (Northern Extended Millimeter Array) Interferometer is a Radio-Astronomical Observatory composed of ten 15-m Antennas (two additional antennas are also being constructed) located on the Plateau de Bure, in the French South Alps. Each of these antennas is equipped with a cryogenic millimeter wave heterodyne quadri-band receiver, covering frequencies from 72GHz to 365GHz. This receiver is composed of four dual-polarizations state-of-the-art side band separating SIS (Supraconductor-Insulator-Supraconductor) modules delivering (for each side band) 4-12GHz IF signals. The four modules covers respectively the 72-116GHz (band 1); 127-179GHz (band 2); 200276GHz (band 3) and 276-365GHz (band 4) bands, and each of the four beams currently looks at a different region of the sky. A future major upgrade planned for these receivers is to combine the 72-116GHz and the 200-276GHz beams with a dichroic filter to enable these two beams to observe simultaneously the same region of the sky. For that purpose IRAM begins to develop in house broadband low loss dichroic filters. Dichroic filters should behave as reflecting mirrors for some wavelengths, and should be transparent for other wavelengths. The dichroic filters currently designed are band-pass: they are designed to transmit the 72116GHz band and to reflect the 200-276GHz band. As NOEMA receivers are very sensitive, the dichroic must introduce as lower losses as possible into the system. The dichroics we developed are made out of several metal layers of periodic structures (square patches and grids), printed on two inches and four inches quartz substrates, and assembled together in a clamp. We present the design, simulations, manufacture process and test results of these dichroic filters.