Polarization Maintaining Dichroic Beam-splitter and Its Surface Shape Control by Back Side AR Coating

Dichroic beam-splitter (DBS) with polarization-maintaining took an important role in the free space quantum telecommunication tests on the Micius satellite of China. In this presentation, we designed and prepared a 50 layer polarization-maintaining DBS coating by a dual ion beam sputtering deposition (Dual-IBS) method. In order to solve a stress problem, an 18 layer special anti-reflection (AR) coating with similar physical thickness ratio was deposited on the backside. By stress compensation, the surface flatness RMS value of the DBS sample decreased from 0.341 λ (@632.8 nm) to 0.103 λ while beam splitting and polarization maintaining properties were almost kept unchanged. Further, we discussed the mechanism of film stress and stress compensation by equation deduction and found that total stress had a strong relationship with the total physical thickness and the ratio of layer materials.