I'm trying to split a high power laser (1064nm, 20W, beam diameter @ 1/e^2 intensity: 3.2mm) into two beams and couple them after some additional optics into two SMF's (single mode fiber). So far I tried a PBS (polarizing beam splitter) cube and a NPBS (non polarizing beam splitter) cube. I can split the beam but the cubes seem to destroy the mode in the reflected port at high power. I conclude this because I couple approx. 20% less efficient in the SMF. Does anybody know how to split(50:50 would be sufficient although polarisation dependent would be preferred) a high power beam without aggravating the mode?
Note1: Plate beam splitters are not preferable as they usually have the same dielectric beam splitter coating as cubes and the reflected port tends to interfere with reflections from the back side of the plate Note2: I'm aware of Glan-Laser Calcite, Glan-Thompson and Glan-Taylor polarizers but as they are, similar to PBS/NPBS cubes, two cemented prisms and I personally suspect this layer to be the reason. I would like to know if anybody has experience with them concerning high power.