Beam splitter and beam combiner with wedge-shaped substrate
Substrate without wedge: possible intensity instabilities
Beams are reflected twice at the media/air interfaces. The reflected beam is finally parallel with the unreflected beam. Both beams interfere, which causes intensity instabilites (Etalon effect). If they are coupled into a fiber, they are both focused onto the same spot and are both coupled.The intensity is not stable due to the interference of the beams.