Lossless beam combiners for nearly equal laser frequencies

We discuss three ways to combine two laser beams with equal linear polariza tions and very closely spaced frequencies into a single output beam contain ing up to 100% of the input power of each beam. One setup, a modified Mach- Zehnder interferometer, is examined in detail; it allows to adjust the comb ined output power electronically with the help of a simple servo loop. With off-the-shelf optical components we obtained a 98% efficiency. (C) 2000 Am erican Institute of Physics. [S0034-6748(00)03802-8].