Lossless beam combiners for nearly equal laser frequencies

We discuss three ways to combine two laser beams with equal linear polarizations and very closely spaced frequencies into a single output beam containing 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 combined output power electronically with the help of a simple servo loop. With off-the-shelf optical components we obtained a 98% efficiency.