As usual, John has correctly guessed that I'm using an interferometer and
controlling the relative phase of the two laser input beams (using the
feedback signal) to null one output port. The snag is I'm using kilowatt
beams, which is why I'm very reluctant to insert a beam sampler into the
"on" output port. In response to Curtis, I'll add that I'm working with cw
beams. The application is simply a high-power combiner with a switched
output.