Abstract: A modular three-dimensional (3D) optical switch that is scalable and that provides monitor and control of MEMS mirror arrays. A first switch module includes an array of input channels. Light beams received from the input channels are directed toward a first wavelength selective mirror. The light beams are reflected off of the first wavelength selective mirror and onto a first array of moveable micromirrors. The moveable micromirrors are adjusted so that the light beams reflect therefrom and enter a second switch module where they impinge upon a second array of moveable micromirrors. The light beams reflect off of the second array of moveable micromirrors and impinge upon a second wavelength selective mirror. The light beams reflect off of the second wavelength selective mirror and into an array of output channels. The alignment or misalignment of a data path through the switch is detected by directing two monitor beams through the data path, one in the forward direction and one in the reverse direction.