a closed loop alignment system for cross - connect switches in optical telecommunications systems . arrays of small tiltable mirrors within the switch direct the signal beams between optical fiber arrays connected to the switch . an external alignment system is incorporated into the cross - connect switch and is employed to provide an alignment beam of a wavelength different from the signal beams . beam splitters such as dichroic mirrors , which pass the signal beams but pass only a portion of the alignment beams , are placed in the beam paths to image the ends of the optical fibers of the arrays . a combined image of the ends of the optical fibers from each array is reflected onto a photodetector array , output signals of which are sent to a feedback control system which compares the position , size , and shape of the combined image to a stored tolerance range of acceptable positions , sizes , and shapes , and consequently moves the tiltable mirrors so that the signal beams correctly enter the ends of the optical fibers in the opposite optical fiber array .