A system and method for fabricating structures in an optical substrate. An optical element produces first and second write beams that intersect at a first intersection location at the optical substrate. The first intersection location includes a fringe pattern produced by the first and second write beams. The optical element also produces first and second reference beams that intersect and are recombined at a second intersection location in substantially the same plane as the first intersection location. A controller then controls relative positioning between the optical substrate and the fringe pattern based on a signal derived from the recombined first and second reference beams.