apparatus for producing an interference pattern from an input optical beam , including a first optical element for separating the input optical beam into a plurality of divergent optical sub - beams and a second optical element including a first surface and a second surface . the first surface is optically coupled to the first optical element to receive at least two of the plurality of sub - beams . in addition , the second optical element is capable of redirecting via total internal reflection at least one of the sub - beams received at the first surface such that at least two sub - beams emerge from the second surface along respective paths intersecting one another outside the second optical element at a distance from the second surface . advantageously , the use of total internal reflection in the second optical element for sub - beam redirection removes any requirement for a mirror to perform this function , results in only minimal power losses as the sub - beams reflect internally and the location at which they intersect is easily adjustable .