An optical device includes two prisms and a beamsplitter configuration. A first of the prisms has a first surface associated with a source and a second surface oblique to the first surface. A second of the prisms has a first surface associated with a detector and a second surface oblique to the first surface. The second surface of the first prism overlaps with the second surface of the second prism to define an interface region that partially extends along at least one of the second surfaces. The prisms are optically attached at the interface region, and the beamsplitter configuration overlies the interface region. A beam emitted by the source propagates through the prisms along two optical paths and reaches the detector as two coherent beams. Beams that propagate along the two optical paths are reflected from the beamsplitter configuration and transmitted by the beamsplitter configuration exactly once.