Abstract: Some example embodiments of an interferometer for fiber optic communication systems include a pair of identical beam splitting prisms. Each of the beam splitting prisms includes a first total-internal-reflection surface, a second total-internal-reflection surface parallel to the first total-internal-reflection surface, and a beam splitting interface parallel to the first total-internal-reflection surface. An interferometer embodiment may optionally include a thermo-optic compensator disposed between the two beam splitting prisms. A beam splitting plate may optionally be included in some example embodiments to provide four spatially-separated output ports, two from each of two delay line interferometers sharing the two beam-splitting prisms.