Abstract: A pair of fiber optic cables (12, 14) are connected at one end to a single light emitting diode (10). The second ends of the cables (12, 14) are connected to a first end of a polarized polyvinylidene fluoride film member (16), in a symmetrical relationship on opposite sides of a longitudinal centerline. The second end of the film member (16) includes a triangular recess formed by intersecting 45.degree. edges (82, 84). A beam splitter (18) is positioned in the recess, with opposite end portions, one positioned to receive a beam of light from cable (12) and the other positioned to receive a beam of light from cable (14). The beam splitter redirects the two beams towards each other and towards a diagonal split (72) at the center of the beam splitter (18). A change in length of the film member (16) on one side of the longitudinal centerline will change the phase relationship of the two light beams where they meet at the split (72). A fiber optic cable (20 ) is connected to the beam splitter (118).