A coaxial multiple-mode antenna system (10) is disclosed, including a coaxial input section (14) for coaxially collecting, for example, MMW and IR electromagnetic radiation. The IR radiation is focused to a point at the input section's focal plane, while the MMW beam has a relatively large width in this plane. An interface section (16) includes an IR section (18) and MMW section (20) that allow the two beams to be separately processed with a minimum of losses. In that regard, the MMW section includes a lens (42), having an incident surface, the center of which is positioned at the focal plane. An optical fiber (46) extends axially through the lens and has an incident end that is aligned with the incident surface of the lens. The lens quasi-optically transmits the MMW beam to a detection circuit (26), while the optical fiber transmits the IR beam to detection circuit (22). Because the optical fiber receives the IR beam before it is processed by the lens, IR losses are limited. Further, the small diameter of the optical fiber and its axial alignment in the lens cause minimal disruption of the MMW beam.