Abstract: A cryogenic optical/electrical interconnect module has optical fibers captivated by an optical fiber fixture. The optical fiber fixture includes a stress support section that captivates the buffers protecting the optical fibers in V-grooves, while the bared optical fibers are laid in V-grooves in a carrier portion that has a slant edge at the end. A photodiode fixture of the same material as the optical fiber fixture has a photodiode array mounted thereon, and the optical fiber fixture is aligned with the photodiode array so that optimal optical energy is coupled from the optical fibers to the photodiodes. The fixtures are mounted in a cavity of a conductive substrate and a circuit board is mounted on the top surface of the substrate surrounding the cavity. The circuit board has microstrip lines, and the photodiodes are electrically coupled to the microstrip lines by flexible microstraps. A thermal transition block may be mounted in the cavity between the substrate and the fixtures.