Optical feedthrough for pressure vessels

Each of two optical fibers to be mated is cemented to one end of a respective quarter-pitch segment of graded-index rod, at the axis of the rod. The opposite end of each of the two quarter-pitch segments is polished flat, perpendicular to the axis. The two polished ends are faced toward each other and held in precise parallelism, but not necessarily touching, by a suitable connector housing. An optical signal from one optical fiber is collimated by its attached graded-index rod segment for transmission across the gap to the other rod, which in turn focuses the parallel rays onto the receiving tip of its attached optical fiber. A planar window, such as a pressure window of a deep-underwater craft, may be interposed between the two sides of the connector without disturbing performance.