Abstract: A coupler for a concentric core optical fiber comprises a step index fiber having an inner core diameter equal to the diameter of the inner core of the concentric fiber. The inner cores of both fibers are aligned and face each other. When positioned in this manner, the cladding layer of the step index fiber is also aligned with the isolation zone of the concentric core fiber and the outer core of the concentric core fiber is aligned with the substrate of the step index fiber. A large glass rod or cane is then fused to the outer periphery of the step index fiber to cause light propagating in the substrate to couple to the cane. The light thus propagating is indicative of the light that is transmitted by the concentric fiber in the outer core. The signal propagating in the inner core of the step index fiber represents the signal propagating in the inner core of the concentric core fiber and can be detected by an ordinary detector.

Abstract: A concentric core fiber having a low numerical aperture has an end face aligned with the end face of a graded index fiber having a high numerical aperture. The inner core of the concentric core fiber is aligned with and faces the central region of the graded index fiber, while the outer core of the concentric core fiber is aligned with the outer periphery of the inner core of the graded index fiber. In this manner, low order modes which propagate in the inner core of the concentric core fiber are coupled to the central region of the graded index fiber. Higher order modes which propagate in the outer core of the concentric core fiber are applied to the inner core of the graded index fiber and propagate at the edges thereof.