Structural optimization for two-mode fiber: Theory and experiment

Structural design of two-mode optical fiber is investigated. It is clarified theoretically that the operatingV-value V0with zero group delay time differenceΔ the LP01and LP11modes is extended to the quasi-two-mode region above the cutoffV-value of the next higher order LP21mode. The optimum operatingV-valueV₀ₘis numerically determined to be 6.45 with the optimum index profile parameterαₒₚₜ = 2.24forΔ = 0.3percent. It is also shown theoretically that an index depression at the core center increasesΔ to theV-value deviation from V0, and that V0becomes smaller than that with no central index depression for the same α. A low-loss two-mode test fiber designed for operating in the 1.3 μm wavelength region is prepared: the core diameter is 21μm withα = 2.65andΔ = 0.31percent, and the test fiber has a precisely controlled graded-index profile with a small central index depression. Experimental results show that the two-mode propagation is maintained in the quasi-two-mode spectral region. Furthermore, smallΔ againstV-value deviation V0are verified experimentally. It is also shown that the two-mode test fiber has about two times larger tolerance for the misalignment at a splice than a step-index single-mode fiber.