Structural optimization for two-mode fiber: Theory and experiment

Structural design of two-mode optical fiber is investigated. It is clarified theoretically that the operating -value V0with zero group delay time difference between the LP01and LP11modes is extended to the quasi-two-mode region above the cutoff -value of the next higher order LP21mode. The optimum operating -value is numerically determined to be 6.45 with the optimum index profile parameter for percent. It is also shown theoretically that an index depression at the core center increases due to the -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 with and percent, 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, small characteristics against -value deviation from 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.