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 V -value V 0 with zero group delay time difference \Delta\tau between the LP 01 and LP 11 modes is extended to the quasi-two-mode region above the cutoff V -value of the next higher order LP 21 mode. The optimum operating V -value V_{0m} is numerically determined to be 6.45 with the optimum index profile parameter \alpha_{opt} = 2.24 for \Delta = 0.3 percent. It is also shown theoretically that an index depression at the core center increases \Delta\tau due to the V -value deviation from V 0 , and that V 0 becomes 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 \alpha = 2.65 and \Delta = 0.31 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 \Delta\tau characteristics against V -value deviation \DeltaV from V 0 are 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.