مرکز منطقه ای اطلاع رساني علوم و فناوري - Structural optimization for two-mode fiber: Theory and experiment

Abstract :

Structural design of two-mode optical fiber is investigated. It is clarified theoretically that the operating $V$ -value V0with zero group delay time difference $\\Delta \\tau$ between the LP01and LP11modes is extended to the quasi-two-mode region above the cutoff $V$ -value of the next higher order LP21mode. 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 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 $\\alpha = 2.65$ 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 $\\Delta V$ 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.