Structural design and characteristics of dual-mode fibers with equalized group velocity

We investigate how the index profile of a few-mode fiber (FMF) can be designed so that group velocities of the two lowest-order modes can be equalized at a normalized frequency, which is below the cut-off frequency of the LP₂₁ mode. This can be achieved using a single-clad power-law profile with a sufficiently large profile exponent or a double-clad profile consisting of a graded-core surrounded by a sufficiently thick depressed inner cladding without index jump at their interface. The fabrication tolerances, effective index differences, intramodal dispersion differences, and effective mode areas of various single- and double-clad profiles are compared. The results show that, in comparison to single-clad fibers, double-clad fibers are capable of producing higher fabrication tolerances and reduced sensitivity of group delay difference to wavelength by three and two orders of magnitude, respectively. Our analyses provide insights into the design of FMFs, which will facilitate future development of high-capacity mode division long-haul transmission systems.