Comparison of bandwidth and sensitivity of long-period gratings in single-mode and few-mode fibers.

We present a comparative study of the bandwidth and the sensitivity of the resonance wavelength of long-period gratings (LPGs) to external perturbation fabricated in single-mode fibers (SMFs) and few-mode fibers (FMFs), and their dependencies on the group indices and the dispersion properties of the phase-matched modes. Unlike SMFs, a relatively large core size of FMFs invariably leads to nonuniform index modulation across the fiber cross section under UV exposure, enabling the coupling between modes having dissimilar azimuthal symmetry. Simple analytical formulas for the group/effective index difference, dispersion difference, bandwidth, and wavelength sensitivities are derived for the case of SMFs where light is coupled from the fundamental core mode to the symmetrical cladding modes.

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 LP 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.

Mode coupling dynamics and communication strategies for multi-core fiber systems.

The propagation dynamics of 7-core multi-core fibers (MCFs) with identical and three-types of cores are analytically derived based on the coupled-mode theory. The mode coupling dynamics can be aperiodic with transmission distance for MCF with identical cores. For MCFs with heterogeneous cores, it is found that even though signals from different core groups will not couple with each other, the coupling within their own group is significantly affected by the presence of other core groups.

Coupled-mode analysis of Bragg-reflection filters based on asymmetric nonlinear dual-core fibers.

We demonstrate a Bragg-reflection grating coupler using a nonlinear dual-core fiber with a long-period grating (LPG) and a fiber Bragg grating (FBG) inscribed in different cores. The LPG couples light from the primary core to the cladding, while the FBG operating in reflection acts to drop the channel from the secondary core. The coupler is nonreflective along the launching core.

Design of wavelength tunable long-period grating couplers based on asymmetric nonlinear dual-core fibers.

We demonstrate a grating coupler using an asymmetric nonlinear dual-core fiber with a pair of long-period gratings (LPGs). Simple formulas for the achievement of 100% coupling efficiency and small sidelobes are derived. The passband exhibits steep band transitions.