Tetsuya Hayashi, in Optical Fiber Telecommunications (Sixth Edition), 2013
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Multi-Core Optical Fibers
Particles a ferrite toroid is a continuous magnetic material with variability effects undiluted by air gaps. This means that tight tolerances such as required for wave filters are not attainable in a toroid, but will generally require a gapped structure such as an E core, pot core, or slug. POWER INDUCTORS.
9.2.8.1 Suppression of the mode coupling coefficient
The mode coupling coefficient can be suppressed by reducing the overlap of the modes, so it can be suppressed by confining the modes to the cores strongly and/or enlarging the core-to-core distance. Of course, the core-to-core distance should be shortened as much as possible, in accordance with the core density. Accordingly, high confinement core design is important for the suppression of the mode coupling coefficient. High-index and small-diameter core structure is one of the options, but it degrades the effective area Aeff and increases the nonlinear noise [49,50]. Specially designed refractive index profilesâsuch as trench- or hole-assisted core structures, shown in Figure 9.7âcan confine the mode strongly while preserving a large Aeff[45,51â54]. One trench/hole layer can either surround one core or be shared between neighboring cores. Photonic-crystal structures can also be leveraged for the strong mode confinement [15â17].