Mode Coupling

• After certain initial length, the pulse distortion increases less rapidly because of mode The energy from one mode is coupled to other mods because of:
  • Structural
  • Fiber diameter
  • Refractive index variations.
  • Microbends in
• Due to the mode coupling, average propagation delay become less and intermodal distortion
• Suppose certain initial coupling length = L_c, mode coupling length, over L_c = Additional loss associated with mode coupling = h (dB/ km).

Therefore the excess attenuation resulting from mode coupling = hZ. The improvement in pulse spreading by mode coupling is given as :

where, C is constant independent of all dimensional quantities and refractive indices. σ_c is pulse broadening under mode coupling.

σ₀ is pulse broadening in absence of mode coupling.

• For long fiber length’s the effect of mode coupling on pulse distortion is significant. For a graded index fiber, the effect of distance on pulse broading for various coupling losses are shown in 2.8.1.

• Significant mode coupling occurs of connectors, splices and with other passive components of an optical