Optical Fiber Mode Coupling

. After certain initial length. The pulse distortion increases less rapidly because of mode coupling. The energy from one mode is coupled to other modes because of :
- Structural imperfection
- Fiber diameter variations.
- Refractive index variations.
- Microbends in cable.
. Due to the mode coupling, average propagation delay becomes less and intermodal distortion reduces.
. Suppose certain initial coupling length = Lc, mode coupling length, over Lc = Z. 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.
σc 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 broadening for various coupling losses are shown in Fig. 1.

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


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