Determine a normalized frequency at 820nm for a step index fibre having 25µm core radius. Given n₁=1.48 and n₂=1.46.Also calculate how several modes propagate within this fibre at 820nm.

      Normalized frequency,

              f = c/λ

Substituting c=3 x 10⁸ m/s

                    Λ= 820nm

             f = 3.658 x 10¹⁴ Hz

Given  n₁=1.48

            n₂=1.46

            r = 25µm

We have

                 V= Πd (n₁²- n₂²)^½  / λ                                                            

     Substituting the values, we get

         V=46.45

Take V=46

No. of modes= V²/2 =46²/2 = 1058