Question

The core diameter of multimode step index fibre is 50 μm.The numerical aperture is 0.25. Calculate the number of guided modes at an operating wavelength of 0.75 μm.

Answer 1

d = 50 x 10^-6 m 

N.A = 0.25

λ = 0.75 x 10^-6 m 

`V = (πd)/λ x N.A`

`= (π  x  50  x  10^(−6))/(0.75  x  10^( −6) `

= 52.36

`Nm = (V^2)/2`
= 1370.8
= 1370

Number of modes = 1370