Dispersion in optical fibres with stairlike refractive-index profiles

In multimode circularly symmetric fibres whose index distribution is a stairlike approximation of an optimum profile, the modal dispersion increases as the number of steps decreases. For a fibre with Δn/n=0.02 and a core radius of 40 μm, numerical calculations based on wave optics show that the r.m.s. impulse response width at λ=1 μm increases from 0.075 ns/km for the smooth optimum profile to 0.23 ns/km for 40 steps of equal areas. Thus an important conclusion of the analysis is that one should avoid introducing steps in the refractive-index profile of fibres for optimum results.