A step-index fibre used in a harbour communications cable has core index and cladding…

PHYSICS · Multi-part · Fermi University Physics · Optics & Photonics · Beams, resonators and guided-wave photonics · Beams, resonators and guided-wave photonics

Optics and Photonics, Ch Optical fibres in communication and endoscopy

Problem

A step-index fibre used in a harbour communications cable has core index

n_1 = 1.50

and cladding index

n_2 = 1.47

. Take

c = 3.00\times10^8

m s

^{-1}

. ((a)) Find the numerical aperture and the acceptance angle

\theta_a

in air. ((b)) Find the modal spread

\Delta t/L

in nanoseconds per kilometre, and the total spread of an initially short pulse after travelling 2 km. ((c)) The fibre's attenuation is 0.5 dB km

^{-1}

. Find the fraction of the launched power that remains after 8 km.

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