A demonstration light guide is made from a bare rod of glass of refractive index
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 demonstration light guide is made from a bare rod of glass of refractive index n1=1.50, surrounded only by air (n0=n2=1.00). A commercial step-index fibre, also with core index n1=1.50, has a glass cladding of index n2=1.47 fused to the core, and is likewise used in air.
((a)) Find the numerical aperture of the bare rod, and say what its value implies about the acceptance angle in air.
((b)) Find the numerical aperture and acceptance angle in air of the clad fibre.
((c)) A technician argues that because the bare rod's numerical aperture is nearly four times larger, it must be the better light guide for carrying a signal over a real, handled length of cable. Say whether this reasoning is correct, and give the physical reason the clad fibre, not the bare rod, is the one actually used.