A step index optical fiber is used to carry control signals for a federation shu

Question

A step index optical fiber is used to carry control signals for a federation shuttle craft. The core index n_1 = 1.602, cladding index n_2 = 1.542, and outside index n_3 = 1.000. The signal wavelength x = 1 micron. The core diameter is 60 microns. a. How many modes can this fiber support? b. Find the maximum beam entrance cone angle 20 c. A lens of focal length f is used to couple a laser with a 4 mm diameter beam into the fiber. Find the focal length f that will allow the focused input beam to match the acceptance cone angle of the fiber calculated in part b above. d. What is the minimum skip distance for this fiber?

Explanation / Answer

a) Normalized frequency parameter or V number V=(2n1a N.A)/

Here, n1 = refractive index of the core; a = radius of the core; = wavelength of the light propagating through the fiber; N.A = Numerical Aperature.

for the given problem V number =81.82212

Since V number is greater than 2.405 it is a multimode fibre.

Number of modes is V2/2

(81.822)2/2 =3348 modes

b) Sin =N.A = Square root(n core-n cladd)

=sin-1N.A =0.4492 for this problem

Maximum beam entrance cone angle =2 =.8984

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