5) The wavelength of a red helium-neon (He-Ne) laser beam in air is ?0 = 632.8 n

Question

5) The wavelength of a red helium-neon (He-Ne) laser beam in air is ?0 = 632.8
nm. The laser light is focused onto the surface of a pure fused quartz optical fiber
core in air. (a) What primary principal in optics enables the use of fiber optic
cables to transmit light around a corner with radius R?; b) What is the frequency
of the He-Ne laser beam in fused quartz? (c) What is the wavelength of the He-
Ne laser beam in fused quartz? (d) What is the speed of the laser beam in the
optical fiber? (e) The optical fiber made from pure fused quartz has a fiber
diameter d = 100 ?m, and index of refraction n = 1.458. The minimum outside
bend radius for the optical fiber is given by Rmin = nd/(n-1). Calculate the
minimum outside bend radius for the optical fiber, Rmin. (f) What happens as the
fiber diameter is decreased?

Explanation / Answer

? = ?(air) / ?(medium) ........(1)
where ? is the refractive index of the liquid and ? is the wavelength of light.

? = c/v
c is the speed of light in air/vacuum and v is the speed of light in medium

v = distance/time
v = 33* 10^-2 / 1.48* 10^-9
v = 2.23* 10^8 m/s

? = 3* 10^8 / 2.23* 10^8
? = 1.34

using equation (1)
1.34 = 633*10^-9 / ?(medium)
?(medium) = 472.38 nm

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