# Ch2Sec5BW - THE FIBER FORUM Fiber Optic Communications...

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THE FIBER FORUM Fiber Optic Communications JOSEPH C. PALAIS PRESENTED BY

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Joseph C. Palais 2.5 2 Section 2.5 Diffraction Theory Some experiments do not exactly follow ray theory. Diffraction is the deviation from the predictions of ray theory. As an example of the difference between ray theory and diffraction, consider focusing by a lens of a uniform beam of light.
Joseph C. Palais 2.5 3 D f Central spot with surrounding rings of light d d is the diameter of the central spot. Uniform beam of light Lens DIFFRACTION THEORY

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Joseph C. Palais 2.5 4 DIFFRACTION THEORY 6 2.44 2 , 1 2.44 10 2 4.88 f d D If f D m then D d m D λ λ μ μ - = = = = = The diameter of the central spot is found from: (2.14) Example:
Joseph C. Palais 2.5 5 GAUSSIAN BEAM An important non-uniform intensity pattern is the Gaussian intensity distribution. r ( μ m) I/I o = exp(-2r 2 /w 2 ) I/I o = exp(-2r 2 /w 2 ) w = 10 μ m 0.135

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Joseph C. Palais 2.5 6 2 2 2 r w o I I e - = GAUSSIAN BEAM where w = spot size and r = radial distance from the center of the beam pattern.
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