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Ch10Sec3

# Ch10Sec3 - THE FIBER FORUM Fiber Optic Communications...

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Dr. Joseph C. Palais 10.3 1 THE FIBER FORUM Fiber Optic Communications Dr. JOSEPH C. PALAIS PRESENTED BY

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Dr. Joseph C. Palais 10.3 2 Section 10.3 Analog Modulation Formats
Dr. Joseph C. Palais 10.3 3 10.3.1 AM/IM Subcarrier Modulation AM: Amplitude modulation AM example application: broadcast radio IM: Intensity modulation Consider electrical AM modulation of signal frequency ϖ m and subcarrier frequency ϖ SC Note that ϖ SC >> ϖ m i = I S (1 + m cos ϖ m t) cos ϖ SC t m = modulation index Expanding this equation for the AM current yields:

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Dr. Joseph C. Palais 10.3 4 Spectrum of the AM signal: ϖ SC - ϖ m ϖ SC ϖ SC + ϖ m ϖ 2 ϖ m i = I S cos ϖ SC t + {[(mI S ) / 2] [ cos( ϖ SC t - ϖ m t) + cos( ϖ SC t + ϖ m t)]}
Dr. Joseph C. Palais 10.3 5 i ϖ m ϖ SC t Add a dc bias current I 0 and apply the resulting current to a light source. The current is: i = I 0 + I s (1 + mcos ϖ m t) cos ϖ SC t

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Dr. Joseph C. Palais 10.3 6 The optical power emitted is: P = P 0 + P s (1 + mcos ϖ m t) cos ϖ SC t The detected current is : I R = ρ P Its electrical spectrum is : ϖ SC - ϖ m 0 ϖ SC ϖ SC + ϖ m ϖ
Dr. Joseph C. Palais

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