Ch10Sec6_1

# Ch10Sec6_1 - Dr Joseph C Palais 10.6 1 THE FIBER FORUM...

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Unformatted text preview: Dr. Joseph C. Palais 10.6 1 THE FIBER FORUM Fiber Optic Communications Dr. JOSEPH C. PALAIS PRESENTED BY Dr. Joseph C. Palais 10.6 2 Section 10.6 Optic Heterodyne Receivers Dr. Joseph C. Palais 10.6 3 10.6.1 Heterodyne Detection (Coherent Detection) Consider the electric field of an optical wave traveling in the z-direction: E SIG = E S (t) cos[ ϖ C t - kz + θ (t) ] ϖ C = optical frequency = 2 π f = (2 π /T ) T ~ 10-14 seconds θ = phase of the wave For intensity modulation, E S varies with time. We will only consider angle modulation and On-Off Keying (OOK) digital signals. Dr. Joseph C. Palais 10.6 4 For optical angle modulation (FM or phase modulation), θ varies with time. Consider the field at z = 0 (i.e. the detector is located at z = 0) Then E SIG = E S (t) cos[ ϖ C t + θ (t) ] (10-30) The instantaneous intensity is I = = cos 2 [ ϖ C t + θ (t)] I = [ /2 ][ 1 + cos( ϖ C t + 2 θ29 ] 2 SIG E 2 ( ) S E t 2 ( ) S E t Dr. Joseph C. Palais 10.6 5 The average intensity (averaged over many optical cycles) is: ( 29 ( 29 2 1 1 cos 2 2 2 s c E t I t dt τ ϖ θ τ < = + + ∫ where the averaging time obeys τ >> T Ex. T = 10-14 s, τ = 10-10 s. The detector’s integration time is τ . It is still short compared to variations in E S (t) and θ (t). Dr. Joseph C. Palais 10.6 6 Thus, E S (t) and θ (t) are (nearly) constant over the integration time τ . Example showing the relationship between the integration time τ and variations in the square of the field amplitude....
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Ch10Sec6_1 - Dr Joseph C Palais 10.6 1 THE FIBER FORUM...

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