Ch3: Lightwave Ch3: Lightwave Fundamentals Fundamentals E = E E = E o sin( sin( wt-kz wt-kz ) ) k: propagation factor = w/v k: propagation factor = w/v wt-kz wt-kz : phase : phase kz kz : phase shift owing to travel : phase shift owing to travel z z length length Plane wave: phase is same over a plane Plane wave: phase is same over a plane k = w/v = wn/c, k k = w/v = wn/c, k o =w/c, k=k =w/c, k=k o n n , , = = v/f, k v/f, k =2 =2/ /Lossy medium: E = E Lossy medium: E = E o e e -z sin( sin( wt-kz wt-kz ) )
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Ch3: Lightwave Fundamentals E = E o sin( wt-kz ) E = E o sin( wt-kz ) k: propagation factor = w/v k: propagation factor = w/v wt-kz : phase wt-kz : phase.
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Information rate: RZ Digital Information rate: RZ Digital SignalSignal
Compare to analog, using 3dB electrical BW to be Compare to analog, using 3dB electrical BW to be conservative: conservative:
RRRZRZ=1/T, by comparison T=1/f, =1/T, by comparison T=1/f, RRRZRZ=f=f3dB3dB (elect) =0.35/ (elect) =0.35/
by considering power spectrum of pulse: f ≤ 1/T, by considering power spectrum of pulse: f ≤ 1/T, and we can substitute as above to end with resultand we can substitute as above to end with result
Information rate: NRZ Digital Information rate: NRZ Digital SignalSignal Compare to analog, using 3dB electrical BW to be Compare to analog, using 3dB electrical BW to be
conservative: conservative:
RRNRZNRZ=1/T, by comparison f=1/2T, =1/T, by comparison f=1/2T, RRNRZNRZ=2f=2f3dB3dB (elect) =0.7/ (elect) =0.7/
by considering power spectrum of pulse: f by considering power spectrum of pulse: f ≤ 1/2T, and we can substitute as above to ≤ 1/2T, and we can substitute as above to end with resultend with result
a standing wavea standing wave Although all power is Although all power is
reflected, a field still reflected, a field still exists in 2exists in 2ndnd medium medium carrying no power carrying no power called evanescent fieldcalled evanescent field
It decays exponentiallyIt decays exponentially
ii close to close to cc, field penetrates deeper , field penetrates deeper inside 2inside 2ndnd medium and decays slower medium and decays slower