Ch3Sec2 - THE FIBER FORUM Fiber Optic Communications JOSEPH...

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Unformatted text preview: THE FIBER FORUM Fiber Optic Communications JOSEPH C. PALAIS PRESENTED BY Joseph C. Palais 3.2 2 Section 3.2 Dispersion, Pulse Distortion, Information Rate When we write E = E o sin ( t kz), we imply a single frequency source. Frequency 2 = f Radio oscillators approximate this pretty well. Optical sources do not produce single frequencies. Joseph C. Palais 3.2 3 SPECTRUM OF AN OPTICAL SOURCE 1 .5 Example Emission Spectrum of an Optical Source f Frequency f = source bandwidth (range of frequencies emitted by the source). Normalized Power f f 1 f 2 Joseph C. Palais 3.2 4 SPECTRUM OF AN OPTICAL SOURCE 1 .5 Alternatively, we can plot the wavelength emission spectrum as follows: Wavelength Normalized Power = linewidth or spectral width 2 1 Joseph C. Palais 3.2 5 SPECTRUM OF AN OPTICAL SOURCE Example: If = 0.82 m, = 30 nm , 037 . 820 30 = = so we have 3.7% bandwidth. The conversion between wavelength and frequency is: = f f (3.9) Joseph C. Palais 3.2 6 SPECTRUM OF AN OPTICAL SOURCE 1 2 2 1 2 1 1 2 1 2 c c f f f c f c - =- =- = = Proof: Joseph C. Palais 3.2 7 SPECTRUM OF AN OPTICAL SOURCE = = 2 2 1 c f then The mean frequency is: f = c/ Define the mean wavelength as: Joseph C. Palais 3.2 8 SPECTRUM OF AN OPTICAL SOURCE f f f f = = Thus, completing the proof of (3.9) . Now, we have Joseph C. Palais 3.2 9 SPECTRUM OF AN OPTICAL SOURCE Spectral Widths for Typical Light Sources Source Spectral Width (nm) LED 20-100 Laser Diode 1-5 Nd:YAG-Laser 0.1 HeNe Laser 0.002 Joseph C. Palais 3.2 10 SPECTRUM OF AN OPTICAL SOURCE If = 0, ( f = 0), the source is perfectly coherent. It is monochromatic . Laser diodes are more coherent than LEDs, but are not perfectly coherent. We will see how source bandwidth limits the information capacity of fiber transmission lines. Joseph C. Palais 3.2 11 3.2.1 Material Dispersion and Pulse Distortion Recall that v = c/n. For glass, n varies with wavelength. Thus, waves of different wavelengths (frequencies) travel at different speeds. Dispersion : Wavelength dependent propagation velocity. Joseph C. Palais 3.2 12 Material Dispersion and Pulse Distortion Material Dispersion : Dispersion caused by the material. Waveguide Dispersion : Dispersion caused by the structure of the waveguide. Joseph C. Palais 3.2 13 Material Dispersion and Pulse Distortion Consider a pulse of light emitted by a source which contains a range of wavelengths (say 1 , 2 , 3 ) as illustrated on the next slide. Joseph C. Palais 3.2 14 Material Dispersion and Pulse Distortion Input Power Output Power t t t t t t t 1 t T 1 2 2 3 3 T + Fastest wavelength Slowest wavelength Arrives last Arrives first Joseph C. Palais 3.2 15 Material Dispersion and Pulse Distortion Because of dispersion, the components of the input pulse at...
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Ch3Sec2 - THE FIBER FORUM Fiber Optic Communications JOSEPH...

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