Ch5Sec6 - 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 5.6 2 Section 5.6 Pulse Distortion And Information Rate In Fibers Signals are distorted by: Material Dispersion Waveguide Dispersion Modal Distortion Polarization-Mode Dispersion Joseph C. Palais 5.6 3 Multimode (Modal) Distortion In the previous chapter, we found the modal pulse spread for the slab waveguide to be: 5.6.1 Distortion in SI Fibers 2 2 1 1 ) ( n c n n n L- = This result applies to the highly multimode SI fibers (V > 10). Joseph C. Palais 5.6 4 Substitute c n L = 1 (5.15) 1 2 1 n n n- = ) ( 1 2 1 = n n c n L 2 1 If n n 2245 Then Distortion in SI Fibers Joseph C. Palais 5.6 5 Example: n 1 = 1.48, n 2 = 1.46 67 / ns km L = Distortion in SI Fibers 0135 . 48 . 1 46 . 1 48 . 1 =- = 24 . 46 . 1 48 . 1 2 2 =- = NA 11 8 (1.48)(0.0135) 6.7 10 / 3 10 s m L - = = & Joseph C. Palais 5.6 6 We will see that this is much greater than material and waveguide dispersion in typical fibers. Distortion in SI Fibers Joseph C. Palais 5.6 7 Note that modal distortion does not depend on source wavelength or spectral width. The total pulse spread is: dispersion material and waveguide dispersive pulse spread = Distortion in SI Fibers dispersion 2 modal 2 ) ( ) ( + = modal modal pulse spread = Joseph C. Palais 5.6 8 Distortion in SI Fibers Now ns 9 . 19 ) 2 . 2 ( ) 20 ( ) ( 2 2 modal =- = Example: Consider fiber length L = 1 km, = 20 ns (measured spread), = 0.82 m LED, = 20 nm. We previously found (in Chapter 3) that ( ) material = 2.2 ns. Compute the modal pulse spread assuming that the waveguide dispersion is negligible. Conclusion: The modal spread dominates. Joseph C. Palais 5.6 9 Waveguide Dispersion From (4.24) source spectral width = Distortion in SI Fibers - = - = g eff M n c L Joseph C. Palais 5.6 10 +- = ) ( g disp M M L Distortion in SI Fibers The total dispersive pulse spread is: Joseph C. Palais 5.6 11 Distortion in SI Fibers 1.3 1.55 0.82 110-20 M (ps/nm . km) ( m) Material Dispersion for Pure Silica Joseph C. Palais 5.6 12 km nm ps M g 0.6 1.3 1.8 5 ( m) 3 1 Distortion in SI Fibers Waveguide Dispersion in a Step-Index Fiber Joseph C. Palais 5.6 13 Near 0.8 m, the material dispersion is on the order of 100 ps/nm km, which is much more than the waveguide dispersion (on the order of 2 ps/nm km)....
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Ch5Sec6 - THE FIBER FORUM Fiber Optic Communications JOSEPH...

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