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ece228 lecture 7

ece228 lecture 7 - Bowers ECE 228A Polarization and Raman...

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Bowers ECE 228A Polarization and Raman Lecture #7 John Bowers ECE 228A R d Ch t 2 Al A l N li Fib O ti ( Read Chapter 2. Also, Agrawal: Nonlinear Fiber Optics (on reserve)
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Bowers ECE 228A Dispersion Management We’ve discussed Material dispersion Waveguide dispersion Modal dispersion Modal dispersion is solved by using single mode fiber. The fiber dispersion (material plus waveguide) is solved by dispersion management: Use low dispersion fiber and compensate at the end for zero total dispersion. Net dispersion Distance ECE 228A This can be repeated many times over a long distance. The final compensation amount differs for each wavelength.
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Bowers ECE 228A Origin of Dispersion and Nonlinearities The refractive index of the fiber depends upon both the frequency and the power of the signal n ( Z , P ) Chromatic Nonlinearities (1)S lf h M d l ti (SPM) Dispersion (1)Self-phase Modulation (SPM) (2)Cross-phase Modulation (XPM) (3)Four-wave Mixing (FWM) (4)Stimulated scattering
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Bowers ECE 228A Effect of Dispersion * FWM and CPM require phase matching of two co-propagating channels. *Dispersion destroys phase matching!!! Dispersion destroys phase matching!!! D = 0 channels propagate together D = 0 channels propagate together. D ő 0 channels walk-off and don t i t t interact.
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Bowers ECE 228A Four Wave Mixing and Dispersion Z Z ± Z ² Z Z ± Z ² ²Z ² ³Z ± ²Z ± ³Z ² DSF D ~ 0 Into Fiber Out of Fiber DSF D 0 Degradation on Bit Stream (Equal Channel Spacing) Degradation on Optical Spectrum (Unequal Channel Spacing) D = -0.2 (ps/nm/km) -10 -20 dBm) D = -1 (ps/nm/km) D = -2 -30 -40 Power (d 16 ns 21 ns 26 ns (ps/nm/km) 1542 1544 1546 1548 Wavelength (nm) D. Marcuse, JLT, 1994 USC Optical Laboratory Communications K.M.F. ofcshortcourse.kmf.dispersion.3
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Bowers ECE 228A Dispersion Management Concept D z 0 D z 0 D total = 0 Chromatic dispersion time time SMF D = 0 XPM and time time time DCF SMF Effects are minimized FWM time time DSF USC Optical Laboratory Communications
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Bowers ECE 228A Methods for Dispersion Compensation Linearly-Chirped Fiber Bragg Grating Dispersion Compensating Fiber Incident D 17 ps/(nm.km) O 1 O 3 O ± O ² O ´ O O µ -80 ps/(nm.km) Reflected No obvious method to tune dispersion USC Optical Communications
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Bowers ECE 228A Fiber Bragg Grating: Concept and Configuration T UV Light Photosensitive fiber Interference Pattern Chirped FBG Uniform FBG on n on O Reflection O Transmissio Reflection Transmissio O ² O ´ O 1 O 1 O ² O ´ Incident R fl d T O O T Chirped Pitch Reflected Uniform Pitch USC Optical Laboratory Communications K M F
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Bowers ECE 228A A Typical Chirped Fiber Bragg Grating dB) ctivity (d ay (ps) d Reflec ime Dela Slope (ps/nm) = rmalized Ti dispersion Wavelength (nm) Nor USC Optical Laboratory Communications K.M.F.
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