EE 230 Optical Fiber Communication Lecture 6EE 230光纤通信讲座.pptVIP

EE 230: Optical Fiber Communication Lecture 6 Polarization In molecules, P=μ+αE+βE2+γE3+… In materials, P=X(o)+X(1)E+X(2)E2+X(3)E3+… If multiple electric fields are applied, every possible cross term is generated. At sufficiently high values of E, quadratic or higher terms become important and nonlinear effects are induced in the fiber. Polarization Non-linear Polarization Nonlinear Effects Stimulated Raman scattering Stimulated Brillouin scattering Four-wave Mixing Self-phase Modulation Cross-phase Modulation Index of Refraction Imaginary part of index: absorption For a sample of absorbance A and thickness d, the imaginary part of the refractive index is equal to Index of Refraction vs Wavelength Nonlinear index of refraction Real part of index is best described as a power series n=n1+n2(P/Aeff) Term in parentheses is the intensity. For silica fiber, n2?2.6x10-11 μm2/mW Interaction Length where α (in cm-1) is the loss coefficient of the fiber. 0.1 dB/km=2.3x10-7 cm-1. Nonlinear parameter Propagation constant is power-dependent Propagation in Single Mode Fiber Effective Length and effective area Single Mode Gaussian Approximation Gaussian Pulse Mode Field Diameter Mitigation If P is high in a fiber application, the nonlinear component of the index is minimized by increasing the effective area of the fiber. Fiber designed for this purpose is called LEAF fiber (Large Effective Area). Phase modulation Self-modulation: φNL= γPLeff Cross-modulation: φNL= 2γPotherLeff Effect of these phase changes is a frequency chirp (frequency changes during pulse), broadening pulse and reducing bit rate-length product Self Phase Modulation Pulse Spreading due to Self Phase Modulation Gaussian Pulse in a Kerr Medium Solitons Nonlinear scattering Signal photon scatters off oscillation that is present in the material, gains or loses frequency equivalent to that of the material oscillation At high powers, beating of signal frequency and scattered frequency generates fr