EE 230: Optical Fiber Communication Lecture 13 Dispersion Compensation From the movie Warriors of the Net

Pulse Dispersion

Definition of chirp The chirp C is defined by the change in frequency d due to the rate of change of the phase:  is the initial 1/e duration of the pulse

Spread of Gaussian Pulse

Dispersion Power Penalty at different Bit Rates

Degradation of a 40 Gb/s Signal

Ideal Dispersion Compensation Device Large negative dispersion coefficient Low attenuation Minimal nonlinear contributions Wide bandwidth Corrects dispersion slope as well Minimal ripple Polarization independent Manufacturable

Various Dispersion Compensation Techniques

Propagation of Gaussian Pulses Input Pulse Output Pulse chirped and broadened b2<0 for standard single mode silica fiber and Ld ~ 1800 km at 2.5 Gb/s and ~115 km at 10 Gb/s Input Pulse Already Positively Chirped After some distance the chirp is removed and the pulse assumes its minimum possible width Upon further propagation the pulse will continue to broaden and acquire chirp. Optical Networks a Practical Perspective-Ramaswami and Sivarajan

Spectral Shaping at the Transmitter Optical Fiber Telecommunications IIIA

Compensation at Receiver Adjust decision point on the fly based on previous few bits Mathematically extrapolate signal back to what it presumably was at origin These techniques can be used only if calculations can be done much faster than bit rate

Dispersion Properties of Various Fibers

Chromatic Dispersion Properties of Various Fibers

Conventional Dispersion Compensating Fiber Fiber Optic Communications Technology- Mynbaev & Scheiner

Dispersion Compensating Fiber

Use of Dispersion Compensating Fiber Understanding Fiber Optics-Hecht

Problem with Conventional Dispersion Shifted Fiber

Importance of Slope Matching

Link Distance Dependence on Slope Matching

Higher order Mode DispersionProperties LaserComm

High-Order-Mode Dispersion Compensation Device

Compensation with Optical Filters

Chirped fiber Bragg grating dispersion where  is the difference between Bragg wavelengths at ends of grating. For n=1.45 and =0.2 nm, D=4.8x107 ps/(km-nm) as compared to 18 for fiber

Chirped Fiber Bragg Gratings Optical Networks A Practical Perspective-Ramaswami & Sivarajan

Pulse Spreading due to Self Phase Modulation

Four-wave Mixing

Taylor Series expansion of β(ω) Through the cubic term: where

Importance of Taylor Series terms Group velocity Vg, dispersion D, and dispersion slope S

Four-Wave Mixing Phase-Matching Requirement Phase mismatch M needs to be small for FWM to occur significantly

Spectral Inversion Add pump signal whose wavelength is ideally at zero-dispersion point Four-wave mixing generates phase conjugate signal at 2p-s Phase conjugate undoes both GVD and SPM over second half of link Filter out pump beam at end

Mid-Span Spectral Inversion Optical Fiber Telecommunications IIIA

Dispersion Managed Network

Summary of Techniques At transmitter: prechirping, coding At receiver: signal analysis, decision point adjustment Fiber: DCF, DSF, dual-mode fiber Filters: Bragg gratings, Mach-Zehnders Spectral inversion