Measurement of the nonlinear refractive index (n2) and stimulated Raman scattering in optical fibers as a function of germania content, using the photorefractive beam coupling technique
Federated Physics Department of NJIT and Rutgers-Newark
Doctor of Philosophy
Johnson, Anthony M.
Federici, John Francis
Murnick, Daniel Ely
Thomas, Gordon A.
DiGiovanni, David J.
Nonlinear index of refraction
Stimulated raman scattering
Germania doped fiber
Erbium doped fiber
One of the greatest challenges in optical communication is the understanding and control of optical fiber nonlinearities. While these nonlinearites limit the power handling capacity of optical fibers and can cause noise, signal distortion and cross talk in optically amplified transmission systems, they have been equally harnessed for the development of new generations of optical amplifiers and tunable laser sources. The two prominent parameters that characterize the nonlinear properties of an optical fiber are the nonlinear refractive index n2 and the Raman gain coefficient gR. These parameters are related to the third order nonlinear susceptibility [x(3)].
In this work, the photorefractive beam coupling technique, also called induced grating autocorrelation (IGA), has been used to measure the nonlinear refractive index (n2) and the Raman gain coefficient (gR) of short lengths (z ~ 20 m) of optical fibers. In the IGA experiment, a transform limited Gaussian pulse is propagated through a short length of an optical fiber, where it undergoes self-phase modulation (SPM) and other nonlinear distortions, and the output pulse is split into two. The two-excitation pulses are then coupled into a photorefractive crystal, where they interfere and form a photorefractive phase grating. The IGA response is determined by delaying one beam (the probe) and plotting the diffracted intensity of the probe versus the relative delay (τ).Analysis of the IGA response yields information about the nonlinear phase distortions and other calibration parameters of the fiber. Using the IGA technique the author has measured the nonlinear refractive index in several types of fibers, including pure silica, Er-Al-Ge doped fibers, DCF (dispersion compensating fiber) and the recently developed TrueWave Rs fiber, and investigated the dependence of n2 on the doping profiles of Er, Al, and Ge in optical fibers.
The standard IGA model for n2 measurements was derived from the solution of the nonlinear wave equation for pulse propagation in the limit of pure self-phase modulation. This model assumed that GVD (group velocity dispersion) and other nonlinear processes such as SRS (stimulated Raman scattering) are negligible. This model has been successfully used to fit the experimental data and determine the n2 of the fiber from the time dependent phase shift. However, SRS has been observed to distort the IGA trace, thus leading to a breakdown of the standard IGA model. A new IGA model has been developed in this study from the solution of the coupled-amplitude nonlinear Schrodinger equation, using both analytical and numerical approaches. This new model successfully accounts for the SRS effects on the IGA trace, in the limit of zero GVD, and allows the direct determination of the Raman gain coefficient from the fit of the SRSdistorted IGA trace. The measured nonlinear refractive index and Raman gain coefficients are in good agreement with published results. It was also shown that in the limit of zero GVD and no Raman, the IGA technique reduces to the widely accepted spectral domain SPM technique pioneered by Stolen and Lin, but is readily applicable to shorter lengths of fiber and is sensitive to smaller phase shifts.
njit-etd2003-114 (214 pages ~ 12,631 KB pdf)
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Created July 1, 2004