Origin of the third-order nonlinear optical response in silica fibers.

摘要

The development of erbium-doped fiber amplifiers, high-bit-rate multi-wavelength communications systems, and the prospect of future soliton communications systems provides motivation to develop a complete understanding of the mechanisms underlying the nonlinear refractive index in optical fibers. In this thesis we present a unified description of the tensor properties of the nonlinear refractive index in silica fibers, including contributions from the nonresonant electronic response, as well as the Raman and electrostrictive responses that originate in interactions of the optical field with vibrations of the material lattice.;We develop a theory to explain the electrostrictive contribution to the nonlinear refractive index and present results of the first experiment designed to directly measure the slow electrostrictive contribution to the nonlinear response. Through our cross-phase modulation experiments we are able to map out the electrostrictive frequency response from 10 MHz to 1 GHz for a variety of fiber types. Experimental results of the electrostrictive frequency response function are in excellent agreement without theory of this response. Our results indicate that the magnitude of the electrostrictive nonlinearity is fiber-type dependent, but is approximately equal in magnitude to the fast Kerr nonlinearity.;The Raman response is essentially instantaneous for all but sub-picosecond optical pulses. In addition to describing the Raman contribution to the nonlinear refractive index, we experimentally demonstrate a unique influence of Raman gain on the propagation of ultrahigh-bit-rate (177 Gbit/s) soliton trains: the intra-pulse Raman scattering causes a gradual frequency shift of the soliton train, and this frequency shift also leads to a migration in the phase between successive solitons in the train. We show how this Raman-induced phase migration limits long distance transmission of coherent soliton trains, while demonstrating successful transmission of ultrahigh-bit-rate trains over a distance greater than 120 soliton periods.;We conclude the thesis with a summary of the tensor properties of the nonlinear refractive index that includes all three physical mechanisms.