Nonlinear mode coupling in optical fibers and VCSELs and some applications to communication systems

Nonlinear phenomena are relatively easy to observe in optical fibers and semiconductor laser cavities. In optical fibers, nonlinear effects can be seen even at low power due to the high intensities in the small fiber cores and long propagation distances possible in low loss fibers. Semiconductor lasers, in particular, vertical cavity surface emitting lasers (VCSELs), have cavities with very high Q-factors, which result in high intra cavity intensities even at low facet powers. In this thesis we will examine how these nonlinear effects are responsible for coupling between the different modes present in the medium and some applications to optical communication systems.; In optical fibers, the nonlinear processes that dominate are self-phase matched processes that automatically satisfy the phase matching condition. These processes include self phase modulation (SPM) and cross phase modulation (XPM). SPM is responsible for the stability and interaction between propagating pulses known as solitons. XPM is responsible for coupling signals with different polarizations or wavelengths propagating in a fiber. In this thesis we have investigated the possibility of controlling the non-linear interaction between solitons, using XPM from another pulse and using this mechanism for pattern recognition in an optical data stream. We demonstrate high-speed (63Gb/s) recognition of 8-bit header words which is a useful function at an add-drop node in an optical network.; Both SPM and XPM can be explained in terms of a nonlinear refractive index—a simplification that is made possible by the extremely fast relaxation times in silica (∼40fs). In semiconductors the carriers exhibit both fast intra band (∼50fs) and spin-flip relaxation (∼2ps), and slow inter band dipole relaxation (∼Ins). Due to this hierarchy of relaxation times, the interaction of light with the semiconductor medium cannot be described by a single effective refractive index and the carrier dynamics have to be accounted for explicitly. Using this detailed description, we have compared the contribution of spin-flip relaxation to the non-linear coupling between the polarization modes near threshold. We show that the detailed structure of the mode spectrum observed when the VCSEL cavity is probed with an injected optical field is accounted for by the slow dipole relaxation times.