Light guided by dark spatial solitons

Abstract

A finite width pulse in tim e or space always broadens as it propagates in a linear medium due to dispersion or diffraction respectively. In a nonlinear medium, however, characterised by an intensity dependent refractive index, dispersion or diffraction can be exactly balanced by the nonlinear response resulting in stable pulse or beam propagation and the formation of optical solitons. A large number of optical solitons exist including dark or bright pulses in time; dark and bright soliton beams in two or three spatial dimensions; and solitons in tim e and space (light bullets). Dark spatial solitons are the main topic of this thesis. All spatial solitons can be thought of as self-guided waves in the nonlinear medium since the interaction between the light fields and the material results in the soliton ” writing” an axially uniform waveguide in that medium. Great interest in spatial solitons and their interactions has grown up in the past few years since there are potential applications in all-optical switching for optical communications systems. This thesis reports investigations of spatial solitons and their ability to “write” waveguides that can be used to guide other weak information carrying probe beams via cross-phase modulation. The main focus of interest is dark spatial solitons and it is shown that these may be used to write various structured waveguides in the medium. For example a pair of dark (or bright) spatial solitons can be used to write a waveguide x-junction and the properties of the junction as a function of the wavelength of the probe beam are studied. Steerable waveguides can be formed by utilising the sensitivity of the direction of a dark spatial soliton to the soliton phase or to the relative intensity of the beam on either side of the soliton. High nonlinearity is required to generate solitons and materials with suitably large nonlinearity may also suffer from saturation or nonlinear absorption. Such perturbations will inevitably be encountered in practice when waveguide devices are formed from dark spatial solitons and the effects of these perturbations have been studied. As examples two-photon absorption and saturation of the nonlinearity as well as the effects of the finite width of the background beam have all been analysed. It has been found that dark spatial solitons, and hence the waveguides induced by them , are rather robust in the face of these perturbations. Several dark soliton induced waveguide structures including adiabatic tapers, waveguide y-junctions and steerable waveguides have been studied experimentally by generating dark spatial solitons in a thermally nonlinear medium. Since in ultrafast nonlinear media the effects of four wave mixing on the propagating beams must be taken into account, a discussion of its role in guiding light by light using spatial solitons is included. The work on four wave mixing is expanded to include a discussion of a way to extend phase matching for the four wave mixing process in fiber couplers or soliton induced couplers.