B. Costantini

Collisions between type II two-dimensional quadratic solitons.

We report experimental and numerical results that describe collisions between two-dimensional type II quadratic solitons excited in a KTP crystal by fundamental waves of orthogonal polarization. Our results provide experimental evidence of the possibility of both inelastic collision (when two quadratic solitons merge at input into a single soliton at output) and quasi-elastic collision.

Polarization-multiplexed χ (2) solitary-wave interactions

We investigate interactions between vectorial solitary waves through typeII second-harmonic generation. We demonstrate that one can tune the collision distance of two initially codirectional solitary waves changing their polarization state. Moreover, by adiabatic perturbation analysis, it is possible to predict such a collision distance as a function of solitary-wave separation, in good agreement with numerical simulations.

Threshold energy of vectorial spatial solitary waves in non-linear quadratic crystals

Excitation and propagation of spatial solitary waves due to three-wave mixing in quadratic non-linear media lead to a rich variety of interesting phenomena some of which have already experimental evidence. Analytical and numerical modelling plays a key role in gaining insight in the experimental data, in completing the theoretical understanding of the process and in predicting new possible applications. In this context, we present an extensive analytical and numerical study on the multi-parameter family of vectorial spatial solitary waves in bulk quadratic crystals. We focus our attention particularly on the threshold energy necessary to drive solitary waves on available crystals exhibiting a second order non-linearity and in describing how this threshold depends on the crystal parameters.

Space and/or polarization diversity multiplexing with type II second-harmonic generation

We describe the interplay among solitary waves steering and polarization switching in a KTiOPO/sub 4/ crystal with type II second-harmonic generation in the presence of walkoff. We show that this nonlinear interaction could be used in addressing a space and/or polarization diversity device, driven by the input polarization state.

Variational analysis of nonlinear channel waveguides

The dynamics of a two-dimensional optical beam propagating in a nonlinear channel waveguide is investigated by means of a variational technique. Through this analysis, one can determine the eigenmodes (self-trapped modes) of the structure. Their stability has also been checked numerically. >

Interactions of Type II vectorial spatial solitary waves in materials with quadratic non-linearity

We present analytical, numerical and experimental results of the interactions between Type II vectorial spatial solitary waves (QSSW) in second-order non-linear media. We show that the outcome of the interaction can be changed from beam coalescence to steering, crossing or repulsion through a non-coherent control (i.e. the control does not rely on the relative phase between the interacting QSSW). We also provide experimental evidence that both quasi-elastic and inelastic collision between QSSW are feasible in second order non-linear media, depending on the relative velocity of the interacting QSSW. We determine (analytically, numerically and experimentally) the critical transverse velocity for elastic collision to be observed, when the fundamental frequency component of the two interacting beams are orthogonally polarized at the input face of the non-linear crystal.

Generation and interactions of two-dimensional vectorial second-order spatial solitary waves

We review recent experimental, theoretical and numerical results dealing with the generation and the interaction of solitary waves in second-order nonlinear crystals. Particular emphasis is devoted to the issues related to the collisions between two-dimensional type II second-order solitons of orthogonal polarization in a KTP crystal. We report the experimental evidence that both quasi elastic and inelastic collision (when two solitons at input merge into a single one at output) are feasible, depending on the relative transverse velocity of the interacting beams.

A wavelength-selective directional coupler based on a boundary-condition-modulation grating: proposal, theory, and preliminary results

A nonconventional wavelength-selective grating-assisted vertical coupler is proposed and analysed. Its novelty consists in the grating position: neither between nor above the waveguide cores, but in a lateral region. Analytic and numerical results indicate that, for devices that are not yet optimized, frequency selectivity and/or device length are at least comparable with those of conventional grating-assisted filter-couplers. Potential advantages of the new device, in terms of performance and of manufacturing technology, are discussed.

Second-order perturbation theory of rectangular waveguides and directional couplers

Perturbation theory is applied to rectangular-core coupled waveguides, whose cladding can be homogeneous or not. Second-order corrections are evaluated for the phase constants of the guided modes and for the coupling coefficient. This requires the full spectrum of radiation modes, which are found within the same approximation as Marcatilis guided modes. It is found that the second-order terms are negligible in most cases of practical interest. However, there are cases for which these terms are so large that the usual first-order theory collapses.

SELF-TRAPPED, STABLE SOLUTIONS IN NONLINEAR CHANNEL WAVEGUIDES

It is well known that a 2-D optical beam propagating along a given direction (say z) in a homogeneous nonlinear medium (HNLM) will either spread out or collapse within a finite distance, depending on whether its power is below or above a critical value, PC [1] . The next question then is whether or not it is possible to obtain stable, stationary (i.e., z-independent) beams by adding a linearly guiding structure, like for example a channel-waveguide. The linear refractive index has to decrease monotonically as a function of distance from the z axis.