Backward optical parametric efficiency in quasi-phase-matched GaN waveguide presenting stitching faults.

We model a backward mirrorless optical parametric oscillator in a fragmented GaN waveguide consisting of a sequence of submicronic periodically poled elements separated by uniformly polarized connection sections representing stitching errors. We find that the generated coherent phase of the backscattered wave locks the phases of the forward propagating waves in such a way that the dynamics is nonintuitivelly as efficient as for a uniform quasi-phase-matched waveguide. The best coherence transfer to the backward wave, obtained for perfect group-velocity matching of the forward propagating waves, requires a nanoscale poled periodicity, which is achieved in GaN through epitaxy.

Highly efficient coupling in lithium niobate photonic wires by the use of a segmented waveguide coupler.

The purpose of this article is to show that efficient light coupling in lithium niobate waveguides presenting a strongly confined mode, such as photonic wires, is possible with the use of a periodically segmented waveguide coupler. The coupler consists in an input periodically segmented waveguide whose mode size is adapted to the mode of a standard single-mode fiber coupled to a photonic wire whose mode size is of the same order of the wavelength. The periodic segmentation of the input waveguide allows fulfilling the phase matching condition necessary to achieve an efficient light transfer between these waveguides. The coupling efficiency is typically 5 times higher than the butt-coupled configuration.

Unexpected light behaviour in periodic segmented waveguides.

In this article, it is shown that multimode periodic segmented waveguides (PSW) are versatile optical systems in which properties of wave chaos can be highlighted. Numerical wave analysis reveals that structures of quantum phase space of PSW are similar to Poincaré sections which display a mixed phase space where stability islands are surrounded by a chaotic sea. Then, unexpected light behavior can occur such as, input gaussian beams do not diverge during the propagation in a highly multimode waveguide.

High performance mode adapters based on segmented SPE:LiNbO3 waveguides.

We propose a new mode adapter which allows more efficient launching of the optical power selectively in the fundamental mode of a multimode waveguide. Theoretical and experimental results confirm that such a mode adapter increases the performances in terms of coupling efficiency, coupling tolerances and transmitted power with respect to previously proposed solutions. Proof of principle of device operation is obtained with a simple Coupled Mode Theory model. Experimental results are obtained at a wavelength of 840 nm in Lithium Niobate Soft Proton Exchanged waveguides and agree very well with theoretical predictions.

Influence of dispersion on the resonant interaction between three incoherent waves.

We study the influence of group-velocity dispersion (or diffraction) on the coherence properties of the parametric three-wave interaction driven from an incoherent pump wave. We show that, under certain conditions, the incoherent pump may efficiently amplify a signal wave with a high degree of coherence, in contrast with the usual kinetic description of the incoherent three-wave interaction. The group-velocity dispersion is shown to be responsible for a spectral filtering process, in which the coherence of the generated signal increases, as the coherence of the pump wave decreases. As a result, the coherence acquired by the signal in the presence of an incoherent pump, is higher than that acquired in the presence of a fully coherent pump. The mechanism underlying this intriguing result is based on the emergence of a mutual coherence between the incoherent pump and the generated idler wave. We calculate explicitly the degree of mutual coherence between the pump and idler waves and show that the two incoherent waves become completely correlated in the full incoherent regime of interaction. The theory is in quantitative agreement with the numerical simulations. To motivate the experimental confirmation of our theory, we characterize the dispersion properties of an actual quadratic nonlinear optical crystal in which the process of signal coherence enhancement induced by pump incoherence may be studied experimentally.

Elastic light scattering with a LiNbO3 waveguide.

We present the measurements of hemispherical elastic scattering of light guided by different modes of a planar LiNbO3 waveguide. It is shown that the fundamental and the lowest-order modes are more sensitive to the scattering properties of the air-core interface, whereas higher-order modes are more sensitive to the optical inhomogeneities of the core-substrate interface. We also demonstrate that because of static polarization of LiNbO3 crystal, the air-core interface is sensitive to the presence of dust particles in air, which causes a change in the scattered light based on time of observation. This sensitivity could be used to elaborate compact sensors of air contamination.