Pierre Aschieri

Soft proton exchange on periodically poled LiNbO3: A simple waveguide fabrication process for highly efficient nonlinear interactions

We report a simple fabrication process for realizing waveguides on periodically poled lithium niobate which preserves both the nonlinearity and the domain inversion. This so-called soft proton exchange has been used to generate highly efficient optical parametric fluorescence in the 1.48–2.01 μm region using a pump around 830 nm. The measured normalized efficiency is 130% W−1 cm−2 for an effective interaction length of 1.3 cm. This experimental figure is very close to the maximum theoretically predicted value of 140% W−1 cm−2.

Quasi-phase-matched parametric interactions in proton-exchanged lithium niobate waveguides

We present a review of parametric fluorescence with bulk and guided geometries in quasi-phase matched lithium niobate. Whereas bulk experiments have yielded results close to theoretical predictions, waveguided versions have shown strongly reduced efficiencies. Attributing the observed conversion efficiency reductions to a loss of the material nonlinearity, to a destruction of the inverted domains during the waveguide fabrication, or to both, we carefully studied the influence of the proton-exchange process on the nonlinear and structural properties of the periodically poled lithium niobate. We found that an annealed proton-exchange process can essentially conserve the nonlinearity but will erase the periodic domain structure. This erasure can be avoided by use of a highly diluted proton-exchange melt. This direct proton-exchange process perfectly preserves all the nonlinear optical and structural properties of periodically poled LiNbO3.

Modeling and experimental observation of parametric fluorescence in periodically poled lithium niobate waveguides

We present a theoretical and experimental study of parametric generation in a guided wave quasi-phase-matched configuration. A numerical model taking into account inverted domain shape, electrooptic and photorefractive effects, as well as waveguide profiles allows calculation of the observed phase matching curves. The observed parametric fluorescence efficiencies can be explained by assuming a slight reduction of nonlinear coefficient and/or a partial degradation of the nonlinear grating. Optimization of the grating depth profile would permit room temperature parametric oscillation using pump wavelengths and powers attainable with diode lasers. >

Soft-Proton-Exchange Tapers for Low Insertion-Loss

We present the results of an experimental study of waveguide tapers realized in LiNbO3 using the soft-proton-exchange process and the waveguide segmentation technique. Measurements show that they can be favorably introduced between the nonlinear or electrooptic part of the components, which require strong mode confinement to increase efficiency, and the coupling section with a standard optical fiber, which requires low mode confinement in order to have low coupling losses. These tapers allow almost lossless mode-shape transformation. Preliminary results show that they allow reducing the coupling losses by up to 0.7 dB, thus proving the practical interest of the approach.

\hbox{LiNbO}_{3}

One of the unique features of mirrorless optical parametric oscillators based on counterpropagating three-wave interactions is the narrow spectral width of the wave generated in the backward direct ...

Devices

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.

Temporal coherence in mirrorless optical parametric oscillators

We consider the long-term evolution of a random nonlinear wave that propagates in a multimode optical waveguide. The optical wave exhibits a thermalization process characterized by an irreversible evolution toward an equilibrium state. The tails of the equilibrium distribution satisfy the property of energy equipartition among the modes of the waveguide. As a consequence of this thermalization, the optical field undergoes a process of classical wave condensation, which is characterized by a macroscopic occupation of the fundamental mode of the waveguide. Considering the nonlinear Schr¨ odinger equation with a confining potential, we formulate a wave turbulence description of the random wave into the basis of the eigenmodes of the waveguide. The condensate amplitudeiscalculatedanalyticallyasafunctionofthewaveenergy,anditisfoundinquantitativeagreementwith the numerical simulations. The analysis reveals that the waveguide configuration introduces an effective physical frequency cutoff, which regularizes the ultraviolet catastrophe inherent to the ensemble of classical nonlinear waves. The numerical simulations have been performed in the framework of a readily accessible nonlinear fiber optics experiment.

High performance mode adapters based on segmented SPE:LiNbO3 waveguides

We have studied the effect of various proton exchange (PE) processes on ferroelectric domain structure in periodically poled lithium tantalate using a selective chemical etching technique. We show that proton exchange carried out using the highly diluted melt can produce waveguides in periodically poled lithium tantalate without any erasure of domain structure.

Condensation and thermalization of classsical optical waves in a waveguide

This letter presents a modeling technique for quasiphase-matched parametric processes in segmented channel waveguides and the results of measurements of infrared parametric fluorescence in segmented KTiOPO4 channel waveguides. We demonstrate both theoretically and experimentally a strong dependence of optical parametric oscillator tuning curves and parametric conversion efficiency (parametric gain) on the segmentation duty cycle. We show that the optimum conversion efficiency for parametric interactions in segmented waveguides is obtained for duty cycles greater than 50%. Our results validate the use of an effective continuous waveguide model for segmented channel waveguide calculations over a wide range of wavelengths and duty cycles.

Effect of proton exchange on periodically poled ferroelectric domains in lithium tantalate

In this article it is shown that for specific initial conditions an input beam injected in a multimode periodic segmented waveguide does not diffract and remains collimated all along the waveguide, whereas a speckle-like pattern is expected at the output of a multimode structure. This nonintuitive behavior can be explained with the help of ray and wave chaos properties. A modal analysis developed in this article reveals that this nondiffractive beam regime is due to a specific superposition of modes with regularly spaced propagation constants. A discrepancy with the commonly used equivalent continuous waveguide model is also identified.