Proton-exchanged Lithium Niobate Waveguides Research Articles

Reduction of lattice defects in proton-exchanged lithium niobate waveguides

It is shown that the formation of water at the surface of protonated LiNbO3 can lead to the precipitation of disordered phases, causing undesirable degradation of the electro-optic properties of the material. These structural defects are observed to increase the conductivity, which leads to a slow build-up of space charges, believed to be a key reason for voltage bias drifts in LiNbO3 electro-optic devices. Annealing under a controlled partial pressure of water vapor is shown to minimize such damage to the crystal lattice. An additional reverse exchange process is proposed that further improves the stability of the crystal and its electro-optic properties.

A Novel Configuration for Phase-Matched Second-Harmonic Generation in LiNbO$_3$ Waveguides

We present a feasibility study of a novel technique for continuously phase-matched second-harmonic generation in proton-exchanged lithium niobate waveguides. We demonstrate that a periodic variation of the nonlinear coefficient along the transverse direction with respect to the propagation one permits for efficient conversions. Comparison with the conventional first-order quasi-phase-matching interactions are then presented in front of unavoidable imperfections of real devices whenever Czochralski off-center technique is exploited in the fabrication process

Surface Hexagonally Poled Lithium Niobate Waveguides

We report on surface hexagonally poled lithium niobate for two-dimensional non-linear interactions in optical waveguide structures. A method for surface inversion of ferroelectric domains has been applied for the fabrication of a hexagonal two-dimensional periodic domain structure with a period of 7.6 μm on an annealed proton exchanged lithium niobate waveguide. This periodic pattern is suitable for quasi-phase-matched second harmonic generation at the fundamental wavelength of 1.06 μm by means of the first order (G10) reciprocal lattice vector.

Surface Domain Inversion in Ferroelectric Lithium Niobate

Periodic inversion is reported for ferroelectric domains near the surface of z-cut lithium niobate crystals by a modified electric field poling technique. The depth of the inverted domain region extends to values of order a few μm (5–10 μm) below the surface of the crystal, thereby removing the high aspect ratio instability problems associated with bulk poling, and therefore allowing the fabrication of fine period ferroelectric domain structures. Using this method periods as short as 1 μm have been achieved. Such periodic domain distributions have been fabricated in Ti-indiffused and proton exchanged lithium niobate waveguides and our first quasi-phase-matching results are shown.

Structural and optical characterization for vapor-phase proton exchanged lithium niobate waveguides

We have studied the crystalline phase transition for vapor-phase proton exchanged (VPE) lithium niobate waveguide by using pure benzoic acid. The VPE layers in the z-cut lithium niobate crystal have been characterized by X-ray rocking curves, OH− bond infrared absorption, and prism coupling measurements. The crystalline quality assessment by the X-ray rocking curve measurement shows desirable κ2 phase formation for nonlinear optical waveguide applications. It has been observed that the crystalline phase makes a transition from the κ2 to the β phase with the increase in the vapor-phase exchange time. The prism coupling measurement also showed a step-like index profile for our VPE samples. From the step index profile, we deduced an effective diffusion coefficient of 0.14μm2h−1, which is twice than that of the previously reported value. Shift in the OH− bond absorption wavelength with crystalline phase change and the increase in absorption with the exchange time were also observed from our spectrophotometer measurement.

Efficient frequency doubling in reverse proton exchanged lithium niobate waveguides

We propose a novel approach for efficient frequency doubling of near infrared light, using the coupled system of buried and surface waveguides obtainable by reverse proton exchange in z-cut lithium niobate. In such guides, supporting TM and TE polarizations, respectively, in spite of the use of the d/sub 31/ nonlinear element, we predict conversion efficiencies as high as 90 or 14% μm/W cm in planar structures excited at 1.32 μm.

Second harmonic generation in reverse proton exchanged Lithium Niobate waveguides

We investigate efficient second harmonic generation in reverse proton exchanged Lithium Niobate waveguides. In z-cut crystals, the resulting buried and surface guides support TM and TE polarizations, respectively, and are coupled through the d 31 nonlinear element. Numerically estimated conversion efficiencies in planar structures operating at 1.32microm reach 90% in 2cm or a normalized 14% microm/Wcm.

The SHG-response of different phases in proton exchanged lithium niobate waveguides

Reflection second-harmonic generation from the polished waveguide end face is used to investigate the second-order nonlinear optical properties of as-exchanged and annealed proton-exchanged (PE) waveguides in different H/sub x/Li/sub 1-x/NbO/sub 3/ phases. A detailed correlation is done between the nonlinear properties, the processing conditions, the refractive index changes, and the optical losses of the waveguides. It is found that for the direct PE samples, where the /spl beta//sub 4/, /spl beta//sub 3/, and /spl beta//sub 1/ phases are generated at the surface, the nonlinearity in the guide is strongly reduced by more than 85% of its bulk value, while for waveguides prepared in the /spl beta//sub 2/ phase, the nonlinear coefficient is about 55% of the bulk one. A consequence is that the step-like /spl beta//sub i/-phase PE LiNbO/sub 3/ waveguides with large refractive index increase are advantageous for efficient SHG in Cherenkov configuration. The nonlinearity, strongly reduced after the initial proton exchange, is found to be restored and even increased after annealing. However, this apparent increase of the nonlinearity is accompanied by a strong degradation of the quality of the second-harmonic generation reflected beam in the region of initial waveguides due to beam scattering. The graded proton exchange technique and dilute melt proton exchange have been shown to produce high-quality waveguides with essentially undergraded nonlinear optical properties. It has been also shown that the nonlinear properties of annealed proton exchanged LiNbO/sub 3/ waveguides can be effectively recovered by the reverse proton exchange technique. The results obtained are important for the design, fabrication, and optimizing of guided-wave nonlinear optical devices in LiNbO/sub 3/.

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.

Biaxial characteristics of planar proton exchanged lithium niobate waveguides

Results of a theoretical and experimental study on proton exchanged lithium niobate (LiNbO/sub 3/) planar waveguides are presented. Experimental results are successfully explained by the theoretical model only if the guiding layer is assumed to be anisotropic with biaxial characteristics, this biaxiality being induced by the proton exchange process in the uniaxial lithium niobate substrate.

Characteristics of hybrid modes in proton-exchanged lithium niobate waveguides

A rigorous numerical model, verified by experimental results, gives an explanation of the particular electromagnetic behaviors observed in x-cut proton-exchanged lithium niobate waveguides. This approach, which allows an exact calculation of the weights of the coupled ordinary and extraordinarywaves that make up the hybrid modes, provides deeper insight into the study of the strains induced by the proton-exchange process in the waveguide itself, showing that the optical axis of the exchanged layer is not parallel to the waveguide plane.

Fully planar proton-exchanged lithium niobate waveguides with grating taps

A two-step proton-exchange process to produce waveguides and gratings in lithium niobate is described. In both steps, mixtures of lithium benzoate and benzoic acid are used. The fabricated components are characterized at 0.633 and 0.422 µm. In particular, we observe that the light intensity diffracted into the air by the grating is ~0.5% of that in the waveguide.

Quantitatively measured photorefractive sensitivity of proton-exchanged lithium niobate, proton-exchanged magnesium oxide-doped lithium niobate, and ion-exchanged potassium titanyl phosphate waveguides

The photorefractive sensitivities of proton-exchanged lithium niobate waveguides and Rb-ion-exchanged potassium titanyl phosphate waveguides are quantitatively measured, and their influence on waveguide applications is estimated.

Characterization of sulfuric acid proton-exchanged lithium niobate

A series of bulk proton-exchanged lithium niobate waveguides has been prepared by using concentrated sulfuric acid as the proton source and varying the time and temperature of the exchange process. The optical and physical properties of the proton-exchanged lithium niobate layer have been measured and found to be essentially identical to proton-exchanged layers obtained by other workers using the benzoic acid process. Sulfuric acid offers a number of advantages over benzoic acid as a proton source, including a wider temperature liquid range and a greater ease of handling in a clean-room environment.

Lithium niobate waveguides and their susceptibility to optical damage

Abstract The laser power-handling characteristics of diffused lithium niobate waveguides are reviewed in terms of laser-power-coupled, transmission length, crystal orientation, and method of preparation. Laser-induced optical damage is recorded for the first time in the new proton-exchanged lithium niobate waveguides. Optical damage is not observed, however, for propagation along the z-axis of titanium-in-diffused waveguides.