Proton-exchanged LiNbO3 Waveguides Research Articles

Subsurface disorder and electro-optical properties of proton-exchanged LiNbO3 waveguides produced by different techniques

It has been established, that proton-exchanged LiNbO3 waveguides have a marked subsurface layer with structural disorder inducing degradation of electro-optical properties of these waveguides. At the same time, such a subsurface disorder is found to be less pronounced in soft proton-exchanged (SPE) waveguides in comparison with annealed proton-exchanged (APE) ones. The experimental samples of phase modulators fabricated by SPE technique exhibit a better electro-optical efficiency compared to the LiNbO3 modulators produced by the standard and improved APE techniques.

Design of strain-introduced MZI interleaver on LiNbO3 substrate

A strain-introduced Mach-Zehnder interferometer (MZI) interleaver on lithium niobate (LiNbO3) is proposed. The structure of the strain-introduced waveguide is designed in detail, and is produced by depositing a SiO2 film on the annealed proton-exchanged LiNbO3 waveguide. Considering the sensitivities of the edge strain to the deposition temperature and the thickness of the SiO2 film, an optimum design of 50 GHz interleaver on this structure is given through analyzing the effective index changes for Epqx mode by finite difference method (FDM). The length of the bending waveguide in this interleaver is just two thirds of that in the conventional interleaver due to the high refractive index difference.

Photorefractive effect in LiNbO3-based integrated-optical circuits at wavelengths of third telecom window

Original results on investigation of the photorefractive effect in straight channels and integrated-optical circuits such as a directional coupler, Y-splitter and Mach–Zehnder interferometer, exploiting titanium-indiffused and proton-exchanged LiNbO3 waveguides, are presented. It has been found that the photorefractive damage is non-negligible for IR radiation with wavelengths near 1.5 μm in all circuits studied. The new methods for accurate evaluation of small extents of photorefractive effect are proposed.

Light intensity dependence of holographic response and dark decays in α-phase PE:LiNbO3waveguides

The influence of the light intensity on the recording/erasing of photorefractive gratings has been studied in α-phase proton exchanged LiNbO3 waveguides. The saturating refractive index modulation increases in a non-linear way as a function of the recording intensity and reaches a region of erratic values above a certain intensity threshold, which increases with the exchange time used for preparing the guides. Erratic values are attributed to photorefractive self-defocusing of the recording beams, while the thresholding effect is attributed to the sharp index modulation increase with the intensity associated to the intrinsic defect NbLi. It has also been confirmed that the proton exchange time affects the photorefractive response and that the dark conductivity can be disregarded with regard to the photoconductivity in the high intensity range near the intensity threshold.

Optical waveguides in LiNbO3 and stoichiometric LiNbO3 crystals by proton exchange

The formation of optical planar waveguides in LiNbO3 and stoichiometric LiNbO3 crystals by proton exchange was reported. The prism-coupling method was used to characterize the dark-line spectroscopy at the wavelength of 633 and 1539 nm, respectively. The mode optical near-field outputs from proton-exchanged LiNbO3 and SLN waveguides at 633 nm were presented. The mode field from stoichiometric LiNbO3 (SLN) waveguide is lighter and more uniform than that from LiNbO3 waveguide, which means the quality of the waveguide in SLN crystal is better than that of the LiNbO3 waveguide. For proton-exchanged LiNbO3 waveguides, the evolution of the refractive index profile with annealing was presented. The disorder profiles of Nb atoms in proton-exchanged LiNbO3 waveguides were obtained by Rutherford backscattering/channeling technique. It is shown that the longer the exchange time, the larger the displacement of Nb atoms.

Light-intensity measurements in optical waveguides using prism couplers

In view of its central role in many optical waveguide applications, the absolute measurement of the actual light intensity propagating within a planar waveguide has been critically revised. Using prism couplers and proton exchanged LiNbO3 waveguides, several approaches have been experimentally checked and compared. A method is proposed and demonstrated, which combines the measurements of the light power out coupled from the waveguide edge and from the output prism. Reliable power measurements are obtained within a 5% error. A simplified version consists in a previous calibration of the setup in order to measure only the power coming from output coupler. Although inaccuracy rises up to ∼15%, it can be useful in many practical applications because of its simplicity. To assess and compare both methods, they have been applied to the measurement of light-intensity dependences of photovoltaic currents and optical damage thresholds in proton exchanged LiNbO3 guides.

Fabrication and characterizations of proton-exchanged LiNbO3 waveguides fabricated by inductively coupled plasma technique

This Letter reports the use of an inductively coupled plasma technique for fabrication of proton-exchanged (PE) LiNbO3 (LN) waveguides. Planar and stripe waveguides have been formed in Y-cut LN which are difficult to obtain with the conventional molten acid method due to the occurrence of surface damage. Secondary ion mass spectrometry, scanning electron microscopy, and infrared absorption spectrum characterization results revealed that a uniform vertical PE profile with a single low order crystal phase has been directly obtained as a result of this unique process. X-ray photoelectron spectroscopy characterization of the treated surface revealed the existence of NbO as the cause for a sometimes darkened surface and confirms the ability to completely restore the surface to LN by oxygen plasma treatment. Atomic force microscopy measurement confirms that good surface quality has been maintained after regeneration of the surface to LN.

Photorefractive fixing phenomena in alpha-phase proton-exchanged LiNbO3 waveguides

Fixing experiments have been carried out in undoped alpha-phase proton-exchanged LiNbO3 waveguides at room temperature and 110°C. Above room temperature, the recording kinetics of the diffraction efficiency exhibits a maximum eta_max and then decays to zero. Upon raising the temperature, etamax decreases and the associated rise and decay times become progressively shorter. After fixing at 90°C and subsequent optical developing at room temperature, the developed value rises up to eta_dev approximately 2.5 eta_max, showing a lifetime of a few hours in the dark and a few days under homogeneous illumination. Time constants for recording, dark decay, and developing have been measured as a function of temperature. Experimental dependences are reasonably simulated by using the standard model for fixing with the same parameters as with bulk LiNbO3, except the electron thermal ionization rate and the proton concentration, which are much greater in alpha-phase guides.

Temperature effects in proton exchanged LiNbO3 waveguides

Temperature effects on single, homogeneous phases of proton exchanged LiNbO3 waveguides have been studied. The refractive index of α-phase guides on z-cut and x-cut congruent substrates monotonically increases with temperature, the thermo-optical coefficient being similar to that of the substrate. In κ2-phases on z-cut congruent substrates and in β1-phases on z-cut and x-cut congruent and quasi-stoichiometric substrates, the thermo-optical coefficient is smaller than that of the substrate below ∼60 °C and higher above this temperature. A rather similar effect has been observed in β1-phases on z-cut and x-cut congruent substrates in the linear expansion coefficient. The microscopic origin of the phase transition is not known for the moment.

Superlinear photovoltaic currents in LiNbO3: analyses under the two-center model

Superlinear photovoltaic currents in LiNbO3 are theoretically studied by using a two active center model, with Fe2+/Fe3+ as primary center and NbLi4+/NbLi5+ as secondary center. Analytical instead of numerical results are provided, including close-form expressions for most common experimental situations. Recent photovoltaic parameters obtained for α-phase proton-exchanged LiNbO3 waveguides (very similar to the substrate) are used for applying the model and comparing with published experimental results. Thoroughly studied aspects are: the redistribution of donor/acceptor states for each species as a function of the light intensity, their contribution to the photovoltaic current density, the effect of the temperature, and the role of the center concentrations and their reduction state. This provides a detailed understanding of the photovoltaic current function shapes versus light intensity, predicts new features of interest for experimentalists and suggests further experiments to better determine the material parameters. Photovoltaic measurements and modeling appear a simpler and safer way of understanding the role of the two-center photovoltaic effect in photorefractive phenomena as well as for determining important photorefractive parameters.

H–Li correlation and stoichiometry of mixed phases in proton-exchanged LiNbO3 waveguides

Elastic recoil detection with a time of flight discrimination method using I127 ions has been applied to the simultaneous determination of the H and Li depth profiles as well as of the stoichiometry of the mixed phases formed by thermal annealing in proton-exchanged LiNbO3 waveguides. For both, as-exchanged and annealed waveguides, a one-to-one local correlation between the H and Li profiles has been obtained so that [H]+[Li]=constant. Moreover, the stoichiometry of the HxLi1−xNbO3 compound, where x is the exchange rate, has been correlated with x-ray diffraction (rocking curves) data in order to identify the generated phases after the exchange and annealing processes. In as-exchanged guides, the value x≃0.85 is mainly associated with a broad β2-phase peak. In annealed guides, a well-defined β2-peak is associated with x≃0.5, a broad κ2-peak is associated with x≃0.40, a broad κ1-peak to x≃0.19 and 0.21, and a α-shoulder to x≃0.09 and 0.12.

Superlinear photovoltaic currents in proton-exchanged LiNbO3 waveguides

Photovoltaic currents along the c axis have been measured in α-phase LiNbO3 proton-exchanged waveguides at several visible wavelengths for a guided-beam configuration. The light-intensity dependence is superlinear and all experimental curves are very well fitted by computer simulations using a two-centre model, with Fe2+/Fe3+ as primary and NbLi4+/NbLi5+ as secondary photovoltaic centres. The superlinear behaviour arises from a much higher effective photovoltaic length of NbLi4+ (small polaron) compared with that of Fe2+. In β1-phase guides, the photocurrents are much smaller than in α-phase guides and apparently do not show superlinear behaviour.

Direct imaging of multimode interference in a channel waveguide.

By use of a near-field scanning optical microscope in collection mode, multimode interference was directly measured in an annealed proton-exchanged LiNbO3 waveguide. Periodic transitions from a single-peaked Gaussianlike intensity distribution to a double-peaked intensity distribution were observed. The intensity distribution along the waveguide was calculated, and the results agree well with the experimental observation.

Dark developing of photorefractive proton-exchanged LiNbO3 waveguides

Abstract Photorefractive measurements have been performed in proton-exchanged (PE) LiNbO3 waveguides as unannealed α-phase layers. Holographic recording and light and dark erasures have been studied. The saturation index change is independent of the light intensity within the studied range 0.3–50 W/cm 2 . The time dependencies of Δn are well represented by the sum of two exponential components. After a complete optical erasure, Δn increases in the dark (dark developing) up to Δn≅3×10−6 and then decays to zero in about 4 h. All experimental results are reasonably well simulated using a model in which the Fe2+/Fe3+ light-induced charge distribution is compensated by a light insensitive species (ionic charges or holes) mobile at room temperature.

Comparison of the electro-optic coefficient r33 in well-defined phases of proton exchanged LiNbO3 waveguides

The electro-optic coefficient, r33, of proton exchanged LiNbO3 waveguides has been measured in well-defined phases of the exchanged layer. Namely, in two low index-jump α-phases, i.e. unannealed soft-exchanged (SPE) and annealed (APE) guides, and in two high index-jump phases, i.e. β1 guides and, for comparison, ordinary proton exchanged (PE) guides (having a mixture of phases). The following values have been obtained (in pm/V): 22.1±0.6 for SPE; 20.9±0.7 for APE; 0.33±0.01 for β1; and 0.76±0.04 for PE. Differences between these values are discussed in terms of the structure of the phase involved.

Dispersion of the ordinary refractive-index change in a proton-exchanged LiNbO3 waveguide

The wavelength dependence of the ordinary refractive-index change of a proton exchanged LiNbO3 waveguide is determined by applying a recently proposed method based on radiation modes. A peculiar behavior is observed since the absolute value of the ordinary index change increases with the wavelength, while the extraordinary one decreases. The Sellmeier curves for the dispersion of both the index changes are provided in the wavelength range from λ=0.514 μm to λ=1.554 μm.

Proton exchanged LiNbO3 optical waveguides: Nonlinear, electrooptical and photorefractive properties

We show that proton exchanged LiNbO3 waveguides exhibit very complex structural chemistry. Seven HxLi1−xNbO3 crystallographic phases have been identified in PE LiNbO3 waveguides. A correlation is done between the electroptical, nonlinear and photorefractive properties, the processing conditions and the refractive index changes of the waveguides.

Second harmonic generation in the strong absorption regime

A detailed theoretical analysis of the second harmonic generation in the strong absorption regime (great absorption index K 2w) is presented. Inside the nonlinear medium, it is found that only the harmonic wave travelling in the same direction as the fundamental one survives. Outside the nonlinear medium, the transmitted and reflected intensities of the harmonic waves show a peculiar dependence on k 2w- The ratio between transmitted and reflected intensities ranges from 5 to 75. Reflected and transmitted harmonic waves appear to be generated in the outer layers within a thickness of a fraction of wavelength. Experimental support is reported with application to a proton-exchanged LiNbO3 waveguide.

Photorefractive charge compensation in α-phase proton-exchanged LiNbO_3 waveguides

Photorefractive recording and light and dark erasures have been measured in unannealed α-phase proton-exchanged LiNbO3 waveguides. The saturation index change, Δns≅9×10-6, is independent of the light intensity within the studied range, 0.3–50 W/cm2. The time dependencies are well represented by the sum of two exponential components. After complete optical erasure, diffraction efficiency η increases in the dark (i.e., dark developing) up to ∼17% of the saturation value ηs≅0.12 and then decays to zero in ∼4 h. All experimental results are reasonably well simulated by a model in which the Fe2+/Fe3+ light-induced charge distribution is compensated for by a light-insensitive species (ionic charges or holes) that is mobile at room temperature.

Effect of SiO2 cladding on properties of annealed proton-exchanged LiNbO3 waveguides

We show SiO2 cladding effects on the optical properties of annealed proton-exchanged LiNbO3 waveguides as well as on the kinetics of annealing process. Annealing with previously deposited SiO2 film reduces the optical losses in annealed proton-exchanged LiNbO3 waveguides. The structural phase diagram of HxLi1−xNbO3 is also modified if annealing has been performed with a previously deposited SiO2 layer. The physical mechanisms, including evaporation of H2O and/or Li2O during annealing without deposited film and, as a result, formation of defects and a lithium deficit phase, are discussed.