J. M. Cabrera

Understanding light intensity thresholds for catastrophic optical damage in LiNbO 3

The appearance of light intensity thresholds for catastrophic optical damage in LiNbO3 is satisfactorily explained by using a photorefractive model based on the Fe(2+)?Fe(3+) and NbLi(4+)?NbLi(5+) defect pairs. Model simulations of the photorefractive amplification gain as a function of the light intensity present sharp threshold behavior. A similar behavior is shown by the saturating refractive index change. In agreement with experiments, predicted thresholds appear shifted towards higher intensities (up to a 10(4) factor) when the Nb(Li) concentration is decreased or the temperature is increased. The model also explains very recent data on the threshold enhancement with the Fe(2+)/Fe(3+) ratio in optical waveguides.

Preparation of proton-exchange LiNbO 3 waveguides in benzoic acid vapor

Either α- or β-phase LiNbO3 guides of high quality were obtained by proton exchange of x- and z-cut substrates in benzoic acid vapor. The second-harmonic-generation efficiency of z-cut samples was found to be similar to that of the substrate for 0.01⩽Δne⩽0.1.x-cut samples show second-harmonic-generation efficiency that is good for Δne⩽0.03 and negligible for Δne⩾0.03. All cuts exhibit low optical losses ( 0.03.

Optical damage inhibition and thresholding effects in lithium niobate above room temperature

Abstract High-intensity single-beam photorefractive data on LiNbO 3 show the occurrence of a well-defined intensity threshold for enhanced deterioration (fanning) of the output beam. That threshold markedly rises with temperature in the range 20–160°C. Below threshold, the beam profile is only slightly modified during propagation and it remains essentially independent of the light intensity. These results offer relevant information on beam degradation in applications involving high light density powers. The data indicate that the thresholding effect is thermally activated with an activation energy of 0.24 eV within the range of values found for the photoconductivity.

Accurate interferometric measurement of electro-optic coefficients: application to quasi-stoichiometric LiNbO3

Abstract A new modulation interferometric technique for accurate measurement of electro-optic coefficients is presented. The accuracy is achieved by detecting the signal both at first and second harmonics of the modulation frequency, while the phase is electronically scanned along two interference orders. This eliminates the uncertainty encountered on measuring specified intensities and phase differences. The method has been applied to the precise measurement of the unclamped electro-optic coefficients at λ =632.8 nm of quasi-stoichiometric ( r 33 =29.4±0.2 pm/V, r 13 =9.25±0.07 pm/V) and congruent ( r 33 =31.4±0.2 pm/V, r 13 =10.49±0.07 pm/V) LiNbO 3 .

Erasure kinetics and spectral dependence of the photorefractive effect in Fe:LiNbO 3

The erasure kinetics of holographic gratings has been studied as a function of the erasing wavelength in Fe:LiNbO3. Sample self-absorption has been rigorously taken into account and is a relevant point. After such a correction, even those apparently nonexponential decays can be nicely fitted to simple exponential ones. This correction also results in a more reliable spectral dependence of the erasing efficient transitions. The photorefractive writing spectrum has been obtained by the compensator technique. Writing and erasing show different spectral behaviors, which are briefly discussed.

Optical damage in x-cut proton exchanged LiNbO3 planar waveguides

The optical damage of different proton exchanged LiNbO3 planar waveguides has been experimentally studied by measuring the intensity output of a single beam as a function of the intensity input. Parallel measurements of photovoltaic currents (sometimes referred to as photocurrents) have been carried out with the same setup as a function of the input intensity and they have been correlated to the optical damage data. The following proton exchanged phases have been studied and compared with the substrate: α, β1, β2, and reverse proton exchanged (RPE). The greatest intensity thresholds for optical damage, about 2×103 times greater than that of the substrate, have been obtained in RPE guides supporting ordinary polarization and in β1,2 guides which support extraordinary polarization. On the other hand, the lowest photovoltaic currents have been measured in β1,2 phases. As a function of the light intensity, the strong superlinear behavior exhibited by the photovoltaic current in the α phase is almost absent in...

Optical absorption spectra of nickel doped lithium niobate

Abstract Optical absorption spectra of LiNb03: Ni have been obtained in the range 0.6-3.5 eV, between 24°K and room temperature (RT). The dependence on the polarization azimuths of the incident light has been also investigated. The results are qualitatively discussed in terms of the energy level scheme resulting from trigonal symmetry. Fine structure observed at about 1.7 eV is thought to be of vibronic origin, associated to the 3A1 (3 A2-3F) → 3A1 (3T1 – 3F) transition.

Spectral dependence of photorefractive erasure in Bi12GeO20 and Bi12SiO20

A holographic method similar to that previously used with LiNbO3 to study the spectral behavior of photorefractive centers has been applied to Bi12SiO20 (BSO) and Bi12GeO20 (BGO). By erasing with incoherent light a previously written grating, measurements of the grating decay constants as a function of the erasing wavelength have been carried out. The grating erasure exhibits decays made up of a sum of two exponential terms for all wavelengths between 390 and 620 nm. In terms of recent photorefractive models which use two active species, our results imply that there exist two different photorefractive centers in BGO and BSO. By numerically fitting the observed spectral dependencies of the decay constants to the model equations, the absorption bands associated to these centers have been obtained. Their peak positions and half‐widths are the following: 2.7 eV (0.27 eV) and 3.1 eV (0.4 eV) for BSO; 2.8 eV (0.3 eV) and 3.1 eV (0.4 eV) for BGO.

Analysis of photorefractive optical damage in lithium niobate: application to planar waveguides.

Photorefractive optical damage of single beams in LiNbO(3) crystals is analyzed within a framework of two photoactive centres (Fe(2+)/Fe(3+) and Nb(Li) (4+)/Nb(Li) (5+)). It compares model simulations and significant experimental measurements in LiNbO(3) waveguides. A good agreement is found in the performed comparisons: photovoltaic currents, refractive index changes and, especially relevant, in degraded beam-profiles. The progress of the degraded wavefront has been simulated by implementing a finite-difference beam-propagating method which includes the model equations. These results, together with previous ones on grating recording, provide a comprehensive, satisfactory explanation of most important questions on photorefractive optical damage.

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.