Optical waveguides in LiNbO<Subscript>3</Subscript> and stoichiometric LiNbO<Subscript>3</Subscript> 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. Supported by the National Natural Science Foundation of China (Grant No. 10475052) and the Scientific Research Start-up Financing of Qufu Normal University     

Temperature-dependent Sellmeier equation in the MIR for the extraordinary refractive index of 5% MgO doped congruent LiNbO<Subscript>3</Subscript>

We present a temperature-dependent Sellmeier equation for the extraordinary refractive index of 5 mol % MgO doped congruent lithium niobate. This equation is adapted for wavelengths in the range of 1.3–5 μm and temperatures between 40 °C and 200 °C. The calculation of the appropriate Sellmeier coefficients is based on the wavelengths of the signal and idler radiation measured for quasi-phase-matched optical parametric generators excited by 10-ns-long, 1064-nm pulses of a Q-switched Nd:YVO₄ laser. PACS 42.65.Ky; 42.70.Mp; 42.65.Yj     

Mode-selective coupler for wavelength multiplexing using LiNbO<Subscript>3</Subscript>:Ti optical waveguides

A mode-selective directional coupler based on titanium in-diffused channel waveguides in lithium niobate is investigated. This coupler may be utilized as a key part of an add-drop multiplexer for dense wavelength division multiplexing in optical network nodes. The proposed coupler is based on evanescent coupling of the fundamental mode of a single-mode channel to the first higher mode of a parallel bi-modal waveguide. Our experimental results show that a compact directional coupler with coupling efficiencies larger than 90%, large bandwidth around 1550 nm, and with negligible polarization dependence can be realized using electro-optic lithium niobate substrates. Presented at 9-th International Workshop on Nonlinear Optics Applications, NOA 2007, May 17–20, 2007, Świnoujście, Poland     

Temperature effects in proton exchanged LiNbO<Subscript>3</Subscript> waveguides

Temperature effects on single, homogeneous phases of proton exchanged LiNbO₃ waveguides have been studied. The refractive index of -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 ₂-phases on z-cut congruent substrates and in ₁-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 ₁-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. PACS Provide PACS codes please.     

Anomalies in the pyroelectric properties of LiNbO<Subscript>3</Subscript> crystals of the congruent composition

A number of anomalies are revealed in the temperature dependence of the primary pyroelectric coefficient for single-domain crystals of lithium niobate LiNbO₃ in the temperature range 310–330 K. It is shown that these anomalies are associated with the transformation of the domain structure of the lithium niobate crystals. The problem regarding the correct performance of the pyroelectric measurements and the possible influence of the observed effects on the optical properties of lithium niobate crystals and on the recording of optical phase holograms are discussed.     

Cr<Superscript>3+</Superscript> spectroscopy study in ZnO-codoped and Zn-in-diffused congruent LiNbO<Subscript>3</Subscript>:Cr crystals

This paper reports on the spectroscopy properties, absorption and luminescence, of Cr³⁺ ions in singly doped, ZnO-codoped, and Zn in-diffused LiNbO₃:Cr crystals. In addition to the broad absorption, inter-ionic transitions ascribed to Cr³⁺ ions located in Li⁺ and Nb⁵⁺ sites; [Cr]_Li and [Cr]_Nb centres two absorption bands at higher energy are reported and ascribed to the charge transfer transitions of the Cr³⁺ ions of the two defect centres. The charge transfer transitions are used as optical probe to study the role of the Zn ions in the Zn in-diffused LiNbO₃:Cr samples. It has been observed that the Zn-in-diffused processes created [Cr]_Nb centres in the diffusion zone. The location of the diffused Zn²⁺ ions is considered to be in Li⁺ site, displacing the Cr³⁺ ions from the Li⁺ sites, [Cr]_Li, to the Nb⁵⁺ positions, [Cr]_Nb.     

Light scattering in single crystals of nonlinear-optical photorefractive LiNbO<Subscript>3</Subscript>:Cu and LiNbO<Subscript>3</Subscript>:Zn

Comparative studies are made of the photorefractive scattering of light in nonlinear single crystals of lithium niobate with congruent compositions (LiNbO₃) doped with "photorefractive" Cu [0.015 mass %] and "nonphotorefractive" Zn [0.5 mass %] cations. For the first time it is found that single crystals doped with "photorefractive" and "nonphotorefractive" cations have different indicatrices for photorefractive light scattering. The aperture angle for photorefractive scattering reaches its steady state value more rapidly with high laser powers than with low. However, at high powers laser induced heating of the crystal is greater, and this leads to a narrowing of the scattering indicatrix. It is also found that photorefractive scattering in these single crystals depends on the region of the boule from which a sample has been cut. This indicates that there is a nonuniform distribution over the boule of the imperfections with localized electrons which determine the magnitude of the photorefractive effect.     

Broadband and low-driving-power LiNbO<Subscript>3</Subscript>electrooptic modulators

Microwave and optical properties of lithium niobate electrooptic modulators are investigated in this paper. The effect of simultaneous matching of optical and microwave velocities and impedance matching, conductor loss, dielectric loss on the optical bandwidth of an ultra-high-speed lithium niobate modulator are presented here. The metal electrode design, buffer thickness, ridge depth, and the gap between electrodes at different operating frequencies on device performance are thoroughly investigated by using the finite element method.     

Near-infrared properties of periodically poled KTiOPO<Subscript>4</Subscript> and stoichiometric MgO-doped LiTaO<Subscript>3</Subscript> crystals for high power optical parametric oscillation with femtosecond pulses

A thorough theoretical analysis of the near-infrared properties of periodically poled KTiOPO₄ (PPKTP) and stoichiometric MgO-doped LiTaO₃ (MgO:PPSLT) crystals for optical parametric oscillation with excitation at 1 μm is presented. To the best of our knowledge, the optical, phasematching, wavelength tuning, and dispersive properties of these crystals for parametric interactions are discussed in detail for the first time. In addition, a new design for high power parametric devices based on PPKTP or MgO:PPSLT crystals with ultrafast Yb-ion laser pump is proposed. The results form a useful reference for the selection of materials and operating conditions for the practical design of high power femtosecond optical parametric oscillators with excitation at 1 μm.     

Photoconductivity in LiNbO<Subscript>3</Subscript> crystals codoped with MgO and Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>

Polymer stabilized cholesteric texture cells with various chiral and monomer concentrations were fabricated. Electro-optical measurement indicates that the chiral concentration and the monomer concentration significantly influence the hysteresis, threshold voltage and field-on and field-off response times of the cell. The chiral dopant is found to increase the hysteresis width and to decrease the field-off response time of the cell. The monomer is found to decrease the hysteresis width and to increase the field-off response time of the cell. The field-on response time is independent of the monomer concentration and the chiral concentration of the cell. A thinner cell has a smaller hysteresis width. PACS 61.30.-v; 42.70.Df; 42.79.Kr     

A Fe[Nb]-Li center in stoichiometric LiNbO<Subscript>3</Subscript> crystals: Mechanism of formation

A new iron center in stoichiometric lithium niobate crystals has been studied by the EPR method. The angular dependences of the EPR spectrum of the center have been used to derive the parameters of its spin Hamiltonian. The data amassed on the variation in the concentrations of two iron centers in lithium niobate crystals annealed in a Li₂CO₃ powder have provided an insight into the mechanism of formation of the new center, as well as corroborated its model proposed by us earlier. According to this model, the center represents a complex of two defects aligned with the polar axis in the crystal: the iron ion at the niobium site and an interstitial lithium ion filling the nearest structural vacancy (Fe³⁺[Nb]-Li⁺[V]). The structure of other Fe³⁺ centers revealed earlier in LiNbO₃ crystals, in which the iron ion occupies the niobium site, has been discussed.     

Superlinear photovoltaic currents in proton-exchanged LiNbO<Subscript>3</Subscript> 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 Nb_Li ⁴⁺/Nb_Li ⁵⁺ as secondary photovoltaic centres. The superlinear behaviour arises from a much higher effective photovoltaic length of Nb_Li ⁴⁺ (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. Received: 22 October 2002 / Revised version: 6 January 2003 / Published online: 12 May 2003 RID="*" ID="*"Corresponding author. Fax: +34-91/3978-579, E-mail: m.carrascosa@uam.es     

Origins of waveguiding in femtosecond laser-structured LiNbO<Subscript>3</Subscript>

Femtosecond laser-induced structural changes in LiNbO₃ are studied. Depending on the laser processing parameters two different types of modification are identified and their origin is discussed. Both types of modification can be described within the framework of induced lattice defects. For strong material damage a refractive index increase can be obtained due to the induced stress field. By appropriate tailoring of this stress field thermally stable and highly symmetric waveguides can be obtained well suited for nonlinear integrated-optical applications. PACS 61.80.Ba; 77.84.Dy; 42.65.Re; 42.82.Et     

LiNbO<Subscript>3</Subscript>:Yb<Superscript>3+</Superscript> crystals for optical temperature sensors

Possibilities of using lithium niobate crystals, doped with ytterbium (LN:Yb³⁺) as materials for optical temperature sensor (OTS), are discussed. We consider both the radiative and absorptive characteristics of the crystals, assuming that their temperature dependences are caused by the Boltzmann factor of the initial population of Stark sublevels of the ion. It is shown that the crystals can be used as materials for OTS in the temperature range 150–400 K.     

Crystal LiNbO<Subscript>3</Subscript>-Ho<Superscript>3+</Superscript>: Material for optical cooling

The possibilities of LiNbO₃-Ho³⁺ crystals for optical cooling based on the anti-Stokes luminescence in the wavelength range 2000-2200 nm are investigated. The efficiency and cooling temperature under the continuous wave (CW) excitation at the wavelengths 2035-2071 nm by ~100 W power are estimated. It is shown that under the CW excitation at 2035 nm wavelength the maximum cooling temperature is equal to 2.5 K, and at 2071 nm wavelength is equal to 10.9 K.     

Controlling optical inhomogeneity of LiNbO<Subscript>3</Subscript> single crystals

Different ways of inducing changes in the optical inhomogeneity of single crystals of lithium niobate are described. These include the application of a conductive coating in combination with the heating of samples and exposure to the field of a corona discharge, to UV radiation from a Nd:YAG laser, and to the electron beam of an electron microscope. All such effects can be used to record images.     

Fabrication of polaritonic structures in LiNbO<Subscript>3</Subscript> and LiTaO<Subscript>3</Subscript> using femtosecond laser machining

Fabrication of patterned materials in ferroelectric LiNbO₃ and LiTaO₃ crystals using femtosecond laser micromachining is presented and discussed. Damage feature sizes in the 10–100 μm range were achieved using 800-nm, 50-fs (FWHM) ultra-fast laser pulses with energies ranging from 10 μJ up to 350 μJ. Fabrication of polaritonic devices such as waveguides, resonators, focusing reflectors, diffractive and dispersive elements, photonic band gap materials, and other microstructures is demonstrated. PACS 77.84.Dy; 42.62.Cf; 71.36.+c