Light-induced refractive index changes in LiNbO3:Ti waveguides

Holographic measurements of light-induced refractive index changes in LiNbO₃Ti waveguides are reported. The results demonstrate that Fe²⁺ centers are stabilized against oxidation in the region of large Ti concentration near the surface thus increasing the sensitivity to optical damage considerably. This undesired effect may be largely abolished by additional in-diffusion of protons.     

Raman characterization of H:LiNbO3 waveguides

In this work we present polarized Raman measurements of z-cut multimode H:LiNbO₃ waveguides in the κ_i- and β_i-phase states. A spectrum with unexpected form and behaviour, originating from the layer at the very surface, was observed in samples with various degrees of Li–H substitution. Possible reasons for the presence of this strange spectrum are considered. Received: 16 May 2001 / Revised version: 7 August 2001 / Published online: 23 October 2001     

MgO:LiNb0.3与LiNb0.3质子交换光波导光学及结构特征的比较

利用退火的质子交换法在MgO:LiNbO.3和LiNbO.3两种材料上制作了光波导，得到了质子交换的扩散特性以及光波导折射率分布的退火规律．通过x射线衍射和OH-红外吸收谱方法研究了 MgO:LiNbO.3和LiNbO.3质子交换光波导的结构特征及其与退火参数的关系，对MgO :LiNbO.3和LiNbO.3光波导光学和结构特征的结果进行了分析和比较． 关键词： 退火质子交换光波导 晶格结构 x射线衍射 红外吸收     

Characterization of LiNbO3 waveguides fabricated by proton exchange in water

Proton exchange of LiNbO₃ substrates has been achieved by immersing the samples in distilled water in sealed ampoules at temperatures between 190 °C and 300 °C. The samples have been studied by infrared spectra analysis, dark mode reflectivity and X-ray rocking curve diffraction. The results obtained show that exchange rates as high as with common acids (85%) are easily reached. The structure of the exchanged waveguides varies from the low index -phase to the high index -phases depending on the temperature and time length of the exchange process, as well as on the distilled water amount used. Optical waveguides made up of phases of high, sharp index jump (n_e0.12) as well as of low index jump and very low propagation losses (0.5 dB/cm) have been obtained and characterized. PACS 42.82.Et; 42.70.Mp     

Praseodymium-doped Ti:LiNbO3 waveguides

3 by diffusion doping is investigated by means of secondary neutral mass spectrometry and secondary ion mass spectrometry. The diffusion of praseodymium in LiNbO₃ can be described by Fick’s laws of diffusion with a concentration-independent diffusion coefficient and a limited solubility of praseodymium in LiNbO₃ increasing exponentially with rising temperature. The diffusion depends on the Li₂O content of the LiNbO₃ crystal. For LiNbO₃ crystals with a nominal slight difference in the congruent composition, the diffusion constants and activation energies for Z-cut LiNbO₃ are 3.28×10^-5 cm²/s and 2.27 eV, and 1.39×10^-5 cm²/s and 2.24 eV, respectively. Titanium-doped waveguides are formed in Pr:LiNbO₃ and characterised in relation to waveguide loss and absorption in the visible and near infrared. Received: 17 September 1998 / Accepted: 11 November 1998     

Ti:LiNbO3 optical waveguides

Various experiments on Ti diffused optical waveguides in LiNbO₃ have been carried out in order to determine precisely the character of the diffusion process. The required guide parameters and the effective mode indices could be controlled by adjusting only the diffusion time under fixed temperature and film thickness. Therefore the dependence of the guide characteristics on the diffusion time has been investigated in detail. On the basis of the data obtained, a two-stage diffusion model is proposed. In the first stage, the Ti diffusant profile is described by a erfc-function, and the second stage is characterized by a modified Gaussian form.     

Phase transition study of H:LiNbO3 waveguides by Raman spectroscopy

New results on Raman spectra of protonated LiNbO₃ are reported in order to clarify the structural and compositional changes in waveguide layers during proton exchange and post-exchange annealing processes. Samples with different degree of Li-H substitution exchanged in either neat or buffered benzoic and pyrophosphoric acid melts are investigated providing Raman and optical measurements simultaneously. The question of phase state of protonated layers at slow and rapid cooling after the annealing procedure is discussed. Qualitative and quantitative agreement between the structural data reported earlier for proton-exchanged powders and the Raman data obtained for protonated single crystals is established. A new band at 69 cm^–1 is observed which is strongly intensity dependent upon the hydrogen presence in waveguide layers. By using this band the degree of proton exchange is determined for different treatment conditions of the samples.     

Thermooptical characteristics of H+:LiNbO3:Ti proton-exchanged optical waveguides

An experimental investigation was carried out on the temperature coefficients for phase angles during excitation of output waves in proton-exchanged planar optical waveguides based on lithium niobate. The functional dependence of these coefficients on the time following annealing was studied for various waveguide modes and light propagation directions. On the basis of experimental results and the analytical expressions obtained for the temperature coefficients of input angles, conclusions were drawn about the significantly lower temperature coefficients for the extraordinary refractive index in the waveguide region.Tomsk State Academy of Control Systems and Radioelectronics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 84–90, July, 1995.     

Surface acoustic wave distribution and acousto-optic interaction in proton exchanged LiNbO3 waveguides

The efficiency of acoustooptic (AO) interaction in YZ-cut proton exchanged (PE) LiNbO₃ waveguides is theoretically analysed by determining the overlap between the optical and acoustic field distributions. The present analysis takes into account the perturbed SAW field distribution due to the presence of the PE layer on the LiNbO₃ substrate determined by the rigorous layered medium approach. The overlap is found to be significant upto very high acoustic frequencies of the order of 5 GHz, whereas in the earlier analysis by vonHelmolt and Schaffer [6] for diffused waveguides, it was shown that the overlap integral rolls down to nearly zero at this high frequency range.     

Fabrication and characterization of titanium indiffused proton exchanged optical waveguides in Z-cut LiNbO3

We report a systematic study on the fabrication and characterization of planar optical waveguides in Z-cut LiNbO₃ made by a process which combines the titanium indiffusion and proton exchange techniques. Mode effective index measurements have been made on a large number of such TIPE guides as well as guides using proton-exchange alone with various exchange temperatures and times. The data have been analyzed to calculate the values of the diffusion coefficient and activation energy for the proton exchange process. The presence of a titanium layer reduces the diffusion coefficient by about 20%. m-line scans indicate that there is a considerable amount of scattering in TIPE guides. The quality, however, improves and becomes comparable with that of PE guides as the exchange time increases.     

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. Received: 18 May 2001 / Revised version: 22 August 2001 / Published online: 30 October 2001     

Refractive index profiles of LiNbO3: Ti waveguides

Eight LiNbO₃: Ti waveguides have been fabricated under differing conditions. The effective refractive indices of guided modes have been measured and successfully fitted to Gaussian index profiles. The change of refractive index and titanium concentration are related by a power law, the exponents being different for the extraordinary and the ordinary beam.     

Visible and infrared dispersion of the refractive indices in periodically poled and single domain Nd:Mg:LiNbO3 crystals

3 crystals are measured in the transparency region (with the accuracy ±0.0002) and for the upper phonon polariton branch (with the accuracy ±0.003–±0.05), from 0.44 μm up to 10.5 μm. The method of spontaneous parametric light scattering is used for measurement of the ordinary refractive index dispersion in the mid-infrared region and for determination of the domain grating period d=5.6±0.2 μm in the periodically-poled crystal. Received: 29 January 1997/Revised version: 10 July 1997     

Site-selective spectroscopy of Er3+:Ti:LiNbO3 waveguides

Starting from previous investigations in LiNbO₃ bulk crystals, we studied the optical properties of Er³⁺ ions in Ti:LiNbO₃ channel waveguides and investigated the waveguide-specific lattice environment of the Er³⁺ ions (“sites”) caused by the doping method used and the presence of a large number of Ti⁴⁺ ions. For that purpose the method of combined excitation–emission spectroscopy was applied for the first time to waveguides at low temperatures. Comparing the spectroscopic results obtained for the green, red, and near-IR luminescence (λ≈550, ≈650 and ≈980 nm) under direct (450 nm), 2-step (980 nm), and 3-step (1.5 μm) laser excitation, we found several distinguishable Er³⁺ sites which in terms of energy levels and relative numbers are similar to those in bulk material, but exhibit significantly different up-conversion efficiencies and strongly inhomogeneously broadened transitions. Moreover, we were able to distinguish isolated and cluster Er³⁺ sites by their characteristic excitation and emission transition energies and studied the respective excitation/relaxation channels. The cluster sites are most efficient in the up-conversion process, especially under 3-step excitation. Using accepted microscopic models for Er³⁺ and Ti⁴⁺ incorporation into the LiNbO₃ crystal lattice, the site distribution and up-conversion mechanisms are elucidated and their consequences for laser applications in different spectral regions are discussed. Received: 16 November 2000 / Published online: 21 March 2001     

Generation of radiation in tunnel-coupled waveguides with positive and negative refractive indices

The possibility of generating microwave radiation in tunnel-coupled planar waveguides, one of which is made of a metamaterial and is characterized by a negative refractive index in the corresponding frequency range, while the other waveguide is active and exhibits positive refractive index, is demonstrated. The model of a tunable distributed feedback laser, in which the dependence of the refractive index of the metamaterial on the external magnetic field allows controlling the oscillation frequency, is proposed.     

Optical waveguides in LiNbO3 formed by ion implantation

Abstract

Ion implantation in LiNbO₃ can be used to modify the refractive index. The change in indices, n ₀ n _e results from the damage formed by energy deposited by the (dE/dx)_nuclear collisions between the ions and the lattice and is independent of the ion species. A saturation change in index of some ?7% occurs after a deposition of 10²³ keV cm^?3 at 300K, greater changes of ?9 % occur with implants at 77K. Annealing studies indicate the optical absorption formed during irradiation is removed below 200°C whereas the index changes exist up to 400°C. For optical waveguide production a negative change in the index is not ideal as the damaged layer cannot directly act as the region of optical confinement. However the (dE/dx)_electronic term is unimportant and so we have been able to form optical waveguides by ion implantation with light energetic ions (e.g. MeV He⁺ ions) because the damaged layer is then formed beneath an unchanged high index surface layer. This retains the desirable electrooptic properties of the single crystal LiNbO₃. The experimentally observed waveguide modes are in accord with our theoretical predictions of the refractive index profiles.     

Photoconductivity and photovoltaic behaviour of LiNbO3 and LiNbO3 waveguides at high optical intensities

The photoconductivity and photovoltaic currents of pure LiNbO₃ and proton exchanged waveguides in LiNbO₃ have been measured as a function of the optical intensity up to about several kW/cm² by the use of surface electrodes. For pure LiNbO₃ the observed dependences are a simple extrapolation of the well known low intensity behaviour. The photoconductivity of proton exchanged waveguides is considerably increased compared with pure LiNbO₃ and the curves are strongly nonlinear in the high intensity region. These results can explain, at least qualitatively, the previously observed characteristic time and intensity dependence of light-induced refractive index changes in this type of waveguides. Both the time and temperature behaviour of the dark conductivity of all proton exchanged waveguides give strong evidence of ionic charge transport in the proton exchanged region.     

Phase-matched SHG in periodically poled LiNbO3 waveguides: A novel configuration

We present a feasiblity study of a new method for enhancing the continuously phase matched second harmonic generation in 2D PPLN optical waveguides fabricated using the off-center Czochralski technique and proton-exchange. We show that a periodic variation of the nonlinear coefficient along the transverse coordinate permits for efficient energetic exchanges.     

Periodic domain structures formed under electron-beam irradiation in LiNbO3 plates and Ti:LiNbO3 planar waveguides of the Y cut

Regular domain structures on the Y cuts of the LiNbO₃ substrates and Ti:LiNbO₃ waveguide structures based on these substrates have been fabricated under electron-beam irradiation. It has been revealed that the domains in undoped and titanium-doped LiNbO₃ crystals are formed as a result of different processes. It has been demonstrated using chemical etching and waveguide second-harmonic generation that regular domain structures in the Ti:LiNbO₃ waveguides are formed at a depth of approximately 8 μm from the surface, where the titanium concentration does not exceed 2 mol %. The quasi-synchronous waveguide optical second harmonic generation with an efficiency of 8.8% has been obtained using the fabricated structures.