Lattice micro-modifications induced by Zn diffusion in Nd:LiNbO<Subscript>3</Subscript> channel waveguides probed by Nd<Superscript>3+</Superscript> confocal micro-luminescence

The luminescence properties of Neodymium ions in Zn diffused LiNbO₃ channel waveguides have been studied with sub-micrometric spatial resolution. The analysis of the luminescence spectra suggests the existence of a local expansion and disordering associated to the presence of Zn in the LiNbO₃ lattice after the diffusion process. By taking this cause–effect relation into account, it has been possible to determine both the in-depth and lateral Zn diffusion lengths, as well to elucidate the spatial location of the channel waveguide. PACS 42.65.Wi; 87.64.Tt; 42.55.Rz     

Compositional characterisation of Zn-diffused lithium niobate waveguides

Rutherford backscattering (RBS) and secondary-ion mass spectrometry techniques have been used to investigate the two-step process involved during waveguide fabrication in LiNbO₃ using Zn-vapour diffusion. Compositional analysis (O, Nb, Li and Zn) in the two steps has been characterised. RBS analysis reveals that the first step, involving a heating of the substrate under a metallic Zn atmosphere, gives rise to a partial exchange between the Nb and Li ions from the crystals and the Zn from the vapour source. The second treatment at higher temperature in an open atmosphere diffuses the Zn deeper into the substrate, thus forming an optical waveguide, while the Nb and Li ions recover their bulk values. Received: 16 May 2001 / Revised version: 7 September 2001 / Published online: 30 October 2001     

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.     

Design and comparison of single-mode planar and ridge type electro-optic waveguide modulators

Electro-optic waveguide modulators utilizing phase retardation of two orthogonally polarized optical modes in LiNbO₃ and LiTaO₃ waveguides have been designed taking into consideration the optical field distribution in the waveguides and the electrical properties of the electrodes. The analysis has revealed that a driving-voltage to frequency-bandwidth ratio of 1 V/GHz is attainable at the wavelength of 1.05 m using presently available embedded and ridge waveguides. Improvement in waveguide fabrication techniques may reduce the ratio by at least a half. Thus, LiNbO₃ and LiTaO₃ waveguide modulators are considered to be promising candidates for practical application to single-mode optical-fibre transmission systems of higher than 1 Gbit/s.     

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     

Variational analysis of Er:Ti:LiNbO3 waveguide lasers based on Gaussian/two-half Gaussian mode field distribution

Starting with two dimensional, scalar wave equation, a variational equation was established for the fundamental TE and TM modes guided in Ti:LiNbO₃ waveguides on the basis of assuming a symmetric Gaussian mode field function in the width direction and two-half Gaussian trial functions in the depth direction. The controllable waveguide fabrication parameters, including channel width, diffusion temperature, initial Ti-strip thickness and diffusion time, dependent of fundamental mode size, effective pump area, coupling efficiency between pump and laser modes, and the coupling loss between a Ti:LiNbO₃ waveguide and a fiber were numerically calculated for Z-cut Er:Ti:LiNbO₃ channel waveguide lasers at three possible emission wavelengths 1532,1563 and 1576 nm and two possible pump wavelengths 1480 and 980 nm. The calculated results were compared with those of Gaussian/Hermite–Gaussian mode field distribution in detail.     

Evidence of different β-phases in highly protonated z-cut H:LiNbO3 waveguides by raman scattering

3 waveguides is demonstrated by the Raman scattering technique. Multimode waveguides have been produced by using pyrophosphoric, benzoic, and diluted benzoic acids as a proton source. The proton-exchanged (PE) layers are subjected to annealing (APE samples) with different rates of cooling: slow (s) and quick (q). The effect of Li-H replacing is considered in the frequency range below 800 cm^-1 and about 3500 cm^-1 (OH modes). The analysis of the spectral data leads to some important conclusions about the changes in the H:LiNbO₃ lattice. We demonstrate that applying (s) ? (q) procedures on H_xLi_1-xNbO₃ waveguides with x>0.56 leads to phase transitions between different states. The high-temperature phase modifications (metastable at room temperature) are characterized by strongly broadened q-Raman bands which imply a high degree of disorder. Although the Raman measurements support the presence of different phases in H:LiNbO₃ waveguides, some conclusions, different from those reported elsewhere, have been made. Received: 16 November 1998 / Accepted: 14 December 1998 / Published online: 24 February 1999     

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.     

Raman study on vapor-phase equilibrated Er:LiNbO3 and Er:Ti:LiNbO3 crystals

Raman spectra of Er:LiNbO₃ crystal and Ti-diffusedEr:LiNbO₃ strip waveguide, in which the Li/Nb ratio was altered using a vapor-phase equilibration (VPE) technique, were measured at room temperature in the wave-number range 50–3500 cm^-1. Both 488 and 514.5 nm radiations were used to excite Raman scattering, A₁(TO) and E(TO) modes were recorded at backward scattering geometry. The results indicated that the lattice vibrational spectra of the as-grown Er:LiNbO₃ are almost the same as those of pure LiNbO₃ except for the little shift of the peak position and the change of relative intensity of some peaks. In comparison with the spectra of as-grown Er:LiNbO₃ crystal the vapor-phase equilibrated Er:LiNbO₃ and Er:Ti:LiNbO₃ crystals in the lattice vibrational region exhibit the following features: firstly, Raman peaks become narrow, indicating that the VPE process has brought Er:LiNbO₃ and Er:Ti:LiNbO₃ crystals closer to a stoichiometric composition; secondly, relative intensity of some peaks varies with the VPE time; and finally, slight blue shifting in peak position was observed. Some of these features were correlated with the NbO₆ octahedra and with the site distribution of the doped Er ions. In addition, green fluorescence peaks and/or bands associated with the electron transitions ² H _11/2?⁴ I _15/2 and ⁴ S _3/2?⁴ I _15/2 of the doped Er³⁺ were also observed. For 488 nm excitation they appear in the wavenumber range of 1200–3000 cm^-1 and are well separated from lattice vibrational region; for 514.5 nm excitation, however, these fluorescence peaks shift towards the low wavenumber region and overlap partially with the lattice vibrational spectra. Received: 24 May 2000 / Accepted: 29 May 2000 / Published online: 13 September 2000     

Defect structure and optical damage resistance of Zn:Fe:LiNbO<Subscript>3</Subscript> crystals

A series of Zn:Fe:LiNbO₃ crystals were prepared by the Czochralski technique with 0.015 wt. % Fe₂O₃ content and various concentrations of ZnO. The ultraviolet-visible absorption spectra and the infrared absorption spectra of the Zn:Fe:LiNbO₃ crystals were detected in order to investigate their defect structure. Their optical damage resistance was characterized by the photoinduced birefringence change and transmission facula distortion method. The optical damage resistance of the Zn:Fe:LiNbO₃ crystals increases remarkably when the concentration of ZnO is over its threshold concentration (more than 6.0 mol. %). The effects of defects on the optical damage resistance of the Zn:Fe:LiNbO₃ crystals are discussed in detail. Received: 25 October 2002 / Revised version: 6 January 2003 / Published online: 22 May 2003 RID="*" ID="*"Corresponding author. Fax: +86-451/2300-926, E-mail: zzxxhhdoctor@sina.com     

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.     

Design of tunable asymmetric directional coupler filter using periodically segmented Ti:LiNbO3 waveguides

A tunable wavelength filter is designed using Ti:LiNbO₃ asymmetric directional coupler. One arm of the directional coupler is made of continuous waveguide while the other arm is periodic segmented. This segmentation reduces the lithographic steps and hence the fabrication cost required for such asymmetric coupler based filter. The propagation constant of periodical segmented and continuous waveguides are analysed using Effective-Index-based Matrix Method. The tunability is achieved using electro-optic property of LiNbO₃.     

Modelling the formation of optical waveguides produced in LiNbO3 by laser induced thermal diffusion of lithium ions

Analytical and numerical modelling of the fabrication of optical waveguides in lithium niobate (LiNbO₃) through direct writing with a continuous wave ultraviolet (UV) laser has been performed. It is proposed that the UV illumination induces heating of the surface and subsequent thermal diffusion of lithium which alters the refractive index in accordance with the lithium concentration dependence of the Sellmeier equations. PACS 61.80.Ba; 66.30.Dn; 77.84.Dy     

Photorefractive properties of LiNbO3:Zn crystals related to the defect structure

LiNbO₃:Zn single crystals and powders were studied by precise X-ray diffraction methods. Structural refinement yielded new models of the intrinsic defect structure valid for different Zn concentration ranges. For concentrations up to 7 at. % in the crystal, Zn ions incorporate onto Li sites; at higher concentrations Zn ions are found on both Li and Nb sites. Photorefractive properties of LiNbO₃:Zn are discussed in the context of the deduced defect models. A smooth increase in the photoconductivity up to 7 at. % Zn is accounted for by a decrease of Nb antisites. Steeper growth of the photoconductivity at higher Zn concentrations is related to vanishing Li vacancies. Received: 17 November 2000 / Revised version: 23 January 2001 / Published online: 20 April 2001     

Modelling of optical amplification in Er/Yb co-doped LiNbO3 waveguides

The optical amplification of LiNbO₃:Er³⁺/Yb³⁺ channel waveguides has been modelled in the small signal regime using the overlapping integrals method and the rate-equation formalism. It has been found that Yb³⁺ -sensitisation improves the pump efficiency at 980 nm and a higher gain is achievable in the high power-limit compared to singly-doped LiNbO₃:Er³⁺ amplifiers.     

Nonvolatile hologram storage properties of tri-doped Zn:Ce:Cu:LiNbO3 crystals

Congruent Zn(7 mol%):Ce:Cu:LiNbO₃ single crystal was grown by the Czochralski method in air. The occupation mechanism of the Zn²⁺ was discussed by an infrared transmittance spectrum. The nonvolatile holographic recording in Zn(7 mol%):Ce:Cu:LiNbO₃ single crystal was measured by two-photon fixed method. Zn(7 mol%):Ce:Cu:LiNbO₃ single crystals present the faster recording time and higher light-induced scattering resistance ability comparing with Ce:Cu:LiNbO₃ single crystals.     

Deposition of ZnO multilayer on LiNbO3 single crystals by DC-magnetron sputtering

Zinc oxide (ZnO) thin films were deposited on LiNbO₃ (LN) single crystals with 200 nm thicknesses by three different ways, where coating of zinc (Zn) film was followed by thermal oxidation for four, two, and one steps with 50, 100, and 200 nm thicknesses repeatedly. Sample, which was produced at 4-step of deposition and oxidation of Zn layer, showed high transmittance and low structural defect due to a lower photoluminescence intensity and Urbach energy. Average grain size in X-ray diffraction (XRD), scanning electron microscopy (SEM) micrograph, and atomic force microscopy (AFM) images for multilayer of ZnO was lower than monolayer of ZnO thin films. Applying multilayer coating technique leads to decrease of surface roughness and scattering on light on surface and fabrication of LiNbO₃ waveguides with lower optical loss.     

Damage profiles in LiNbO3 by low-energy H+ implantation

Low-energy 120 and 150 keV H⁺ was implanted in z-cut LiNbO₃ at room temperature. The fluence of H⁺ is 5?×?10¹⁶ ions/cm². The damage profiles in LiNbO₃ induced by implantation were investigated using Rutherford backscattering/channelling. The damage profiles were extracted using the channelling results. The experimental damage profiles in LiNbO₃ were analyzed and compared to the simulated results from TRIM. The results show a good consistency between experimental and simulated results. The present results are useful for the fabrication of H-implanted waveguides of oxide crystals, especially LiNbO₃.     

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.