Loss in low-finesse Ti:LiNbO3 optical waveguide resonators

We propose to measure the contrast of Fabry-Perot resonances in low-finesse (end face polished) integrated optical resonators to determine an upper limit of the waveguide attenuation coefficient. The method is discussed; its absolute accuracy increases with decreasing loss. As example, experimental results are presented for 3 and 10 m wide Ti: LiNbO₃ strip guides; the attenuation coefficients are 0.46 dB/cm (EH₁₁) in the narrower guide at =0.63 m, respectively 0.19 dB/cm (EH₁₁) and 0.09 dB/cm (HE₁₁) in the 10 m wide guide at =1.15 m.     

Propagation characteristics of Ag+ ion-exchanged stripe waveguides

A detailed experimental study of stripe waveguides formed by Ag⁺/Na ion-exchange has been carried out. Propagation constants were precisely measured by excitation of single output pq-lines. The resulting data allowed us to approximate the transverse silver distribution by a rectangular profile. The effective index method is used for calculating theoretical propagation constants, assuming no sideways diffusion. A comparison between experiment and theory is made over the range of mask widths studied.     

Electromagnetic propagation at interfaces and in waveguides in uniaxial crystals

The use of surface-impedance and surface-admittance concepts for analyzing reflection and refraction at an isotropic dielectric interface (first developed about 1938) is extended to include an interface between uniaxial birefringent dielectrics. Total internal reflection and the polarizing (Brewster) angle at an anisotropic interface are shown to be naturally explainable in terms of surface impedance (for TM polarization) and surface admittance (for TE polarization). The allowable modes in an integrated optical uniaxial asymmetric dielectric slab waveguide are also shown to be directly obtainable using the surface impedance/admittance approach. Numerical examples are presented.     

Optical beam splitter and focuser in Ti:LiNbO3 planar waveguide

A -type two-electrode system on a Ti diffused optical waveguide in LiNbO₃ has been studied in symmetrical and in asymmetrical operation regimes. In the first case the device acts as a beam splitter or focuser, depending on the polarity of the applied voltage. In the second case the device acts as a scanner. The driving voltage is relatively low. Possible applications are optical communication systems and optical processing of information.     

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     

A planar electrooptic beam splitter/modulator with a pair of parallel electrodes

A new planar electrooptic beam splitter with a pair of parallel electrodes is proposed and its characteristics are demonstrated on LiNbO₃. At a voltage up to 80 V and a 9.5 m electrode separation, the two output beams can be scanned over a total angle of 3° in the guide plane. The structure can also be applied to amplitude modulation, requiring 15 V for complete on-off modulation. The device permits convenient fabrication and has a potential to operate at high speeds.     

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     

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.     

Optical losses in ion-implanted waveguides

In this paper we first introduce complex refractive indices to describe the optical losses in the buffer layer of optical waveguides obtained by ion implantation. It is shown that the color-center absorption in the buffer layer can change significantly the mode-spacing function. We carried out a detailed analysis of the influence of the imaginary component of the buffer layer refractive index on the propagation characteristics.     

The concept of quasi-discrete levels for ion-implanted waveguides

In this paper, we introduce the concept of quasistationary states for ion-implanted waveguides. It can be employed for a certain range of implanted barrier parameters. It is shown that for a rather large thickness of the barrier, the waveguide modes can be regarded as quasidiscrete lines with some widths. In order to estimate these widths the refractive index profile has been modelled by a four-layer step index function.     

Second harmonic generation in planar optical waveguides

We study optical second harmonic generation (SHG) in planar waveguide structures composed of several layers with different dielectric constants. We develop a general formalism for the calculation of mode generation by a planar antenna embedded in the waveguide. As an application we consider a monolayer of high second-order susceptibility adsorbed at the interface between two layers of the waveguide structure. Periodic modulation of the nonlinear susceptibility allows phase matching leading to dramatically enhanced second harmonic intensities. We investigate the SHG-efficiency of various experimentally realizable geometries.     

Spectral line broadening mechanism of Er3+ transitions in Er:Ti:LiNbO3 channel waveguides

In order to elucidate the interaction effects among the various defects present in a LiNbO₃-based integrated optical device, we investigated the change of the optical properties of Er³⁺ ions under the application of an external electric field and hydrostatic pressure. We obtained for stoichiometric bulk material a complete picture of the field-induced spectral shifts as a function of transition and site. As a first important application of these results we were able to clarify the mechanism of spectral broadening of the Er³⁺ transitions in Ti:Er:LiNbO₃ channel waveguides. By selecting different waveguide modes for excitation and using highly selective double-resonance excitation with two lasers, we found that the [Ti⁴⁺] concentration gradient caused by the indiffusion results in an internal E-field gradient. This translates, due to the averaging within the guided mode, into mode-dependent spectral line broadening. Received: 24 May 2001 / Published online: 23 October 2001     

Determination of refractive index profiles in diffused optical waveguides

An improved method for the reconstruction of diffused waveguide index profiles from experimental mode spectra is proposed. The method uses a parabolic approximation between successive points. It is tested on the example of an exponential profile, and applied to the reconstruction of index profiles in Ag⁺ ion-exchanged waveguides from measured mode spectra. This method appears to work well also in the cases of very few mode wave-guides.     

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     

Permanent narrow-band reflection holograms for infrared light recorded in LiNbO3:Ti:Cu channel waveguides

Permanent refractive-index gratings are generated by thermal fixing of holograms in photorefractive lithium niobate channel waveguides. The guides are fabricated by successive indiffusion of titanium stripes and thin layers of copper. After high-temperature recording with green light, refractive-index modulations exceeding Δn=8×10^-5 for light of the telecommunication wavelength 1550 nm appear without the need of any development process of the written holograms. The gratings are stable in the dark and no compensation mechanism via dark conductivity is observed. Thus this method may be well suited for long-time applications in holography and integrated optics. Received: 2 October 2000 / Revised version: 25 January 2001 / Published online: 22 March 2001     

Effects of a lossy thin plasma-film on metal-diffused LiNbO3 optical waveguides

I intend to investigate the effects of different kinds of partially reflecting metal film (plasma layer) on the wave characteristics of a stratified titanium-diffused LiNbO₃ optical waveguide. The mode number, the equivalent indices, the extinction coefficients, the field distributions and the powers carried in each layer are given as a function of the free-space wavelength, of the diffusion parameters, and of the opticc-axis orientation. Of course, the power of TM-like hybrid waves flows in opposite directions in the plasma and the other regions.For ac-axis varying in the transverse plane, the ith-order hybrid mode is rather complicated in the guide with an aluminum layer. There, in fact, the field mapping can be seen as an overlap of a TE _i with a TM _i+1, mode because of a sudden transformation of the TM₀ mode into a superficial plasma wave (SPW). On the contrary, with a silver layer, the hybrid field is a simple combination of a TE _i with a TM _i mode with no SPW growing.The guide losses assume the lowest values (1 dB/cm) for an Ag layer which is the prime candidate for making electro-optical or acoustic-optical devices. On the other hand, the nickel film causes the highest losses (66 dB/cm).     

Effect of stresses on the OH libration bands in proton-exchanged LiNbO3 optical waveguides

i -H_xLi_1-xNbO₃ phases generated on x-cut LiNbO₃ the new anisotropic libration mode at 890–940 cm^-1 is observed in addition to the known isotropic mode at 960–970 cm^-1, whereas the same phases on z-cut exhibit only latter isotropic libration mode. The observed phenomena are explained as a result of anisotropic stresses generated in the x-cut waveguides, whereas the stresses in the structures on z-cut are isotropic. The OH-bond librations are clearly anisotropic in the α, κ₁, and κ₂ phases generated on both z- and x-cuts, which indicates a fundamental difference of the local symmetry of the proton’s position in the β_i phases compared to the α, κ₁, and κ₂ H_xLi_1-xNbO₃ phases. Received: 4 May 1998/Revised version: 26 June 1998     

Thermal fixing of holographic gratings in planar LiNbO3:Ti:Fe waveguides

3 :Ti:Fe waveguides are thermally fixed during hologram recording at elevated temperatures. Different guides are fabricated by titanium indiffusion using iron-doped and nominally pure y-cutLiNbO₃ substrates and characterization is performed by dark-mode spectroscopy. The refractive index modulation of gratings written and simultaneously fixed at 180 °C is investigated as a function of propagation depth, titanium and iron concentration. The experimental results are compared with those obtained for unfixed holograms recorded at room temperature. Received: 21. July 1997/Revised version: 13 October 1997     

Efficient cascaded difference frequency conversion in periodically poled Ti:LiNbO3 waveguides using pulsed and cw pumping

Using an FM-mode-locked Ti:Er:LiNbO₃ waveguide laser as the fundamental source, wavelength conversion by cascaded χ⁽²⁾:χ⁽²⁾-difference frequency generation with a conversion efficiency of up to +3(-4.6) dB was demonstrated at a pulse repetition rate of about 2 (10) GHz. In addition, multi-channel conversion was demonstrated with a fully packaged wavelength converter using a continuous fundamental source. Received: 29 May 2001 / Revised version: 10 August 2001 / Published online: 2 November 2001     

Second-harmonic generation in Zn-diffused periodically poled LiNbO3 channel waveguides

In this work second-harmonic generation by quasi-phase matching (QPM) in Zn-diffused periodically poled lithium niobate channel waveguides is presented. A stable TM?TE conversion by QPM has been found. The results are in good accordance with theoretical estimations obtained by the phase-matching condition, either for the polarisation character of the second-harmonic wave as well as for the spectral range, taking into account the periodicity of the domains. Received: 16 May 2001 / Revised version: 7 September 2001 / Published online: 30 October 2001