S. Tonchev

Polyimide thin-film waveguides : optical and Raman spectroscopic studies

Polyimide films deposited on fused quartz, silicon, and ceramic substrates are examined. Waveguide techniques are used to determine the guide modes from which the index of refraction is inferred. Film thicknesses are determined from transmission interferences and, independently, from a luminescence‐induced reflection interference pattern. Using a waveguide configuration we also observed and analyzed Raman scattering features in the region above 1000u2009cm−1.

Optical and structural properties of annealed PE:LiNbO3 waveguides formed with pyrophosphoric and benzoic acids

The optical and structural properties of protonated lithium niobate waveguides have been investigated. The proton‐exchange process was performed by immersing Z‐cut substrates in molten pyrophosphoric and benzoic acids at 240u2009°C for 8 h. Surface refractive index changes of 0.148 (pyrophosphoric acid) and 0.124 (benzoic acid) were obtained at λ=632.8 nm. The exchanged samples were subjected to annealing for 1 h through each 20° starting from 200u2009°C. Reflection high‐energy electron diffraction, optical measurements, and vibrational (IR and Raman) spectra were used at each stage of annealing in order to study the guiding characteristics. Immediately after the exchange process and at low‐temperature annealings a two‐step index profile was detected. The dependence of the profile parameters on the annealing temperature up to 360u2009°C was followed and correlated with the phase transformations in the LiNbO3‐HNbO3 system. The hydrogen concentration in the waveguide layers was estimated by using optical measurements a...

Raman scattering of proton exchanged LiNbO3 waveguides

Proton exchanged LiNbO3 waveguides are studied using Raman spectroscopy. Depending on the waveguide mode, the spectra resemble those of pure LiNbO3 at high temperatures. A characteristic peak appears at 69 cm-1 which is probably due to second-order Raman scattering but strongly associated with the presence of protons. It is proposed that the change in the Raman spectra is due to the coexistence of the ferroelectric and paraelectric phases of LiNbO3 at room temperature, induced by the proton exchange deformations on the lattice.

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.

The influence of the cooling rate on the stability of z-cut H: waveguides

The influence of cooling rate on the refractive index of proton-exchanged z-cut waveguides has been investigated. After short annealing at about C, the waveguide mode indices of as-exchanged and annealed samples are different for quick and slow cooling. A hypothesis explaining this effect by assuming a diffuse phase transition in the protonated crystal lattice is given. The possible changes in the macroscopic parameters responsible for the index differences are analysed.

Raman study of Cs:KTiOPO4 waveguides

Planar waveguides were produced by K+-Cs+ exchange of Z-Cut KTiOPO4 single domain crystals. The analysis of the refractive index profile has shown an exponential Cs+ distribution in the exchanged layers. By exciting different waveguide modes, Raman spectra were obtained from guide depths with varying Cs doping. Significant changes have been observed between Raman spectra of Cs:KTiOPO4 and those of pure KTiOPO4 only in the region of external vibrations while the internal modes, due to vibrations of the TiO6 and PO4 groups, are practically the same. The bands corresponding to the external vibrations were found to be strongly influenced by the substitution indicating strong disorder in the lattice, and possibly a structural phase transition. Furthermore, Raman spectroscopy proves to be an alternative technique for determining variation in Cs composition.

Raman study of protonated H : LiNbO3 thin film waveguides

Abstract Multimode proton exchanged waveguides HxLi1−xNbO3 have been formed in congruent z-cut substrates using benzoic acid for proton source. The H concentration was changed by annealing, applying two different rates of cooling – quick (q) and slow (s). The samples were characterized by measuring their extraordinary refractive index profiles and waveguide Raman scattering. At high proton concentrations (x>0.56), where the βi phases are supposed to be existing the s- (equilibrium β2/β1 combination) and q- (metastable β1 phase) type of samples show very different Raman spectra, being also different from the scattering of the pure substrate. This is an evidence for lattice transformations probably related to the large differences in the electrooptic and nonlinear properties of the phases.

Waveguide Mach-Zehnder Intensity Modulator produced via Proton Exchange Technology in LiNbO3

Guided-wave Mach-Zehnder intensity modulators are key elements for a variety of applications in optical communications. They usually present a broad operation band and low drive voltages. Two general ways for fabrication of such modulators on LiNbCO3 are known - by standard Ti-indiffusion and by proton exchange with additional annealing.

Raman scattering and phase state of H:LiTaO3 waveguide thin films

Raman scattering of optical waveguides formed by proton-exchange in lithium tantalate is studied. The effect of a post-exchange annealing up to 420°C on the optical properties and Raman spectra are also investigated. Two types of Raman bands are observed: those which are very strongly influenced by the presence of protons (“proton-related” bands) and those characteristic of the pure crystal (“bulk” bands). The intensity of the two kinds of bands depends in an opposite way on the annealing temperature, with the “proton-related” bands following the anomalous behaviour of ne. The spectra from samples annealed up to the temperature of the maximum value of ne are remarkably similar to the high-temperature paraelectric spectra of a LiTaO3 single crystal.