Proton-exchanged LiNbO3 Research Articles

Hydrogen diffusion in proton-exchanged congruent Lithium Niobate during post-annealing

We investigated the diffusion of hydrogen during post annealing of proton-exchanged congruent LiNbO3 single crystals in the temperature range between 200 and 400 °C. Proton exchange was done in benzoic acid with 3.6 mol% lithium benzoate added. This process results in a 180 nm thin surface layer with a sharp interface where Li is substituted by H for about 45%. Depth profile analysis was carried out by secondary ion mass spectrometry. From the experimental results, diffusivities were extracted which follow the Arrhenius law with an activation enthalpy of diffusion of (1.39 ± 0.17) eV. The diffusivities are about two orders of magnitude lower than hydrogen tracer diffusivities in pure and in proton-exchanged LiNbO3 but are in good agreement to effective interdiffusion coefficients, governing the proton exchange. Also the activation energy for the latter case of (1.24 ± 0.04) eV is identical within error limits. The results indicate that the diffusional redistribution of high amounts of hydrogen in LiNbO3 is an interdiffusion process between H and Li that is governed by the migration of Li.

Optical transmitter module with hybrid integration of DFB laser diode and proton-exchanged LiNbO3 modulator chip

In this work, a hybrid integrated optical transmitter module was designed and fabricated. A proton-exchanged Mach–Zehnder lithium niobate (LiNbO3) modulator chip was chosen to enhance the output extinction ratio. A fiber was used to adjust the rotation of the polarization direction caused by the optical isolator. The whole optical path structure, including the laser chip, lens, fiber, and modulator chip, was simulated to achieve high optical output efficiency. After a series of process improvements, a module with an output extinction ratio of 34 dB and a bandwidth of 20.5 GHz (from 2 GHz) was obtained. The optical output efficiency of the whole module reached approximately 21%. The link performance of the module was also measured.

Tracer diffusion in proton-exchanged congruent LiNbO3 crystals as a function of hydrogen content.

The proton-exchange process is an effective method of fabricating low-loss waveguides based on LiNbO3 crystals. During proton-exchange, lithium is replaced by hydrogen and Li1-xHxNbO3 is formed. Currently, mechanisms and kinetics of the proton-exchange process are unclear, primarily due to a lack in reliable tracer diffusion data. We studied lithium and hydrogen tracer diffusion in proton-exchanged congruent LiNbO3 single crystals in the temperature range between 130-230 °C. Proton-exchange was done in benzoic acid with 0, 1, 2, or 3.6 mol% lithium benzoate added, resulting in micrometre thick surface layers where Li is substituted by H with relative fractions between x = 0.45 and 0.85 as determined by Nuclear Reaction Analysis. For the diffusion experiments, ion-beam sputtered isotope enriched 6LiNbO3 was used as a Li tracer source and deuterated benzoic acid as a H tracer source. Isotope depth profile analysis was carried out by secondary ion mass spectrometry. From the experimental results, effective diffusivities governing the lithium/hydrogen exchange as well as individual hydrogen and lithium tracer diffusivities are extracted. All three types of diffusivities can be described by the Arrhenius law with an activation enthalpy of about 1.0-1.2 eV and increase as a function of hydrogen content nearly independent of temperature. The effective diffusivities and the lithium tracer diffusivities are identical within a factor of two to five, while the hydrogen diffusivities are higher by three orders of magnitude. The results show that the diffusion of Li is the rate determining step governing the proton-exchange process. Exponential dependencies between diffusivities and hydrogen concentrations are determined. The observed increase of Li tracer diffusivities and effective diffusivities as a function of hydrogen concentration is attributed to a continuous reduction of the migration enthalpy of diffusion by a maximum factor of about 0.2 eV. Simulations based on the determined diffusivities can reproduce the step-like profile of hydrogen penetration during proton-exchange.

Lithium tracer diffusion in proton-exchanged lithium niobate

We studied lithium tracer diffusion in proton-exchanged congruent LiNbO3 single crystals in the temperature range between 150 and 230 °C. Proton exchange was done in benzoic acid with 1 mol% lithium benzoate added, resulting in a surface layer where Li is substituted by H for about 60% (about 12 at.% H within LiNbO3). For the diffusion experiments, ion-beam sputtered isotope enriched 6LiNbO3 was used as a tracer source. Isotope sensitive depth profile analysis was carried out by secondary ion mass spectrometry in depth profile mode. From the experimental results effective diffusivities governing the lithium/hydrogen exchange and lithium tracer diffusivities are extracted. Both types of diffusivities are identical within error limits and follow the Arrhenius law with an activation enthalpy of about 1.2 eV. In contrast, the lithium tracer diffusivities are lower by three orders of magnitude than hydrogen tracer diffusivities which were recently determined by our group. The results indicate that the diffusion of Li is the rate determining step for the proton-exchange process.

Hydrogen diffusion in proton-exchanged lithium niobate single crystals

Hydrogen diffusion in proton-exchanged congruent LiNbO3 single crystals is investigated in the temperature range between 140 and 200 °C. The proton-exchange process carried out in a mixture of benzoic acid and lithium benzoate (1 mol. %) results in an up to 2 μm thick surface layer where Li is substituted by H for about 60% (about 12 at. % H within LiNbO3) as determined by nuclear reaction analysis. For the diffusion experiments, deuterated benzoic acid is used as a tracer source and the hydrogen/deuterium isotope-exchange occurs at temperatures of 200 °C and below. Isotope sensitive depth profile analysis is done by secondary ion mass spectrometry. From the experimental results, effective diffusivities governing the lithium/hydrogen exchange and tracer diffusivities of deuterium within the exchanged layer are extracted. Both types of diffusivities can be described by the Arrhenius law with an activation enthalpy of about 1.2 eV, while the effective diffusivities are lower by three orders of magnitude. This result shows that the diffusion of hydrogen is not the rate-determining step for the proton-exchange process. Possible diffusion mechanisms of hydrogen tracer diffusion are discussed.

Phase composition of channel proton-exchanged waveguides in different near-congruent LiNbO3

Micro-Raman spectroscopy and method of Raman data calibration are proposed for the quantitative study of the proton-exchanged LiNbO3 channel waveguides, which contain the different HxLi1–xNbO3 phases, depending on the fabrication conditions. The spectroscopic parameters, phase composition and electro-optic properties of these waveguides have been found to be depending on the small variation of stoichiometry in the near-congruent lithium niobate substrates used for waveguides fabrication.

Liquid crystal-based widely tunable integrated optic wavelength filters

We report widely tunable wavelength filters using liquid crystal (LC)-based long-period waveguide gratings (LPWGs). The high birefringence and electrically tunable refractive index of the LC have been used to achieve a tuning range covering most of the S, C, and L bands of an optical communication system. E7 LC has been used as a cover layer in an LPWG rib waveguide structure. The LPWG structure couples light from the fundamental mode to the slab modes of the waveguide. This results in transmission dips at resonance wavelengths. The mode constants of the coupling modes can be changed by controlling the refractive index of the LC and hence the resonance wavelengths by using the externally applied electric field. We have carried out numerical simulations to analyze the performance of E7 LC-based LPWGs on various substrates such as ion-exchanged BK7 glass and proton-exchanged LiNbO3 and silicon-on-insulator and silicon nitride substrates. The potential applications of the device are electrically tunable wavelength filters for optical communication systems.

Transmission Spectral Characteristics of Photonic Crystals Milled in Annealed Proton-Exchange LiNbO3 Waveguide**Supported by the National Natural Science Foundation of China under Grant Nos 50872089, 61077039 and 61377060, the Research Grants Council of the Hong Kong Special Administrative Region of China under Grant No 11211014, the Key Program for Research on Fundamental to Application and Leading Technology of Tianjin Science and

Transmission spectra of triangular lattice photonic crystals milled in the top surface of an annealed proton-exchange waveguide are numerically simulated. The effects of the finite depth, conical shape, trapezoidal shape and hybrid shape of holes are theoretically analyzed. Due to the difficulty of milling high aspect-ratio cylindrical holes in lithium niobate (LiNbO3), a compromised solution is proposed to improve the overlap between shallow holes and the waveguide mode, and useful transmission spectra with strong contrast and sharp band edges are achieved.

Surface modification of optical materials with hydrogen plasma for fabrication of Bragg gratings.

We investigate the hydrogen plasma process as a route for creating Bragg gratings (BGs) on optoelectronic materials such as undoped lithium niobate (LiNbO(3)), proton-exchanged LiNbO(3), and soda-lime glass. Photopatterns (periodic modulations, Λ=323-2000 nm) were created on those substrates and the hydrogen plasma process was investigated for its ability to transfer the microstructures and the underlying mechanisms involved in this process. The diffraction efficiency and surface topology of the BG were characterized, as well as the optical properties of corresponding bulk materials undergoing the same plasma treatment. It is shown that the hydrogen plasma treatment changes the complex refractive index and modifies the surface topology with a volume expansion in the near-surface region, and both features are connected to the appearance of structural defects in the materials. The hydrogen plasma offers unique flexibility and advantages that can be explored for the fabrication of integrated photonic components.

Green-induced infrared absorption in annealed proton-exchanged MgO:LiNbO_3 waveguides

The green-induced infrared absorption (GRIIRA) effect in annealed proton-exchanged (APE) MgO-doped LiNbO3 (MgO:LiNbO3) waveguides was investigated experimentally. By directly measuring the green-light-induced transmittance change in the waveguides, the GRIIRA coefficients of the waveguides were studied at a light power density level of several MW/cm2. GRIIRA effects in APE MgO:LiNbO3 and nondoped MgO:LiNbO3 waveguides were compared. The GRIIRA-induced temperature gradient from the waveguide core to the ambient environment was simulated using the measured results. Potential impacts of such temperature gradients on MgO:LiNbO3-based green lasers have been discussed. Optimization temperature control methods have also been discussed.

Subsurface disorder and electro-optical properties of proton-exchanged LiNbO 3 waveguides produced by different techniques

Subsurface disorder and electro-optical properties of proton-exchanged LiNbO 3 waveguides produced by different techniques

Subsurface disorder and electro-optical properties of proton-exchanged LiNbO3 waveguides produced by different techniques

It has been established, that proton-exchanged LiNbO3 waveguides have a marked subsurface layer with structural disorder inducing degradation of electro-optical properties of these waveguides. At the same time, such a subsurface disorder is found to be less pronounced in soft proton-exchanged (SPE) waveguides in comparison with annealed proton-exchanged (APE) ones. The experimental samples of phase modulators fabricated by SPE technique exhibit a better electro-optical efficiency compared to the LiNbO3 modulators produced by the standard and improved APE techniques.

Design of strain-introduced MZI interleaver on LiNbO3 substrate

A strain-introduced Mach-Zehnder interferometer (MZI) interleaver on lithium niobate (LiNbO3) is proposed. The structure of the strain-introduced waveguide is designed in detail, and is produced by depositing a SiO2 film on the annealed proton-exchanged LiNbO3 waveguide. Considering the sensitivities of the edge strain to the deposition temperature and the thickness of the SiO2 film, an optimum design of 50 GHz interleaver on this structure is given through analyzing the effective index changes for Epqx mode by finite difference method (FDM). The length of the bending waveguide in this interleaver is just two thirds of that in the conventional interleaver due to the high refractive index difference.

Effects of swift argon-ion irradiation on the proton-exchanged LiNbO3 crystal

A proton-exchanged LiNbO3 crystal was subjected to 70-MeV argon-ion irradiation. The lattice damage was investigated by the Rutherford backscattering and channeling technique. It was found that the lattice disorder induced by the proton exchange process was partially recovered and the proton-exchanged layer was broadened. It indicated that the lithium ions underneath the initial proton-exchanged layer migrated to the surface during the swift argon-ion irradiation and supplemented the lack of lithium ions in the initial proton-exchanged layer. This effect was ascribed to the great electronic energy deposition and relaxation. The swift argon-ion irradiation induced an increase in extraordinary refractive index and formed another waveguide structure beneath the proton-exchanged waveguide.

Lattice damage and waveguide properties of a proton-exchanged LiNbO3 crystal after oxygen-ion implantation

A z-cut LiNbO3 crystal was immersed in a molten benzoic acid for 10min and then was implanted with 6-MeV oxygen ions at a fluence of 6×1014 ions/cm2. Lattice damage in this crystal was measured by a Rutherford backscattering and channeling technique and was compared with lattice damage in a proton-exchanged LiNbO3 crystal and an oxygen-ion-implanted LiNbO3 crystal. A totally amorphous layer was formed at the crystal’s surface after both proton exchange and oxygen-ion implantation processes were performed, even though either process alone never led to a relative disorder of the lattice up to 0.2. It indicates that the crystal lattice in the proton-exchanged layer is unstable and can be easily damaged by ion implantation subsequently. The waveguide structure formed by proton exchange was destroyed by oxygen-ion implantation. Oxygen-ion implantation induced an increase in extraordinary refractive index and formed another waveguide structure underneath the amorphous surface layer.

Analytical study of phase-matched modes of diffused, proton-exchanged [formula omitted] waveguides

In this paper, we present an analytical model for guided modes determination in diffused proton-exchanged LiNbO 3 waveguides. The dispersion equation and guided mode shapes are obtained. These results calculated at different wavelengths permit for the phase matching exploration in phenomena such as second harmonic generation.

Photorefractive effect in LiNbO3-based integrated-optical circuits at wavelengths of third telecom window

Original results on investigation of the photorefractive effect in straight channels and integrated-optical circuits such as a directional coupler, Y-splitter and Mach–Zehnder interferometer, exploiting titanium-indiffused and proton-exchanged LiNbO3 waveguides, are presented. It has been found that the photorefractive damage is non-negligible for IR radiation with wavelengths near 1.5 μm in all circuits studied. The new methods for accurate evaluation of small extents of photorefractive effect are proposed.

A finite-difference frequency-domain method for full-vectroial mode solutions of anisotropic optical waveguides with arbitrary permittivity tensor

A new finite-difference frequency-domain (FDFD) method based eigenvalue algorithm is developed for analyzing anisotropic optical waveguides with an arbitrary permittivity tensor. Yee's mesh is employed in the FD formulation along with perfectly matched layer (PML) absorption boundary conditions. A standard eigenvalue matrix equation is successfully derived through considering simultaneously four transverse field components. The new algorithm is first applied to the mode solution of a proton-exchanged LiNbO(3) optical waveguide and the results agree with those obtained using a full-vectorial finite-element beam propagation method. Then, the algorithm is used to study modes on a liquid-crystal optical waveguide with arbitrary molecular director orientation. This arbitrary orientation may cause the loss of transverse-axis symmetries of the waveguide with symmetric background structure. Asymmetric mode-field profiles under such situations are clearly demonstrated in the numerical examples.

Optical waveguides in LiNbO3 and stoichiometric LiNbO3 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.

Gamma-Ray Irradiation-Enhanced Wet-Etching of Proton-Exchanged Lithium Niobate

Gamma-ray irradiation-enhanced wet-etching of proton-exchanged Z-cut Y-propagating LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> is presented. The etched ridge height of a sample with gamma-ray irradiation of intensity 650 krad/h on a LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> substrate for 30 min is three times larger than that without. Moreover, experimental results show the propagation loss values of Ti-indiffused LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> waveguides fabricated on the irradiated ridge structures are only slightly larger than those on the nonirradiated ones.