Proton-exchanged Waveguides Research Articles

Buried annealed proton-exchanged waveguide in periodically-poled MgO:LiTaO3 fabricated by surface-activated bonding for high-power wavelength conversion

Watt-class UV laser lights with symmetric spatial modes are suitable for high-resolution industrial applications. Channel waveguides with a guided mode of several ten microns diameter in MgO:LiTaO3 enhance integration capability while avoiding photorefractive damages during high-power wavelength conversion. We focused on buried waveguides fabricated by proton exchange, surface-activated bonding (SAB), and proton diffusion processes in periodically-poled (PP) MgO:LiTaO3. In this work, the mode profiles were simulated using the proton diffusion coefficients estimated by secondary ion mass spectrometry. Using the simulation results, the buried waveguides with a mode diameter larger than 30 μm were fabricated. By adopting designed PP structures, second harmonic generation (SHG) devices at a wavelength of 532 nm were fabricated. The nonlinear coupling coefficient was estimated to be 0.15 W−1/2 cm−1. Compared to the conventional annealed proton-exchanged waveguide SHG device without SAB, symmetric guided modes were obtained while maintaining the nonlinear coupling coefficient.

The impact of various factors on the surface of X-cut lithium niobate and properties of proton-exchanged waveguides

This work describes the effects of various impacts (thermal, plasma, chemical) on X-cut lithium niobate surface and on the structure and optical characteristics of proton-exchanged waveguides. Pre-annealing at 500 °C improved the structural homogeneity of congruent lithium niobate and thus enabled fabricating waveguides of a higher quality. Ar-plasma pretreatment damaged crystal surface and increased lattice deformations. Chemical treatment influenced the surface composition of lithium niobate and the refractive index in annealed proton exchange waveguides. The results provide better understanding of the role of crystal surface in the fabrication of diffusion waveguides and photonic integrated circuits.

Visualization of κ 1-phase in proton-exchange waveguides on the surface of lithium niobate crystal

This work is devoted to the visualization and evolution of κ 1-phase particles in annealed proton exchange (PE) waveguides. It is well-known that the presence of κ 1-phase particles in optical waveguides leads to an increase in their optical losses and instability of operation. It has been found that precipitated κ1 -phase particles propagate through the whole waveguide depth and have perpendicular orientation to the surface of the lithium niobate crystal. These results are important for fast quality control of annealed PE waveguides.

Influence of Thermal Pretreatment of Lithium Niobate Plates on the Characteristics of Proton-Exchange Waveguides

Influence of Thermal Pretreatment of Lithium Niobate Plates on the Characteristics of Proton-Exchange Waveguides

Long-period gratings on lithium-niobate waveguides: phase-matching turning points for ultra-broadband mode conversion

We analyze a polymer long-period grating (LPG) formed on the surface of a z-cut lithium-niobate (LN) few-mode waveguide for TM00–TM10 mode conversion. We identify and explore the turning points along the phase-matching curves of the LPGs for the achievement of ultra-broadband mode conversion. We fabricate a number of gratings with annealed proton-exchanged LN waveguides and our experimental results agree well with the theoretical results. A typical fabricated grating operating near the turning point provides a mode extinction ratio larger than 20 dB from 1550 nm to a wavelength longer than 1700 nm, and another one provides a mode extinction ratio larger than 10 dB from 1464 nm to a wavelength longer than 1700 nm. Our gratings show good thermal stability. The structure of the LPG on LN waveguide could be applied to the realization of ultra-broadband high-speed mode switches for mode-multiplexing applications.

Formation of channel proton-exchange waveguides in YB3+, ER3+:PPLN

Formation of channel proton-exchange waveguides in YB3+, ER3+:PPLN

Enlargement of mode size in annealed proton-exchanged periodically-poled MgO doped stoichiometric LiTaO3 waveguide for high power second harmonic generation

Watt-class wavelength conversion is expected by enlarging the mode size of the channel waveguides to dozens of micrometers to avoid optical damage with high-power pump lasers. In this work, uniform periodically-poled (PP) structures in MgO doped stoichiometric LiTaO3 were fabricated by applying voltage with a SiO2 insulation layer. Annealed proton-exchanged waveguides with a full width at 1/e 2 maximum in a width of 43 μm with an in-depth of 28 μm were obtained by proton exchanging for 4 h and thermal diffusion for 75 h. This waveguide can be excited by pump waves with a power of 8.6 W considering the optical damage threshold. Second harmonic generation with a wavelength of 515 nm was conducted in the low power region of about 35 mW to investigate the basic characteristics. Normalized wavelength conversion efficiency in the 15 mm long PP structures was estimated to be 4.3% W−1.

Reduction in DC-Drift in LiNbO3-Based Electro-Optical Modulator

This study involves the results of research on short-term and long-term DC-drifts in electro-optical modulators based on annealed proton exchange waveguides in LiNbO3 crystals after wafer pre-annealing. The relaxation time of the DC-drift of the operating point for a short-term drift is measured in minutes, and for a long-term drift it is measured in hours and days. DC-drift was measured by applying bias voltage and changing crystal temperature. The obtained results show significant impact on the stability of operating point in EO-modulators after treatment of defective structure of the near-surface layer of a LiNbO3 crystal. Treatment of the disturbed near-surface layer of a LiNbO3 crystal results in the simultaneous reduction in short-term DC-drift and increase in operation stability of electro-optical modulators during long-term measurement of temperature by activation energy calculation.

Подавление дрейфа рабочей точки интегрально-оптических схем на основе ниобата лития

This work is devoted to the study of the drift of the operating point of integrated-optical circuits based on proton-exchange waveguides in lithium niobate crystal with a recovered structure of the near-surface layer. Recovered of the damaged near-surface layer of lithium niobate wafer was carried out using pre-annealing at temperature of 500 °C. Drift of operating point is characterized by a constant change in the optical output power of the integrated-optical circuits when a bias voltage is applied to the electrodes or temperature changes. Recovered of the damaged near-surface layer of lithium niobate wafer leads to a decrease in the short-term and long-term drifts of the operating point of integrated-optical circuits. Crystal structure factor was investigated on the drift of operating point of integrated-optical circuits based on lithium niobate crystal.

An Optical Switch Based on Electro-Optic Mode Deflection in Lithium Niobate Waveguide

We propose a 1 × 2 electro-optic miniature optical switch based on mode deflection of lithium-niobate (LN) annealed proton exchange waveguides with microstructured electrodes. Our fabricated device consists of LN waveguides covered with isosceles-triangle-shaped array electrodes, a horn-shaped input waveguide, and two tapered output waveguides. At a driving voltage of 40 V, the two-modes can switch between two output ports with an extinction ratio of 17.39 dB at the wavelength of 1530 nm. The switching speed is 22 ns. This miniature mode switch could find applications in signal processing and high-speed optical communication networks.

Influence of surface treatment on the structure and properties of proton-exchanged waveguides in lithium niobate

The influence of reverse proton exchange (RPE) and some types of surface pretreatment (exposure to Piranha or RCA-1 solutions or to HNO3+HF mixture) on the structure and properties of proton-exchanged layers has been studied in X-cut lithium niobate (LN). It has been shown that the rate of phase transitions in PE layers during RPE is greater than that during annealing at the same temperature. Pretreatment of LN surface before PE by Piranha or RCA-1 solutions slightly increases refractive index change Δne in the resulting waveguide, whereas pretreatment by HNO3+HF mixture decreases it.

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.

Моделирование кривых дифракционного отражения рентгеновских лучей от протонообменных слоев монокристалла ниобата лития

In this work, within the framework of the dynamic theory of the X-ray scattering, a model is constructed that describes the diffraction reflection curves obtained from the proton-exchange layers of a lithium niobate monocrystal subjected to the post-exchange annealing. Planar proton-exchange waveguides based on the X-cut of the lithium niobate monocrystal are obtained experimentally. It was shown that the proton implantation leads to the formation of the new crystalline phases with a larger lattice parameter. The structure changes (by the method of the diffraction structural analysis) and the optical properties (by the method of the mode spectroscopy) of the obtained waveguides are investigated in the various temperature regimes. The microstrain of the crystal lattice caused by еру proton implantation was estimated by the analysis of X-ray diffraction line broadening. It is shown the thickness of the waveguide layer and the number of new crystalline phases depend not only on the temperature regime of the proton exchange, but also on the duration of post-exchange annealing. The simulation of the experimentally obtained curves of the diffraction reflection is carried out within the framework of the described model. As a result of the modeling, the depth of the waveguide layer was determined, which is consistent with the data obtained by the method of the mode spectroscopy. The models of the assumed profiles of the crystal lattice deformation caused by proton implantation are presented. The average values of the lattice microstrain and the phase composition of proton-exchange lithium niobate layers are determined.

Fabrication and characterization of annealed proton exchanged long period waveguide grating in x-cut LiNbO3

In this paper, a long-period waveguide grating was fabricated in x-cut lithium niobate substrate by patterned annealed proton exchange waveguide fabrication process. The waveguide mode characteristic was evaluated using a charge-coupled device (CCD) camera. It shows that the waveguide is single mode transmission at a wavelength of 1 550 nm. The transmission spectra of the long period waveguide gratings were measured by optical spectrum analyzer (OSA) and show an extinction ratio of ~17 dB and a 3 dB bandwidth of ~10 nm at the resonant wavelength. The resonant wavelength moves toward to the long wavelength direction as the waveguide width-difference increases in the same period, and also shifts toward to the long wavelength direction with the increase of the period in the case of the same waveguide width-difference. The method of fabricating a long period waveguide grating based on a patterned annealed proton exchange technique simplifies the fabrication process, and at the same time, reduces the fabrication cost.

Polarization mode converters in ZnO-diffused LiNbO3 waveguides

The linear electro-optic effect in lithium niobate is capable of realizing a variety of polarization transformations, including TE-to-TM and left-to-right circular polarization conversion. Several types of devices have been demonstrated in Ti-diffused waveguides. LiNbO3 devices have historically been based on either Ti-diffused or proton-exchanged waveguides. The proton-exchanged waveguides only guide light polarized along the optic axis, and therefore, are not applicable to polarization transforming devices. Zinc oxide diffusion is an alternative waveguide fabrication technology that guides both e- and o-waves with much higher power-handling capability than Ti : LiNbO3 waveguides. ZnO : LiNbO3 waveguides exhibit a highly circular mode field with lower anisotropy than Ti-diffused waveguides. We report the design, fabrication, and testing of ZnO : LiNbO3 devices for polarization mode conversion.

Effect of pre-annealing of lithium niobate on the structure and optical characteristics of proton-exchanged waveguides

The influence of pre-annealing of lithium niobate in the range of 400–800 °C was studied at each step of formation of proton-exchange waveguides. Pre-annealing affects the structure and optical properties of lithium niobate. There is a relationship between the initial structure of lithium niobate and the optical characteristics of proton-exchange waveguides. The linear dependence between the deformations of the annealing proton exchange waveguides and the refractive index is established. Pre-annealing leads to an improvement in the structure of the subsurface layer of lithium niobate, as well as a slight decrease in the refractive index due to an increase in the homogeneity and a lower concentration of protons inside the crystal. The crystal structure becomes most uniform at a temperature of 500 °C. The results can be useful for different optical crystals used for applications.

Electro-optic deflection in a lithium niobate quasi-single mode waveguide with microstructured electrodes.

We propose an electro-optic mode deflection device based on an annealed proton exchange (APE) waveguide in lithium niobate, associated with isosceles-triangle-shaped array electrodes and a horn-shaped input waveguide. The input waveguide is tapered down to ensure that the output of the device has a good beam quality, i.e., a quasi-single mode in this case. This new device allows beam deflection at a relative low voltage and large deflection angle. At an APE-waveguide width of 80 μm, mode deflections of 0.265 and 0.240 μm/V are obtained for 1064 and 980 nm, respectively. This beam deflection device can be applied in high-speed optical switch, and beam smoothing of a high-power laser, etc.

Estimation of Crystal Lattice Deformations with Depth of Proton-Exchanged Optical Waveguides on Lithium Niobate Single Crystal Substrates

A method for determining the crystal phase and estimating the crystal lattice deformations in HxLi1-xNbO3 proton-exchanged optical waveguides from an analysis of the Raman light scattering spectra measured with high spatial resolution (the so-called method of micro-RS spectroscopy) is suggested.

ОСОБЕННОСТИ СТРУКТУРНО-ФАЗОВЫХ ПРЕВРАЩЕНИЙ ПРИ ОТЖИГЕ ПРОТОНООБМЕННЫХ СЛОЕВ НА Z-СРЕЗЕ КРИСТАЛЛА НИОБАТА ЛИТИЯ

С использованием методов оптической микроскопии в поляризованном свете, модовой спектроскопии, ИК-спектроскопии и рентгеноструктурного анализа подтверждена последовательность фазовых превращений, происходящих при отжиге протонообменных слоев на Z-срезе кристалла ниобата лития: b2-, b1-фазы ® k2-фаза ® k1-фаза ® α-фаза. Установлено, что в процессе отжига в ходе фазовых превращений когерентность слоев, образованных из различных фаз, не нарушается, что обусловлено меньшей величиной деформаций (напряжений) формирующихся протонообменных фаз по сравнению с деформациями (напряжениями) на X-срезе кристалла. 
 Работа выполнена при финансовой поддержке гранта РФФИ (проект № 17-43-590309 р_а).

Analysis of Waveguides on Lithium Niobate Thin Films

Waveguides formed by etching, proton-exchange (PE), and strip-loaded on single-crystal lithium niobate (LN) thin film were designed and simulated by a full-vectorial finite difference method. The single-mode condition, optical power distribution, and bending loss of these kinds of waveguides were studied and compared systematically. For the PE waveguide, the optical power distributed in LN layer had negligible change with the increase of PE thickness. For the strip-loaded waveguide, the relationships between optical power distribution in LN layer and waveguide thickness were different for quasi-TE (q-TE) and quasi-TM (q-TM) modes. The bending loss would decrease with the increase of bending radius. There was a bending loss caused by the electromagnetic field leakage when the neff of q-TM waveguide was smaller than that of nearby TE planar waveguide. LN ridge waveguides possessed a low bending loss even at a relatively small bending radius. This study is helpful for the understanding of waveguide structures as well as for the optimization and the fabrication of high-density integrated optical components.