Nanosecond response of Bragg deflectors in periodically poled LiNbO3

For the first time, transients are measured for a bulk-optical Bragg-deflector based on an electrooptically (EO) induced grating in periodically poled LiNbO₃ (PPLN). Optical pulse risetimes of 1.2 ns are reported with a first-order diffraction efficiency of 76% for circular Gaussian beams. The fast risetime is accounted for by a substantially suppressed excitation of acoustic thickness modes and careful electrode design. For many applications, the device could replace both acoustooptical Bragg-deflectors and conventional EO amplitude modulators due to a fivefold reduction in risetime and fourfold reduction in drive power, respectively     

Cutoff wavelength of periodically segmented waveguides in Ti:LiNbO3

The cutoff wavelength for Ti:LiNbO₃ periodic segmented waveguides (PSWs) with different duty-cycles and of continuous waveguides has been measured for both polarizations, PSWs are described by an equivalent continuous waveguide and, using this model, a universal relationship between the cutoff wavelength of PSWs with different duty-cycles is proposed and verified experimentally     

CW periodically poled LiNbO3 optical parametricoscillator model with strong idler absorption

An analytical model is described for predicting the gain and performance of CW singly resonant optical parametric oscillators with strong idler absorption, strong pump depletion, and thermally induced dephasing. Pump and signal beam focusing effects are included by using the Guha et al. h_sm parameters. It is shown that significant parametric gain can be generated with an idler absorption-length product exceeding 10, and in some cases idler absorption is seen to enhance the pump depletion by impeding back conversion. The model accurately predicts current experimental results with weak idler absorption. Validation with strong idler absorption awaits experimental measurements to be completed     

Second-harmonic generation in periodically poled lithium niobate single-crystal fibres

Periodic ferroelectric domain structures were introduced into lithium niobate (LiNbO/sub 3/) single-crystal fibres by the electric field poling method using a configuration of direct electrode contact. The fibres were pumped with a 1064 nm Nd:YAG laser with a CW input power as high as 1 W. The maximum generated second-harmonic signal was 1.6 mW, thus corresponding to an overall conversion efficiency of 0.016%.     

Increased acceptance bandwidth for quasi-phasematched secondharmonic generation in LiNbO3 waveguides

The authors demonstrate waveguide second harmonic generation using a variable-spaced phase-reversed quasi-phasematched grating that has an acceptance bandwidth 15 times greater than a uniformly quasi-phasematched interaction of the same length     

Electro-optic effect in proton exchanged LiNbO3 andLiTaO3 waveguides

Electro-optic properties of proton-exchanged (PE) waveguide layers in LiTaO₃ and LiNbO₃ are studied and related to their optical characteristics. The proton-exchange process induces a degradation of the electro-optic activity in both types of waveguides, PE LiNbO₃ and PE LiTaO₃. The measured electro-optic effect is close to the detection sensitivity even when the exchange regime is performed at low temperatures for short periods of time. The PE samples have been annealed (APE waveguides) and the changes of their r₃₃ electro-optic coefficient has been followed at successively higher temperatures and periods of time. Subjected to annealing at temperatures between 265-420°C, the LiTaO₃ layers show a partially recovered r₃₃ coefficient, the recovering being different for quick and slow cooling of the samples. In thin APE LiNbO₃ waveguiding layers a restoration of r₃₃ up to 75% of the bulk value is observed due to the annealing at temperatures between 200-340°C     

Design parameters of Ti:LiNbO3 waveguides for optimumacoustooptic interaction efficiency

A computer model of optical and surface acoustic wave modes in titanium indiffused Y-cut LiNbO₃ guiding structures is used to produce several graphs for the design of guided-wave acoustooptic devices with optimum interaction characteristics. The design concentrates on waveguide fabrication parameters in order to achieve the optimum acoustic frequency and the acoustooptic interaction bandwidth. The optimum acoustic frequency for a given value of Ti film thickness and diffusion depth is defined as the frequency causing the maximum value of the overlap integral     

Ti:LiNbO_3质子交换光波导

本文研究了在Ti扩散LiNbO_3衬底上用苯甲酸质子交换制备光波导的方法,以及单模条件、折射率分布、交换系数、表面状态和热稳定性等问题。     

Second-harmonic generation of green light in periodically poled planar lithium niobate waveguide

A periodically poled, planar waveguide in lithium niobate was used to generate 532 nm radiation at room temperature by continuous-wave frequency-doubling with a conversion efficiency of 5% per W cm/sup 2/. Quasi-phase-matching allowed generation of the second harmonic using the d/sub 33/ nonlinear coefficient.<>     

Fabrication of LiNbO3 channel waveguides using water

A channel waveguide formed in LiNbO₃ was fabricated by using distilled water instead of acids for proton exchange and a self-aligned SiO₂ cap for diffusion. It was demonstrated that the water reaction is an important reaction to cause the index change in proton exchange     

Fabrication tolerance of Ti:LiNbO3 waveguides

A simulation program based on the three-dimensional beam propagation method (BPM) is used to study the fabrication conditions of single-mode Ti:LiNbO₃ waveguides. The calculated cutoff wavelengths are in good agreement with experimental data. The thickness of titanium needed to support the fundamental and first-order modes for three Ti strip widths (6, 7, and 8 μm) is calculated. The two-dimensional BPM is used to calculate the coupling length from the fabrication conditions. Results are consistent with measured data. The fabrication tolerances of the coupling length of directional couplers are also presented     

Ti扩散LiNbO_3单模波导参数的测量

对Ti扩散LiNbO_3波导分别采用双曲正割和抛物线分布来近似其折射率的高斯分布,在两个波长下由相应的两种近似本征值方程分别推得了单模波导参数的计算公式,并在4880(?)和6328(?)激光波长下利用棱镜耦合器测量了单模波导参数,对测量结果给出必要的理论分析。     

LiNbO3 optical waveguides formed in a new proton source

Proton-exchanged planar waveguides have been fabricated on Z-cut and X-cut lithium niobate crystals by using a new proton source formed by a mixture of benzoic and adipic acids. Waveguide index profiles and optical characteristics have been obtained at different values of the adipic-benzoic acid concentration ratio. The samples have been structurally characterized by Raman and infrared (IR) absorption spectroscopy and double-crystal X-ray diffraction. Good quality samples have been fabricated by using 30 mol% ratio dilution, showing very low scattering levels (<0.1 dB/cm), relatively high electrooptic coefficient (r₃₃=0.88 pm/V), and low relative percentage of interstitial protons (26%). The main factor limiting the waveguide optical properties is the substitutional-interstitial proton ratio, which can be easily controlled to produce good quality waveguides. A demonstration of the repeatability of the exchange process in the acid mixture is also provided     

Second-harmonic and sum-frequency generation in KTiOAsO4

Tungstate flux-grown KTiOAsO₄ (KTA) has been found to be phase matchable for type-2 SHG and SFG down to 0.5174 and 0.4193 μm at 20.0°C. Sellmeier's equations and nonlinear optical constants of this crystal are reported     

Finite element analysis of LiNbO3 waveguides with Si orSi/SiO2 overlay

LiNbO₃ waveguides with Si overlays are emerging as a basic building block for a variety of integrated-optic components, including modulators, high-efficiency gratings, and narrowband WDM filters. However, the development and optimization of these devices are, in large part, hindered by the lack of understanding of the specifics of the Si-on-LiNbO₃ structure which appear to differ dramatically from those of the Si and LiNbO₃ waveguides, considered separately. In this work, we provide a specific insight into the waveguiding properties of vertically stacked Si-on-LiNbO₃ waveguides. In particular, we present a detailed theoretical analysis of the effect of the Si film on the modal characteristics (propagation constant and field distribution) of the structure. The vectorial finite element method (VFEM) is used to numerically investigate a step-index and graded-index single-mode channel waveguide in LiNbO₃, with a Si or Si/SiO₂ multimode overlay. We show that for ~70% of all Si thicknesses, in the range from 0 to 1.6 μm, the highest order normal mode of the entire structure has more than 99.9% of the total energy confined in the LiNbO₃ region, i.e., beneath the Si overlay. This fact is quite intriguing given the fact a planar Si layer of submicron thickness on bulk LiNbO₃ is already multimoded. Furthermore, we show that the effective mode index of the structure is considerably modified compared to that of the LiNbO₃ waveguide while the propagation loss is, on the other hand, practically unaffected (~0.3 dB/cm) even in the presence of the lossy Si film, as confirmed by our previous experimental results. Evidently, large modulation of the effective index and low-loss propagation provide an ideal combination of properties suitable for the fabrication of high-reflectance corrugated waveguide gratings, essential for a number of practical devices, in particular, WDM filters     

Buried annealed/proton-exchanged LiNbO3 waveguides fabricated by direct bonding technique

A new method for fabrication of buried annealed/proton-exchanged LiNbO₃ waveguides using direct bonding technique is demonstrated. The buried waveguides were fabricated and tested at 1.55 m wavelength, and guided modes with excellent symmetry were obtained. The waveguides are suitable for efficient coupling to optical fibres and/or efficient nonlinear-optic wavelength conversion devices.     

Ti扩散LiNbO_3平面波导的折射率分布测量

用反射干涉技术测量了Ti扩散LiNbO_3平面波导的折射率分布。简介了测试原理、样品制备方法,并给出了测试结果。     

Theoretical modeling of optical amplification in Er-doped Ti:LiNbO3 waveguides

An accurate theoretical analysis is presented describing optical amplification in Er-diffused Ti:LiNbO₃ channel waveguides. It follows as far as possible the theory already developed for Er-doped fibers. As optical pumping around λ_p≈1.48 μm is considered, a quasi-two-level model for the Er³⁺ ions is used with wavelength-dependent cross sections. The optical gain in the 1.53 μm<λ<1.64-μm wavelength range is evaluated. The characteristic parameters, as Er concentration profile, cross sections, pump, and signal mode distributions and waveguide (scattering) losses are taken from experiments. Examples of numerically calculated pump-, small-signal-gain-, and ASE-evolutions are presented. The model has been tested by comparing computed and experimentally observed gain characteristics for Xˆ- and Yˆ-cut LiNbO₃; an almost quantitative agreement has been obtained     

Anomalous refractive index change in proton exchanged LiNbO3 waveguides after annealing

An anomalous refractive index change in proton-exchanged LiNbO₃ optical waveguides formed at the benzoic acid melt after annealing has been observed and explained     

Noncritical quasi-phase-matched second harmonic generation in anannealed proton-exchanged LiNbO3 waveguide

We report the demonstration of dimensional noncritical phase matching, a phase-matched interaction length exceeding 10 mm, and an internal conversion efficiency of 204%/W for second harmonic generation of 976 nm radiation in a periodically poled, annealed proton-exchanged LiNbO₃ waveguide. Using models for the linear and nonlinear optical properties of annealed proton-exchanged LiNbO₃ waveguides and the observed ferroelectric domain grating, the phase-matching wavelength was predicted to within several nm and the conversion efficiency to within ≈20% of the measured values. Optimization of waveguide second harmonic generation devices is discussed