Performance and modeling of proton exchanged LiTaO3branching modulators

Proton-exchanged LiTaO₃ branching modulators are demonstrated experimentally and modeled using a normal mode coupled-mode theory. The normal modes are approximated by ordinary coupled-mode theory parameters which are experimentally obtained. Agreement between the model and the experimental results is demonstrated. Crosstalk in these linear branch devices is found to be linearly proportional to the voltage-length product, a result which leads to an analytic expression for the output power     

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     

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     

Electrooptic coefficient measurements in LiTaO3 andLiNbO3 waveguides

The magnitudes of linear electrooptic coefficients r₁₃ and r₃₃ in Zn:LiTaO₃ repoled channel waveguides are reported. The measurements were made at 0.633-μm wavelength using a Fabry-Perot interferometer. The waveguides were produced by diffusion from the vapor phase at a temperature above the Curie temperature. For full recovery of the Pockels effect, an electric field of 200 V/cm is needed during repoling. The measured values of r₁₃ and r₃₃ at 32-MHz modulation frequency are 7.2 and 30.3 pm/V, respectively. The difference between unclamped and clamped coefficients is comparable to that from bulk crystals. Measurements were also made on Ti:LiNbO₃ waveguides that did not require repoling, and good agreement with bulk crystal values was obtained     

Hybrid modes in proton exchanged waveguides realized in LiNbO3, and their dependence on fabrication parameters

In this paper, we present a relation between material structure, hybrid modes, and propagation losses in proton exchanged (PE) LiNbO₃ waveguides. Rutherford backscattering (RBS) and X-ray diffraction studies are used to show that there are two essential reasons for losses and complex behavior in high δn_e PE waveguides. RBS studies show that using benzoic acid melt temperatures below 300°C leads to distorted waveguide layers and propagation losses higher than several dB/cm. At high temperature, the X-ray studies prove that the crystalline order is preserved, but induced strain leads to biaxial exchanged layers, which results in hybrid modes that can have very high losses. Finally, fabrication parameters allowing the realization of high quality, high δn_e, PE waveguides in LiNbO₃ are identified     

Efficient frequency doubling in reverse proton exchanged lithiumniobate waveguides

We propose a novel approach for efficient frequency doubling of near infrared light, using the coupled system of buried and surface waveguides obtainable by reverse proton exchange in z-cut lithium niobate. In such guides, supporting TM and TE polarizations, respectively, in spite of the use of the d₃₁ nonlinear element, we predict conversion efficiencies as high as 90 or 14% μm/W cm in planar structures excited at 1.32 μm     

Crystalline and optical quality of proton exchanged waveguides

In this paper we demonstrate the possibility of obtaining high-quality proton-exchanged (PE) lithium niobate guides by performing the exchange at high temperatures (300°C) in benzoic acid diluted with lithium benzoate. Rutherford backscattering studies suggest that the higher exchange temperatures avoid the production of niobium dislocations, and, IR absorption measurements suggest that the use of lithium benzoate diluted melts prevents the creation of interstitial hydrogen.     

Anomalous attenuation and depolarization scattering in y-cut LiNbO3proton exchanged waveguides

A large anomalous attenuation and depolarization scattering have been observed iny-cut,x-direction propagating LiNbO3proton exchanged waveguides at 0.632 μm wavelength. We observe this phenomenon only when the mode effective index is smaller than the ordinary indexn_{0} (n_{0} = 2.286). The effect is caused by phase-match scattering into the substrate. Exchange solutions with various mole percentages of lithium benzoate have been used and the same phenomenon has been consistently observed.     

Fabrication of LiTaO3 optical waveguides by Ni diffusionbelow the Curie temperature

Using Ni diffusion into LiTaO₃ below the Curie temperature for the fabrication of optical waveguides was presented for the first lime. Ordinary and extraordinary polarization waveguide modes were obtained. The index distribution profiles of both modes were measured by a prism coupler. Loss in planar waveguides at 0.6328 μm is 0.7±0.1 dB/cm for the ordinary mode and 1.3±0.2 dB/cm for the extraordinary mode. Moreover, a Mach-Zehnder interferometer was fabricated by this method for electrooptic characterization. The measured half-wave voltage is 5.5 V and the extinction ratio is greater than 29 dB     

质子交换LiNbO_3平面光波导中的非简并四波混频

用棱镜耦合法在质子交换LiNbO_3平面光波导两端激发相对传播的光导波,在导波交迭处观察到垂直于波导表面出射的倍频光,实现了非简并四波混频.     

Characterization of annealed proton exchanged LiNbO3waveguides for nonlinear frequency conversion

A thorough and detailed characterization of annealed proton-exchanged (APE) waveguides in Z-cut LiNbO₃ is described. The mode index measurements in planar waveguides as a function of wavelength and annealing time are reported, including useful analytical relations for the refractive index change, its dispersion, and the depth profile as a function of annealing parameters. Analytical expressions for the mode propagation characteristics are presented and experimentally verified with reasonable accuracy. It is shown that the planar waveguide characterization results can be used to model the channel waveguide characteristics accurately. The model provides closed-form expressions for the mode index and the mode field profile, and the theoretical results are in excellent agreement with the measured data. The technique is used to accurately predict the phase mismatch between the fundamental and second harmonic modes in frequency-doubling experiments using APE channel waveguides. An optimum waveguide geometry for which the phase mismatch is relatively insensitive to the waveguide nonuniformity was predicted and verified experimentally     

Optimizing KTP and LiTaO3 channel waveguides forquasi-phase-matched second harmonic generation with high conversionefficiency

The conversion efficiency of quasi-phase-matched second harmonic generation (QPM-SHG) in waveguides can be enhanced significantly by optimizing the linear properties of the guiding structure. We describe here a method for fast and accurate computation of the modal properties of continuous, periodically poled, ferroelectric channel waveguides and a multiparameter optimization algorithm which we have used to maximize normalized internal SHG conversion efficiency. We present the phase-matching characteristics of these designs and discuss the possibility of high SHG conversion efficiency between lowest order transverse modes at the fundamental and second harmonic wavelengths near noncritically phase-matched regimes of operation     

Interferometric polarization-independent modulator in LiTaO3

Polarization-independent Mach-Zehnder modulators that utilize the off-diagonal r₅₁ electrooptic coefficient are produced in LiTaO₃. The waveguides are formed by Zn vapor diffusion. Extinction values of 85% at 0.633-μm wavelength are demonstrated for both transverse electric and transverse magnetic polarization using a single control voltage and thermal tuning. The relatively large voltage-length product values (VL⩾15 V-cm) attained in these devices are attributed to a weak overlap between optical and electrical field profiles. Electrode alignment and dielectric loading are critical factors for the optimization of device operation     

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     

Polarization-independent LiTaO3 guided-wave electroopticswitches

Polarization-independent LiTaO₃ optical switches that utilize the off-diagonal electrooptic coefficient r₅₁ are discussed. The waveguides are formed by Zn vapor diffusion. Interguide transfer efficiencies of 84 and 88% for TM and TE polarizations, respectively, have been obtained. Modulation efficiency of 99.8% for TM and 95% for TE have been demonstrated at nearly equal voltages by thermally tuning to 20°C. The relatively high voltage-length product (30-35 V-cm) attained in these initial devices at a 0.633-μm wavelength is attributed to a weak overlap between optical and electrical fields in the periodic electrode structure     

Full-wave analysis of coplanar waveguides for LiNbO3optical modulators by the mode-matching method considering nonidealconductors on etched buffer layers

Rigorous analysis of traveling-wave coplanar waveguide electrodes for LiNbO₃ optical modulator applications is presented by using an extended full-wave mode-matching method. The microwave propagation characteristics under the composite influence of substrate anisotropy, uniform or etched buffer layers, finite electrode thickness and conductivity, and metallization undercutting are accurately assessed by employing a network equivalent formulation. Variations of the coplanar waveguide microwave effective index and the characteristic impedance at low frequencies due to finite electrode conductivity are illustrated, and are important even though the mode is quasi-TEM in nature. The effect of etching the SiO₂ buffer layer is shown to be one possible method for lowering the microwave effective index while keeping the conductor loss at a fixed level     

Integrated optical proton exchanged TM-pass polarizers inLiNbO3: modelling and experimental performance

Integrated optical TM-pass polarizers operating at wavelengths around λ=1.5 μm have been realized by introducing proton exchanged regions adjacent to a Ti-indiffused waveguide on X-cut, Y-propagating lithium niobate. Structural investigations of the proton exchanged regions have been carried out by raster electron microscope and optical methods, in order to characterize the index profile created by the proton exchange. Several polarizers with different geometries of the proton exchanged regions have been analysed numerically by employing the beam propagation method (BPM) and the finite element method (FEM). The device performance is shown to depend strongly on the geometrical shape of the outer boundaries of the proton exchanged regions and on their distance to the channel waveguide. Experimental results are given for several samples with different gaps between the optical waveguide and the proton exchanged regions as well as for different annealing times. For a proper device design a TE-extinction of -26 dB and a TM-excess loss of only 1.2 dB have been obtained experimentally     

Photorefractive effect and high power transmission in LiNbO/sub 3/ channel waveguides

The photorefractive index changes in singlemode annealed proton exchanged channel waveguides is measured for guided powers of more than 100 mW at 1064 nm. Wave guidance up to 2 W is demonstrated.     

Ion exchange model for α phase proton exchange waveguides inLiNbO3

An H⁺/Li⁺ exchange model is found to be applicable to describe the diffusion of protons when optical waveguides are formed in LiNbO₃ by proton exchange methods where the proton doped crystal structure stays in the pure α phase. The H ⁺ and Li⁺ self-diffusion coefficients in the ion exchange model are determined as a function of the proton exchange temperature both for x-cut and z-cut LiNbO₃. In this way a very useful tool for predicting the proton concentration profiles and hence the refractive index profiles of α phase proton exchange LiNbO₃ waveguides is achieved     

Quasi-phase-matched second harmonic generation in a LiTaO3 waveguide

The authors report third-order quasi-matched (QPM) second harmonic generation (SHG) in a LiTaO₃ channel waveguide. A deep domain-inverted region is first fabricated by a proton-exchange and heat treatment technique. Then a uniform and low-pass channel waveguide is fabricated by pyrophosphoric acid proton exchange. Consequently, 12 mW of blue light is obtained at 424 nm wavelength with a conversion efficiency of 6%. The observed FWHM (full-width half maximum) temperature acceptance width for SHG power is 3.2°C and FWHM wavelength acceptance bandwidth for that is 0.2 nm. It is also shown that diffraction limited focusing of the generated blue light may be obtained