Buried optical waveguide polarizer by titanium indiffusion andproton-exchange in LiNbO3

Buried optical waveguide polarizers on LiNbO₃ have been realized by titanium indiffusion, followed by proton-exchange and annealing. The proton-exchange process decreases the ordinary refractive index and so modifies the index profile of the titanium indiffused waveguide. The measured intensity profile is in good agreement with calculation. An aluminum film absorbs the surface TM mode on z-cut LiNbO₃, leaving a buried nearly symmetric TE mode with lower optical loss than surface-guided TE modes. The extinction ratio obtained is estimated to be greater than 50 dB/cm at 0.633 μm     

Iterative simplex-finite difference method for the characterizationof optical waveguides

A new method is presented for reconstructing smooth refractive index profiles of optical waveguides from measured effective indexes. It is based on the semivectorial finite difference method to solve the polarized wave equation for a given refractive index profile. An iterative simplex algorithm is used to find the best refractive index parameters that give, as a solution, effective indexes close to the measured ones. The method is applied successfully to Ag⁺-Na ⁺ ion-exchanged glass slab waveguides and to diffused Mg/Ti:LiNbO₃ slab waveguides. Dopant concentration profiles are obtained by using secondary ion mass spectrometry. The relationship between the refractive index change and the dopant concentration is determined for both cases. The iterative simplex algorithm-finite difference method (ISA-FDM) is compared to other index profile reconstruction methods, and the advantages with respect to WKB-based methods are pointed out     

Fabrication, characterization and index profile modeling ofhigh-damage resistance Zn-diffused waveguides in congruent andMgO:lithium niobate

A study of the fabrication and optical properties of planar waveguides fabricated in MgO:LiNbO₃ and LiNbO₃ substrates by diffusion of a ZnO film is presented. Transmission electron microscopy was used to show that using ZnO instead of metallic zinc as a source, and maintaining the ZnO film thickness below a prescribed value, greatly reduces second phase precipitation and produces usable waveguides. Dopant and refractive index profiles were characterized by electron microprobe analysis and interference microscopy, respectively. The dependence of the Zn diffusion coefficient on temperature and the dependence of the refractive-index change on Zn concentration are inferred from these measurements. A simple model is also reported which predicts the index profile of the waveguide given the film thickness, diffusion time and temperature. The validity of the model is demonstrated by comparison between calculated profiles and profiles measured by prism coupling and IWKB analysis     

Activation energy of DC-drift of x-cut LiNbO3 opticalintensity modulators

The DC-bias-induced drift phenomenon in LiNbO₃ optical intensity modulators is a main cause of device wearout failure. In order to estimate the device lifetime, an activation energy value Ea of the drift is needed, and Ea=1.0 eV is already known for z-cut LiNbO₃ modulators. However, Ea of x-cut LiNbO₃ modulators is not known even though there is a possibility that the Ea depends on the crystal orientation. Here, Ea=1.4 eV is obtained experimentally for the x-cut LiNbO₃ modulator with a SiO₂ buffer layer from their drift measurements between 50°C-140°C     

Time-domain optical response of an electrooptic modulator usingFDTD

A time-domain analysis of an LiNbO₃ electrooptic modulator using the finite-difference time-domain (FDTD) technique is performed. This allows for the calculation of optical modulation and the time-domain optical response of an electrooptic modulator. The electromagnetic fields computed by FDTD are coupled to standard electrooptic relations that characterize electrooptic interactions inside the embedded Ti diffused LiNbO₃ optical waveguides. The electric field-dependent change in the index of refraction inside these optical waveguides and resulting minute phase shifts imparted to optical signals propagating along the device are determined in time, allowing for the simulation of optical intensity modulation. This novel approach to LiNbO₃ electrooptic modulators using a coupled FDTD technique allows for previously unattainable investigations into device operating bandwidth and data transmission speed     

An optical integrated circuit for time-division 2-D velocitymeasurement

An LiNbO₃ optical integrated circuit pigtailed with two single-mode fibres, which allows time-division two-dimensional velocity measurement, is discussed. To detect time-division multiplexed beat signals corresponding to velocity components v_X and v_γ of a moving object, a waveguide switch is integrated on a Z-propagating LiNbO₃ substrate of 28×7 mm² in addition to a waveguide interferometer with a frequency shifter. In the optical IC, either v_X or v_γ could be measured selectively with signal-to-noise ratio of 20 dB by driving an electronic gate placed after a photodiode in synchronization with the waveguide switch     

Analytical expressions of the guided mode field distribution:analysis and comparison

Methods for determining the parameters of the three different approximate solutions for the fundamental mode field distribution of Ti:LiNbO₃ optical waveguides by using the variational technique are presented. This analysis gives closer approximation not only to the field distribution, but also to the propagation constant. Comparison with the exact results for the exponential index distribution in depth shows that these trial functions are closer to the real solution. It is also shown that these solutions are as accurate as the effective index method for Ti:LiNbO₃ strip waveguides. The analysis provides an approach for comparing and assessing these solutions     

Improved dispersion equation for MgO:LiNbO3 crystal inthe infrared spectral range derived from sum and difference frequencymixing

Phase matching angles for sum and difference frequency mixing in a 7% MgO:LiNbO₃ crystal of the radiation of a Ti:Sapphire laser and the radiation of 2.2 to 3.4 μm from a Raman shifter are used to determine the refractive indices of the crystal. A dispersion equation accurate in the mid-infrared spectral range as well as in the visible and near infrared range is firstly given. Based on this result the phase matching characteristic of a mid-infrared optical parametric generator using 7% MgO:LiNbO₃ crystal is described     

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     

Evolution of the intensity profile of Cerenkov second-harmonicradiation with propagation distance in planar waveguides

Using coupled mode theory, we have studied the output intensity profile of Cerenkov second-harmonic radiation from planar waveguides as a function of the propagation distance. In particular, we have obtained the variation of the intensity profile taking into account the effect of prism coupling as well as propagation loss, and have shown that the second-harmonic radiation evolves into a beam-like output. Results of the measured intensity profile of the second-harmonic radiation in proton-exchanged planar waveguides in Z-cut LiNbO₃ are also presented that are consistent with the theory     

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     

LiNbO3晶体纳秒级光开关的研究

LiNbO3晶体由于其透光范围广，响应速度快，不易潮解等特点而广泛用于高精度和高速光开关快门。为了将LiNbO3光开关曝光时间拓宽到纳秒领域从而弥补现有的高速光开关的不足，文章阐述了LiNbO3作为光开关理论原理，对多波段通光的晶体长宽比进行了优化设计，并对LiNbO3纳秒级光开关的可行性进行了实验验证。实验半波电压与理论符合很好，实验光信号和电信号在纳秒级范围内能实现同步响应。研究结果证实了LiNbO3光开关在纳秒级范围内的可行性，并为LiNbO3纳秒级光开关的制作提供了理论和实验依据。     

3.6 Gb/s all laser-diode optical soliton transmission

3.6-Gb/s optical soliton transmission using a gain-switched 1.55-μm distributed-feedback laser diode and a Ti:LiNbO₃ intensity modulator is demonstrated. An Er³⁺-doped fiber amplifier and a Raman amplifier, both pumped by 1.48-μm laser diodes, are used for achieving intense optical pulses and fiber-loss compensation, respectively. The intensity-modulation direct-detection optical receiver of a commercial F-1.6 G system is used to measure the bit-error rate     

Prepatterned optical circuits in thin ion-sliced single-crystalfilms of LiNbO3

Crystal ion slicing was used in conjunction with conventional annealed proton exchange in Z-cut LiNbO₃ to result in prepatterned microns-thick single-crystal LiNbO₃ films with channel guides and a measured waveguide propagation loss of 0.2-0.7 dB/cm. Full optical circuit transfer, including a buffer layer and a patterned metal electrode structure for active control was demonstrated     

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     

Evaluation of absorption and emission cross sections of Er-dopedLiNbO3 for application to integrated optic amplifiers

The polarization-dependent absorption and emission spectra of the ⁴I_13/2-⁴I_15/2 transition (λ~1.5 μm) in single crystal bulk Er:LiNbO₃ have been measured. Low-temperature (10 K) measurements of the Stark split energy levels of these two manifolds indicate at least two Er³⁺ sites. McCumber theory is applied to determine the Er:LiNbO₃ absorption and emission cross sections. These values are used to calculate the gain characteristics of Er:LiNbO₃ channel waveguides. Calculations indicate that a gain of 10 dB is achievable in a waveguide of several centimeters using ~20-mW pump power     

Measurement of uniformity of driving voltage inTi:LiNbO3 waveguides using Mach-Zehnder interferometers

A method for measuring the spatial variation of electrically induced index change (electrooptical effect) in Ti-diffused LiNbO₃ optical waveguides is demonstrated. By segmenting the electrodes of Mach-Zehnder modulators, electrically induced index changes at as many as 45 positions on x-cut and z-cut substrates have been measured. The result confirms that the spatial variation of index change is within ±3%     

Modeling of broad-band traveling-wave optical-intensity modulators

In this paper, an accurate simulation tool for the electrical and optical response of broad-band traveling-wave optical intensity modulators is presented, which takes into account multisectional electrical transmission lines. This model is applied to analyze a high-speed fully packaged LiNbO₃ Mach-Zehnder interferometer     

Fabrication of magnesium-oxide-induced lithium outdiffusionwaveguides

A short-time high-temperature diffusion process for the fabrication of the lithium niobate (LiNbO₃) optical waveguide with a magnesium-oxide (MgO) strip-loaded structure is presented. The deposited MgO layer causes the lithium to diffuse out of the LiNbO₃ substrate. The secondary ion mass spectroscopy (SIMS) data of the fabricated waveguide show that the lithium diffuses out of the substrate and gets into the MgO cover layer, which results in an increase in the extraordinary refractive index of the substrate. In particular, it is found that the diffused atoms are piled up near the LiNbO₃ and MgO interface. Also, the electrooptic effect of the fabricated waveguide is better than that of the conventional titanium-indiffused waveguide. Moreover, single-mode and single-polarization waveguides can be easily obtained without any significant surface guiding or surface damage on the crystalline substrate     

Integrated-optical gigahertz frequency shifter for 1.5 μmsignals

A single-sideband modulator for gigahertz modulation frequencies is demonstrated in Ti:LiNbO₃. At a frequency shift of Δf 5.9 GHz, the optical bandwidth is δλ=20 nm and the electrical bandwidth δf=0.25-1 GHz