A comparison of the performance of LiNbO3, and LiTaO3, traveling-wave phase modulators

A comparison of the phase modulators made by Ti:LiNbO₃ and Ti:LiTaO₃ optical waveguides is presented. Of particular interest is their halfwave voltages at the wavelength 0.6328 μm and the frequency responses for the same electrode structure. For the Ti:LiNbO₃ phase modulator, a halfwave voltage of 6.6 V and a band-width of 13 GHz are obtained. Whereas, the Ti:LiTaO₃, a 6.8 V halfwave voltage and 12 GHz bandwidth are obtained.     

Reduction of optical damage effects in LiNbO3 and LiTaO3

Optical damage effects in electro-optic crystals such as LiNbO₃ and LiTaO₃ result from light-induced refractive index changes which bring about decollimation and scattering of light beams. The origin of the index changes is discussed. Although it is currently impossible to avoid undesired optical damage completely, various possibilities are proposed to reduce the effects.     

Laser induced memory bits in photorefractive LiNbO3 and LiTaO3

We study experimentally the formation of refractive index voxels (volume elements) in photorefractive LiNbO₃ and LiTaO₃ crystals illuminated with high irradiance femtosecond laser pulses. We used 150 fs pulses at 800 nm wavelength (energy 6–50 nJ) tightly focused inside the crystals in a single shot regime. This resulted in a formation of a micrometer size region of elevated refractive index, which may be used as memory bits in information storage/retrieval application. The maximum refractive index change of 5×10⁻⁴ was recorded in undoped LiNbO₃ at an average light intensity of ∼TW/cm² that is close to the breakdown threshold. A simple setup for photorefractive recording and in situ monitoring of the refractive index changes has been proposed. M. Sūdžius leaves from: the Institute of Materials Science and Applied Research of Vilnius University, Lithuania.     

An EXAFS study of dopant substitution in LiNbO3 and LiTaO3

Abstract

We report an EXAFS study of Co and Fe doped LiNbO₃ and LiTaO₃. Our results show that the dopant ions occupy Li sites in both materials.     

Chemically assisted femtosecond laser machining for applications in LiNbO3 and LiTaO3

We introduce and optimize a fabrication procedure that employs both femtosecond laser machining and hydrofluoric acid etching for cutting holes or voids in slabs of lithium niobate and lithium tantalate. The fabricated structures have 3 μm lateral resolution, a lateral extent of at least several millimeters, and cut depths of up to 100 μm. Excellent surface quality is achieved by initially protecting the optical surface with a sacrificial silicon dioxide layer that is later removed during chemical etching. To optimize cut quality and machining speed, we explored various laser-machining parameters, including laser polarization, repetition rate, pulse duration, pulse energy, exposure time, and focusing, as well as scanning, protective coating, and etching procedures. The resulting structures significantly broaden the capabilities of terahertz polaritonics, in which lithium niobate and lithium tantalate are used for terahertz wave generation, imaging, and control. The approach should be applicable to a wide range of materials that are difficult to process by conventional methods.     

Refined modeling of traveling-wave Ti:LiNbO3 channel waveguide modulator

A refined computation model for traveling-wave operation of external electro-optical phase modulators made of a single titanium in-diffused lithium niobate channel waveguide (Ti:LiNbO₃WG) is described. The modulator circuit model outlined as two traveling-wave coupled microstrips includes microwave losses and dispersion. In the optimization procedure, we consider different circuit load conditions. The influence of a dielectric overlay covering the electrodes is examined. The consolidated techniques of conformal mapping, multiple image, and finite element are used to investigate the microwave characteristics of the structure, and the refractive effective index method is used in calculating the characteristics of the diffused anisotropic optical channel. The performances of Y-cut and Z-cut substrate configurations are compared. We have verified that the Y-cut substrate modulator gives the best overall performance, and that the overlay presence causes significant improvements in the evaluated modulation efficiency and in the bandwidth. A model of the modulator that takes electrode thickness into account permits one to obtain a wider bandwidth.     

Assessment of damage ranges by measurements of the refractive index in ion implanted LiNbO3 and LiTaO3

Ion implantation in LiNbO₃ and LiTaO₃ produces radiation damage by nuclear collisions. The amorphisation of the lattice reduces the refractive index of the material. In the case of fast ion bombardment with helium this damage layer is buried below the surface. The refractive index profile which then exists is suitable for an optical waveguide on the surface which supports several modes. Analysis of the refractive index profile yields the damage distribution in the crystal and this in turn can be compared with theoretical estimates of the damage production and ion ranges.

Our analyses show that in the high energy range from 0.5 to 2.0 MeV the depth of the damage is predominantly controlled by the electronic stopping.     

Figure of merit for near-velocity-matched traveling-wave modulators

We derive a relationship between the bandwidth and active length and a figure of merit for velocity- and near-velocity-matched lithium niobate traveling-wave electro-optic modulators. The figure of merit is given by the bandwidth per unit drive voltage squared and is independent of the length of the device. Alternatively, this figure of merit can be described by its inverse, which is proportional to the device's switching energy.     

LiTaO3晶体高压结构相变的理论研究

利用基于密度泛函理论的平面波赝势结合局域密度近似的从头算方法,计算了LiTaO3晶体在0~200 GPa压力范围内的冷压曲线(P-V/V0)和零温焓,以研究它的高压结构相变.参照同构体LiNbO3的高压相结构,对LiTaO3的菱形相(R3c对称群,室温大气压结构)和正交相(Pbnm对称群)进行计算.结果表明,菱形相压缩线与低压冲击实验数据和静压结果符合较好,而正交相压缩线与扣除热压贡献的高压冲击实验数据相符;正交相更难压缩且各轴向的压缩率不同,对应的常态密度比菱形相高约24%.理论预测的相变起始压力约为23 GPa.由此可见LiTaO3的冲击高压相具有正交对称性,与LiNbO3的室温高压相类似.     

Micro‐structured integrated electro‐optic LiNbO3 modulators

In this paper, we will review the state‐of‐the‐art of LiNbO₃ based integrated electro‐optic modulators and will show how micro‐structuring techniques such as etching, domain inversion and thin film processing can be used to realize new configurations which can take the performance to unprecedented levels. In particular, we will review recent results on the use of domain inversion on a micron scale and we report on the fabrication of a chirp‐free modulator having ∼ 2 V switching voltage and bandwidth of 15 GHz designed by placing the waveguide arms of the Mach‐Zehnder interferometer in opposite domain oriented regions. We also review some of the new modulation formats (e.g. DQPSK) that can represent an application development of the presented micro‐structured devices. Finally, we address the issue of the integration of the modulator chip in a transmitter board comprising tunable laser, bias‐control electronics and RF driver. The requirements of integration can even push further the reduction in size of modulator chips, thus making more crucial the use of micro‐ and nano‐structuring techniques.     

Comparison of two coplanar waveguide electrodes for Ti:LiNbO3 interferometric modulators

The coplanar waveguide travelling-wave electrodes widely used for Z-cut Ti:LiNbO₃ Mach-Zehnder optical modulators can be divided into two types. One has a wide centre electrode and small electrode gaps, while the other has a narrow centre conductor, wide gaps, and a relatively thick buffer layer. An accurate method for analysing the operational efficiency of the devices with these two electrode structures has been developed using the Galerkin method and the point matching method. Simulation results show that the interaction efficiency of the interferometric modulator with a narrow centre conductor is slightly lower than that with a wide centre electrode. However, the former is more promising because it can achieve a 50 transmission line matched with the drive circuit. The optimization of the optical waveguide position and the fabricational tolerance on the alignment of the electrode pattern are also discussed.     

Raman scattering and phase state of H:LiTaO3 waveguide thin films

Raman scattering of optical waveguides formed by proton-exchange in lithium tantalate is studied. The effect of a post-exchange annealing up to 420°C on the optical properties and Raman spectra are also investigated. Two types of Raman bands are observed: those which are very strongly influenced by the presence of protons (proton-related bands) and those characteristic of the pure crystal (bulk bands). The intensity of the two kinds of bands depends in an opposite way on the annealing temperature, with the proton-related bands following the anomalous behaviour of n _e. The spectra from samples annealed up to the temperature of the maximum value of n _e are remarkably similar to the high-temperature paraelectric spectra of a LiTaO₃ single crystal.     

坩埚下降法生长的LiNbO3,Fe:LiNbO3,Zn:Fe:LiNbO3单晶的吸收光谱特性

用紫外可见光谱(UV／Visible Spectra)测试并研究了坩埚下降法生长的LiNbO3、Fe：LiNbO3，以及Zn：Fe：LiNbO3晶体的吸收特性。分析了产生这些吸收特性的原因以及与工艺生长方法的内在联系。研究结果表明：LiNbO3单晶沿晶体生长方向，其紫外吸收边向长波方向移动，且在350—450nm波段的吸收也逐渐增大，这是由于Li的分凝与挥发，逐渐产生缺锂所造成的；在Fe：LiNbO3单晶中观察到Fe^2 离子在480nm附近的特征吸收峰，并发现沿生长方向，Fe^2 离子的浓度逐渐增加，这与提拉法生长得到的晶体不同；在Fe：LiNbO3单晶中掺入质量分数为1．7％ZnO后，吸收边位置发生蓝移，而掺杂质量分数达到3．4％时，观察到有红移现象。Fe^2 离子在Zn：Fe：LiNbO3单晶中的浓度与ZnO掺杂量有密切关系。在掺杂质量分数1．7％ZnO的Fe：LiNbO3单晶中，Fe^2 离子从底部到顶部的浓度变化比在掺杂质量分数3．4％ZnO晶体中大，这是由于Zn^2 抑制Fe^2 离子进入Li位的能力随掺杂量的增加而逐渐减弱造成的。就该下降法工艺技术对Fe^2 离子在晶体中的浓度分布的影响作了分析。     

Non-equilibrium population and slow relaxation of Fe2+ in57Co:LiNbO3 and57Co:LiTaO3 single crystals

The Fe²⁺ fraction observed in the Mössbauer emission spectra of⁵⁷Co:LiNbO₃ and⁵⁷Co:LiTaO₃ exhibits both slow electronic relaxation and nonthermal populations of them _s sublevels of the⁵A_lg orbital singlet ground state at low temperatures (T<15 K) in high magnetic fields. The relaxation rates depend on temperature and on the angle ? between the magnetic field and the crystallographicc-axis.     

Zn∶Fe∶LiNbO_3晶体二波耦合和相位共轭效应的研究

采用二波耦合和四波混频实验,研究了Zn:Fe:LN(Fe:0.03wt.%,Zn:5.0mol)晶体的光折变二波耦合衍射效率和长时间照射下透射光强随时间的动态变化行为,以及其相位共轭效应,分别测试了其二波耦合衍射效率和其相位共轭反射率,结果表明该晶体的衍射率可达到56%,且其衍射效率随着其信号光的减小先增大后减小,相位共轭反射率随信号光的增大呈减小的趋势,最高共轭反射率可达到650%。