Optical beam splitter and focuser in Ti:LiNbO3 planar waveguide

A -type two-electrode system on a Ti diffused optical waveguide in LiNbO₃ has been studied in symmetrical and in asymmetrical operation regimes. In the first case the device acts as a beam splitter or focuser, depending on the polarity of the applied voltage. In the second case the device acts as a scanner. The driving voltage is relatively low. Possible applications are optical communication systems and optical processing of information.     

Loss in low-finesse Ti:LiNbO3 optical waveguide resonators

We propose to measure the contrast of Fabry-Perot resonances in low-finesse (end face polished) integrated optical resonators to determine an upper limit of the waveguide attenuation coefficient. The method is discussed; its absolute accuracy increases with decreasing loss. As example, experimental results are presented for 3 and 10 m wide Ti: LiNbO₃ strip guides; the attenuation coefficients are 0.46 dB/cm (EH₁₁) in the narrower guide at =0.63 m, respectively 0.19 dB/cm (EH₁₁) and 0.09 dB/cm (HE₁₁) in the 10 m wide guide at =1.15 m.     

Ti:LiNbO3 optical waveguides

Various experiments on Ti diffused optical waveguides in LiNbO₃ have been carried out in order to determine precisely the character of the diffusion process. The required guide parameters and the effective mode indices could be controlled by adjusting only the diffusion time under fixed temperature and film thickness. Therefore the dependence of the guide characteristics on the diffusion time has been investigated in detail. On the basis of the data obtained, a two-stage diffusion model is proposed. In the first stage, the Ti diffusant profile is described by a erfc-function, and the second stage is characterized by a modified Gaussian form.     

Q-switched Ti:Er:LiNbO3 waveguide laser

Q-switched operation of a Ti:Er:LiNbO3 waveguide laser has been achieved with up to 2.4 W pulse peak power (0.18J) at 2kHz repetition frequency with an intracavity Mach–Zehnder-type amplitude modulator. A theoretical model has been developed to describe the laser operation. Good agreement between numerical and experimental results has been achieved. Significant potential for improvement is predicted.     

极化子荧光及其断层扫描对Ti:LiNbO3光波导表征研究

小极化子(NbLi4+)荧光处于700-950 nm谱段,对不同化学还原程度同组分的纯铌酸锂晶体的研究表明荧光的强度反映了化学还原程度的强弱.从小极化子浅层能级出发,提出以"1中心3级态跃迁"来描述小极化子的受激荧光过程.钛扩散铌酸锂光波导(Ti:LiNbO3)表面还原程度的均匀性可能在波导制造的热扩散过程中被破坏.在对铌酸锂波导表面、深部的扫描结果中,荧光的强度呈现出表面与深部、波导内与外的差异,表面的强度是深部的6-8倍.研究表明,利用非损伤性的极化子荧光谱,对加工过程热处理进行检控是非常有效的.同时,波导内与外的荧光强度差异间接反映了波导的横切面的外形.     

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.     

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.     

900-nm Nd:Ti:LiNbO(3) waveguide laser

We report what is to our knowledge the first laser operation of Nd(3+) -doped LiNbO(3) near 900 nm. An absorbed power threshold of 26 mW was obtained when the device was pumped at 814 nm. The design of the waveguide geometry to favor laser operation at this wavelength is demonstrated.     

Fully packaged high-speed Ti:LiNbO3, waveguide mach-zehnder intensity modulator

A fully packaged high-speed Ti:LiNbO₃, Mach-Zehnder intensity modulator at a 1.5μm wavelength has been designed and fabricated. The design has been considered with respect to optical insertion loss, switching voltage, and modulation bandwidth. Re fiber-pigtailed device has a 6.7 dB fiber-to-fiber loss and an optical modulation bandwidth of 8 GHz. The voltage required to switch the device from the on to off state is measured to be 7.3 V, with an on-off extinction ratio of 21 dB.     

A waveguide polarizer based on Si-coated Ti:LiNbO3 planar structure

A novel design of a transverse magnetic(TM)-pass waveguide polarizer based on Si on an X-cut,Y-propagatiou Ti:LiNbO3 planar waveguide is presented.The eigenvalue functions of such structure havebeen obtained by using Wenzel-Kramers-Brillouin(WKB)method with modified Airy functions.Theintervals of the silicon thickness have been found which result in transverse electric(TE)light sufferingstrong attenuation while TM wave propagating with fairly low loss.A planar waveguide polarizer isfabricated and its polarization-dependent measurements lead to the best polarization extinction ratio～34dB and the insertion loss<0.4 dB.     

Spectral noise analysis of Er3+:Ti:LiNbO3 curved waveguide amplifiers

We report simulations concerning optical amplification in Er:Ti:LiNbO₃ curved waveguides. The derivation and the evaluation of the spectral optical gain, the spectral noise figure, the amplified spontaneous emission photon number, and the signal to noise ratio are performed under the small gain approximation. The simulations show the evolution of these parameters under various pump regimes, Er concentration profiles and waveguide lengths. The results obtained are of significant interest for the design of complex, rare earth-doped integrated optics structures involving bent waveguides.     

钛扩散铌酸锂矩形波导中的双折射耦合对传播常数的影响

 从亥姆霍兹方程出发，得到了各向异性光波导在弱导近似下的耦合模理论。耦合系数包括偏振耦合项和双折射耦合项。并用马卡提里近似下的模式作为零级近似。用一级微扰法计算了折射率渐变分布的钛扩散铌酸锂矩形波导的传播常数。在给定参数的情况下，得到双折射项引起的传播常数的改变约为主微扰项的3%，因此双折射耦合对钛扩散铌酸锂矩形波导的性能影响不可忽略，这对设计和分析与它相关的光波导器件具有指导意义。     

Effects of a lossy thin plasma-film on metal-diffused LiNbO3 optical waveguides

I intend to investigate the effects of different kinds of partially reflecting metal film (plasma layer) on the wave characteristics of a stratified titanium-diffused LiNbO₃ optical waveguide. The mode number, the equivalent indices, the extinction coefficients, the field distributions and the powers carried in each layer are given as a function of the free-space wavelength, of the diffusion parameters, and of the opticc-axis orientation. Of course, the power of TM-like hybrid waves flows in opposite directions in the plasma and the other regions.For ac-axis varying in the transverse plane, the ith-order hybrid mode is rather complicated in the guide with an aluminum layer. There, in fact, the field mapping can be seen as an overlap of a TE _i with a TM _i+1, mode because of a sudden transformation of the TM₀ mode into a superficial plasma wave (SPW). On the contrary, with a silver layer, the hybrid field is a simple combination of a TE _i with a TM _i mode with no SPW growing.The guide losses assume the lowest values (1 dB/cm) for an Ag layer which is the prime candidate for making electro-optical or acoustic-optical devices. On the other hand, the nickel film causes the highest losses (66 dB/cm).     

Praseodymium-doped Ti:LiNbO3 waveguides

3 by diffusion doping is investigated by means of secondary neutral mass spectrometry and secondary ion mass spectrometry. The diffusion of praseodymium in LiNbO₃ can be described by Fick’s laws of diffusion with a concentration-independent diffusion coefficient and a limited solubility of praseodymium in LiNbO₃ increasing exponentially with rising temperature. The diffusion depends on the Li₂O content of the LiNbO₃ crystal. For LiNbO₃ crystals with a nominal slight difference in the congruent composition, the diffusion constants and activation energies for Z-cut LiNbO₃ are 3.28×10^-5 cm²/s and 2.27 eV, and 1.39×10^-5 cm²/s and 2.24 eV, respectively. Titanium-doped waveguides are formed in Pr:LiNbO₃ and characterised in relation to waveguide loss and absorption in the visible and near infrared. Received: 17 September 1998 / Accepted: 11 November 1998     

In‐band pumped Ti:Tm:LiNbO3 waveguide amplifier and low threshold laser

The fabrication by diffusion doping and a detailed optical investigation of a Ti:Tm:LiNbO₃ waveguide amplifier and of a Fabry‐Pérot type Ti:Tm:LiNbO₃ laser are reported. Both devices are in‐band pumped by a laser diode at 1650 nm. The wave‐guide amplifier shows broad‐band optical gain in the wavelength range 1750 nm < λ < 1900 nm. The laser emits at 1890 nm, the longest emission wavelength of a Tm:LiNbO₃ laser reported so far; also 1850 nm emission could be demonstrated. Laser threshold (1890 nm) is at 4 mW coupled pump power; the slope efficiency is ∼13.3%. Properties and potential of both devices are analyzed by extensive modeling.     

Theoretical analysis of output statistics of Er3+:Ti:LiNbO3 M-mode straight waveguide amplifiers

In this paper we present a theoretical analysis of some statistical parameters which characterize the Er³⁺-doped Ti:LiNbO₃ M-mode straight waveguides. The derivation and the evaluation of the spectral optical gain, the spectral noise figure, the Fano factor, the statistical fluctuation and the spontaneous emission factor are performed under the small gain approximation. The simulations show the evolution of these parameters under various pump regimes and waveguide lengths. The obtained results can be used for the design of complex rare earth doped integrated circuits.     

Epitaxial growth of yttria-stabilised zirconia buffer layers on X-cut LiNbO3 for superconducting electrodes

We report on the epitaxial growth of yttria-stabilised zirconia (YSZ) buffer layers on X-cut LiNbO₃ (LNO) single crystals by pulsed laser deposition. Despite the low chemical stability of the substrates at high temperature, high quality fully reproducible films were obtained over a relatively broad range of processing conditions. The films were (00h) out-of-plane single oriented and the in-plane edge of the YSZ unit cell was aligned with the polar axis of the LNO. However, the YSZ deposition also promoted the formation of the compound LiNb₃O₈. This compound is epitaxial and located at the interface. The homogeneous YSZ film presents a uniform surface, free of outgrowths and with a low roughness. These characteristics are suitable for the epitaxial growth of other oxides, as has been demonstrated with the preparation of YBa₂Cu₃O₇/CeO₂/YSZ/LNO heterostructures. The superconducting YBa₂Cu₃O₇ films were epitaxial, with the c axis perpendicular to the surface and single in-plane orientation, and presented good transport properties (critical temperatures around 86 K and critical current densities close to 10⁶ A/cm² at 77 K). Received: 5 April 2001 / Accepted: 30 July 2001 / Published online: 30 October 2001     

Single-frequency Ti:Er:LiNbO3 distributed Bragg reflector waveguide laser with thermally fixed photorefractive cavity

The first single-frequency Ti:Er:LiNbO₃ distributed Bragg reflector waveguide laser with two thermally fixed photorefractive gratings as resonator mirrors is reported. The optically pumped (λ_p=1480 nm,120-mW incident power) laser emits up to 1.1 mW at λ_s=1561.1 nm. The threshold pump power is 70 mW. Received: 7 June 2001 / Published online: 30 October 2001     

Raman characterization of H:LiNbO3 waveguides

In this work we present polarized Raman measurements of z-cut multimode H:LiNbO₃ waveguides in the κ_i- and β_i-phase states. A spectrum with unexpected form and behaviour, originating from the layer at the very surface, was observed in samples with various degrees of Li–H substitution. Possible reasons for the presence of this strange spectrum are considered. Received: 16 May 2001 / Revised version: 7 August 2001 / Published online: 23 October 2001     

Acoustic properties of proton exchanged LiNbO3 investigated by Brillouin scattering

The propagation of surface acoustic waves at microwave frequencies (10¹⁰ Hz) was studied on proton exchanged LiNbO₃ crystals by means of Brillouin scattering. The proton exchange causes a large velocity reduction for surface acoustic waves propagating in the x–y plane of ay-cut crystal as well as for longitudinal bulk acoustic waves travelling in the proton exchanged sub-surface region. The velocity reduction amounts to about 20% for both types of waves. The corresponding elastic constants are reduced even by about 40% since the density remains almost constant. This softening seems to involve both the shear and compressional elastic constants, but in an anisotropic way.Thus by proton exchange it is possible to build acoustic waveguides adjacent to the surface, similar to the construction of optical waveguides. By a lateral control of the proton exchange rate optical elements for ultrasonic waves, for example, acoustic lenses can be produced without deformation of the flat surface.The absorption of surface acoustic waves on proton exchanged surfaces is stronger than on pure LiNbO₃ indicating a novel absorption mechanism becoming active in the proton exchanged material.