Proton exchange in Y-cut LiNbO3

In the present work, planar waveguides in Y-cut LiNbO₃ were obtained using modified proton exchange (PE) conditions of: PE and subsequent annealing (APE), PE in buffered melts (BMPE), APE followed by PE (APE+PE), and PE in vapours (VPE). Benzoic acid was used as the proton source in the PE, BMPE, and (APE+PE) experiments. Cinnamic acid was used for obtaining VPE-waveguides. The main aim was to prevent surface damage of Y-cut crystals due to the strains introduced by proton exchange. The investigations performed showed that the surface etching is probably due to lattice deformation anisotropy leading to higher strains in PE Y-cut samples. Most encouraging results were observed, when an optimized (APE+PE)-procedure was used for waveguide formation. This method is very attractive for the fast preparation of deep high-index and low-loss waveguides in Y-cut LiNbO₃. This procedure allows passive and active elements to be produced in one and the same Y-cut substrate of LiNbO₃. Similar preliminary results were obtained under VPE conditions.     

Modeling of Ti:LiNbO3 waveguide directional couplers

A simple and accurate method is presented for studying the coupling characteristics of 3D diffused channel waveguide directional couplers. The method is based on the scalar variational principle and differs from other methods as it does not require any assumption for the functional form of the trial field. The authors also propose an equivalent 2D directional coupler that also gives accurate results and, hence, saves considerable computational time, Comparison with experimental and other theoretical results is also presented     

钛扩散铌酸锂波导多电极■β倒相定向耦合器

本文报道了我们研制的四对电极的定向耦合器.利用电极连接的不同方式,分别测试了一对电极、两对电极和四对电极三种工作状态下器件的工作特性.采用四对电极工作时,器件实现了交叉和直通两种工作状态的电调制.     

Fluctuational crosstalk in Ti:LiNbO3 directional couplerswitches

It has been found that the fluctuations in the Ti concentration along waveguides of Ti:LiNbO₃ directional coupler switches degrade the switch extinction ratio and introduce an unwanted crosstalk between both channels. We discuss analytically the effect of effective optical index fluctuations on the transfer matrix of the optical switch. We demonstrate the sensitivity of the mismatch to the statistics of the fluctuations of the Ti concentration     

Zn indiffusion waveguide polarizer on a Y-cut LiNbO3 at1.32-μm wavelength

A polarization-dependent loss measurement of Zn indiffusion (ZI) waveguide on a Y-cut LiNbO₃ substrate is firstly reported. The measured results show that the waveguides support either a single extraordinary polarization or both extraordinary and ordinary polarizations depending on the fabrication process parameters. For the single extraordinary polarization waveguide, the measured propagation loss at 1.32-μm wavelength is 0.9 dB/cm and the best measured polarization extinction ratio is 44 dB at a distance of 1.5 cm from the input end, which are quite good for being a waveguide polarizer. Moreover, the voltage-length product measured for the ZI Mach-Zehnder modulator shows that the substrate electrooptic coefficient is not degraded     

LiNbO3 optical intensity modulator packaged with monitorphotodiode

The assembly technique to simply install a monitor photodiode (PD) with a LiNbO₃ (LN) Mach-Zehnder modulator in the same package is proposed. A light radiated from the Mach-Zehnder Y-branch toward the LN output facet is lead into a glass bead, which is bonded to the LN facet as a fixture of the optical fiber. The end of the glass bead is cut to have an angled face, and the radiated light is reflected normal to the LN waveguide axis and toward the PD fixed on the side of package. The curved surface of the glass bead is effective to roughly focus the light onto the PD. We demonstrate that such monitor PD outputs an intensity modulation exactly reverse to the LN output modulation with a sensitivity higher than 0.03 A/W per optical output power of LN, indicating a usability of the PD output to monitor the drift of LN modulator     

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。     

Ionization-induced refractive index and polarization effects inLiNbO3:Ti directional coupler waveguides

Exposure of an LiNbO₃:Ti directional coupler to pulsed 16-MeV electrons was observed to nonequivalently affect the transverse electric-transverse magnetic (TE-TM) polarized transmission of a continuous wave 1300-nm optical signal through the channel waveguides. Two distinct ionization-induced attenuation responses were observed to depend on the magnitude of the optical power in the guides and on the polarization states. An hypothesis for these phenomena is advanced based on the presence of localized space charge fields arising from ionization-induced transient color centers, free carriers within or near the light guiding regions, and polarization conversion initiated through decreased mode confinement and scattering. Transient space charge fields causing localized changes to the refractive indexes in and near the guiding regions are believed to have caused both the loss of guiding and the onset of photorefractive-like processes     

Traveling wave optical modulator using a directional coupler LiNbO3waveguide

Fabrication methods for low drive voltage and broad-band LiNbO3waveguide directional coupler optical modulator are described. Optical waveguides were prepared by conventionaly Ti in-diffusion into LiNbO3c-cut plate. To obtain wide-band frequency response, traveling wave electrodes were used. Electrode characteristic impedance measured by time domain reflectometry method coincided well with the calculated value by conformal mapping. Measured electrode conductor losses followed square root of frequency. To reduce electrode conductor losses, asymmetric and 3-μm thick Al electrodes were used. Directional coupler optical modulator frequency response was analyzed, using the phase difference average value along the propagation directions. Calculated value by this method coincided well with measured value obtained by a swept frequency technique. The characteristics of this modulator at the 1.317-μm wavelength are as follows: 100 percent modulation voltage is 4 V, extinction ratio is 17 dB, optical insertion loss is 5.4 dB, 3-dB bandwidth is 3.6 GHz, and rise time is about 400ps.     

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     

Broad-band reflection traveling-wave LiNbO3 modulator

A velocity matched traveling-wave Mach-Zehnder interferometer (MZI) in LiNbO₃ operating in reflection, is reported and drive voltages are compared to those for single-pass devices of the same length. The device achieves double-pass operation by simultaneously reflecting the optical and RF waves, Broad-band operation to 20 GHz with drive voltages from 0.5 to 1 V (<0.5 GHz) to ~4.5 V (at 20 GHz) is observed with the reflection device. This performance is superior to that of the single-pass device, which has drive voltages of 2.1 V (dc) to ~4.5 V (at 20 GHz)     

An LiNbO3 integrated coherence modulator

The authors describe the principle of operation and the performance of a newly integrated LiNbO₃ modulator specially designed to generate optical delays of several millimeters. This is achieved by using the simultaneous propagation of the TE and TM modes in a waveguide and by taking benefit of the high natural birefringence of LiNbO₃. This modulator is well-suited to coherence modulation of light in fiber-optic transmissions. When powered by a short coherence source, the large optical delays induced by a cascade of such modulators can be used as information carriers to transmit several signals simultaneously. A demonstration is reported using a single source and two modulators in cascade to transmit signals simultaneously     

Examination of LiNbO3/nitride heterostructures

Oxides such as LiNbO₃, PbTiO₃, etc. have large polarization effects arising from the ferroelectric and piezoelectric effects. Recent work on nitride heterostructures has shown that polar charge can be used to substitute for dopant charge and controlled to create a two-dimensional electron (and hole) gas, resulting in ohmic contacts and diode like current–voltage characteristics. In this paper, we examine the potential of using ferroelectrics with traditional semiconductors for applications in novel electronic devices. In particular, we examine their use in creating very high sheet charge densities of mobile charge and tailorable current–voltage characteristics. It is seen that owing to hysteresis based effects in the ferroelectric layer, the direction of variation of voltage has a marked effect on the charge induced as well as the variation of that charge with voltage.     

Further evidence of strong coupling effects in three-arm Ti:LiNbO3 directional couplers

The switch characteristics of three-arm integrated optic directional couplers with both symmetric and antisymmetric drive voltages are examined. The symmetry properties of these devices, predicted by previous theory, are shown to be violated and the reasons for this are discussed in terms of scalar strong coupling coefficients normally neglected as small. It is also shown that the inclusion of modal overlaps introduces additional coupling terms and causes significant power redistribution in the bend sections. Experimental measurements for the antisymmetric configuration are presented, and it is shown that different electrode loading of the center and outer guides affects device operation     

钛扩散LiNbO_3波导干涉仪调制器

研制成Ti扩散LiNbO_3波导干涉仪调制器。在0.6328微米的光波长下加1千赫方波信号。50千赫的脉冲信号和100千赫～30兆赫的正弦波信号进行了调制实验。实验样品的最大调制深度87％,半波电压V_π=45伏,电容8微微法,3分贝带宽可达800兆赫。     

A broadband Ti:LiNbO3 optical modulator with a ridgestructure

We describe the design, fabrication, and characteristics of a Ti:LiNbO₃ optical modulator with a ridge structure. The structure keeps microwave propagation loss low and enables a large interaction between microwaves and optical waves under the conditions of velocity-matching and impedance matching, resulting in a large modulation bandwidth and low driving voltage. Using this structure, we have developed an optical intensity modulator with an optical 3-dB bandwidth of 75 GHz and a driving voltage of 5.0 V at a wavelength of 1.5 μm     

Comparison between strong coupling theory and experiment forthree-arm directional couplers in Ti:LiNbO3

The derivation of scalar coupled mode theory for TEM waveguide arrays is examined. It is demonstrated that four previously published methods are identical at this level of approximation. The accuracy of the theory and its application to a slab-waveguide model of a three-arm coupler is considered. It is shown that the coupled mode equations require modification for slowly varying arrays. Analytic expressions for the matrix coefficients are presented, and it is demonstrated that device input-output transitions have significant effect on the response. The model is compared with experimental measurements of three-arm couplers fabricated in Ti:LiNbO₃, and excellent qualitative agreement is obtained     

掺铁铌酸锂(LN:Fe)的各向异性自衍射

本文研究了单光束辐照下LN:Fe晶体中的变偏振现象,观察到了e光变ο光的光环;ο光变e光的扇形光像.观测了光强随时间变化的弛豫曲线和温度效应.用时间分辨简并四波混频解释了这一自衍射现象,也对产生这一现象的噪音栅形成机制作了探讨.     

用Fe:LiNbO_3晶体进行实时双曝光干涉的研究

1.在激光全息无损检测中希望找到一种新的实时记录介质,可以重复使用且衍射效率与分辨率都不低于银盐干板.除了银盐干板外,光色材料、光导热塑料、明胶、电光晶体等都可以作记录介质.本文采用了Fe:LiNbO_3晶体作为实时记录材料进行双曝光的实时干涉.由于Fe:LiNbO_3晶体记录的是位相全息图,所以有很高的衍射效率.二次曝光后得到清晰的干涉条纹.     

Polarization selective phase modulator in LiNbO3

An integrated optical phase modulator which selectively modulates only one mode of polarization is discussed. Selectivity is accomplished by controlling the ratio between the voltages applied to two different electrode sections. A method for determining the correct ratio and evaluating the polarization selectivity of the phase modulation Γ has been developed. Experimentally, a Γ of 46 dB for selective modulation in the TM direction and a corresponding value of 40 dB for the TE direction have been obtained. It is also shown that the concept is a possible alternative or complement to polarizers and polarization preserving components in optical sensors