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     

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     

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。     

Optical switching and power control in LiNbO3 coupledwaveguide arrays

A diversity of switching and power-distribution functions in coupled multi-waveguide arrays is discussed and demonstrated. Functionality is achieved by varying the effective index in each waveguide independently by means of the electro-optic effect. The splitting and switching functions are attained by the use of a multi-variable optimization procedure. Specifically we demonstrate [1×2] and [1×5] routing and power division and [2×2] cross-connection. Polarization diversity and polarization-independent switching are attained using a special crystal orientation and electrode configuration. Single-polarization routing was measured in most cases, with crosstalk figures of the order of - 15 dB. Polarization-independent switching was achieved with crosstalk of less than - 19 dB for a three-waveguide [1× ] switch. By addition of dummy guides, a polarization-independent [2×2] cross-connection function could be attained with crosstalk figures better than -30 dB     

Wavelength-selective polarisation analyser with integrated Ti:LiNbO3 acousto-optical TE-TM converter

A wavelength-tunable acousto-optical Ti:LiNbO₃ TE-TM converter, followed by a 45° linear polariser, can be used to measure the four Stokes parameters. A demonstration unit for 1.5 μm wavelength operation with 9 MHz electrical measurement bandwidth, roughly 0.04 rad accuracy and 250 GHz optical bandwidth has been developed     

8×8 symmetric nonblocking integrated acoustooptic spaceswitch module on LiNbO3

An 8×8 symmetric nonblocking integrated acoustooptic space switch module has been realized on a Y-cut LiNbO₃ substrate, 1.0×10.0×37.0 mm in size. The switch module consists of a new hybrid beam expanding-collimating lens, a large aperture focusing lens, and a pair of four-element tilted surface acoustic wave (SAW) transducer arrays in a titanium-in-diffused channel-planar-channel composite waveguide. Experimental results of point-to-point nonblocking switching at the optical wavelength of 0.6328 μm, including an average crosstalk of -12.2 dB, optical switching efficiency of 25% at 125 mW RF drive power, carrier frequency increment of 13.5 MHz for switching between adjacent output channels and reconfiguration time of 0.4 μs have been obtained     

A high-speed 4×4 Ti:LiNbO3 integrated optic switchat 1.5 μm

The operation of a fully packaged and pigtailed polarization dependent Ti:LiNbO₃ 4×4 integrated optic switch with an operating wavelength of 1.5 μm is demonstrated. The switching matrix is fast, with a measured 3-dB small-signal bandwidth greater than 1 GHz. Unwanted cross modulation between channels due to coupling between switching elements was measured and found to be less than -20 dB of the signal strength. The switch is composed of balanced bridge switching elements having a 17-V switching voltage, an 18 dB extinction ratio, and an excess optical loss of 0.6 dB     

Polarization-independent integrated optical, acoustically tunabledouble-stage wavelength filter in LiNbO3

We report a polarization-independent integrated acousto-optical double-stage wavelength filter in X-cut, Y-propagating LiNbO₃. The device consists of a combination of four acousto-optical mode converters, a TE- and a TM-pass polarizer, and two polarization splitters; their specific design and properties are discussed. The filter has a bandwidth of 1.4 nm and a tuning range of 76 nm around λ1.55 μm. Maximum filter transmission is achieved with a RF drive power of only 80 mW; it excites via a single unidirectional transducer a guided surface acoustic wave driving the four polarization converters of both stages. Fiber-to-fiber insertion loss is 4.6 dB for TM- and 4.8 dB for TE-polarized waves. Multiwavelength filtering has been demonstrated     

Millimeter-wave Ti:LiNbO3 optical modulators

The design, fabrication, and characteristics of ridged Ti:LiNbO ₃ optical modulators that work in the millimeter-wave region are presented. A new concept of design under velocity matching is demonstrated for the proposed modulator. It has been shown by calculation that impedance matching is achieved and conductor loss is greatly reduced under velocity matching with wider gaps and a thicker coplanar waveguide electrode in conjunction with a ridge structure. Two types of Mach-Zehnder optical intensity modulators for the wavelength of 1.5 μm are developed. A fully packaged module for 40 Gb/s transmission with a half-wave voltage of 3.5 V and a broadband modulator responsible up to 100 GHz with a half-wave voltage of 5.1 V     

Polarization independent bidirectional fiber link using coherencemulti/demultiplexing LiNbO3 integrated electroopticalcircuits

We report recent results obtained in the area of bidirectional transmissions. We demonstrate the application of coherence modulation to such transmission systems using especially designed LiNbO₃ integrated electrooptic circuits. We describe a system, formed by a pair of matched Mach-Zehnder interferometers integrated in Z-propagating lithium niobate substrate, whose peculiarity is to feature a polarization-independent behavior in the frame of digital transmissions. Considerations on crosstalk are also discussed. The experimental results deal with the demonstration of a bidirectional link with a data bit rate of 25 Mb/s     

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.     

Fabrication tolerance of Ti:LiNbO3 waveguides

A simulation program based on the three-dimensional beam propagation method (BPM) is used to study the fabrication conditions of single-mode Ti:LiNbO₃ waveguides. The calculated cutoff wavelengths are in good agreement with experimental data. The thickness of titanium needed to support the fundamental and first-order modes for three Ti strip widths (6, 7, and 8 μm) is calculated. The two-dimensional BPM is used to calculate the coupling length from the fabrication conditions. Results are consistent with measured data. The fabrication tolerances of the coupling length of directional couplers are also presented     

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.     

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)     

90° reflecting mirrors in Ti:LiNbO3 waveguidestructures

The first integrable 90° reflecting mirrors in LiNbO₃ waveguide structures are reported. Mirror power efficiencies (⩽28%) have been measured experimentally and are compared with theoretical values obtained by mode overlap integration     

Polarisation-independent LiNbO3 4×4 matrix switch

The first polarisation independent LiNbO₃ strictly non-blocking 4×4 matrix switch has been developed. This matrix switch has a 4-6 dB insertion loss at any incident polarisation with 1.3 μm wavelength and about 30 V switching voltage     

High-speed shielded velocity-matched Ti:LiNbO3 opticalmodulator

A shielded velocity-matched Ti:LiNbO₃ optical modulator is investigated, focusing on the characteristics of the traveling-wave (TW) electrode and the optical waveguide. The optical waveguide is analyzed and its parameters are determined using the newly developed modified-step-segment method (MSSM). The TW electrode is analyzed using the second-order triangular element quasi-TEM finite element method (FEM). By taking the thickness of the coplanar waveguide (CPW) traveling-wave (TW) electrode into consideration, it is confirmed that there is an optimum overlaid layer thickness for a given electrode thickness. It is also shown that very wide modulation bandwidth can be attained by using the optimum CPW TW electrode thickness and overlaid layer thickness     

LiNbO_3:Fe中激光损伤所致的微电畴

本文观察到了LiNbO_3:Fe晶体中由光致损伤所致的一种微电畴.它起始于光伤迹线的正P_s边,其指向与P_s相反.用α-径迹显影法显示了Li离子沿负P_s方向的迁移.这种微电畴源于光生伏特效应下,Li~+离子穿越氧三角面的沿P_s轴的定向迁移及产生Li-空位-Nb有序化的局域空间电荷物.     

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。