缓冲质子源LiNbO3波导折射率计算及实验

为实现铌酸锂退火质子交换(APE)波导折射率分布的准确计算,选择含苯甲酸锂的苯甲酸缓冲液作为质子交换质子源,高温退火制作了波导样本.针对该工艺过程建立退火质子交换波导模型,包括非线性扩散模块和光学数值仿真模块,分别计算APE波导折射率及其模式有效折射率.以测得的样本波导模式有效折射率和计算的有效折射率差的均方根构建评价函数(FOM),结合遗传算法提取该工艺条件下质子扩散参数,实现了不同交换深度和退火时间波导折射率分布及其光学特性的一体化计算.实验表明:FOM小于0.001,计算折射率分布同IWKB方法测得结果吻合较好,最大偏差约0.002.     

硅包层质子交换条形光波导的频率响应

退火质子交换工艺已成为一种制备低损耗铌酸锂光波导的重要技术.但目前对该类光波导传输特性的研究还不多.利用半矢量光束传播法,对硅包层X切退火质子交换铌酸锂条形光波导的频率响应特性进行了数值分析.给出了几种波导结构参数下的计算结果.结果表明,波导传输模式的衰减特性与波长相关.波导的衰减特性,可以通过调整波导表面中心处折射率增量,硅包层厚度,及缓冲层的折射率和厚度来控制.硅包层光波导可以用来制作光频滤波器.     

Graded-index planar dielectric waveguides

Properties of planar dielectric waveguides having a diffusion-induced index of refraction distribution are investigated. It is found that the spatial distribution of modes, group velocity, and mode spectrum of these guides differed qualitatively from the corresponding properties found in a slab waveguide. These results were experimentally verified by measuring the mode spectrum of a dielectric waveguide having an assumed complimentary error-function distribution of refractive index.     

Optical Channel Waveguides in $hbox {KTiOPO}_{4}$ Crystal Produced by Proton Implantation

We reported on, to our knowledge the first time, the channel waveguide formation in KTiOPO₄ crystal by the implantation of protons with a special designed photoresist mask. The 2-D refractive index profile was constructed according to the measured dark-mode spectroscopy (for longitudinally planar configuration) as well as the shape of the channel waveguide cross section. Based on this index distribution, the modal profile was calculated through a numerical simulation, which showed reasonable agreement with the near-field light intensity distribution of the guided mode that was obtained by an end-coupling method. After annealing at 200deg C for 30 min in air, propagation loss of the channel waveguides was determined to be as low as ~2.9 dB/cm at wavelength of 632.8 nm.     

Proton-exchanged optical waveguides in Z-cut LiNbO3 using phosphoric acid

Proton-exchanged Z-cut LiNbO₃ planar waveguides formed using phosphoric acid were characterized optically. The refractive index profile and the diffusion parameters were studied systematically. These waveguides have propagation losses of less than 1 dB/cm and exhibit properties that are different from those obtained using benzoic acid. The index profile is not a simple step function and can be modeled accurately by a polynomial expression. A maximum surface index increase of 0.145 was measured at a 0.633-μm wavelength. The diffusion constant D₀ and the activation energy Q for the proton-exchange process using this acid were found to be 6.43×10⁸ μm²/h and 82.91 kJ/mol, respectively. The annealing properties of these waveguides were also established, and the effects of annealing on surface index change and waveguide depth increase were found to follow a power-law relationship     

质子交换LiTaO3平板光波导的表征

本文采用质子交换法制备了Ｚ切ＬｉＴａＯ３晶体平板光波导，并研究了退火前后波导的光学性能。讨论并用实验结果验证了退火前后波导内折射率的分布，实验证明退火前波导层的折射率分布为阶跃型，退火后其分布为费米型。在６３２．８ｎｍ的Ｈｅ－Ｎｅ激光波长下的测量结果表明：质子交换ＬｉＴＡＯ３波导层折射率增量退火前约为０．０１２，退火后为０．０２５。     

十八酸质子交换铌酸锂光波导的性能及其退火性能的研究

报道了采用十八酸作为质子源制作的铌酸锂光波导的性能及其退火对波导性能的影响，给出了退火前后波导层中非常光有效折射率的剖面分布，得出了质子交换的扩散参数和波导层的表面非常光折射率的增量．给出了波导层的表面非常光折射率的增量和扩散深度的增量与退火时间的指数关系．     

Double-layered magnetooptic channel waveguide for waveguideisolator application

TE-TM mode conversion based on the nonreciprocal Faraday effect for waveguides fabricated by liquid-phase epitaxially grown (YBiCa)₃Fe₅O₁₂ films is discussed. The phase mismatch is controlled by constructing a double-layer channel waveguide with an intermediate cladding layer of refractive index slightly lower than the core. Care must be taken to compensate any birefringence of materials, e.g. stress- and growth-induced birefringence. It is shown that such a structure is effective in decreasing the effect of waveguide shape deformation on the phase-matching degree. The analysis of the double-layered channel waveguide and experimental results are described     

Sol-gel glass waveguide and grating on silicon

This paper reports on the fabrication and characterization of hybrid organic-inorganic glass sol-gel slab and channel waveguides by ultraviolet light imprinting in thin films deposited by a one-step dip-coating process. The adjustment of chemical composition of the materials provides precise selection of refractive index from 1.48 to 1.52 at the wavelength of 632.8 mn. The refractive index of the waveguides at 1.55 μm is similar to that of optical fiber, thus reducing the reflection loss between the two to less than 0.01 dB. The effect of ultraviolet light exposure and heat treatment on waveguide refractive index is studied. Fabrication parameters to produce ridge waveguides are optimized to achieve very smooth side walls. Propagation losses in these waveguides are ~0.1 dB/cm. Single mode buried waveguides, at 1.55 μm wavelength, with circular mode profile are demonstrated     

退火质子交换LiNbO3光波导低损耗耦合分析

利用光束传播法分析了退火质子交换铌酸锂光波导模场参数随波导宽度、深度和表面折射率改变的变化情况,指出波导模场受这些参数的综合影响,并讨论了波导结构参数对光纤．波导耦合效率的影响。结果表明,要实现最佳耦合,不同模场直径的光纤对波导结构参数的要求是不同的,这可供相应波导器件的设计和制备参考。     

Exact derivation of the coupling coefficient in corrugatedwaveguides with arbitrary cross-section: application to optical fibers

Expressions for the coupling coefficient in corrugated waveguides with rectangular tooth shape are derived based on local mode expansion. The expressions are exact to first order in the tooth height (or index difference) for any waveguide cross-section. When applied to slab waveguides, the familiar expressions are obtained. Specializing to weakly guiding fibers, with step index profile, result in simple expressions in terms of the fiber parameters, which are compatible with the effective index method     

Collinear acoustical TM-TE mode conversion in proton exchangedTi:LiNbO3 waveguide structures

Collinear acoustooptic TM-TE mode conversion is experimentally investigated in proton exchanged Ti:LiNbO₃ optical waveguides. Their birefringence is adjusted by an appropriate annealing to allow phase matching at surface acoustic wave frequencies of about 90 MHz and 180 MHz, respectively. A planar mode converter with an efficiency of up to 90 percent is presented (87 MHz, 175 MHz). Furthermore, a combined acoustical/optical strip waveguide structure is developed as mode converter, leading to strongly reduced power requirements; only 0.3-mW acoustic power is sufficient to achieve a conversion efficiency of 50 percent     

Guided Mode Characteristics of Metal-Clad Planar Optical Waveguides Produced by Diffusion (Short Paper)

Analytical and numerical results for the guided mode characteristics of metal-clad planar waveguides produced by diffusion are developed. Values of the complex propagation constants are obtained numerically and are shown to be in good agreement with the analytical results. These give insight into how waveguide and material parameters determine the loss. Since the profile of the waveguide represents the variation of the refractive index of the diffused-channel waveguide with the depth dimension, the results obtained can be used to reduce the dimensionality of the diffused-channel waveguide and facilitate the application of the effective-index method.     

Towards the optimization of dielectric branching waveguides: theeffect of longitudinal variations in the branching angle

Mathematical models that describe mode propagation in planar Y -branch waveguides indicate there is a region in the branch where mode conversion occurs, provided asymmetry exists in the waveguide geometry or refractive index profile. Since the mode conversion is influenced by the degree of asymmetry and by the branching angle, the Y-branch can be designed to act as a power divider or as a mode splitter. The problem of identifying constant width Y-branch structures where longitudinal variations in the branching angle are introduced to optimize the power division or mode-splitting properties of a branching waveguide is considered. This constrained optimization is performed using forward dynamic programming and a compatible method of mode propagation analysis, the step approximation technique. The propagation characteristics of the optimized waveguide structure are compared to those of conventional constant-slope waveguides     

Efficient coupling between annealed K+-Na+ion-exchanged channel waveguides and 10/125 single-mode fibers atλ=1.321 μm

The process of thermal annealing of K⁺-Na⁺ ion-exchanged channel waveguides has been studied with the aim of optimizing their coupling efficiency with commercial single-mode fibers at λ=1.321 μm. Waveguides obtained in soda-lime glass slides, with mask apertures ranging between 13.4 and 2.6 μm, were characterized before the annealing by combining nearfield measurements and an etching procedure. The experimental results were successfully compared with a theoretical model based on the variational principle. The refractive index distribution of K⁺-Na⁺ ion-exchanged channel waveguides supporting one or a low number of modes was given: compared to the corresponding slab case, the refractive index step Δn_o remained constant, while the waveguide depth was lower. The thermal annealing process of the channels was then performed and modeled by means of the standard diffusion theory. As a result, the channel fabrication parameters for optimum guide-fiber coupling could be predicted: 0.23-dB mode mismatch losses were measured between the optimized channel and a commercial 10/125 single-mode fiber, at λ=1.321 μm     

A new method for producing graded index PMMA waveguides

We report a new method for producing graded index polymethyl methacrylate (PMMA) waveguides using optical densification by angled X-ray exposure. The resulting larger refractive index near the surface of the waveguide serves to better isolate the guided light from potentially absorbing or scattering substrates than in waveguides of constant refractive index. We calculate that a 65° angle exposure in 6-μm-thick planar PMMA waveguide shifts the peak of the fundamental mode profile ~0.5 μm away from the substrate. Consistent with this result, we measure a reduction in the propagation loss of PMMA planar waveguides on a Ni substrate from 1.5 dB/cm to 0.5 dB/cm after angle X-ray exposure     

Numerical field solution for an arbitrary asymmetrical graded-index planar waveguide

We present a numerical solution for the field amplitudes in an asymmetrical graded-index planar waveguide with an arbitrary index profile. For small index differences between the surface and bulk indexes and for large differences between the cover and surface indexes, the modes of the diffused waveguides can be described entirely in terms of normalized mode index, diffusion depth, effective modal width, and theVnumber. The results show a high degree of accuracy when checked against exact published results for the parabolic and exponential index profiles. Universal charts for the modal fields in terms of the normalized quantities are presented for profiles of practical interest, namely, Gaussian and complimentary error function index profiles. We show that the modal width, while somewhat sensitive to theVnumber, is surprisingly insensitive to the index profile. Tailoring of the index profile, therefore, does not seem important for the efficient fiber-waveguide endfire coupling. Error involved in the estimation of the phase shift at the cover-surface boundary as function of the asymmetry parameter is shown to be rather small for practical cases. Also, the discrepancy in the field distribution between an almost symmetrical and a highly asymmetrical waveguide at the cover-surface boundary is illustrated with a practical example.     

Analysis of planar optical waveguide fabrication by ion exchange in glass

Fabrication of planar optical waveguides by silver-ion exchange in glass, and the resulting refractive index profiles related to the fabrication conditions, are investigated. The second-order polynomial profile model proposed by Stewart et al. [3] is confirmed, and the dependence of the model characteristics on fabrication conditions is analyzed. Use of the resulting relationships is demonstrated by the compilation of a design table of expected waveguide mode structures for given fabrication conditions. Methods of exchanged-ion concentration profile determination are reviewed and a simple method based on sample angle lapping is proposed.     

Network analysis of eigenvalue problem for multilayer dielectric waveguide consisting of arbitrary number of layers

In this paper, the eigenvalue problem of a multilayer dielectric waveguide consisting of arbitrary number of layers is solved by the microwave network method. A general program with the function of computer graphics has been developed for analyzing the dispersion characteristics and the electromagnetic field distributions of an N layer dielectric waveguide. As examples of practical applications of the program, first, the dispersions and field patterns for the planar waveguides with refractive index of parabolic and exponential profiles are analyzed. Secondly, the procedure of mode conversion and mode separation in dielectric branching waveguides is vividly demonstrated through analyzing the field distributions of asymmetric multilayer dielectric structures and the general rules of mode conversion are discussed. The examples show that the present method possesses the advantages of versatility, rapidity, simplicity and accuracy.