Aging effect in proton-exchanged optical waveguides based on lithium niobate crystals

The optical characteristics of planar waveguides produced by proton exchange in lithium niobate crystals in different acids (with and without additives) are unstable with time (“aging” effect). The aging effects in LiNbO₃ optical waveguides are investigated at room and elevated (50, 60, or 70°C) temperatures. In all cases, variations in the refractive index of the waveguide layer with time, i.e., for two or three years at room temperature and for several weeks at elevated temperatures, are determined for waveguides prepared in different media. The dependences of the optical characteristics of waveguides on the nature of proton sources and the aging conditions are obtained experimentally.     

Guiding of a one-dimensional optical beam with nanometer diameter

The concept of a one-dimensional optical wave and its waveguides are proposed for what is to our knowledge the first time. The proposed waveguides are principally new and named for one-dimensional optical waveguides. One-dimensional optical waveguides make it possible to guide very thin optical beams in the visible or the near-infrared region with a diameter in the nanometer range. The propagation properties are analyzed theoretically. The applications of the waveguides to optical devices in the nanometer range are discussed.     

光脉冲在微环耦合谐振光波导中传输线性特性的数值仿真

利用传输矩阵法得到了微环耦合谐振光波导色散关系的一般表达式,并由色散关系出发讨论了光脉冲在微环耦合谐振光波导中传输时的线性特性,包括带宽、群速度、色散和线性相位变化,这些特性对微环耦合谐振光波导在光通信和光传感领域的应用有重要意义. 关键词： 微环光波导 光脉冲线性特性 传输矩阵法 数值仿真     

Low cost fabrication of SiO2 optical waveguides by laser direct writing on Ti-doped Sol-Gel films

A commercial direct laser writing (DLW) system operating at 1070 nm was used to fabricate SiO₂ optical waveguides on silicon wafers. A Ti-doped SiO₂ Sol-Gel film was deposited on the SiO₂/Si substrate by the dip-coating technique, based on which SiO₂ optical waveguides were patterned by DLW using a Ytterbium fiber laser and followed by chemical etching. The effects of laser parameters and the preheated temperature of Sol-Gel films on the dimensions of optical waveguides were studied systematically. The differences of etching rate between laser irradiated and non-irradiated areas in Sol-Gel films preheated at various temperatures are characterized by measuring the thickness of the films. Results demonstrate that the available laser power density range for laser densification and the width of the patterned optical waveguides are influenced strongly by the preheated temperature of the Sol-Gel films. The width of the optimized optical waveguide in this work is 25 μm. The minimum propagation loss of the fabricated optical waveguides is 1.7 dB cm⁻¹ at the wavelength of 1550 nm.     

小尺寸低折射率差硅基二氧化硅阵列波导光栅

随着AWG型器件在光通信系统中的大规模应用,对低成本AWG芯片的需求越来越多。在各种降成本方案中,减小AWG芯片的尺寸是最有效的方法之一。本文介绍了一种新型小尺寸低折射率差硅基二氧化硅阵列波导光栅（AWG）的设计。在该AWG中,输入波导/输出波导与平板波导连接的部分制作成两侧为空气槽的高折射率差波导,所以在与输出平板波导连接处的相邻输出波导间距较小,这样可以在设计上缩短平板波导的长度、减少阵列波导的数量,实现较小的AWG芯片尺寸。该AWG的其它部分,如输入/输出波导与光纤耦合的部分、阵列波导光栅等均采用常规的低折射率波导工艺,所以就同时具有与常规的低折射率波导AWG相同的优点：如低耦合损耗、较好的串扰以及光学特性等。根据这个原理,设计了一种40通道100 GHz频率间隔的低折射率差硅基二氧化硅AWG,其芯片尺寸只有23.88 mm?10.5 mm,是传统相同材料制作的AWG尺寸的1/6。     

Optical propagation of the HE11 mode and Gaussian beams in hollow circular waveguides

The propagation of the HE₁₁ mode and Gaussian beams in hollow oversized circular waveguides is analyzed using optical theories. Different types of waveguides are considered : hollow dielectric or conducting waveguides, dielectric-lined waveguides, corrugated waveguides. General formulas are derived which give the power transmission through these different guides. The best wall materials and structures are determined from a comparison of the waveguide transmissions, at the infrared and millimeter wavelengths. The question of the coupling between the HE₁₁ mode and Gaussian beams is discussed and from a review of coupling coefficients derived before, an optimum value is pointed out. The problem of matching a Gaussian beam into circular waveguides in order to achieve the maximum power transmission is analyzed. These results are of interest for infrared lasers or waveguide applications and for Electron Cyclotron Wave (ECW) systems at the millimeter wavelength.     

Analysis of electrooptically induced waveguides in strontium barium niobate and optical modulators

An electrooptically induced optical waveguide can be formed by applying a DC voltage between electrodes fabricated on the surface of a crystal with strong electrooptic coefficients (for example, strontium barium niobate). Calculations of the refractive index profile and the optical mode size for such waveguides are presented. Approximate solutions for the fundamental mode are used to analyse the power coupling between a single-mode step-profile fibre and these waveguides. The power transmission losses in the straight and S-shaped waveguides are estimated by using a finite-difference beam propagation method (FD-BPM). The calculations predict that a new kind of modulator/switch can be realized with the induced waveguides. The important characteristics of these devices are discussed and compared with LiNbO₃ optical waveguide modulators.     

Spatial Optical Solitons in Planar Waveguides Based on Electrooptical Crystals

The characteristics of spatial optical solitons in planar waveguides based on electrooptical crystals can differ essentially from the corresponding characteristics for a three-dimensional medium. The main experimental findings relating to the effects of the formation of spatial optical solitons and their interaction in optical waveguides in lithium niobite and strontium – barium niobite crystals at very low light intensities are discussed.     

Optical loss measurements in femtosecond laser written waveguides in glass

The optical loss is an important parameter for waveguides used in integrated optics. We measured the optical loss in waveguides written in silicate glass slides with high repetition-rate (MHz) femtosecond laser pulses. The average transmission loss of straight waveguides is about 0.3 dB/mm at a wavelength of 633 nm and 0.05 dB/mm at a wavelength of 1.55 μm. The loss is not polarization dependent and the waveguides allow a minimum bending radius of 36 mm without additional loss. The average numerical aperture of the waveguides is 0.065 at a wavelength of 633 nm and 0.045 at a wavelength of 1.55 μm. In straight waveguides more than 90% of the transmission loss is due to scattering.     

Dispersion characteristics of SOI-based slot optical waveguides

The invention of the slot waveguide had enabled a number of interesting novel linear or nonlinear optical applications by guiding light in nanometers-wide low index slots guarded by high-index slabs. As one of its key characteristics, the low modal index for this kind of waveguides has been demonstrated by many studies. However, their higher order dispersion properties have not been thoroughly investigated yet, while the growing size and complexity of these devices and their potential nonlinear optical applications involving short optical pulses demand further understanding in their dispersion behavior. We here carry out a numerical study on the higher order dispersion characteristics of the SOI-based slot structures around the 1550 nm wavelength. Our results show that they could have significantly different second order dispersion properties in contrast to the traditional channel SOI waveguides. Their potentially large normal dispersion could have an impact on various nonlinear or linear applications. The relationship between the dispersion performance and the waveguide design is also investigated, and the results could show further venues to optimize or control the dispersion properties of such waveguides.     

New method for calculating the spectra and radiation losses of leaky waves in multilayer optical waveguides

The efficiency of a new method for calculating the spectrum and attenuation coefficient of leaky electromagnetic modes is demonstrated with multilayer planar optical waveguides the guiding properties of which are determined by antiresonant reflection from the multilayer cladding (antiresonant reflecting optical waveguides) rather than by total internal reflection from the core-cladding interface as in standard optical waveguides. The new method applies to calculation of both electromagnetic modes in dielectric waveguides and electron quantum states in multibarrier semiconductor heterostructures. The characteristics of multilayer waveguides calculated by the new method are compared with published data obtained from a complex dispersion relation by the transfer matrix method. As an example, the wavelength dependence of the radiation losses for the first TE mode of a planar optical waveguide containing 52 pairs of layers is calculated.     

Fabrication of fluorinated polyimide optical waveguides by micropen direct writing technology

A novel and cheap direct writing method based on the micropen has been developed to fabricate fluorinated polyimide stripe optical waveguides on Si/SiO₂ wafers. The overall design, starting material, micropen direct writing system and fabrication processes of the stripe optical waveguides are presented. The effects of the key direct writing parameters, such as the tip-to-substrate distance, extrusive gas pressure, writing speed and viscosity of the polyamic acid, on the dimension and morphology of the stripe optical waveguides are discussed in detail. After deposition by the micropen system and baking process, the fluorinated polyimide stripe optical waveguides with good morphology and surface quality can be fabricated using the optimal parameters. The propagation losses at the wavelength of 1.55 μm are in the range of 1.4-3.5 dB cm⁻¹ as characterized by different length combinations of the strip optical waveguides.     

Polymer-based waveguides with low propagation loss and polarization-dependent loss

Polymeric optical waveguides based on a new synthesized bisphenol A–aldehyde polymer (PA-1 resin) have been fabricated using photolithography and reactive ion etching technique. The polymer is novel with relatively high refractive index, low birefringence and absorption at the optical communication wavelengths. The single-mode channel waveguides exhibit a propagation loss of 0.41±0.05 dB/cm at a wavelength of and 0.5±0.05 dB/cm at for both the TE and TM polarizations. The polarization-dependent loss of the waveguides is 0.1±0.05 dB/cm at these wavelengths.     

Vertically coupled photonic crystal optical filters

A novel design of optical filters, based on the vertical coupling between channel waveguides and photonic crystal microcavities, is proposed. The choice of channel waveguides can be arbitrary because of the flexibility of the device geometry, which makes it possible to utilize conventional low-loss waveguides. The separation between waveguides and microcavities, which determines filter bandwidths and drop efficiencies, can be accurately controlled by material growth (or deposition). The filters might also be switchable, e.g., by mechanical movements of microcavities. An example of a reflection-type optical filter is demonstrated in numerical experiments using the finite-difference time-domain method.     

平面光波导用于实时测试生化反应新方法的研究

在分析经典消逝波光波导生化传感器的基础上,为寻找性能更佳的传感器,探讨用更为直观的截止特性来进行生化传感。分别对三层及四层平板波导进行了研究,指出了作为基于截止特性的生化传感器三层波导结构存在的局限性。在理论上研究了可以在四层波导中使用变折射率材料来进行基于截止特性的生化传感,并给出了由空气隙、极化聚合物、高折射率薄膜、待测物组成的传感器结构示意图。用数值分析的方法绘出了此装置的灵敏度与高折射率薄膜厚度的关系曲线,并将它同经典消逝波光波导生化传感器灵敏度与薄膜厚度关系图进行了比较。结果表明,这种新型传感器理论上可以在较厚的薄膜结构中达到很高的灵敏度。     

Self-aligning method of fiber-to-waveguide pigtailing

An innovative self-aligning technique for the pigtailing of optical fibers to buried channel planar waveguides is presented, based on selective etching. This technique utilizes a plug-and-socket mechanism that is intrinsically self-aligning and mechanically stable. The processes involved have been specifically designed to facilitate the bulk manufacture of pigtailed single or multiple fibers and waveguides. An optimized alignment geometry for the physical connection of fibers to waveguides is presented.     

Periodic poling of optical waveguides produced by swift-heavy-ion irradiation in LiNbO<Subscript>3</Subscript>

The generation of periodically poled structures in waveguides prepared by swift-heavy-ion (SHI) irradiation, i.e. in the electronic stopping power regime, has been achieved following two different strategies. In one of them we have prepared bulk PPLN samples by an applied electrical field, followed by irradiation with F ions at 22 MeV. After the ion irradiation, a waveguide showing a high optical confinement is obtained, preserving the original PPLN structure. The second strategy consisted of electric periodic poling of previously fabricated swift-ion-irradiated waveguides. To our knowledge this method has not been, so far, successful for conventional implanted waveguides. The successful fabrication of PPLN structures on novel waveguides prepared by SHI irradiation offers a promising potential for nonlinear integrated optical devices.     

Principal waves in multiply connected waveguide light concentration on subwavelength areas

The use of multiply connected waveguides for localizing and concentrating optical electromagnetic radiation onto areas of subwavelength transverse dimension is substantiated. In optical microscopes, this makes it possible to reach subwavelength resolution along all three spatial coordinates. In experiments where interaction of laser radiation with matter is studied, the subwavelength multiply connected waveguides allow the field intensity in the optical wave to be significantly increased. Applications of subwavelength waveguides in submicrometer photolithograpy, submicrosecond microelectronics, and photogrammetry is discussed, as well as for optical information recording and readout.     

Design and modeling of microwave photonic devices

Different types of finite element methods (FEMs) for microwave and optical waveguides are reviewed and are utilized for modeling of a traveling-wave (TW) optical modulator, as one of the typical microwave photonic devices. Using the quasi-TEM and the full-wave vector FEM solvers for microwave waveguides and the scalar FEM solver for optical waveguides, the behaviour of a TW Z-cut Ti:LiNbO3 Mach–Zehnder optical modulator with a ridge structure is investigated.     

Step-like structure and assisted tunneling in two coupled modulated waveguides

The propagation of an initially-localized optical beam along two coupled modulated waveguides is investigated. A universal step-like structure emerges in the propagation process once |d|\delta v/v₀|v_0| ~ 1 and T = 2 π/m v₀m v_0, both for the linear and nonlinear optical waveguides. The reason is that the coupling between the two waveguides can be completely suppressed at z = m T or (2 m+1)T/2. When the nonlinearity is strong, assisted tunneling can be found for the initially-localized optical beam by slightly increasing the strength of the modulation.