Tunable electro-optical 2 × 2 photonic crystal beam splitter

A tunable electro-optical 2?×?2 beam splitter based on two-dimensional rod-type photonic crystals is presented. The beam splitter consists of two orthogonally crossed linear waveguides and a single center rod in square lattice photonic crystals. In order to create a linear waveguide, the radius of a line of rods is reduced. A single center rod is positioned at the intersection of the linear waveguides to divide the input lightwave into output ports. The switching mechanism is a change in the conductance of the waveguide region and hence modulating the guided modes. The tunable beam splitter can be applied to photonic integrated circuits.     

Theoretical study on sensing performance of hydrogen annealed silicon waveguides

Due to the surface diffusion movement of Si atom in hydrogen annealing process, the sharply formed corners of waveguides will be rounded and its sidewall profile could be reformed. In this paper, the performances of microring sensors based on three different gradual annealed structures, strip with large/small round corners and cylinder waveguide, are investigated theoretically. Characteristic parameters of sensors based on cylinder waveguide, sensitivity, Q factor, and measuring range are analyzed and compared with that of sensors based on the widely-used strip and slot waveguides. Simulation results demonstrate that the sensitivity of microring is significantly increased after annealing with comparable Q factor and measuring range. The hydrogen annealing process promises a feasible and effective method to improve the performance of biosensors in the future.     

玻璃基掩埋式光波导堆栈的制作与表征

在硅酸盐光学玻璃基片上制作了光波导堆栈, 这种光波导堆栈通过Ag⁺/Na⁺熔盐离子交换和电场辅助离子扩散技术顺次制作了两层掩埋式光波导. 对光波导堆栈的横截面显微结构进行了观察, 并对堆栈中两层波导的损耗特性进行了测试. 所获得的光波导堆栈中的上、下两层波导芯部分别位于玻璃表面以下14和35 μm处; 上层光波导芯部尺寸约为12 μm×7 μm; 下层光波导芯部尺寸约为9 μm×8 μm. 通光测试显示两层波导在1.55 μm工作波长下均为单模光波导, 且两者之间没有相互耦合. 损耗测试分析结果显示: 堆栈中两层光波导的传输损耗均约为0.12 dB/cm,与单模光纤之间的耦合损耗分别为0.78和0.73 dB. 分析表明, 这种光波导堆栈在玻璃基集成光芯片的高密度集成方面具有很好的应用前景.     

Analysis and design of a rib-like-based slot waveguide for nonlinear silicon nanophotonics

Design of efficient nonlinear optical waveguides is an essential requirement for development of silicon nanophotonics. These waveguides should have a unique capability to play the main role in realization of both passive and active optical devices. A high-performance dielectric rib-like-based slot waveguide is proposed for nonlinear silicon nanophotonics. Its slot region can be filled with Si-nc:SiO₂ which exhibits a high third-order nonlinear effect. Study of numerical results shows that this new slot waveguide has a nonlinear parameter of the same order of magnitude as an equally sized silicon nanophotonics strip-based slot waveguide. The new waveguide can be fabricated easily by etching a slot in the core region of the silicon-on-insulator (SOI)-based rib-like waveguide.     

Transverse electric modes in planar slot waveguides

Transverse electric (TE) modes in symmetrical planar slot waveguides are analytically solved, and a comprehensive knowledge about them is provided. It has been shown that there are four types of TE modes in planar slot waveguides just as the TM modes. Their field solutions, characteristic equations and cut-off conditions are derived. The cut-off conditions and the power confinement factors of the TE and TM modes are compared. It is demonstrated that there may be degeneration points between TE and TM modes. Because a three-dimensional waveguide has similar modal characteristics as its planar counterpart, our work presents a foundation of recognizing and utilizing the three-dimensional slot waveguides.     

阵列波导光栅解复用器光谱响应特性分析

基于单模光波导本征模场及其端面衍射场分布的高斯近似表达,推导出两个端面非接触光波导耦合的耦合系数的函数表达式,并基于此推导出阵列波导光栅解复用器的简单光谱响应效率的函数表达式。建立了分析阵列波导光栅解复用器光谱响应特性的简洁数学模型,阐明了阵列波导光栅解复用器对通道中心频率的响应度和相邻信号通道间的串扰与基本参数的关系,为设计和分析阵列波导光栅解复用器提供了理论基础。     

Polarization properties of two-dimensional slot waveguides

We propose and study slot waveguide geometries where both quasi-TE and quasi-TM modes may propagate highly confined within the same low-index slot region. Conventional horizontal and vertical slot waveguides can only provide high slot confinement for either the quasi-TM or quasi-TE modes, respectively. Different two-dimensional slot waveguide structures are analyzed in terms of their mode characteristics, such as the effective index, the confinement factor, and the overlap of quasi-TE and -TM modes within the slot. Attention is also paid to practical manufacturability. Various waveguide structures can be tailored to have zero birefringence or equal confinement at both polarizations. Values for the confinement factors and the overlap of the two polarizations, in the slot region, can reach 0.4 to 0.5.     

Reduction of sidewall roughness, insertion loss and crosstalk of polymer arrayed waveguide grating using vapor-redissolution technique

An efficient vapor-redissolution technique is used to greatly reduce sidewall scattering loss in the polymer arrayed waveguide grating (AWG) fabricated on a silicon substrate. Smoother sidewalls are achieved and verified by a scanning electron microscopy. Reduction of sidewall scattering loss is further measured for the loss measurement of both straight waveguides and AWG devices. The sidewall loss in straight polymer waveguide is decreased by 2.1 dB/cm, the insertion loss of our AWG device is reduced by about 5.5 dB for the central channel and 6.7 dB for the edge channels, and the crosstalk is reduced by 2.5 dB after the vapor-redissoluton treatment.     

Matrix method for two-dimensional waveguide mode solution

In this paper, we show that the transfer matrix theory of multilayer optics can be used to solve the modes of any two-dimensional (2D) waveguide for their effective indices and field distributions. A 2D waveguide, even composed of numerous layers, is essentially a multilayer stack and the transmission through the stack can be analysed using the transfer matrix theory. The result is a transfer matrix with four complex value elements, namely A, B, C and D. The effective index of a guided mode satisfies two conditions: (1) evanescent waves exist simultaneously in the first (cladding) layer and last (substrate) layer, and (2) the complex element D vanishes. For a given mode, the field distribution in the waveguide is the result of a ‘folded’ plane wave. In each layer, there is only propagation and absorption; at each boundary, only reflection and refraction occur, which can be calculated according to the Fresnel equations. As examples, we show that this method can be used to solve modes supported by the multilayer step-index dielectric waveguide, slot waveguide, gradient-index waveguide and various plasmonic waveguides. The results indicate the transfer matrix method is effective for 2D waveguide mode solution in general.     

Silicon on silicon dioxide slot waveguide evanescent field gas absorption sensor

Several trace gases such as H₂O, CO, CO₂, NO, N₂O, NO₂ and CH₄ strongly absorb in the mid-IR spectral region due to their fundamental rotational and vibrational transitions. In this work, we propose an evanescent field absorption gas sensor based on silicon/silicon dioxide slot waveguide at 3.39 μm for sensing of methane gas. These waveguides can provide the highest evanescent field ratio (EFR) > 47% with adequate dimensions. Higher EFR values often come at an expense of higher propagation losses. These waveguides have relatively higher losses as compared to conventional waveguides, such as rib and slab waveguides, as these fundamental losses are static and the proposed sensing mechanism is established on the incremental loss due to the absorption of the gas. Therefore, incident power can always be incremented to compensate the waveguide losses.     

CMOS compatible on-chip telecom-band to mid-infrared supercontinuum generation in dispersion-engineered reverse strip/slot hybrid Si3N4 waveguide

A silicon nitride (Si₃N₄)-based reverse strip/slot hybrid waveguide with single vertical silica slot is proposed to acquire extremely low and flat chromatic dispersion profile. This is achieved by design and optimization of the geometrical structural parameters of the reverse hybrid waveguide. The flat dispersion varying between ±10 ps/(nm·km) is obtained over 610 nm bandwidth. Both the effective area and nonlinear coefficient of the waveguide across the entire spectral range of interest are investigated. This led to design of an on-chip supercontinuum (SC) source with ?30 dB bandwidth of 2996 nm covering from 1.209 to 4.205 μm. Furthermore, we discuss the output signal spectral and temporal characteristic as a function of the pump power. Our waveguide design offers a CMOS compatible, low-cost/high yield (no photolithography or lift-off processes are necessary) on-chip SC source for near- and mid-infrared nonlinear applications.     

Direct integration of nanoscale conventional and slot waveguides

Slot waveguides can provide high optical confinement in a nanoscale low-index layer. While a conventional waveguide has a Gaussian-like Eigenmode profile, the Eigenmode profile of a slot waveguide is quite non-Gaussian type, due to the large discontinuity of refractive indices and thus the transverse electric field component between the high and low index layers of a slot waveguide. Although the field profiles of the two types of waveguides seem different, here we show that direct integration of conventional and slot waveguides yields efficient coupling of light into and out of slot waveguides using the rigorous finite-difference time domain method. The proposed direct coupling method has comparable performance to recently proposed taper based coupling methods, while having advantages in easier integration with conventional waveguide optics and higher integration density. We also show that coupling efficiency is not sensitive to the symmetricity of the slot waveguide, resulting in good manufacturing tolerance. The proposed direct coupler may have a number of applications in lightwave interconnects, sensing and data storage.     

Numerical analysis of a miniaturized design of a Fabry–Perot resonator based on silicon strip and slot waveguides for bio-sensing applications

A miniaturized design of a Fabry–Perot resonator based on silicon strip and slot waveguide is presented. The corrugated Bragg gratings and a cavity are structured on both sides of a waveguide. The spectral characteristics of both the resonator designs are studied. The variation of the medium refractive index is detected by monitoring the shift of the spectral maxima of the Fabry–Perot cavity. The transmission spectrum and electric field distribution of the sensor structures are simulated using finite-element method. The resonator based on slot-waveguide shows better sensitivity and figure of merits as compared to strip waveguide. Our proposed sensor design has the potential to find further applications in biomedical science and nano-photonic circuits.     

Highly efficient coupling in lithium niobate photonic wires by the use of a segmented waveguide coupler

The purpose of this article is to show that efficient light coupling in lithium niobate waveguides presenting a strongly confined mode, such as photonic wires, is possible with the use of a periodically segmented waveguide coupler. The coupler consists in an input periodically segmented waveguide whose mode size is adapted to the mode of a standard single-mode fiber coupled to a photonic wire whose mode size is of the same order of the wavelength. The periodic segmentation of the input waveguide allows fulfilling the phase matching condition necessary to achieve an efficient light transfer between these waveguides. The coupling efficiency is typically 5 times higher than the butt-coupled configuration.     

High-index-contrast waveguides and devices

We present a theoretical and experimental study of high-index-contrast waveguides and basic (passive) devices built from them. Several new results are reported, but to be more comprehensive we also review some of our previous results. We focus on a ridge waveguide, whose strong lateral confinement gives it unique properties fundamentally different from the conventional weakly guiding rib waveguides. The ridge waveguides have distinct characteristics in the single-mode and the multimode regimes. The salient features of the single-mode waveguides are their subwavelength width, strong birefringence, relatively high propagation loss, and high sensitivity to wavelength as well as waveguide width, all of which may limit device performance yet provide new opportunities for novel device applications. On the other hand, wider multimode waveguides are low loss and robust. In addition, they have a critical width where the birefringence is minimal or zero, giving rise to the possibility of realizing intrinsically polarization-independent devices. They can be made effectively single mode by employing differential leakage loss (with an appropriate etch depth) or lateral mode filtering (with a taper waveguide). Together these waveguides provide the photonic wire for interconnections and the backbone to build a broad range of compact devices. We discuss basic single-mode devices (based on directional couplers) and multimode devices (multimode interferometers) and indicate their underlying relationship.     

A novel design of a cpw‐fed single square‐loop antenna for circular polarization

Abstract

The experimental and simulated results for the proposed antenna are investigated in this article. Moreover, a novel broadband design of a circularly polarized (CP) single square slot antenna fed by a single coplanar waveguide is presented. By appropriately choosing the circumference of the square‐loop, the length of the protruded strip, and the gap, this proposed antenna thus owns good CP radiation and good impedance match simultaneously at the frequency of 2.45 GHz. This proposed antenna has the fundamental resonant frequency of 2.5 GHz with the minimum return loss of ‐39.9 dB. Furthermore, its impedance bandwidth is 460 MHz or 18.4% and 3‐dB axial‐ratio (AR) bandwidth is 360 MHz or 14.4% at 2.5 GHz.     

Dispersion engineering of a silicon-nanocrystal-based slot waveguide for broadband wavelength conversion

A proposal for broadband wavelength conversion using four-wave mixing is presented based on a slot waveguide with silicon nanocrystals (Si-nc's) as the optical nonlinear material. The dispersion of the waveguide is engineered to realize a flat dispersion as well as a small effective mode area for better nonlinear interaction by optimizing the waveguide dimensions. The conversion performance is synthetically analyzed and numerical results show that a bandwidth of over 400?nm and an efficiency of -2.38?dB can be achieved using a pump power of 150?mW in a 4?mm long Si-nc slot waveguide with slot width of 50?nm, slab width of 310?nm, and height of 305?nm.     

Slab waveguide theory for general multi-slot waveguide

Abstract

Optical devices based on slot waveguide are of considerable interest in numerous applications due to the distinct feature of strong electric field confinement in a low-refractive index region. A theoretical model based on multi-slab waveguide theory is used to reveal the physical mechanism of the slot waveguide. The calculation results derived from the basic Helmholtz equation for the conventional single-slot waveguide with a ~2% validation of the effective refractive index are compared to the former experiment results by the Cornell University group. Moreover, we extend the theoretical model to a general multi-slot waveguide. Its electric field distribution and key properties such as optical power confinement factor and enhancement factor in slot are deduced theoretically and fully discussed.     

Monolithic integration of microfluidic channels, liquid-core waveguides, and silica waveguides on silicon

The fabrication of embedded microchannels monolithically integrated with optical waveguides by plasma-enhanced chemical vapor deposition of doped silica glass is reported. Both waveguide ridges and template ridges for microchannel formation are patterned in a single photolithography step. The microchannels are formed within an overlay of borophosphosilicate glass (BPSG), which also serves as the top cladding layer of the silica waveguides. No top sealing of the channels is required. Surface accessible fluid input ports are formed in a BPSG layer, with no additional steps, by appropriate design of template layers. By tightly controlling the refractive index of the waveguide layer and the microchannel-forming layer, fully integrated structures facilitating optical coupling between solid waveguides and liquids segments in various geometries are demonstrated. Applications in liquid-filled photonic device elements for novel nonlinear optical devices and in optical sensors and on-chip spectroscopy are outlined.     

Optimal design method of a low-loss broadband Y branch with a multimode waveguide section

An optimal design method for a low broadband silica-on-silicon Y branc is considered. A multimode waveguide section, which was used earlier to reduce the excess loss, is designed optimally when the light distribution at the end of the multimode waveguide section is matched to the profile of the symmetric supermode for the structure of the two branching waveguides. An optimization method that combines the genetic algorithm and a gradient-based search method is used to obtain the optimal geometrical parameters for the multimode waveguide section as well as the widths for the input and branching waveguides.