Silica glass bend waveguide assisted by two-dimensional photonic crystals

In this paper we report on the modeling of low index contrast silica glass 90° bend ridge waveguides assisted by a two-dimensional photonic crystal. A three-dimensional finite-difference time-domain (3D-FDTD) based computer code has been used in order to evaluate the transmission characteristics and the in-plane losses of the investigated waveguides having different values of the bend radius. The performance of the bend structure surrounded by two-dimensional photonic crystals is compared to that of a classical bend ridge waveguide and the phenomenon of light confinement is critically analyzed. The device design is optimized for quasi-TM polarization at the wavelength of 1.3 μm.     

Numerical investigation of characteristics of laser radiation scattered in an integrated optical waveguide with three-dimensional inhomogeneities

Specifics of theoretical analysis of wave phenomena in irregular integrated optical waveguides are investigated. The object of the investigation and the main types of irregularities (smooth, statistical, and sharp) are described. The goals of the numerical modeling are formulated. The structure of the program and the general structure of the algorithm allowing numerical investigation of guided modes’ scattering from 3D-irregularities of an integrated optical waveguide are described. The dispersion relations of the TE and TM modes of the integrated optical waveguide under investigation, as well as field patterns of the radiating TE modes of the substrate and the laser radiation scattered from the three-dimensional guiding-layer inhomogeneities of an integrated optical waveguide, are presented. The results are analyzed in detail. The methods developed can be used for numerical investigation of the characteristics of laser radiation scattered in various optical waveguides with three-dimensional irregularities.     

Control of femtosecond laser written waveguides in silica glass

Writing conditions for the fabrication of optical waveguides in bulk fused silica glass by use of 1 kHz focused femtosecond laser pulses at 800 nm were systematically determined for different focusing geometries. The results demonstrate that waveguides can be formed based on optical breakdown, filamentation (single or multiple), or a combination of both processes, when using pulse energies lower than the threshold of structural damage. The mechanisms of laser-induced index change are also discussed. PACS 42.65.Jx; 42.70.Ce; 42.79.Gn     

Elastic wave propagation in sinusoidally corrugated waveguides

The ultrasonic wave propagation in sinusoidally corrugated waveguides is studied in this paper. Periodically corrugated waveguides are gaining popularity in the field of vibration control and for designing structures with desired acoustic band gaps. Currently only numerical method (Boundary Element Method or Finite Element Method) based packages (e.g., PZFlex) are in principle capable of modeling ultrasonic fields in complex structures with rapid change of curvatures at the interfaces and boundaries but no analyses have been reported. However, the packages are very CPU intensive; it requires a huge amount of computation memory and time for its execution. In this paper a new semi-analytical technique called Distributed Point Source Method (DPSM) is used to model the ultrasonic field in sinusoidally corrugated waveguides immersed in water where the interface curvature changes rapidly. DPSM results are compared with analytical solutions. It is found that when a narrow ultrasonic beam hits the corrugation peaks at an angle, the wave propagates in the backward direction in waveguides with high corrugation depth. However, in waveguides with small corrugation the wave propagates in the forward direction. The forward and backward propagation phenomenon is found to be independent of the signal frequency and depends on the degree of corrugation.     

Fully three-dimensional accurate modeling of scattering loss in optical waveguides

Scattering loss in high-index-contrast optical waveguides has been modeled by a rigorous 3D numerical algorithm based on volume current method. The electromagnetic field generated by the wire current distribution simulating sidewalls roughness has been calculated by 3D finite element method. The developed modeling technique does not introduce any approximation in radiated power estimation. Numerical results obtained by our model have been compared with some experimental results reported in literature for four typical sub-micrometer high-index-contrast waveguides realized by different technologies and a very good agreement (relative error less than 3%) has been demonstrated. Closed-form expressions for scattering loss in low-index-contrast waveguides have been also derived and discussed. Developed modeling technique has been compared with other three-dimensional algorithms for scattering loss estimation and its advantages in terms of accuracy, computation time and generality have been pointed out. Scattering loss dependence on the parameters of the roughness distribution has been finally discussed.     

A visible-near infrared tunable waveguide based on plasmonic gold nanoshell

A tunable plasmonic waveguide via gold nanoshells immerged in a silica base is proposed and simulated by using the finite difference time-domain （FDTD） method. For waveguides based on near-field coupling, transmission frequencies can be tuned in a wide region from 660 to 900 nm in wavelength by varying shell thicknesses. After exploring the steady distributions of electric fields in these waveguides, we find that their decay lengths are about 5.948-12.83 dB/1000 nm, which is superior to the decay length （8.947 dB/1000 nm） of a gold nanosphere plasmonic waveguide. These excellent tunability and transmittability are mainly due to the unique hollow structure. These gold nanoshell waveguides should be fabricated in laboratory.     

157 nm F2-laser writing of silica optical waveguides in silicone rubber

Silica (SiO2) optical waveguides have been fabricated on the surface of silicone [(SiO(CH3)2)n] rubber by photochemical modification of silicone rubber into silica with 157 nm F2-laser radiation. The 2 mm thick silicone was exposed through a thin (approximately 0.2 mm) air layer to generate oxygen radicals that chemically assisted in the silica transformation. Silica waveguides were defined in 8-16 microm wide exposure strips by a proximity Cr-on-CaF2 photomask. Optimum laser processing conditions are presented for generating crack-free waveguides with good optical transparency at red (635 nm) and infrared (1550 nm) wavelengths. A propagation loss of approximately 6 dB/cm is reported at the 1550 nm wavelength.     

Mode propagation in curved waveguides and scattering by inhomogeneities: application to the elastodynamics of helical structures

This paper reports on an investigation into the propagation of guided modes in curved waveguides and their scattering by inhomogeneities. In a general framework, the existence of propagation modes traveling in curved waveguides is discussed. The concept of translational invariance, intuitively used for the analysis of straight waveguides, is highlighted for curvilinear coordinate systems. Provided that the cross-section shape and medium properties do not vary along the waveguide axis, it is shown that a sufficient condition for invariance is the independence on the axial coordinate of the metric tensor. Such a condition is indeed checked by helical coordinate systems. This study then focuses on the elastodynamics of helical waveguides. Given the difficulty in achieving analytical solutions, a purely numerical approach is chosen based on the so-called semi-analytical finite element method. This method allows the computation of eigenmodes propagating in infinite waveguides. For the investigation of modal scattering by inhomogeneities, a hybrid finite element method is developed for curved waveguides. The technique consists in applying modal expansions at cross-section boundaries of the finite element model, yielding transparent boundary conditions. The final part of this paper deals with scattering results obtained in free-end helical waveguides. Two validation tests are also performed.     

Temperature characterizations of silica asymmetric Mach-Zehnder interferometer chip for quantum key distribution

Quantum key distribution (QKD) system based on passive silica planar lightwave circuit (PLC) asymmetric Mach-Zehnder interferometers (AMZI) is characterized with thermal stability, low loss and sufficient integration scalability. However, waveguide stresses, both intrinsic and temperature-induced stresses, have significant impacts on the stable operation of the system. We have designed silica AMZI chips of 400 ps delay, with bend waveguides length equalized for both long and short arms to balance the stresses thereof. The temperature characteristics of the silica PLC AMZI chip are studied. The interference visibility at the single photon level is kept higher than 95% over a wide temperature range of 12 ℃. The delay time change is 0.321 ps within a temperature change of 40 ℃. The spectral shift is 0.0011 nm/0.1 ℃. Temperature-induced delay time and peak wavelength variations do not affect the interference visibility. The experiment results demonstrate the advantage of being tolerant to chip temperature fluctuations.     

Directional coupler wavelength selective filter based on dispersive Bragg reflection waveguide

A new type of wavelength selective filter, based on high differential dispersion between two coupled waveguides, is presented. The Bragg Reflection Waveguide displays high effective refractive index dispersion, due to the interaction of the guided mode with the two confining Bragg reflectors. When coupled with a weakly guided buried channel silica waveguide, a very narrow bandwidth filter (<1 nm) can be easily produced, in a shorter length, with respect to directional couplers made with standard step index channel waveguides. The complete design methodology, fabrication and characterization are presented.     

TM-Wave Properties of HTS''s Rectangular Waveguides with Meissner Boundary Condition

Using Meissner Boundary Condition (MBC), the properties of a high-temperature superconductive (HTS) rectangular waveguide are studied. The results show that the modes properties in a HTS's waveguide is much different from those in a normal conductive waveguide. The investigation indicates that all design formulations for normal mettalic waveguides can not be applied to design HTS's waveguides and related circuits. New design formulations should be developed in future.     

Stress in femtosecond-laser-written waveguides in fused silica

We identify two states of stress induced in waveguides fabricated by femtosecond lasers in fused silica and show how they can be relieved by annealing. In-plane stress and stress concentration are revealed through birefringence and loss measurements. Another kind of laser-induced stress appears in the form of swelling of the glass surface when waveguides are written near the surface and is a manifestation of confined rapid material quenching. By annealing the sample we reduce the losses by approximately 30% (at 633 nm) and decrease the birefringence by a factor of 4 in fused silica.     

Leaky modes of channel waveguides with uniaxial anisotropy

An exact numerical method for analyzing the propagation properties of leaky modes of inhomogeneous channel optical waveguides with a complex uniaxial diagonal permittivity tensor is developed. The method is based on solving the system of integro-differential equations formulated with respect to transversal components of the magnetic field and the longitudinal component of the electric field. Some results of investigation of leaky modes of diffused channel waveguides in LiNbO₃ and LiTaO₃ crystals are given.     

Transverse Writing of Multimode Interference Waveguides inside Silica Glass by Femtosecond Laser Pulses

Multi-mode interference waveguides are fabricated inside silica glass by transverse writing geometry with femtosecond laser pulses. The influences of several writing and reading factors on the output mode are systematically studied. The experimental results of straight waveguides are in good agreement with the simulations by the beam propagation method. By integrating a straight waveguide with a bent waveguide, a 1× 2 multi-mode splitter is formed and 2×3 lobes are observed in the output mode.     

Ultra‐low loss waveguide platform and its integration with silicon photonics

Planar waveguides with ultra‐low optical propagation loss enable a plethora of passive photonic integrated circuits, such as splitters and combiners, filters, delay lines, and components for advanced modulation formats. An overview is presented of the status of the field of ultra‐low loss waveguides and circuits, including the design, the trade‐off between bend radius and loss, and fabrication rationale. The characterization methods to accurately measure such waveguides are discussed. Some typical examples of device and circuit applications are presented. An even wider range of applications becomes possible with the integration of active devices, such as lasers, amplifiers, modulators and photodetectors, on such an ultra‐low loss waveguide platform. A summary of efforts to integrate silicon nitride and silica‐based low‐loss waveguides with silicon and III/V based photonics, either hybridly or heterogeneously, will be presented. The approach to combine these integration technologies heterogeneously on a single silicon substrate is discussed and an application example of a high‐bandwidth receiver is shown.     

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.     

Ultra compact hybrid plasmonic–photonic coupler: configuration of metal–silica–silicon

In this work, a novel and practical configuration as a hybrid plasmonic–photonic coupler based on silicon (Si) nanofibers, silica waveguides and metal nanoparticles is examined and investigated. All of utilized waveguides, fibers and nanoparticles are embedded in an \(\hbox {Mg}_{2}\hbox {F}\) crystal host. Integrated plasmonic–photonic coupler provides significant transmission efficiency during guiding and propagating of light. Utilizing enhanced plasmonic waveguides helps to reduce the inherent losses such as scattering into the far-field and absorption of optical power inside the employed components, especially in nanoparticles. The transmission loss component under transverse electric excitation (TE) for the superstructure has been calculated as approximately \(\gamma _{T}=3\,\hbox {dB}/675\)  nm. Also, we investigate the coupling efficiency at overlapping regions between Si nanofibers and silica ( \(\hbox {SiO}_{2})\) waveguides which is referred to near-field interactions. Transmitted power ratio and the group velocity of the propagated light are computed and depicted for the proposed coupler.     

Anti-resonant reflecting photonic crystal waveguide (ARRPCW): modeling and design

In this paper, we apply an antiresonant reflecting layer concept in photonic crystal based waveguides. We have proposed a thin core ARRPCW with linear waveguide carved in it and hence calculated the transmission spectra for various length of waveguide and have shown that the longer waveguides in ARRPCW yields transmission with significantly high quality factor. Comparison of the transmission characteristics of normal conventional planar ARROW-B waveguides & ARRPCW has also been reported. The 2D FDTD numerical modeling reveals improved transmission for various lengths of planar ARROW and ARRPCW with low losses in long waveguides. Transmittance and quality factor are also calculated to confirm superior performance of the proposed design of ARROW based photonic crystal waveguide.     

Cu2+离子注入的石英光波导的特性研究

利用Cu2+离子注入的方式在熔融石英和石英晶体上分别制备了平面光波导结构.通过棱镜耦合实验测试了两种光波导的导模特性,结果表明:在同样的注入条件下熔融石英上形成了增加型的光波导结构,而石英晶体上形成了位垒型的光波导结构.研究了退火温度对两种光波导导模折射率的影响,熔融石英光波导中导模的折射率随着退火温度的升高而降低,而石英晶体光波导中导模的折射率随着退火温度的升高先增加后降低.为了进一步分析离子注入两种材料形成光波导的微观机理,利用SRIM模拟了Cu2+离子注入两种材料的电子能量损失和核能量损失,并且模拟了两种光波导结构的折射率分布.模拟结果表明:熔融石英光波导的主要形成原因是离子注入表面的折射率大于其体材料的折射率,而石英晶体光波导的主要形成原因是离子射程末端的折射率小于其体材料的折射率.因此,在熔融石英光波导的形成中电子能量损失起主要作用,而在石英晶体光波导的形成中核能量损失起主要作用.     

Fabrication of multimode interference waveguides in glass by use of a femtosecond laser

We report on the fabrication of multimode interference (MMI) waveguides that split single-mode light into multimode light in synthesized silica by use of femtosecond laser pulses. The number of modes at the output facet is dependent on both the width and the length of the MMI waveguides. The output spectral responses of the MMI waveguides in the visible region were examined. The fabricated MMI waveguides can be used as compact power splitters with large fan-outs.