Waveguide Profile Research Articles

Improved Microfluidic Coupled-Cavity Waveguides for Slow Light Transmission

We present a method based on the selective liquid infiltration in air holes to produce slow light in a coupled-cavity waveguide structured by two-dimensional photonic crystal and analyze the slow light propagation in the coupled-cavity waveguide with triangular lattice. The group velocity profile of different coupled-cavity waveguides, obtained by the selective liquid infiltration in the holes between the cavities in waveguide and the increased radius of the first row of holes adjacent to the waveguide, is evaluated by using both the plane-wave expansion method and a tight binding model. We determine the optimal parameters to reduce the group velocity. Using a simpler coupled-cavity waveguide structure we obtain smaller group velocity compared to most investigations.

Finding exact spatial soliton profiles in nematic liquid crystals

Finding exact analytical soliton profile solutions is only possible for certain types of non-linear media. In most cases one must resort to numerical techniques to find the soliton profile. In this work we present numerical calculations of spatial soliton profiles in nematic liquid crystals. The nonlinearity is governed by the optical-field-induced liquid crystal director reorientation, which is described by a system of coupled nonlinear partial differential equations. The soliton profile is found using an iterative scheme whereby the induced waveguide and mode profiles are calculated alternatively until convergence is achieved. In this way it is also possible to find higher order solitons. The results in this work can be used to accurately design all-optical interconnections with soliton beams.

Beam shape and intensity profile of a fiber-bundle prism-coupled waveguide

Design and performance of a two-stage optical beam shaping system based on a plastic fiber-bundle prism-coupled waveguide is described in this study. Such systems offer practical means to modify and change the output beam shape at two stages and also provide quantitative information concerning the output beam intensity profile. It is possible to obtain reflection and image transmittance data by using a light source for the illumination and image analysis by a CCD/digital camera. Using the active lighting, the reflection data and image profiles for the output beam are obtained by a CCD camera and presented in this study. The photograph picture of the illuminating LED beam just at its output point shows a circular shape with a radius of about 5 mm. Picture of the fiber-bundle output beam is also taken, which shows a linear arrangement of circular spot lights almost similar in cross section. The output beam cross section at this stage is in rectangular shape with a dimension of 30 mm width and 3 mm height. In another study CCD images at prism exit face is obtained and image profile is investigated. The final output beam cross section at the second stage of beam shaping is a square with a dimension of about 5 by 5 mm. The transmitted output beam power is also measured at different stages. The conversion efficiency for the fiber bundle at first stage beam shaping is about 70% and for the overall system is about 15%. In another study the relative reflected light off the prism entering face is measured by using two fiber probes in the stripe cable, which shows the ability to monitor such reflections for selecting the prism duct with a lower reflection loss.

Liquid crystal applications in photonics

We developed new optical switches based on nematic and ferroelectric liquid crystal (LC) cells for photonics applications. Certain new LC switches based on the effect of total internal reflection in nematic LC and deformed helix ferroelectric effect ferroelectric LC with very fast response time were developed. Fast bistable optical switches of the light polarization based on ferroelectric liquid crystal cells were proposed. The switches are characterized by 100 is switching time and 26 dB crosstalk at the wavelength of 632.8 nm and bistable, i.e., required zero power consumption in the switch state. High frequency hysteretic free electrically controlled 0–2π phase modulation of light has been proposed using a very short helix pitch (less than 400 nm) deformed helix ferroelectric liquid crystal. The electrically controlled 0–2π hysteretic free phase modulation was achieved at the driving voltage frequency up to 4 kHz and the voltage amplitude of 32 V. The application of fast V-shaped deformed helix ferroelectric ferroelectric LC (DHF-FLC) for new active-matrix liquid crystal display (LCD) and optical data processing devices is envisaged. Photoalignment technology can be very useful for the new generation of liquid crystal devices as well as in new photovoltaic, optoelectronic and photonic devices based on highly ordered thin organic layers. We have investigated the LC photoalignment in superthin tubes, which are basic elements of switchable photonic crystal/liquid crystal structures and obtained the order parameter comparable with usual homogeneous nematic LC cells. We studied LC alignment on silicon surfaces with submicrometer-sized straight and curved waveguide profiles. The liquid crystal cladding refractive index was then varied according to the applied voltage, and subsequently the microresonator resonance wavelengths were tuned. Based on our initial measurements, the free spectral range (FSR) wavelength shift within the range of 20 nm was obtained, which is comparable with a thermooptic effect. The new voltage controllable Si-based add drop filters are envisaged based on this principle.

Measurement of two dimensional refractive index profiles of channel waveguides using an interferometric technique

Two dimensional refractive index profiles of ion exchanged channel waveguides in glass have been measured using an interferometric method. In order to obtain depth data, a shallow bevel is produced in the glass by polishing. A regularization algorithm for the extraction of the phase data from the interferometer image is presented. The method is applied to waveguides formed by the electric field assisted diffusion of Cu+ ions into a borosilicate glass. The index change obtained from the interferometer is in good agreement with that obtained from measurements on planar waveguides.

Design of Planar Waveguides With Prescribed Mode-Profile Using Inverse Scattering Theory

We demonstrate a new method based on inverse scattering theory for designing the refractive index profile of single-mode planar waveguides in order to obtain a desired TE-mode profile. The method enables a direct design of the waveguide profile without the need for iterative optimization algorithms. The design is based on a first order solution to the Gel'fand-Levitan-MarCcaronenko integral equation that gives a simple linear connection between a small change in the scattering data and the corresponding change in the kernel function. This connection reduces the design problem to a simple linear constrained minimization problem which has an explicit solution. Our design method allows adding additional constraints on the refractive index profile such as the waveguide width. The method presented in this paper can be expanded to analyze TM modes and for designing multi-mode planar waveguides.

Determination of Refractive Indices From the Mode Profiles of UV-Written Channel Waveguides in ${\hbox {LiNbO}}_{3}$-Crystals for Optimization of Writing Conditions

We report on a method for the simultaneous determination of refractive index profiles and mode indices from the measured near-field intensity profiles of optical waveguides. This method has been applied to UV-written single-mode optical waveguides in LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> for the optimization of the writing conditions. The results for the waveguides written with light of the wavelengths 275, 300.3, 302, and 305 nm for different writing powers and scan speeds reveal that for optimum writing conditions a maximum possible refractive index change of ~0.0026 can be achieved at a value of 632.8 nm transmitting wavelength. The computation process used in the presented technique may also become useful to extract absolute refractive index values of any slowly varying graded index waveguide.

Inverse Algorithm With Built-In Spatial Filter to Obtain the 2-D Refractive Index Profile of Optical Waveguides From the Propagating Mode Near-Field Profile

We propose a modified inverse regularization algorithm for computing the 2-D refractive index profiles of integrated-optic waveguides from measured near-field data. The algorithm involves embedding of a spatial mean filter to remove impulses and high frequency noises from the near-field intensity as well as from its second derivative. The proposed filtering scheme is built-in to the inverse algorithm and does not require extra computation for filtering. The effectiveness of the proposed method is demonstrated by reconstructing the refractive index profile of a single mode lithium niobate channel waveguide from the propagating mode near-field intensity pattern.

Silicon Waveguide Sidewall Smoothing by KrF Excimer Laser Reformation

A novel laser-reformation technique is presented for sidewall smoothing of silicon waveguides. A KrF excimer laser is used to melt and reform the sidewalls to reduce the surface roughness. Atomic-force-microscopy measurement shows that the root-mean-square (rms) roughness is reduced from 14 to 0.24 nm. The calculated scattering loss is reduced to 0.033 dB/cm. The waveguide profile after laser illumination at an incident angle of 75deg transforms to a shape of arch. The crystal quality of laser-illuminated silicon wafer characterized by microwave reflection photoconductance-decay carrier lifetimes shows 94% less damage than the furnace-treated wafer.

Sol–gel elaboration and structural investigations of Lu2O3:Eu3+ planar waveguides

Europium actived Lu2O3 sol–gel transparent and crack free films were deposited by dipping on silica substrates. The film microstructure was studied by waveguide Raman spectroscopy (WRS) with annealing temperatures from 400 up to 1,000 °C and X-ray diffraction. The WRS results and TEM observations were correlated and showed that crystallization of the lutetium oxide phase into cubic phase occurs at 600 °C and is stable up to 1,000 °C, the crystallite size increasing between ±38 nm with annealing temperature ranging from 600 to 1,000 °C. Opto-geometrical parameters were determined by m-lines spectroscopy using four different wavelengths of laser sources in order to confirm the step-index profile of the as-prepared waveguides. The Eu3+ doped films heat-treated at 1,000 °C presented a constant thickness for the wavelengths 493, 543, 594 and 632.8 nm and a density of 8.4 g cm−3. High-resolution X-ray images were obtained.

Fracture behaviour of microstructured silica optical fibres

Microstructured optical fibres (with a periodic transverse microstructure) are of interest since they offer a simple alternative to controlling the index profile of optical waveguides. Although many types of optical fibres and cables have been developed to meet the needs of communications service providers for long-term performance and reliable operation, the brittle nature, aging and fatigue of these fibres remain as the key materials issues. The flaws on the surface of fibres caused by processing (drawing) or subsequent assembling make the situation more complex. In this work, experimental investigation and fracture mechanics analyses have been conducted to understand the fracture behaviour of these newly-developed optical fibres. The results are believed to be useful for design, fabrication and evaluation of optical fibres for a variety of applications.

Approach for determining the refractive-index profile of graded-index planar waveguides

We measured the modal indices of a planar waveguide made from GeSeSb glass sandwiched between SiO(2) and air by using the prism-coupling technique. Based on the measured indices of the TE modes, we determined the position of the turning point corresponding to each mode of the waveguide by using the inverse WKB method. Using the modified fitting criterion introduced previously [Appl. Opt. 33, 3227 (1994)], we accurately determined the spatial profile of the refractive index for such a waveguide. Such a graded-index profile is probably caused by compositional variation of the GeSeSb guiding layer.

Stress Induced Effects for Advanced Polarization Control in Silicon Photonics Components

We review the use of the oxide cladding stress-induced photoelastic effect to modify the polarization dependent properties in silicon-on-insulator (SOI) waveguide components, and highlight characteristics particular to this high index contrast (HIC) systems. The birefringence in SOI waveguides has its origin in the electromagnetic boundary conditions at the waveguide boundaries, and can be further modified by the presence of stress in the waveguiding materials. With typical stress levels in SiO2 films, which are often used as the upper cladding, the waveguide effective index can be altered anisotropically up to the order of 10−3 for ridges with heights ranging from 1 μm to 5 μm. This effect can be used effectively to counter the waveguide geometrical birefringence, allowing the waveguide cross-section profiles to be optimized for design criteria other than null geometrical birefringence. Design strategies are developed for using stress engineering to achieve a variety of functions. Polarization insensitive arrayed waveguide gratings (AWGs), polarization insensitive ring resonators, and polarization splitters and filters are demonstrated using these design principles.

Refractive-Index Profiling of Buried Planar Waveguides by an Inverse Wentzel–Kramer–Brillouin Method

We propose an inverse Wentzel-Kramer-Brillouin (WKB) method for the construction of the refractive-index profiles of buried graded-index planar waveguides from the measured effective indexes of the guided modes. Our method involves experimental determination of the location of the peak index in the buried profile, measurement of the effective indexes in air, and an index-matching liquid, and application of a searching algorithm to find the best-fit profile through inverting the WKB equations. We present numerical examples and experimental results to demonstrate the performance of the method.

Investigation of refractive index modifications in CW CO 2 laser written planar optical waveguides

This paper presents the measurement of the refractive index profile of buried channel waveguides fabricated by a CW CO 2 direct writing technique. A reflectance method is used to assess the refractive index distribution n( x,y) in these structures as it is a key parameter determining the propagation properties of guided wave devices. Beam propagation method (BMP) is used with experimentally determined cross-sectional data and refractive index profile to model the waveguides. The spatial resolution is 1.3 μm and, 5.10 −4 for the refractive index.

Reconstruction of refractive index profiles of 3 MeV O2+ ion-implanted MgO-doped LiNbO3 using wet etching and ellipsometry

A planar waveguide is fabricated by 3 MeV O2+ ion-implanted in MgO-doped lithium niobate, and the refractive index profiles of waveguides are reconstructed based on etching and ellipsometry techniques. The SRIM2003 code is used to simulate the damage distribution induced by implantation. The etching rate versus the etching depth is extracted and the relation between the etching rate and the damage profile is discussed. The index profile of this kind of waveguide is determined by etching in combination with the following ellipsometric measurements. Both ordinary and extraordinary index profiles in waveguide are obtained. The influence of damage profile on index profiles in waveguide, as well as that on waveguide properties is analysed.

Construction of refractive-index profile of Ag+–Na+ ion-exchange channel waveguide by two-dimensional inverse matrix method

The refractive-index profile of ion-exchange channel waveguide is reconstructed by near-field measurement combined with two-dimensional inverse matrix method, which can avoid the problems brought by multimeasurements and iteration smoothing in the process. The method used here has the advantages of convenient operation and accurate results for the characteristics of the asymmetric channel waveguide. It can be also applied to the refractive-index profile measurements of graded index fibers, polarization maintaining fibers, and single-mode planar waveguides.

Three-dimensional control of optical waveguide fabrication in silicon

In this paper, we report a direct-write technique for three-dimensional control of waveguide fabrication in silicon. Here, a focused beam of 250 keV protons is used to selectively slow down the rate of porous silicon formation during subsequent anodization, producing a silicon core surrounded by porous silicon cladding. The etch rate is found to depend on the irradiated dose, increasing the size of the core from 2.5 microm to 3.5 microm in width, and from 1.5 microm to 2.6 microm in height by increasing the dose by an order of magnitude. This ability to accurately control the waveguide profile with the ion dose at high spatial resolution provides a means of producing three-dimensional silicon waveguide tapers. Propagation losses of 6.7 dB/cm for TE and 6.8 dB/cm for TM polarization were measured in linear waveguides at the wavelength of 1550 nm.

Reconstruction of extraordinary refractive index profiles of optical planar waveguides with single or double modes fabricated by O2+ ion implantation into lithium niobate

A method named intensity calculation method (ICM), which is based on beam propagation method (BPM) and image processing, was carried out to reconstruct the extraordinary refractive index profile (RIP) of single-mode planar waveguide in lithium niobate (LiNbO3), which was fabricated by multi-energy megaelectron-volt (MeV) O2+ ion implantation. In addition, it has been proved reasonable that the alternation of extraordinary refractive index induced by ion implantation into LiNbO3 is mainly due to the degradation of polarization and reduction of material physical density. As a result, the possible extraordinary RIP of the double-mode planar waveguide could be reconstructed using BPM according to such a hypothesis and the calculated guiding mode values. The end-fire coupling and m-line arrangements were carried out to obtain the near-field modal patterns and dark-mode spectra of waveguides, respectively.

Mode formulas for shallow water waveguides using a modified asymptotic approximation

Sound speed profiles in shallow water waveguides are small deviations from isospeed and often decrease monotonically with depth. Approximation formulas for the propagating modes are found using a modified form of the classical WKBJ method. The ocean bottom is taken as homogeneous. The approach is accurate for modes with phase speeds greater than, and even slightly less than, the maximum water sound speed. The validity and accuracy of the approximations over frequency and mode number are illustrated using benchmark numerical calculations. Comparisons with approximations from other methods, including previous WKBJ approaches, are described. The formulas here are typically more compact and convenient. The results demonstrate how changes in parameters, such as frequency, bottom sound speed, and water sound speed profile quantitatively affect the mode functions. Applications for representative waveguide profiles are provided. [Work supported by the Office of Naval Research.]