A waveguide polarization toolset design based on mode beating

A toolset of waveguide elements is examined, which can be combined to produce polarization functional devices in a single contiguous waveguide. In particular, waveguide implementations of an optical isolator and a polarization modulator are discussed. The waveguide elements, i.e., quasi-phase-matched nonreciprocal polarization mode converter, reciprocal polarization mode converter (R-PMC), and a differential phase shifter, are all based on mode beating. A universal 3-dB R-PMC specification is identified, which suffices for all the polarization functional devices considered here. A full-vectorial modesolver is used to determine the modes in a number of example III–V waveguide structures, and the polarization state evolution is considered by using an averaged Stokes vector illustrated on the Poincaré sphere constr

Quasi-phase matching magneto-optical waveguides

<p>Photonic integration has proved remarkably successful in combining multiple optical devices onto a single chip with the benefits of added functionality, and reduction in costs, arising from the replacement of manual assembly and alignment of individual components with lithographic techniques. However, the incorporation of optical isolators and related non-reciprocal devices within standard optoelectronic wafer platforms is exceptionally challenging. Preferred magneto-optic materials cannot be exploited as waveguide core layers on semiconductor wafers due to a lower refractive index. Another difficulty is the phase velocity mismatch as a consequence of the inherent structural birefringence associated with waveguide geometries.</p> <p>Our approach to the integration of an optical isolator with a III-V semiconductor laser involves combining a nonreciprocal mode converter with a reciprocal mode converter, based on an asymmetric profiled rib waveguide, fabricated by Reactive Ion Etching. We demonstrate that suitably tapered waveguides can be employed to connect the mode converter to other sections thereby avoiding problems caused by mode-matching and reflections from the section interfaces.</p> <p>The nonreciprocal mode converter is formed from a continuation of the III-V semiconductor waveguide core with a magneto-optic upper cladding so that Faraday rotation occurs through the interaction of the evanescent tail. The phase velocity mismatch due to the waveguide birefringence is overcome using a quasi-phase-matching approach. Lithography is used to pattern the top cladding so that the film immediately on top of the waveguide core alternates between magnetooptic and a non-magneto-optic dielectric of a similar refractive index. Our first demonstrations used a dielectric (silica or silicon nitride) patterned by etching, or lift-off, on top of a GaAs rib waveguide, over which was deposited a magneto-optic film. This film was deposited by sputtering from a Ce:YIG target and demonstrated magnetic hysteresis, but, as it was not annealed, it was believed to consist of Ce:YIG and/or gamma iron oxide microcrystallites embedded in an amorphous matrix. With quasi-phase-matching periods of 110–160 μm and a waveguide length of 8 mm, we were able to demonstrate up to 12% non-reciprocal TE- to TM-mode conversion around a wavelength of 1.3 μm using the remanent magnetisation.</p> <p>In order to enhance the magneto-optic effect it is desirable to anneal such films. However the mismatch in thermal expansion coefficients results in a catastrophic failure of samples with large area film coverage. This problem has been shown to be alleviated by patterning the YIG film. Unfortunately wet-etching of YIG also etches (Al)GaAs and, therefore, the development of a lift-off process for YIG deposition has been undertaken. Initial results are promising with ∼100 μm×2.5 μm YIG sections deposited on a GaAs layer which remain intact after an anneal in an oxygen atmosphere.</p&gt

Quantum cascade lasers with an integrated polarization mode converter

We discuss the design, fabrication and characterization of waveguide polarization mode converters for quantum cascade lasers operating at 4.6 μm. We have fabricated a quantum cascade laser with integrated polarization mode converter that emits light of 69% Transverse Electrical (TE) polarization from one facet and 100% Transverse Magnetic (TM) polarization from the other facet

Difference frequency generation by quasi-phase matching in periodically intermixed semiconductor superlattice waveguides

Wavelength conversion by difference frequency generation is demonstrated in domain-disordered quasi-phase-matched waveguides. The waveguide structure consisted of a GaAs/AlGaAs superlattice core that was periodically intermixed by ion implantation. For quasi-phase-matching periods of 3.0–3.8 μm, degeneracy pump wavelengths were found by second-harmonic generation experiments for fundamental wavelengths between 1520 and 1620 nm in both type-I and type-II configurations. In the difference frequency generation experiments, output powers up to 8.7 nW were generated for the type-I phase matching interaction and 1.9 nW for the type-II interaction. The conversion bandwidth was measured to be over 100 nm covering the C, L, and U optical communications bands, which agrees with predictions

Quasi-phase-matched Faraday rotation in semiconductor waveguides with a magnetooptic cladding for monolithically integrated optical isolators

Strategies are developed for obtaining nonreciprocal polarization mode conversion, also known as Faraday rotation, in waveguides in a format consistent with silicon-on-insulator or III–V semiconductor photonic integrated circuits. Fabrication techniques are developed using liftoff lithography and sputtering to obtain garnet segments as upper claddings, which have an evanescent wave interaction with the guided light. A mode solver approach is used to determine the modal Stokes parameters for such structures, and design considerations indicate that quasi-phase-matched Faraday rotation for optical isolator applications could be obtained with devices on the millimeter length scale

Civil society and global governance : the possibilities for global citizenship

In this article we reassert the role of governance as well as of civil society in the analysis of citizenship. We argue that to analyse global civil society and global citizenship it is necessary to focus on global governance. Just as states may facilitate or obstruct the emergence and development of national civil society, so too global governance institutions may facilitate or obstruct an emerging global civil society. Our key contention is that civil society at the global level thrives through its interaction with strong facilitating institutions of global governance. We start with a discussion of civil society and citizenship within the nation-state, and from there develop a model of global civil society and citizenship. Through analysing the impacts of various modes of global governance, we identify strategically appropriate forms of political and social engagement that best advance the prospects for global citizenship. <br /

Global, regional, and national comparative risk assessment of 79 behavioural, environmental and occupational, and metabolic risks or clusters of risks, 1990�2015: a systematic analysis for the Global Burden of Disease Study 2015

Background The Global Burden of Diseases, Injuries, and Risk Factors Study 2015 provides an up-to-date synthesis of the evidence for risk factor exposure and the attributable burden of disease. By providing national and subnational assessments spanning the past 25 years, this study can inform debates on the importance of addressing risks in context. Methods We used the comparative risk assessment framework developed for previous iterations of the Global Burden of Disease Study to estimate attributable deaths, disability-adjusted life-years (DALYs), and trends in exposure by age group, sex, year, and geography for 79 behavioural, environmental and occupational, and metabolic risks or clusters of risks from 1990 to 2015. This study included 388 risk-outcome pairs that met World Cancer Research Fund-defined criteria for convincing or probable evidence. We extracted relative risk and exposure estimates from randomised controlled trials, cohorts, pooled cohorts, household surveys, census data, satellite data, and other sources. We used statistical models to pool data, adjust for bias, and incorporate covariates. We developed a metric that allows comparisons of exposure across risk factors�the summary exposure value. Using the counterfactual scenario of theoretical minimum risk level, we estimated the portion of deaths and DALYs that could be attributed to a given risk. We decomposed trends in attributable burden into contributions from population growth, population age structure, risk exposure, and risk-deleted cause-specific DALY rates. We characterised risk exposure in relation to a Socio-demographic Index (SDI). Findings Between 1990 and 2015, global exposure to unsafe sanitation, household air pollution, childhood underweight, childhood stunting, and smoking each decreased by more than 25. Global exposure for several occupational risks, high body-mass index (BMI), and drug use increased by more than 25 over the same period. All risks jointly evaluated in 2015 accounted for 57·8 (95 CI 56·6�58·8) of global deaths and 41·2 (39·8�42·8) of DALYs. In 2015, the ten largest contributors to global DALYs among Level 3 risks were high systolic blood pressure (211·8 million 192·7 million to 231·1 million global DALYs), smoking (148·6 million 134·2 million to 163·1 million), high fasting plasma glucose (143·1 million 125·1 million to 163·5 million), high BMI (120·1 million 83·8 million to 158·4 million), childhood undernutrition (113·3 million 103·9 million to 123·4 million), ambient particulate matter (103·1 million 90·8 million to 115·1 million), high total cholesterol (88·7 million 74·6 million to 105·7 million), household air pollution (85·6 million 66·7 million to 106·1 million), alcohol use (85·0 million 77·2 million to 93·0 million), and diets high in sodium (83·0 million 49·3 million to 127·5 million). From 1990 to 2015, attributable DALYs declined for micronutrient deficiencies, childhood undernutrition, unsafe sanitation and water, and household air pollution; reductions in risk-deleted DALY rates rather than reductions in exposure drove these declines. Rising exposure contributed to notable increases in attributable DALYs from high BMI, high fasting plasma glucose, occupational carcinogens, and drug use. Environmental risks and childhood undernutrition declined steadily with SDI; low physical activity, high BMI, and high fasting plasma glucose increased with SDI. In 119 countries, metabolic risks, such as high BMI and fasting plasma glucose, contributed the most attributable DALYs in 2015. Regionally, smoking still ranked among the leading five risk factors for attributable DALYs in 109 countries; childhood underweight and unsafe sex remained primary drivers of early death and disability in much of sub-Saharan Africa. Interpretation Declines in some key environmental risks have contributed to declines in critical infectious diseases. Some risks appear to be invariant to SDI. Increasing risks, including high BMI, high fasting plasma glucose, drug use, and some occupational exposures, contribute to rising burden from some conditions, but also provide opportunities for intervention. Some highly preventable risks, such as smoking, remain major causes of attributable DALYs, even as exposure is declining. Public policy makers need to pay attention to the risks that are increasingly major contributors to global burden. Funding Bill & Melinda Gates Foundation. © 2016 The Author(s). Published by Elsevier Ltd. This is an Open Access article under the CC BY licens