Birefringence control using stress engineering in silicon-on-insulator (SOI) waveguides

We demonstrate that stress engineering is an effective tool to modify or eliminate polarization dispersion in silicon-on-insulator (SOI) waveguide devices, for a wide range of waveguide cross-section shapes and dimensions. The stress-induced effects on the modal birefringence of SOI waveguides are investigated numerically and experimentally. Finite-element simulations show that while the birefringence of ridge waveguides with both slanted and vertical sidewalls can be effectively modified using cladding stress, the birefringence becomes much less sens

Stress-induced birefringence in silicon-on-insulator (SOI) waveguides

We show that stress engineering can be used to adjust the SOI waveguide birefringence to the stringent polarization tolerances expected of commercial devices, using only standard silicon processes. With decreasing device dimensions and high index contrast the waveguide birefringence becomes increasingly sensitive to device geometry. As a result, it is almost impossible to eliminate waveguide birefringence by adjusting waveguide profile alone. This paper presents, for the first time, a systematic study of the stress-induced birefringence in SOI waveguides. Through full-vectorial finite-element simulations, we investigate the variation of stress-induced birefringence with waveguide core and cladding geometries. It is found that the stress-induced birefringence is determined by the waveguide cross-section, the upper cladding layer thickness, and film stress levels. We develop a waveguide model that predicts the total waveguide birefringence and guides the post-fabrication processing steps. An experimental demonstration of polarization insensitivity in SOI arrayed waveguide grating (AWG) demultiplexers is presented. The polarization dependent wavelength shifts measured experimentally agree well with the simulations

A highly virulent variant of HIV-1 circulating in the Netherlands

We discovered a highly virulent variant of subtype-B HIV-1 in the Netherlands. One hundred nine individuals with this variant had a 0.54 to 0.74 log10 increase (i.e., a ~3.5-fold to 5.5-fold increase) in viral load compared with, and exhibited CD4 cell decline twice as fast as, 6604 individuals with other subtype-B strains. Without treatment, advanced HIV-CD4 cell counts below 350 cells per cubic millimeter, with long-term clinical consequences-is expected to be reached, on average, 9 months after diagnosis for individuals in their thirties with this variant. Age, sex, suspected mode of transmission, and place of birth for the aforementioned 109 individuals were typical for HIV-positive people in the Netherlands, which suggests that the increased virulence is attributable to the viral strain. Genetic sequence analysis suggests that this variant arose in the 1990s from de novo mutation, not recombination, with increased transmissibility and an unfamiliar molecular mechanism of virulence