Evaluation of Attenuation Methods for an Integrated, Weak Coherent Source for Quantum Key Distribution

Quantum key distribution (QKD) systems that use weak coherent states often rely on attenuated lasers to generate signals with an average of less than one photon per pulse. Two ways of attenuating laser light in a weak, coherent, integrated QKD transmitter chip are compared in terms of noise, namely attenuation with Mach-Zehnder (MZ) interferometers and attenuation with semiconductor optical amplifiers (SOAs) biased as attenuators. Results from simulations and experiments on the optical spectrum of the output of the transmitter chip show that under reverse bias conditions the SOAs result in similar noise levels as the MZs. The footprint of the SOAs on the chip, however, is more than 50 times smaller than that of the MZs. This makes them the better candidate for the integrated, weak coherent QKD source

Evaluation of Attenuation Methods for an Integrated, Weak Coherent Source for Quantum Key Distribution

Quantum key distribution (QKD) systems that use weak coherent states often rely on attenuated lasers to generate signals with an average of less than one photon per pulse. Two ways of attenuating laser light in a weak, coherent, integrated QKD transmitter chip are compared in terms of noise, namely attenuation with Mach-Zehnder (MZ) interferometers and attenuation with semiconductor optical amplifiers (SOAs) biased as attenuators. Results from simulations and experiments on the optical spectrum of the output of the transmitter chip show that under reverse bias conditions the SOAs result in similar noise levels as the MZs. The footprint of the SOAs on the chip, however, is more than 50 times smaller than that of the MZs. This makes them the better candidate for the integrated, weak coherent QKD source

A III-V-on-Si ultra-dense comb laser

Optical frequency combs emerge as a promising technology that enables highly sensitive, near-real-time spectroscopy with a high resolution. The currently available comb generators are mostly based on bulky and high-cost femtosecond lasers for dense comb generation (line spacing in the range of 100 MHz to 1 GHz). However, their integrated and low-cost counterparts, which are integrated semiconductor mode-locked lasers, are limited by their large comb spacing, small number of lines and broad optical linewidth. In this study, we report a demonstration of a III-V-on-Si comb laser that can function as a compact, low-cost frequency comb generator after frequency stabilization. The use of low-loss passive silicon waveguides enables the integration of a long laser cavity, which enables the laser to be locked in the passive mode at a record-low 1 GHz repetition rate. The 12-nm 10-dB output optical spectrum and the notably small optical mode spacing results in a dense optical comb that consists of over 1400 equally spaced optical lines. The sub-kHz 10-dB radio frequency linewidth and the narrow longitudinal mode linewidth (<400 kHz) indicate notably stable mode-locking. Such integrated dense comb lasers are very promising, for example, for high-resolution and real-time spectroscopy applications

Design of an integrated hybrid plasmonic-photonic device for all-optical switching and reading of spintronic memory

We introduce a novel integrated hybrid plasmonic-photonic device for all-optical switching and reading of nanoscale ferrimagnet bits. The racetrack memory made of synthetic ferrimagnetic material with a perpendicular magnetic anisotropy is coupled on to a photonic waveguide onto the indium phosphide membrane on silicon platform. The device which is composed of a double V-shaped gold plasmonic nanoantenna coupled with a photonic crystal cavity can enable switching and reading of the magnetization state in nanoscale magnetic bits by enhancing the absorbed energy density and polar magneto-optical Kerr effect (PMOKE) locally beyond the diffraction limit. Using a three-dimensional finite-difference time-domain method, we numerically show that our device can switch and read the magnetization state in targeted bits down to ~100 nm in the presence of oppositely magnetized background regions in the racetrack with widths of 30 to 120 nm, clearly outperforming a bare photonic waveguide. Our hybrid device tackles the challenges of nonlinear absorption in the waveguide, weak PMOKE, and size mismatch between spintronics and integrated photonics. Thus, it provides missing link between the integrated photonics and nanoscale spintronics, expediting the development of ultrafast and energy efficient advanced on-chip applications

HYPERFINE-STRUCTURE IN THE 6SNH (1 = 5) RYDBERG SERIES OF BARIUM

The hyperfine structure in the odd-parity 6snh (9 ≤ n ≤ 40) Rydberg series of barium has been investigated, using a single cw ring dye laser and a beam of neutral atoms in the metastable 5

Analysis of hybrid mode-locking of two-section quantum dot lasers operating at 1.5 micron

For the first time a detailed study of hybrid mode-locking in two- section InAs/InP quantum dot Fabry-Pérot-type lasers is presented. The output pulses have a typical upchirp of approximately 8 ps/nm, leading to very elongated pulses. The mechanism leading to this typical pulse shape and the phase noise is investigated by detailed radio-frequency and optical spectral studies as well as time-domain studies. The pulse shaping mechanism in these lasers is found to be fundamentally different than the mechanism observed in conventional mode-locked laser diodes, based on quantum well gain or bulk material. ©2009 Optical Society of America

Long-term thermal sensitivity of Earth’s tropical forests

The sensitivity of tropical forest carbon to climate is a key uncertainty in predicting global climate change. Although short-term drying and warming are known to affect forests, it is unknown if such effects translate into long-term responses. Here, we analyze 590 permanent plots measured across the tropics to derive the equilibrium climate controls on forest carbon. Maximum temperature is the most important predictor of aboveground biomass (−9.1 megagrams of carbon per hectare per degree Celsius), primarily by reducing woody productivity, and has a greater impact per °C in the hottest forests (>32.2°C). Our results nevertheless reveal greater thermal resilience than observations of short-term variation imply. To realize the long-term climate adaptation potential of tropical forests requires both protecting them and stabilizing Earth’s climate

The global abundance of tree palms

Aim Palms are an iconic, diverse and often abundant component of tropical ecosystems that provide many ecosystem services. Being monocots, tree palms are evolutionarily, morphologically and physiologically distinct from other trees, and these differences have important consequences for ecosystem services (e.g., carbon sequestration and storage) and in terms of responses to climate change. We quantified global patterns of tree palm relative abundance to help improve understanding of tropical forests and reduce uncertainty about these ecosystems under climate change. Location Tropical and subtropical moist forests. Time period Current. Major taxa studied Palms (Arecaceae). Methods We assembled a pantropical dataset of 2,548 forest plots (covering 1,191 ha) and quantified tree palm (i.e., ≥10 cm diameter at breast height) abundance relative to co‐occurring non‐palm trees. We compared the relative abundance of tree palms across biogeographical realms and tested for associations with palaeoclimate stability, current climate, edaphic conditions and metrics of forest structure. Results On average, the relative abundance of tree palms was more than five times larger between Neotropical locations and other biogeographical realms. Tree palms were absent in most locations outside the Neotropics but present in >80% of Neotropical locations. The relative abundance of tree palms was more strongly associated with local conditions (e.g., higher mean annual precipitation, lower soil fertility, shallower water table and lower plot mean wood density) than metrics of long‐term climate stability. Life‐form diversity also influenced the patterns; palm assemblages outside the Neotropics comprise many non‐tree (e.g., climbing) palms. Finally, we show that tree palms can influence estimates of above‐ground biomass, but the magnitude and direction of the effect require additional work. Conclusions Tree palms are not only quintessentially tropical, but they are also overwhelmingly Neotropical. Future work to understand the contributions of tree palms to biomass estimates and carbon cycling will be particularly crucial in Neotropical forests

Mode locked InAs/InP (100) quantum dot lasers at 1.5 micrometer

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Stabilization of an InP-based laser using the Pound-Drever-Hall technique deploying electro-optic tuning for the electrical feedback

In this work, we present results from the stabilization of an integrated single frequency InP diode laser with an extended cavity using the Pound-Drever-Hall locking technique. The laser is a multi-section DBR laser with an intra-cavity ring resonator. It is locked to a 1 MHz wide resonance of a highly stable Fabry-Perot etalon via electrical feedback which is applied on the rear DBR section. The DBR section is used in reverse bias to tune the lasing mode thus avoiding any thermal effects. We show measurements that confirm the drift stabilization of the laser to an etalon resonance for over 10 minutes and linewidth measurements using the delayed self-heterodyne method. The latter show a reduction of the laser linewidth below 50 kHz which is at the moment limited by the time delay of the current feedback loop