Multiplication theory for dynamically biased avalanche photodiodes: new limits for gain bandwidth product

Novel theory is developed for the avalanche multiplication process in avalanche photodiodes (APDs) under time-varying reverse-biasing conditions. Integral equations are derived characterizing the statistics of the multiplication factor and the impulse-response function of APDs, as well as their breakdown probability, all under the assumption that the electric field driving the avalanche process is time varying and spatially nonuniform. Numerical calculations generated by the model predict that by using a bit-synchronous sinusoidal biasing scheme to operate the APD in an optical receiver, the pulse-integrated gain-bandwidth product can be improved by a factor of 5 compared to the same APD operating under the conventional static biasing. The bit-synchronized periodic modulation of the electric field in the multiplication region serves to (1) produce large avalanche multiplication factors with suppressed avalanche durations for photons arriving in the early phase of each optical pulse; and (2) generate low avalanche gains and very short avalanche durations for photons arriving in the latter part of each optical pulse. These two factors can work together to reduce intersymbol interference in optical receivers without sacrificing sensitivity

New Perspective on Passively Quenched Single Photon Avalanche Diodes: Effect of Feedback on Impact Ionization

Single-photon avalanche diodes (SPADs) are primary devices in photon counting systems used in quantum cryptography, time resolved spectroscopy and photon counting optical communication. SPADs convert each photo-generated electron hole pair to a measurable current via an avalanche of impact ionizations. In this paper, a stochastically self-regulating avalanche model for passively quenched SPADs is presented. The model predicts, in qualitative agreement with experiments, three important phenomena that traditional models are unable to predict. These are: (1) an oscillatory behavior of the persistent avalanche current; (2) an exponential (memoryless) decay of the probability density function of the stochastic quenching time of the persistent avalanche current; and (3) a fast collapse of the avalanche current, under strong feedback conditions, preventing the development of a persistent avalanche current. The model specifically captures the effect of the load’s feedback on the stochastic avalanche multiplication, an effect believed to be key in breaking today’s counting rate barrier in the 1.55–μm detection window

Multi-Gain-Stage InGaAs Avalanche Photodiode with Enhanced Gain and Reduced Excess Noise

We report the design, fabrication, and test of an InGaAs avalanche photodiode (APD) for 950-1650 nm wavelength sensing applications. The APD is grown by molecular beam epitaxy on InP substrates from lattice-matched InGaAs and InAlAs alloys. Avalanche multiplication inside the APD occurs in a series of asymmetric gain stages whose layer ordering acts to enhance the rate of electron-initiated impact ionization and to suppress the rate of hole-initiated ionization when operated at low gain. The multiplication stages are cascaded in series, interposed with carrier relaxation layers in which the electric field is low, preventing avalanche feedback between stages. These measures result in much lower excess multiplication noise and stable linear-mode operation at much higher avalanche gain than is characteristic of APDs fabricated from the same semiconductor alloys in bulk. The noise suppression mechanism is analyzed by simulations of impact ionization spatial distribution and gain statistics, and measurements on APDs implementing the design are presented. The devices employing this design are demonstrated to operate at linear-mode gain in excess of 6000 without avalanche breakdown. Excess noise characterized by an effective impact ionization rate ratio below 0.04 were measured at gains over 1000

Physiology and productivity of rice crop influenced by drought stress induced at different developmental stages

Rice is sensitive to moisture stress and in view of the water scarcity in the coming years, it is imperative to evaluate the performance of rice cultivar under moisture deficit. The present study aimed to evaluate the physiological responses of two rice cultivars under drought stress induced at panicle initiation and soft dough stages. The seeds of BAS-385 and KS-282 were soaked in ABA (10-6 M) prior to sowing. Foliar application of ABA (10-6 M) was made at tillering stage, 40 days after sowing (40 DAS) in both cultivars. Drought stress was induced at panicle initiation (PI) (65 DAS) and soft dough (SD) (105 DAS) stages with re-watering at incipient wilting (12% soil moisture). Drought induced significant decrease in endogenous level of IAA, GA, sugar and protein contents in leaves at SD stage, while ABA and proline contents increased significantly as compared to control. In grains, drought induced decreases in IAA, sugar and protein content were greater at PI stage in both cultivars. Stomatal resistance was significantly increased in flag and penultimate leaves at PI stage. ABA treatments ameliorated the adverse effects of drought stress for most of the physiological parameters but were ineffective to restore the drought-induced decrease in GA content. On-rewatering, the recovery of prestressed plants was significantly enhanced under ABA seed soaking and foliar spray treatments as compared to drought alone. It is inferred that the mechanism of ABA-induced tolerance to drought stress appears to be involved in maintenance of water budget by decreasing GA, increasing stomatal resistance and by osmoregulation as observed by increase in proline accumulation, and enhanced grain filling to bring early maturity in rice over control.Key words: Drought stress, rice cultivars, panicle initiation, soft dough

Key priority areas for patient safety improvement strategy in Libya: a protocol for a modified Delphi study

Introduction Patient safety is a global public health problem. Estimates and size of the problem of patient safety in low-income and developing countries are scarce. A systems approach is needed for ensuring that patients are protected from harm while receiving care. The primary objective of this study will be to use a consensus-based approach to identify the key priority areas for patient safety improvement in Libya as a developing country. Design A modified Delphi study. Methods and analysis A three-phase modified Delphi study will be conducted using an anonymous web-based questionnaires. 15 international experts in the field of patient safety will be recruited to prioritise areas of patient safety that are vital to developing countries such as Libya. The participants will be given the opportunity to rank a list of elements on five criteria. The participants will also be asked to list five barriers that they believe hinder the implementation of patient safety systems. Descriptive statistics will be used to evaluate consensus agreement, including percentage agreement and coefficient of variation. Kendall’s coefficient of concordance will be used to evaluate consensus across all participants. Ethics and dissemination Ethical approval has been granted from Imperial College Research Ethics Committee (ICREC: 16IC3598). The findings of the study will be published in a PhD thesis. A manuscript will also be prepared for publication in a high-impact peer-reviewed journal describing the Delphi process and the findings of the study

Access to primary care and the route of emergency admission to hospital: retrospective analysis of national hospital administrative data

BACKGROUND: The UK government is pursuing policies to improve primary care access, as many patients visit accident and emergency (A and E) departments after being unable to get suitable general practice appointments. Direct admission to hospital via a general practitioner (GP) averts A and E use, and may reduce total hospital costs. It could also enhance the continuity of information between GPs and hospital doctors, possibly improving healthcare outcomes. OBJECTIVE: To determine whether primary care access is associated with the route of emergency admission-via a GP versus via an A and E department. METHODS: Retrospective analysis of national administrative data from English hospitals for 2011-2012. Adults admitted in an emergency (unscheduled) for ≥1 night via a GP or an A and E department formed the study population. The measure of primary care access-the percentage of patients able to get a general practice appointment on their last attempt-was derived from a large, nationally representative patient survey. Multilevel logistic regression was used to estimate associations, adjusting for patient and admission characteristics. RESULTS: The analysis included 2 322 112 emergency admissions (81.9% via an A and E department). With a 5 unit increase in the percentage of patients able to get a general practice appointment on their last attempt, the adjusted odds of GP admission (vs A and E admission) was estimated to increase by 15% (OR 1.15, 95% CI 1.12 to 1.17). The probability of GP admission if ≥95% of appointment attempts were successful in each general practice was estimated to be 19.6%. This probability reduced to 13.6% when <80% of appointment attempts were successful. This equates to 139 673 fewer GP admissions (456 232 vs 316 559) assuming no change in the total number of admissions. Associations were consistent in direction across geographical regions of England. CONCLUSIONS: Among hospital inpatients admitted as an emergency, patients registered to more accessible general practices were more likely to have been admitted via a GP (vs an A and E department). This furthers evidence suggesting that access to general practice is related to use of emergency hospital services in England. The relative merits of the two admission routes remain unclear

Combined application of inorganic and organic phosphorous with inoculation of phosphorus solubilizing bacteria improved productivity, grain quality and net economic returns of Pearl Millet (Pennisetum glaucum [L.] R. Br.)

Pearl millet (Pennisetum glaucum [L.] R. Br.) is a climate-smart cereal crop for environments prone to drought and heat stresses. Pearl millet is cultivated in Pakistan on marginal soils with phosphorus (P) deficiency, which significantly decreases its productivity. Moreover, P fixation in the country’s calcareous soils is another major constraint which requires attention. P solubilizing bacteria (PSB) have the potential to improve P availability in the soil. However, the potential of PSB in improving P availability in soil and pearl millet yield has been rarely tested in Pakistan. Therefore, this 2-year field study explored the role of combined application of organic and inorganic P sources along with PSB (i.e., Bacillus sp. MN54) inoculation to improve yield-related traits, P use efficiency (PUE), net economic returns and grain quality of pearl millet grown under semi-arid climatic conditions. Phosphorus was applied through inorganic sources, organic sources (farmyard manure) and 50% inorganic sources + 50% organic sources with or without PSB inoculation. In control treatment, pearl millet was grown without P application. The individual and combined application of P from different sources and PSB inoculation significantly improved yield-related traits and PUE of pearl millet. The highest grain yield was observed with combined (50% inorganic + 50% organic) application of P with PSB inoculation. The same treatments resulted in higher iron, zinc, protein and P contents in the grains during both years. Likewise, P application through organic and inorganic sources combined with PSB inoculation improved soil bulk density, fertility and microbial population during both years. The highest economic returns and benefit–cost ratio was recorded for combined P application (50% inorganic + 50% organic) and PSB inoculation. In crux, the combined application of organic and inorganic P fertilizers along with PSB (Bacillus sp. MN54) inoculation seemed a feasible approach to enhance productivity, grain quality and net economic returns of pearl millet. Therefore, it is recommended that P should be applied through both organic and inorganic sources combined with PSB inoculation to improve P availability and productivity of pearl millet in Pakistan. The current study has explored the potential of combined P application through organic and inorganic sources along with PSB inoculation. Future studies should focus on the determination of mobilized P with the application of PSB

Dependence of the Performance of Single Photon Avalanche Diodes on the Multiplication Region Width

The dependence of the performance of separate-absorption-multiplication (SAM) single-photon avalanche diodes (SPADs) on the width of the multiplication region is theoretically investigated. The theory is applied to SAM SPADs with InP homojunction multiplication regions and InAlAs-InP heterojunction multiplication regions. In both cases the absorber layer is InGaAs. Two scenarios for the dark counts are considered: (i) low-temperature operation, when the number of dark carriers is dominated by field-assisted mechanisms of band-to-band tunneling and tunneling through defects; and (ii) room-temperature operation, when the number of dark carriers in the multiplication region is dominated by the generation/recombination mechanism. The analysis utilizes a generalized theory for breakdown probability, which takes into account the random locations where dark and photogenerated carriers are produced in each layer. Depending upon the detector temperature, as the width of the multiplication region is increased the effects from the reduction in the number of dark carriers due to field-assisted generation mechanisms are counteracted by the effects from the elevation in the number of generation/recombination dark carriers. Thus, there exists an optimal width of the multiplication region that achieves the best performance of the SPAD