Nonlinear optical thresholding in a 4-Channel OCDMA system via two-photon absorption

We demonstrate the use of a Two-Photon Absorption based detector in an OCDMA system. This detector provides a significant performance improvement over standard linear detection

Noise Suppression in OCDMA Networks using Nonlinear Optical Devices

Optical code division multiple access (OCDMA) is a multiplexing technique that has a number of inherent advantages that make it suitable for use in passive optical networks, such as allowing subscribers to transmit information in an asynchronous fashion over a single optical fibre. This form of multiplexing can provide a higher degree of flexibility and simplicity in comparison to other techniques. However, due to the asynchronous nature of transmission, OCDMA networks suffer from multiple access interference (MAI) and optical beat noise which severely impairs system performance. A number of solutions have been proposed to mitigate these noise sources. Increasing the optical code lengths used can reduce the level of optical beat noise, however this is generally at the expense of transmission speed and increased transmitter complexity. MAI suppression can be achieved through the use fibre-based nonlinear thresholders or optical time-gating. One problem with these solutions is the requirement of long lengths of nonlinear fibre that are susceptible to changes in environmental conditions. Therefore, this thesis focuses on the development and testing of a nonlinear optical receiver based on semiconductor devices for the suppression of noise in OCDMA systems. The nonlinear optical process of two-photon absorption (TPA) in a commercially available 1.3 micron Fabry-P´erot laser is investigated as a method for optical thresholding in an OCDMA system. It is shown that the use of a saturable absorber (SA) directly before the TPA-based detector can provide additional suppression of MAI noise. However, the level of beat noise that is present on the optical signal can be increased due to the nonlinear responses of both devices. As a result, a gain-saturated semiconductor optical amplifier (SOA) is demonstrated as a method for the reduction of optical beat noise. It is shown that error-free performance can be achieved in an optical testbed designed to simulate an OCDMA system using an SA-SOA-TPA-based receiver. The performance improvement due to the suppression of MAI and beat noise using an SA-SOA receiver is examined in relation to a current fibre-based thresholding technique; a Mamyshev filter. It is shown that the SA-SOA receiver can offer a similar level of improvement when compared to the performance of a Mamyshev filter

Reduction of MAI and beat noise in OCDMA systems using an SA-SOA-TPA-based receiver

In this letter, we investigate the removal of multiple access interference and optical beat noise from a two-channel optical system that simulates the output from a time-spread optical code-division multiple access system operating at a data rate of 100 Mb/s. Both noise sources were removed using a saturable absorber semiconductor optical amplifier two-photon absorption receiver structure. Experimental results show error-free operation when all three devices are used together

All-optical pulse processing for advanced photonic communication system

This paper investigates the use of a two-photon absorption photodetector for high speed processing of ultrashort optical pulses in advanced photonic communication systems. Specifically the paper describes how the two-photon absorption photodetector maybe employed for chromatic dispersion monitoring in high-speed, wavelength division multiplexed networks, and also for reducing multiple access interference noise in an optical code division multiplexed system

Two-photon-absorption-based OSNR monitor for NRZ-PSK transmission systems

A two-photon absorption microcavity-based technique for monitoring in-band optical signal-to-noise ratio (OSNR) in nonreturn-to-zero phase-shift-keying systems is presented. Experiments using a 10-Gb/s differential phase-shift-keying system showed that accurate measurements ( 1 dB) were possible for OSNRs in excess of 20 dB

Blood pressure lowering and risk of new-onset type 2 diabetes:an individual participant data meta-analysis

Background Blood pressure lowering is an established strategy for preventing microvascular and macrovascular complications of diabetes, but its role in the prevention of diabetes itself is unclear. We aimed to examine this question using individual participant data from major randomised controlled trials. Methods We performed a one-stage individual participant data meta-analysis, in which data were pooled to investigate the effect of blood pressure lowering per se on the risk of new-onset type 2 diabetes. An individual participant data network meta-analysis was used to investigate the differential effects of five major classes of antihypertensive drugs on the risk of new-onset type 2 diabetes. Overall, data from 22 studies conducted between 1973 and 2008, were obtained by the Blood Pressure Lowering Treatment Trialists' Collaboration (Oxford University, Oxford, UK). We included all primary and secondary prevention trials that used a specific class or classes of antihypertensive drugs versus placebo or other classes of blood pressure lowering medications that had at least 1000 persons-years of followup in each randomly allocated arm. Participants with a known diagnosis of diabetes at baseline and trials conducted in patients with prevalent diabetes were excluded. For the one-stage individual participant data meta-analysis we used stratified Cox proportional hazards model and for the individual participant data network meta-analysis we used logistic regression models to calculate the relative risk (RR) for drug class comparisons. Findings 145 939 participants (88 500 [60.6%] men and 57 429 [39.4%] women) from 19 randomised controlled trials were included in the one-stage individual participant data meta-analysis. 22 trials were included in the individual participant data network meta-analysis. After a median follow-up of 4.5 years (IQR 2.0), 9883 participants were diagnosed with new-onset type 2 diabetes. Systolic blood pressure reduction by 5 mm Hg reduced the risk of type 2 diabetes across all trials by 11% (hazard ratio 0.89 [95% CI 0.84-0.95]). Investigation of the effects of five major classes of antihypertensive drugs showed that in comparison to placebo, angiotensin-converting enzyme inhibitors (RR 0.84 [95% 0.76-0.93]) and angiotensin II receptor blockers (RR 0.84 [0.76-0.92]) reduced the risk of new-onset type 2 diabetes; however, the use of beta blockers (RR 1.48 [1.27-1.72]) and thiazide diuretics (RR 1.20 [1.07-1.35]) increased this risk, and no material effect was found for calcium channel blockers (RR 1.02 [0.92-1.13]). Interpretation Blood pressure lowering is an effective strategy for the prevention of new-onset type 2 diabetes. Established pharmacological interventions, however, have qualitatively and quantitively different effects on diabetes, likely due to their differing off-target effects, with angiotensin-converting enzyme inhibitors and angiotensin II receptor blockers having the most favourable outcomes. This evidence supports the indication for selected classes of antihypertensive drugs for the prevention of diabetes, which could further refine the selection of drug choice according to an individual's clinical risk of diabetes

Detection of intrinsic source structure at ~3 Schwarzschild radii with Millimeter-VLBI observations of SAGITTARIUS A*

We report results from very long baseline interferometric (VLBI) observations of the supermassive black hole in the Galactic center, Sgr A*, at 1.3 mm (230 GHz). The observations were performed in 2013 March using six VLBI stations in Hawaii, California, Arizona, and Chile. Compared to earlier observations, the addition of the APEX telescope in Chile almost doubles the longest baseline length in the array, provides additional {\it uv} coverage in the N-S direction, and leads to a spatial resolution of $\sim$30 $\mu$as ($\sim$3 Schwarzschild radii) for Sgr A*. The source is detected even at the longest baselines with visibility amplitudes of $\sim$4-13% of the total flux density. We argue that such flux densities cannot result from interstellar refractive scattering alone, but indicate the presence of compact intrinsic source structure on scales of $\sim$3 Schwarzschild radii. The measured nonzero closure phases rule out point-symmetric emission. We discuss our results in the context of simple geometric models that capture the basic characteristics and brightness distributions of disk- and jet-dominated models and show that both can reproduce the observed data. Common to these models are the brightness asymmetry, the orientation, and characteristic sizes, which are comparable to the expected size of the black hole shadow. Future 1.3 mm VLBI observations with an expanded array and better sensitivity will allow a more detailed imaging of the horizon-scale structure and bear the potential for a deep insight into the physical processes at the black hole boundary.Comment: 11 pages, 5 figures, accepted to Ap

GDNF and Parkinson's Disease : Where Next? A Summary from a Recent Workshop

The concept of repairing the brain with growth factors has been pursued for many years in a variety of neurodegenerative diseases including primarily Parkinson's disease (PD) using glial cell line-derived neurotrophic factor (GDNF). This neurotrophic factor was discovered in 1993 and shown to have selective effects on promoting survival and regeneration of certain populations of neurons including the dopaminergic nigrostriatal pathway. These observations led to a series of clinical trials in PD patients including using infusions or gene delivery of GDNF or the related growth factor, neurturin (NRTN). Initial studies, some of which were open label, suggested that this approach could be of value in PD when the agent was injected into the putamen rather than the cerebral ventricles. In subsequent double-blind, placebo-controlled trials, the most recent reporting in 2019, treatment with GDNF did not achieve its primary end point. As a result, there has been uncertainty as to whether GDNF (and by extrapolation, related GDNF family neurotrophic factors) has merit in the future treatment of PD. To critically appraise the existing work and its future, a special workshop was held to discuss and debate this issue. This paper is a summary of that meeting with recommendations on whether there is a future for this therapeutic approach and also what any future PD trial involving GDNF and other GDNF family neurotrophic factors should consider in its design.Peer reviewe

Monitoring the Morphology of M87* in 2009–2017 with the Event Horizon Telescope

The Event Horizon Telescope (EHT) has recently delivered the first resolved images of M87*, the supermassive black hole in the center of the M87 galaxy. These images were produced using 230 GHz observations performed in 2017 April. Additional observations are required to investigate the persistence of the primary image feature—a ring with azimuthal brightness asymmetry—and to quantify the image variability on event horizon scales. To address this need, we analyze M87* data collected with prototype EHT arrays in 2009, 2011, 2012, and 2013. While these observations do not contain enough information to produce images, they are sufficient to constrain simple geometric models. We develop a modeling approach based on the framework utilized for the 2017 EHT data analysis and validate our procedures using synthetic data. Applying the same approach to the observational data sets, we find the M87* morphology in 2009–2017 to be consistent with a persistent asymmetric ring of ~40 μas diameter. The position angle of the peak intensity varies in time. In particular, we find a significant difference between the position angle measured in 2013 and 2017. These variations are in broad agreement with predictions of a subset of general relativistic magnetohydrodynamic simulations. We show that quantifying the variability across multiple observational epochs has the potential to constrain the physical properties of the source, such as the accretion state or the black hole spin