Frequency drift characterization of a laser stabilized to an optical fiber delay line.

Lasers stabilized to optical fiber delay lines have been shown to deliver a comparable short-term (<1 s) frequency noise performance to that achieved by lasers stabilized to ultra-low expansion (ULE) cavities, once the linear frequency drift has been removed. However, for continuous stable laser operations, the drift can be removed only when it can be predicted, e.g., when it is linear over very long timescales. To date, such long-term behaviour of the frequency drift in fiber delay lines has not been, to the best of our knowledge, characterised. In this work we experimentally characterise the frequency drift of a laser stabilised to a 500 m-long optical fiber delay line over the course of several days. We show that the drift still follows the temperature variations even when the spool temperature is maintained constant with fluctuations below tens of mK. Consequently, the drift is not linear over long timescales, preventing a simple feed-forward compensation. However, here we show that the drift can be reduced by exploiting the high level of correlation between laser frequency and the fiber temperature. In our demonstration, by applying a frequency correction proportional to temperature readings, a calculated frequency drift of less than 16 Hz/s over the several days of our test was obtained, corresponding to a 23-fold improvement from uncorrected values.

The DiaCoVAb Study in South Italy: Immune Response to SARS-CoV-2 Vaccination in Dialysis Patients.

INTRODUCTION: Since the pandemic of COVID-19 started from December 2019, remarkable numbers of infections and deaths associated with COVID-19 have been recorded worldwide. End-stage kidney disease patients on dialysis are particularly at high risk of infections due to impairments in the innate and adaptive immune systems. Vaccination on dialysis patients (DP) still remains challenging because of the variable response and a low seroconversion rate compared with healthy participants (HP). Therefore, it is urgently necessary to establish a different vaccination strategy for DP, in terms of the dose and administration time. METHODS: Here, we report an observational prospective cohort study in which the immunogenic efficacies of SARS-CoV-2 vaccine BNT162b2 on DP and HP were evaluated by absolute quantification of IgG levels in the blood. RESULTS: DP showed a delayed seroconversion after two vaccine doses, with a low absolute IgG levels compared to HP. While HP reached complete seroconversion within 10 days from the administration of a second dose, only 76% of DP were seropositive. After the booster dose, DP had a strongly improved seroconversion rate as well as antibody levels, reaching 97% seropositivity and 50 times enhancement on antibody levels. DISCUSSION/CONCLUSION: These results prompt to suggest an additional vaccine dose in DP, reducing the interval of time from the second dose. Since limited data are available on immune response in DP overtime after three vaccine doses currently, our study is among the first reports demonstrating the improved seropositivity and IgG levels in DP after the booster vaccine dose.

Deep Constraint-Based Propagation in Graph Neural Networks.

The popularity of deep learning techniques renewed the interest in neural architectures able to process complex structures that can be represented using graphs, inspired by Graph Neural Networks (GNNs). We focus our attention on the originally proposed GNN model of Scarselli et al. 2009, which encodes the state of the nodes of the graph by means of an iterative diffusion procedure that, during the learning stage, must be computed at every epoch, until the fixed point of a learnable state transition function is reached, propagating the information among the neighbouring nodes. We propose a novel approach to learning in GNNs, based on constrained optimization in the Lagrangian framework. Learning both the transition function and the node states is the outcome of a joint process, in which the state convergence procedure is implicitly expressed by a constraint satisfaction mechanism, avoiding iterative epoch-wise procedures and the network unfolding. Our computational structure searches for saddle points of the Lagrangian in the adjoint space composed of weights, nodes state variables and Lagrange multipliers. This process is further enhanced by multiple layers of constraints that accelerate the diffusion process. An experimental analysis shows that the proposed approach compares favourably with popular models on several benchmarks.

Thinly coated hollow core fiber for improved thermal phase-stability performance.

The optical phase accumulated when light propagates through an optical fiber changes with temperature. It has been shown by various authors that this thermal phase sensitivity is significantly smaller in hollow core fibers (HCFs) than in standard single-mode fibers (SSMFs). However, there have been considerable differences in the level of sensitivity reduction claimed, with factors in the range ×3 to ×20 improvement for HCFs relative to SSMFs reported. Here we show experimentally that this large variation is likely attributable to the influence of fiber coating, which is exacerbated in HCFs with a relatively thin silica glass outer wall (e.g., the wall thickness is typically just 20 µm in a 125 µm diameter HCF). Further, we show that the coating also causes the optical phase stability to suffer from relaxation effects, which have not been previously discussed in the HCF literature, to the best of our knowledge. In addition to demonstrating these relaxation effects experimentally, we analyze them through numerical simulations. Our results strongly suggest that they originate from the viscoelastic properties of the coating. To minimize the adverse effects of the coating, we have fabricated a HCF with a relatively thick wall (∼50µm) and a very thin coating (10 µm). This results in an almost 30-fold reduction in HCF thermal phase sensitivity relative to SSMFs - a significantly lower sensitivity than in previous reports. Moreover, our thinly coated HCF exhibits no discernable relaxation effects while maintaining good mechanical properties.

Learning in Text Streams: Discovery and Disambiguation of Entity and Relation Instances.

We consider a scenario where an artificial agent is reading a stream of text composed of a set of narrations, and it is informed about the identity of some of the individuals that are mentioned in the text portion that is currently being read. The agent is expected to learn to follow the narrations, thus disambiguating mentions and discovering new individuals. We focus on the case in which individuals are entities and relations and propose an end-to-end trainable memory network that learns to discover and disambiguate them in an online manner, performing one-shot learning and dealing with a small number of sparse supervisions. Our system builds a not-given-in-advance knowledge base, and it improves its skills while reading the unsupervised text. The model deals with abrupt changes in the narration, considering their effects when resolving coreferences. We showcase the strong disambiguation and discovery skills of our model on a corpus of Wikipedia documents and on a newly introduced data set that we make publicly available.

Ultrastable laser interferometry for earthquake detection with terrestrial and submarine cables.

Detecting ocean-floor seismic activity is crucial for our understanding of the interior structure and dynamic behavior of Earth. However, 70% of the planet's surface is covered by water, and seismometer coverage is limited to a handful of permanent ocean bottom stations. We show that existing telecommunication optical fiber cables can detect seismic events when combined with state-of-the-art frequency metrology techniques by using the fiber itself as the sensing element. We detected earthquakes over terrestrial and submarine links with lengths ranging from 75 to 535 kilometers and a geographical distance from the earthquake's epicenter ranging from 25 to 18,500 kilometers. Implementing a global seismic network for real-time detection of underwater earthquakes requires applying the proposed technique to the existing extensive submarine optical fiber network.

Wavelength conversion technique for optical frequency dissemination applications.

We demonstrate coherent wavelength conversion capable of covering the entire C-band by modulating the incoming optical carrier with a compact Fabry-Perot cavity embedded phase modulator and by optical injection locking a semiconductor laser to a tone of the generated optical frequency comb. The phase noise of the converted optical carrier over 1 THz frequency interval is measured to be -40 dBc/Hz at 10 Hz offset and the frequency stability is better than 2 × 10(-17) level for averaging times >1000 s, making this technique a promising solution for comparisons of state-of-the-art optical clocks over complex fiber networks.

Ultralow thermal sensitivity of phase and propagation delay in hollow core optical fibres.

Propagation time through an optical fibre changes with the environment, e.g., a change in temperature alters the fibre length and its refractive index. These changes have negligible impact in many key fibre applications, e.g., telecommunications, however, they can be detrimental in many others. Examples are fibre-based interferometry (e.g., for precise measurement and sensing) and fibre-based transfer and distribution of accurate time and frequency. Here we show through two independent experiments that hollow-core photonic bandgap fibres have a significantly smaller sensitivity to temperature variations than traditional solid-core fibres. The 18 times improvement observed, over 3 times larger than previously reported, makes them the most environmentally insensitive fibre technology available and a promising candidate for many next-generation fibre systems applications that are sensitive to drifts in optical phase or absolute propagation delay.

Optical injection locking-based amplification in phase-coherent transfer of optical frequencies.

We demonstrate the use of an optical injection phase locked loop (OIPLL) as a regenerative amplifier for optical frequency transfer applications. The optical injection locking provides high gain within a narrow bandwidth (<100 MHz) and is capable of preserving the fractional frequency stability of the incoming carrier to better than 10(-18) at 1000 s. The OIPLL was tested in the field as a mid-span amplifier for the transfer of an ultrastable optical carrier, stabilized to an optical frequency standard, over a 292 km long installed dark fiber link. The transferred frequency at the remote end reached a fractional frequency instability of less than 1×10(-19) at averaging time of 3200 s.

Suppression of amplitude-to-phase noise conversion in balanced optical-microwave phase detectors.

We demonstrate an amplitude-to-phase (AM-PM) conversion coefficient for a balanced optical-microwave phase detector (BOM-PD) of 0.001 rad, corresponding to AM-PM induced phase noise 60 dB below the single-sideband relative intensity noise of the laser. This enables us to generate 8 GHz microwave signals from a commercial Er-fibre comb with a single-sideband residual phase noise of -131 dBc Hz(-1) at 1 Hz offset frequency and -148 dBc Hz(-1) at 1 kHz offset frequency.

Dissemination of an optical frequency comb over fiber with 3 × 10(-18) fractional accuracy.

We demonstrate that the structure of an optical frequency comb transferred over several km of fiber can be preserved at a level compatible with the best optical frequency references currently available. Using an optical phase detection technique we measure the noise introduced by the fiber link and suppress it by stabilizing the optical path length. The measured fractional frequency stability of the transferred optical modes is 2 × 10(-18) at a few thousand seconds and the mode spacing stability after optical-microwave conversion is better than 4 × 10(-17) over the same time scale.

High-resolution microwave frequency transfer over an 86-km-long optical fiber network using a mode-locked laser.

We demonstrate the transfer of an ultrastable microwave frequency by transmitting a 30-nm-wide optical frequency comb from a mode-locked laser over 86 km of installed optical fiber. The pulse train is returned to the transmitter via the same fiber for compensation of environmentally induced optical path length changes. The fractional transfer stability measured at the remote end reaches 4×10(-17) after 1600 s, corresponding to a timing jitter of 64 fs.

Common-path self-referencing interferometer for carrier-envelope offset frequency stabilization with enhanced noise immunity.

A nonlinear interferometer design for stabilizing the carrier-envelope offset frequency of a Ti:sapphire frequency comb with superior immunity to air currents and acoustic noise is presented. The scheme uses a pair of Wollaston prisms for group-delay dispersion compensation, providing an all-common-path optical configuration. Out-of-loop phase noise measurements for an unshielded interferometer setup showed up to 15 dB improvement compared to a Michelson interferometer based system. Further simplification of the self-referencing scheme providing a compact single-Wollaston-prism design has been demonstrated.

High-stability microwave frequency transfer by propagation of an optical frequency comb over 50 km of optical fiber.

A high-stability microwave frequency reference is transferred over 50 km of spooled optical fiber by propagation of a 90-nm-wide optical frequency comb centered at 1.56 microm. Environmentally induced fiber phase noise is actively suppressed by returning the optical frequency comb via a separate fiber. The stability of the microwave frequency delivered is measured at the "user" end, eliminating the need for the assumption of correlation between the noise in the forward and return paths. The measured transfer stability is 4.6 x 10(-15)tau(-1), rendering this technique suitable for the transfer of state-of-the-art frequency references.

Hallucinations after abrupt withdrawal of oral and intrathecal baclofen.

Since 1977 several cases of hallucinations after abrupt withdrawal of oral baclofen have been described. There are no reports of hallucinations after gradual withdrawal of oral baclofen. No one has ever described visual hallucinations after abrupt interruption of intrathecal baclofen therapy. We describe five personally observed cases of visual hallucinations occurring after sudden interruption of baclofen (in two of these cases, intrathecal baclofen) therapy. The patients were immediately submitted to routine EEG, visual evoked potentials and standard brain magnetic resonance imaging (MRI). A few days later they also underwent polysomnography, fundus oculi examination and brain MRI of the temporal lobe. All these examinations were normal. We hypothesise that these symptoms could be due to biochemical and molecular changes, chiefly in glutamatergic n-methyl-d-aspartate, GABA-A, and GABA-B receptor response, leading to increased excitability and spontaneous activity as a result of chronic use of baclofen.

Quality of life of patients with telemetric pumps for intrathecal baclofen infusion: data and limitations of assessment scales.

Over the past three decades, telemetric pumps have been used for the infusion of intrathecal baclofen in patients with severe spasticity, but the correlation between pump implantation and quality of life (QoL) has rarely been studied. The aim of this study was therefore to quantify QoL in these patients. We assessed 15 candidates for intrathecal baclofen infusion pump implantation using three scales: Self-Evaluation of Life Function, Quality of Life Index, and Quality of Well-Being Scale. These scales were administered a week before pump implantation and 12 months after reaching the optimal dosage. The first scale revealed a significant increase in QoL, whereas on the other two the increase was not significant. These results encourage us to continue this study in a larger patient sample, considering different types of pathology and presence/absence of caregivers.