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In a context where SARS-CoV-2 population-wide testing is implemented, clinical features and antibody response in those infected have never been documented in Africa. Yet, the information provided by analyzing data from population-wide testing is critical to understand the infection dynamics and devise control strategies. We described clinical features and assessed antibody response in people screened for SARS-CoV-2 infection. We analyzed data from a cohort of 3464 people that we molecularly screened for SARS-CoV-2 infection in our routine activity. We recorded people SARS-CoV-2 diagnosis, age, gender, blood types, white blood cells (WBC), symptoms, chronic disease status and time to SARS-CoV-2 RT-PCR conversion from positive to negative. We calculated the age-based distribution of SARS-CoV-2 infection, analyzed the proportion and the spectrum of COVID-19 severity. Furthermore, in a nested sub-study, we screened 83 COVID-19 patients and 319 contact-cases for anti-SARS-CoV-2 antibodies. Males and females accounted for respectively 51% and 49% of people screened. The studied population median and mean age were both 39 years. 592 out of 3464 people (17.2%) were diagnosed with SARS-CoV-2 infection with males and females representing, respectively, 53% and 47%. The median and mean ages of SARS-CoV-2 infected subjects were 37 and 38 years respectively. The lowest rate of infection (8%) was observed in the elderly (aged > 60). The rate of SARS-Cov-2 infection in both young (18–35 years old) and middle-aged adults (36–60 years old) was around 20%. The analysis of SARS-CoV-2 infection age distribution showed that middle-aged adults accounted for 54.7% of SARS-CoV-2 positive persons, followed respectively by young adults (33.7%), children (7.7%) and elderly (3.8%). 68% (N = 402) of SARS-CoV-2 infected persons were asymptomatic, 26.3% (N = 156) had influenza-like symptoms, 2.7% (N = 16) had influenza-like symptoms associated with anosmia and ageusia, 2% (N = 11) had dyspnea and 1% (N = 7) had respiratory failure, which resulted in death. Data also showed that 12% of SARS-CoV-2 infected subjects, had chronic diseases. Hypertension, diabetes, and asthma were the top concurrent chronic diseases representing respectively 58%, 25% and 12% of recorded chronic diseases. Half of SARS-CoV-2 RT-PCR positive patients were cured within 14 days following the initiation of the anti-COVID-19 treatment protocol. 78.3% of COVID-19 patients and 55% of SARS-CoV-2 RT-PCR confirmed negative contact-cases were positive for anti-SARS-CoV-2 antibodies. Patients with severe-to-critical illness have higher leukocytes, higher neutrophils and lower lymphocyte counts contrarily to asymptomatic patients and patients with mild-to-moderate illness. Neutrophilic leukopenia was more prevalent in asymptomatic patients and patients with mild-to-moderate disease for 4 weeks after diagnosis (27.1–42.1%). In Patients with severe-to-critical illness, neutrophilic leukocytosis or neutrophilia (35.6–50%) and lymphocytopenia (20–40%) were more frequent. More than 60% of participants were blood type O. It is also important to note that infection rate was slightly higher among A and B blood types compared with type O. In this African setting, young and middle-aged adults are most likely driving community transmission of COVID-19. The rate of critical disease is relatively low. The high rate of anti-SARS-CoV-2 antibodies observed in SARS-CoV-2 RT-PCR negative contact cases suggests that subclinical infection may have been overlooked in our setting.

International audience; One fundamental law in integrated photonics (IP) is the decomposition of the guided optical waves into two independent linearly transverse polarization modes: the TE and TM modes. Because of the planar symmetry of on chip devices that induces linear birefringence (LB) in these systems, the use of IP is limited to the area where the relevant polarization modes are the linear ones. Using hybrid organic-inorganic chiral material with high circular biregringence (CB), we susscefuly achived channel sol-gel chirowaveguides via a simple imprint technique. By varying the channel chirowaveguides geometry and consequently, balancing LB and CB, elliptical birefringence could be tuned and eigenmode polarization from linear to nearly circular could be propagated in these unique waveguides. Our study unravels a new way of polarization control in IP, providing a route towards the design of versatile optical chirowaveguides in which light can propagate through eigenmodes with a polarization other than linear.

International audience; Rare earth emitters are promising in integrated optics but require complex integration on silicon. In this work, we have fabricated an Y 2 O 3 :Eu 3+ micro-emitter on SiO 2 on Si substrate without etching. Since pulsed laser deposition produces a high quality layer at room temperature, material can be locally deposited on top of substrates by lift-off processing. After annealing, microstructures exhibit good crystallographic quality with controlled dimensions for light confinement and narrow emission. This works allows envisioning rare-earth doped micro-photonic structures directly integrated on silicon without etching, which opens the way to integration of new functional materials on silicon platform.

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International audience; We present here for the first time experimental bifurcation diagrams of a self-pulsing dual wavelength erbium-doped fiber laser. New type of generalized bistabilities are observed such as 3T-chaos and 1T-chaos. Theoretical calculation based on the two-mode ion-pair model shows a very good agreement with the experimental results.

International audience; Laser spectra are strongly modified by the optical feedback which is provided by external cavities; here we give a preliminary experimental result describing the optical spectrum of a fibre laser having two identical end cavities, which appear between the external mirrors and fibre ends. One observes a channelled spectrum including mode clusters separated by empty regions. We use the optical Airy function defined in the pure frequency domain to study these spectra. An optical Vernier effect allows the description of this structure. The linewidth of each mode is calculated as a function of the gain, by using the same Airy function.

An Er:ZBLAN microspherical laser has several laser lines around 1.55 μm with narrow bandwidths ( 50-100 kHz). Such a laser is used as a master laser to optically inject a slave DFB semiconductor laser. We show that the slave can be easily locked by one microsphere mode only, enabling a spectral narrowing of the laser line (from 30 MHz to 67 kHz). We stress the selective process of light amplification and filtering effect by the DFB slave.

We give an analysis of the spectrum around the laser line of vertical-cavity surface-emitting lasers. We show that they contain sidebands which are not symmetrically located on each side of the laser frequency and that this asymmetry corresponds to a non-degeneracy of relaxation oscillations of the carriers. A model which explains this non-degeneracy is introduced.

The process of amplification of light by a laser set above its oscillation threshold is studied. This laser has a wide bandwidth (typically 100 MHz) and is injected by a signal whose bandwidth is much narrower, which allows to separate the amplified signal from the ordinary laser spectral density. Theoretical calculations are based on simple semiclassical formulas and agree with the experimental results which are performed at 1.55 μm using single-mode semiconductor lasers. The amplification arises from a progressive channeling of energy from the wider laser line onto the narrow injected signal.

Nous montrons que la fonction de transfert generalisee au laser introduite recemment [1] nous permet de decrire l'evolution de la largeur de raie d'un laser libre ou optiquement injecte.

We present a general modeling of a multi-longi-tudinal-mode bipolarized Nd-doped fiber laser which keeps both vectorial (polarization) and longitudinal properties in the dynamical equations. We use an approach based on random orientation of the microscopic dipoles and demonstrate that it is isomorphic to the model derived assuming a transverse two-site distribution function for the ions. This result is also true under the mean-field approximation. At steady-state, the model is the generalization of Rigrod’s theory to the case of a bipolarized laser.

We present a steady-state analysis of a general model for a bipolarized Nd-doped fiber laser including the slow longitudinal variations of the dynamical variables. The combined effects of pump-induced, distributed, and localized anisotropies are studied versus the fiber length. In particular, the evolution of the thresholds versus the pump-input polarization angle, for different fiber lengths, is investigated in depth. Approximate analytical results are derived for the thresholds and the output intensities. A good agreement with the numerical simulations is achieved in a large range of parameters.

The polarization state of a vertical cavity surface emitting laser (VCSEL) output was driven by means of optical injection from another VCSEL. The bistability inherent to polarization locking allowed us to generate a memory effect. The control parameter was the modulated bias current of the maser laser and effects were shown to exist at frequencies up to 1 GHz.

An injected microchip laser is theoretically studied, with the use of two models: in the first model, which is traditional, the field is represented by a single frequency component in the slowly-varying envelope approximation. In the second model, referred to as the Fabry-Perot model, two field components are considered which are respectively centered around the frequency of an eigenmode of the injected laser and the frequency of the injected field. Computation of locking ranges, bistability do mains are performed and the results compared. They show not only an improved precision of the second model but also a necessity to use it to describe some effects such as the bistable domains in the limits of the locking domains.

We study experimentally and theoretically two polarization effects in a vertical cavity surface emitting laser submitted to optical feedback. In a first experiment, we obtain flips between two linearly polarized laser modes up to a frequency of 50 MHz using an external cavity with a polarizer. In a second experiment, polarization self modulation is demonstrated up to a frequency of 2.6 GHz, using a quarter wave plate instead. Numerical calculations, based on a four levels model for the active medium, show a good agreement with the experiments.

We show that the light of a commercially available Er : Yb :Cr :phosphate glass microchip laser end-pumped at 977 nm by an InGaAs laser diode can be switched between two linear polarization eigenstates when it is submitted to a polarized optical feedback. The flip occurs when the feedback phase is scanned by the external cavity length. In our experiments the laser diode light is confined to a fiber which is single-mode at pumping wavelength and the microchip laser operates in a single- longitudinal-and transverse mode. The microchip laser exhibits a self-pulsing behavior. Key words : Solid state laser, Optical switching, Optical polariza- tion, Microstructure, Glass, Phosphate, Feedback, Optical pumping, Experimental study. pompe et le microlaser est dgalement monomode trans- verse et longitudinal. On observe un comportement auto- impulsionnel du laser. Mots el~s : Laser solide, Commutation optique, Polarisation opti- que, Microstructure, Verre, Phosphate, R6troaction, Pompage optique, Etude exp6rimentale. Contents

A Nd:YVO 4 microchip single mode laser whose facets are cut perpendicularly to the optical axis is studied. We have observed that in general the laser field has two frequency components which are linearly polarized and perpendicular to each other. Fluorescence emission spectra together with polarization-resolved laser spectra and relaxation oscillations are also presented.

A semiclassical, nonlinear theory of a single-mode laser is developed so that the transition around threshold can be studied. The field is expressed in the frequency domain which allows us to emphasize the role of the Fabry-P\'erot cavity. A generalized Airy function is obtained for the laser line shape: it contains the source line shape and the empty cavity line shape. It describes the laser both above and below threshold, including the transition region where there is an abrupt increase of intensity and decrease of the laser linewidth. The general arguments are illustrated by detailed numerical calculations for the 3.39-\ensuremath{\mu}m line of the He-Ne laser. The intensity of the spontaneous emission source which plays a central role is computed from first principles.

Vertical-cavity surface-emitting lasers (VCSELs) are known to exhibit a small birefringence and dichroism whose axes are directed along the crystal axes of the quantum wells and which fix the polarization of the oscillating light. In this paper, we consider the dynamics of the polarization of light in such a laser, extended by means of a quarter-wavelength plate in an external cavity. Periodic variations are experimentally observed. A theoretical analysis and numerical results are proposed, using a recent model which includes the carriers' spin interaction. A good match is found between experimental and numerical results.

We give a general equation which allows the description of the evolution of the laser properties when the gain is varied from below to above threshold. The method is general in the context of the semi-classical theory of lasers. It is numerically illustrated in the simplest case of a single mode laser with a source term characterized by a very narrow spectrum. The transformation of the Airy line shape of the passive Fabry-Perot cavity into the laser line shape is described. A classical linewidth which complements the usual quantum limit can thus be introduced, together with its control parameters.

A linear stability analysis of erbium-doped fibre laser dynamics including spontaneous emission is presented. It is shown that spontaneous emission favours the CW solution near threshold and pushes the unstable steady states towards higher values of the ion pair concentration in the case of a bad cavity.

A simple adjustment of the "Lang and Kobayashi" delayed rate-equations which describe the dynamics of an external-cavity semiconductor laser is shown to predict low-frequency resonances in the output intensity spectrum of the system. Numerical simulations with control parameter values-driving current and feedback strength-corresponding to our experiments show excellent agreement with theory. A possible mechanism for the occurrence of low-frequency self-oscillations is also proposed. >