Institut National d'Optique

Higher order Stokes waves are easily generated in optical fibres through nonlinear stimulated Brillouin scattering. Since the Stokes waves have a linewidth limited by the narrow bandwidth of the Brillouin gain of ~20MHz, higher order Stokes waves are near multiples of the first Stokes shift of ~10GHz, generating a comb of frequencies. When Fourier transformed this comb can potentially lead to ultrashort pulse generation. Our investigations showed that short pulses of order 10ps can indeed be generated with a few Stokes waves, starting with a single CW pump. However, in longer cavities, these Stokes orders de-phase and lead to chaotic behaviour. Taming this source could lead to complex instruments being replaced by a simple optical fibre cavity.

This study describes the function, optimization, and demonstration of a new class of passive, low-cost microfluidic flow meters based on birefringent chitosan biomembranes analyzed by polarized microscopy. We subjected the membrane to dynamic flow conditions while monitoring the real-time response of its optical properties. We obtained figures of merit, including the linear response operating range (0 to 65 μL min-1), minimum response time (250 ms), sensitivity (2.03% × 10-3 μL-1 min), and minimum sensor longevity (1 week). In addition, possible sources of interference were identified. Finally, we demonstrate the membrane as a low-cost flow rate measurement device for the close loop control of a commercial pressure-driven pump. Preliminary experiments using a basic PID controller with the membrane-based flow rate measurement device showed that stable control could be achieved and the system could reach steady-state behavior in less than 15 seconds. Analysis of fundamental limits to sensor response time indicate the potential for faster steady-state behaviour.

Since early 2022, routine testing for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) based on symptoms and exposure history has largely ceased in Canada. Consequently, seroprevalence studies, particularly longitudinal studies, have become critical for monitoring the rate of incident SARS-CoV-2 infections and the proportion of the population with evidence of immunity. EnCORE is a longitudinal SARS-CoV-2 seroprevalence study comprising five rounds of serology testing from October 2020 to June 2023, in a sample of 2- to 17-year-olds (at baseline), recruited from daycares and schools in four neighbourhoods of Montreal, Canada. We report on SARS-CoV-2 incidence and seroprevalence among the 509 participants in the fifth and final round of the study. Seroprevalence of antibodies from either infection or vaccination was 98% (95 per cent confidence interval [CI]: 97, 99). The infection-acquired seroprevalence was 78% (95% CI: 73–82), and the incidence rate was 113 per 100 person-years (95% CI: 94–132), compared to the seroprevalence of 58% and the incidence rate of 133 per 100 person-years, respectively, in the fourth round of testing (mid–late 2022). Of the 131 participants newly seropositive for infection in Round 4, only 18 were seronegative for infection in Round 5 (median follow-up: 326 days).

.SignificanceTypical light sheet microscopes suffer from artifacts related to the geometry of the light sheet. One main inconvenience is the non-uniform thickness of the light sheet obtained with a Gaussian laser beam.AimWe developed a two-photon light sheet microscope that takes advantage of a thin and long Bessel-Gauss beam illumination to increase the sheet extent without compromising the resolution.ApproachWe use an axicon lens placed directly at the output of an amplified femtosecond laser to produce a long Bessel-Gauss beam on the sample. We studied the dopaminergic system and its projections in a whole cleared mouse brain.ResultsOur light sheet microscope allows an isotropic resolution of in all three axes of the scanned volume while keeping a millimetric-sized field of view, and a fast acquisition rate of up to . With slight modifications to the optical setup, the sheet extent can be increased to 6 mm.ConclusionThe proposed system’s sheet extent and resolution surpass currently available systems, enabling the fast imaging of large specimens.

We study the nonparaxial diffraction of the well-known Cartesian oval in finite and infinite conjugate configurations. We started by expressing the refraction of the Cartesian oval by analytical closed-form equations. These equations are convenient to obtain the pupil apodization function of the Cartesian oval, which is needed to compute the diffraction pattern using Richard–Wolf theory. A comparison of the diffraction patterns of the Cartesian oval with finite/infinite objects, the aplanatic lens, and the parabolic mirror for radially polarized illumination is presented. From this comparison, we conclude that a Cartesian oval for a far object is not a good candidate for tight focusing and super-resolution applications and the performance of a Cartesian oval in a finite conjugate configuration is similar to the performance of an aplanatic lens and in some scenarios, it can perform better than the aplanatic lens.

Since the demonstration of Davis et al. in 1996, femtosecond laser direct inscription emerged as a powerful tool for the fabrication of three-dimensional photonic circuits. Even today, the performance of calculations based on the volume density of components would greatly benefit from the 3D capability of fs-laser inscription. Although several advanced 3D devices such as photonic quantum circuits and lab-on-a-chip were successfully fabricated, compactness is still limited by the minimum achievable waveguide bend radius. Another growing interest is the laser inscription in materials with transmission up to the mid- and long-wave infrared for applications such as micro-organism detection, environmental monitoring, medical diagnostic and optical communication in the second atmospheric window at 8–12 microns. In this spectral band, materials that can be drawn into fiber optics, such as fluoride and chalcogenide glasses, are expensive and fragile. On the other hand, laser inscription allows the fabrication of waveguides in virtually any material, even crystals, enabling new IR applications, especially for harsh environmental conditions. In this communication, we present our recent progress on these two topics. First, we demonstrate waveguide bend radii down to <400 µm, which is an important improvement over the minimum 10-mm radius reported previously. The high refractive index change allowing such tight bends is attributed to a femtosecond laser induced band gap shift (FLIBGS) in the material. We also report low loss depressed-cladding waveguide (DCW) in crystals for IR applications. We particularly demonstrate the challenging inscription of a large DCW for single-mode operation at 10.6 µm with propagation loss of <0.63 dB/cm. We also describe a technique using a cover slide with optical contact to inscribe waveguides at the bulk surface for refractometric sensing applications.

Abstract. Detailed characterization of the spatially and temporally varying inherent optical properties (IOPs) of sea ice is necessary to better predict energy and mass balances, as well as ice-associated primary production. Here we present the development of an active optical probe to measure IOPs of a small volume of sea ice (dm3) in situ and non-destructively. The probe is derived from the diffuse reflectance method used to measure the IOPs of human tissues. The instrument emits light into the ice by the use of an optical fibre. Backscattered light is measured at multiple distances away from the source using several receiving fibres. Comparison to a Monte Carlo simulated lookup table allows, in theory, retrieval of the absorption coefficient, the reduced scattering coefficient and a phase function similarity parameter γ, introduced by Bevilacqua and Depeursinge (1999). γ depends on the two first moments of the Legendre polynomials, allowing the analysis of the backscattered light not satisfying the diffusion regime. The depth reached into the medium by detected photons was estimated using Monte Carlo simulations: the maximum depth reached by 95 % of the detected photons was between 40±2 and 270±20 mm depending on the source–detector distance and on the ice scattering properties. The magnitude of the instrument validation error on the reduced scattering coefficient ranged from 0.07 % for the most scattering medium to 35 % for the less scattering medium over the 2 orders of magnitude we validated. Fixing the absorption coefficient and γ, which proved difficult to measure, vertical profiles of the reduced scattering coefficient were obtained with decimetre resolution on first-year Arctic interior sea ice on Baffin Island in early spring 2019. We measured values of up to 7.1 m−1 for the uppermost layer of interior ice and down to 0.15±0.05 m−1 for the bottommost layer. These values are in the range of polar interior sea ice measurements published by other authors. The inversion of the reduced scattering coefficient at this scale was strongly dependent on the value of γ, highlighting the need to define the higher moments of the phase function. This newly developed probe provides a fast and reliable means for measurement of scattering in sea ice.

Abstract Sea ice pressure ridges have been recognized as important locations for both physical and biological processes. Thus, understanding the associated light‐field is crucial, but their complex structure and internal geometry render them hard to study by field methods. To calculate the in‐ and under‐ridge light field, we combined output from an ice mechanical model with a Monte‐Carlo ray tracing simulation. This results in realistic light fields showing that light levels within the ridge itself are significantly higher than under the surrounding level ice. Light guided through ridge cavities and scattering in between ridge blocks also results in a more isotropic ridge‐internal light field. While the true variability of light transmittance through a ridge can only be represented in ray tracing models, we show that simple parameterizations based on ice thickness and macro‐porosity allow accurate estimation of mean light levels available for photosynthesis underneath ridges in field studies and large‐scale models.

Oxyfluoride glass-ceramics are considered to be one of the promising materials for anti-Stokes laser cooling. Our previous results showed that glass-ceramics are more appropriate than glassy materials for laser cooling applications. In the present work, we describe the role of optimization of the oxide: fluoride ratio in oxyfluoride glass ceramics with the composition (SiO2-Al2O3)(100-x)(YLiF4)x: (YbF3)1 (x = 35 and 40; in mol.%) in order to obtain high photoluminescence quantum yield (PLQY) and low background absorption focusing on optical refrigeration applications. Glass-ceramics with high transparency (~ 90 % in the infrared region) were synthesized by the conventional melt-quenching process followed by heat-treatment. Near-infrared (NIR) photoluminescence (PL) emission due to the 2F5/2 – 2F7/2 Yb3+ transition, centered at ~1010 nm was observed. An improvement in the quantum efficiency is observed for all the samples, which varies between 64 % and 99 %, depending on the oxide: fluoride ratio. A decrease in the background absorption of the samples was investigated by calorimetry. The enhanced radiative (radiative emission) quantum efficiency is achieved due to the YLiF4 crystals (low phonon energy ~450 cm-1) which minimize non-radiative relaxations. At a laser wavelength of 1020 nm, the glass-ceramics show anti-Stokes PL emission, essential to achieve laser cooling. The proposed composition is an ideal candidate for laser cooling considering the low phonon energy and low background absorption compared to the other oxyfluoride glasses previously investigated.