Laboratory evaluation of the (VIS, IR) scattering matrix of complex-shaped ragweed pollen particles.

Ragweed or Ambrosia artemisiifolia pollen is an important atmospheric constituent affecting the Earth's climate and public health. The literature on light scattering by pollens embedded in ambient air is however rather sparse: polarization measurements are limited to the sole depolarization ratio and pollens are beyond the reach of numerically exact light scattering models mainly due to their tens of micrometre size. Also, ragweed pollen presents a very complex shape, with a small-scale external structure exhibiting spikes that bears some resemblance with coronavirus, but also apertures and micrometre holes. In this paper, to face such a complexity, a controlled-laboratory experiment is proposed to evaluate the scattering matrix of ragweed pollen embedded in ambient air. It is based on a newly-built polarimeter, operating in the infra-red spectral range, to account for the large size of ragweed pollen. Moreover, the ragweed scattering matrix is also evaluated in the visible spectral range to reveal the spectral dependence of the ragweed scattering matrix within experimental error bars. As an output, precise spectral and polarimetric fingerprints for large size and complex-shaped ragweed pollen particles are then provided. We believe our laboratory experiment may interest the light scattering community by complementing other light scattering experiments and proposing outlooks for numerical work on large and complex-shaped particles.

Specific innovative semi-transparent solar cell for indoor and outdoor LiFi applications.

Research in light-fidelity (LiFi), also called visible light communication (VLC), has gained huge interest. In such a communication system, an optical sensor translates the received luminous modulation flux into an electrical signal which is decoded. To consider LiFi as an alternative solution for wireless communication, the receiver must be operational in indoor and outdoor configurations. Photovoltaic modules could appear as a solution to this issue. In this paper, we present signal-to-noise ratio (SNR) response in the frequency of two different kinds of photovoltaic modules. We characterize in detail the SNR by using an experimental setup which connects a software-based direct current optical (DCO)-orthogonal frequency division multiiplexing emitter and receiver to hardware optical front ends. We analyze LiFi performances under different lighting conditions. We prove that the available bandwidth depends drastically on ambient lighting configurations. Under specific lighting conditions, a bandwidth around 4 MHz corresponding a data rate around 8 Mbit/s could be achieved. We present the lighting saturation effects and we prove that the semi-transparent solar cell under study improves their performances (both bandwidth and data rate) in high ambient lighting environments.

LED dynamic electro-optical responses and light-fidelity-application optimization.

Commercial light-emitting diodes (LEDs) from different manufacturers were studied by means of impedance measurements in the frequency range between 75 kHz and 10 MHz. Electrical characteristics of these LEDs, such as impedance and resistance, were proven to be strongly influenced by the applied frequencies, the bias values, and the alternating-signal amplitudes. Through these measurements, a specific bias value, which later could be of great importance, was pointed out. Coupled with the optical signal-to-noise ratio measurements, this frequency-, bias-, and alternating-signal-amplitude-dependent impedance shows a close correlation between optical and electrical responses of LEDs, which turns out to be useful for visible light communication. Hence, a new and simple method of light-fidelity optimization through impedance measurements is proposed in this article.

Selective color imaging using weighted interleaved multiple annular linear diffractive axicons.

Annular linear diffractive axicons are optical devices providing chromatic imaging over an extended depth of focus when illuminated by a white light. To improve their low radiometric performance, multiple annular linear diffractive axicons (MALDAs) have been introduced. Their chromatic properties are well known and constrained by dispersion laws. A first attempt to freely combine colors or wavelength bands has been obtained with interleaved MALDAs (I_MALDAs). However, such optics do not provide a full decoupling between wavelength combination and brightness control required in the CIE color space to address any colors. We present here a new category of I_MALDA providing this capability when illuminated by a white source containing tristimulus (red/green/blue) values. We assess both theoretically and experimentally imaging qualities of such optics with respect to two different interleaving techniques and suggest some potential applications, in particular in the field of anticounterfeit and authentication techniques.

Multispectral imaging axicons.

Large-aperture linear diffractive axicons are optical devices providing achromatic nondiffracting beams with an extended depth of focus when illuminated by white light sources. Annular apertures introduce chromatic foci separation, making chromatic imaging possible despite important radiometric losses. Recently, a new type of diffractive axicon has been introduced, by multiplexing concentric annular axicons with appropriate sizes and periods, called a multiple annular linear diffractive axicon (MALDA). This new family of conical optics combines multiple annular axicons in different ways to optimize color foci recombination, separation, or interleaving. We present different types of MALDA, give an experimental illustration of the use of these devices, and describe the manufacturing issues related to their fabrication to provide color imaging systems with long focal depths and good diffraction efficiency. Application to multispectral image analysis is discussed.

Multiple annular linear diffractive axicons.

We propose a chromatic analysis of multiple annular linear diffractive axicons. Large aperture axicons are optical devices providing achromatic nondiffracting beams, with an extended depth of focus, when illuminated by a white light source, due to chromatic foci superimposition. Annular apertures introduce chromatic foci separation, and because chromatic aberrations result in focal segment axial shifts, polychromatic imaging properties are partially lost. We investigate here various design parameters that can be used to achieve color splitting, filtering, and combining using these properties. In order to improve the low-power efficiency of a single annular axicon, we suggest a spatial multiplexing of concentric annular axicons with different sizes and periods we call multiple annular aperture diffractive axicons (MALDAs). These are chosen to maintain focal depths while enabling color imaging with sufficient diffraction efficiency. Illustrations are given for binary phase diffractive axicons, considering technical aspects such as grating design wavelength and phase dependence due to the grating thickness.