347 research outputs found
Adaptive polarimetric image representation for contrast optimization of a polarized beacon through fog
We present a contrast-maximizing optimal linear representation of
polarimetric images obtained from a snapshot polarimetric camera for enhanced
vision of a polarized light source in obscured weather conditions (fog, haze,
cloud) over long distances (above 1 km). We quantitatively compare the gain in
contrast obtained by different linear representations of the experimental
polarimetric images taken during rapidly varying foggy conditions. It is shown
that the adaptive image representation that depends on the correlation in
background noise fluctuations in the two polarimetric images provides an
optimal contrast enhancement over all weather conditions as opposed to a simple
difference image which underperforms during low visibility conditions. Finally,
we derive the analytic expression of the gain in contrast obtained with this
optimal representation and show that the experimental results are in agreement
with the assumed correlated Gaussian noise model
Depolarization remote sensing by orthogonality breaking
A new concept devoted to sensing the depolarization strength of materials
from a single measurement is proposed and successfully validated on a variety
of samples. It relies on the measurement of the orthogonality breaking between
two orthogonal states of polarization after interaction with the material to be
characterized. The two fields orthogonality being preserved after propagation
in birefringent media, this concept is shown to be perfectly suited to
depolarization remote sensing through fibers, opening the way to real time
depolarization endoscopy.Comment: 5 pages, 4 figure
Optical fiber Sagnac interferometer for sensing scalar directional refraction: application to magnetochiral birefringence
We present a set-up dedicated to the measurement of the small scalar
directional anisotropies associated to the magnetochiral interaction. The
apparatus, based on a polarization-independent fiber Sagnac interferometer, is
optimized to be insensitive to circular anisotropies and to residual
absorption. It can thus characterize samples of biological interests, for which
the two enantiomers are not available and/or which present poor transmission.
The signal-to-noise ratio is shown to be limited only by the source intensity
noise, leading to a detection limit of Df = 500 nrad.Hz-1/2. It yields a limit
on the magnetochiral index nMC < 4 10-13 T-1 at 1550 nm for the organic
molecules tested.Comment: 17 pages, 8 figure
Experimental evidence and theoretical modeling of two-photon absorption dynamics in the reduction of intensity noise of solid-state Er:Yb lasers
A theoretical and experimental investigation of the intensity noise reduction
induced by two-photon absorption in a Er,Yb:Glass laser is reported. The time
response of the two-photon absorption mechanism is shown to play an important
role on the behavior of the intensity noise spectrum of the laser. A model
including an additional rate equation for the two-photon-absorption losses is
developed and allows the experimental observations to be predicted
Theoretical optimal modulation frequencies for scattering parameter estimation and ballistic photon filtering in diffusive media
The efficiency of using intensity modulated light for estimation of
scattering properties of a turbid medium and for ballistic photon
discrimination is theoretically quantified in this article. Using the diffusion
model for modulated photon transport and considering a noisy quadrature
demodulation scheme, the minimum-variance bounds on estimation of parameters of
interest are analytically derived and analyzed. The existence of a
variance-minimizing optimal modulation frequency is shown and its evolution
with the properties of the intervening medium is derived and studied.
Furthermore, a metric is defined to quantify the efficiency of ballistic photon
filtering which may be sought when imaging through turbid media. The analytical
derivation of this metric shows that the minimum modulation frequency required
to attain significant ballistic discrimination depends only on the reduced
scattering coefficient of the medium in a linear fashion for a highly
scattering medium
Modèle de la réponse temporelle de la dispersion chromatique
National audienceNous proposons un modèle numérique de fonction de transfert en z d’un milieu dispersif (fibre, milieux gazeux dense) pour l’enveloppe temporelle (modulations de phase et d’amplitude) de porteuses optiques ou hyperfréquence. Les modélisations existantes (FFT, filtre à réponse impulsionnelle finie, transformée bilinéaire) sont insuffisantes pour des signaux contenant des bruits colorés ou même ne reproduisent pas la dispersion voulue. À partir de développements de Padé des fonctions exponentielle et logarithme, nous obtenons une fonction de transfert sous forme de fraction rationnelle. Les pôles instables sont traités par retournement temporel. Le modèle est étudié pour un objectif de modulation en quadrature à haut débit sur de longues fibres, par exemple plusieurs centaines de mètres de fibre SMF à 2*28 Gb/s en QAM
Estimating the polarization degree of polarimetric images in coherent illumination using maximum likelihood methods
This paper addresses the problem of estimating the polarization degree of polarimetric images in coherent illumination. It has been recently shown that the degree of polarization associated to polarimetric images can be estimated by the method of moments applied to two or four images assuming fully developed speckle. This paper shows that the estimation can also be conducted by using maximum likelihood methods. The maximum likelihood estimators of the
polarization degree are derived from the joint distribution of the image intensities. We show that the joint distribution of polarimetric images is a multivariate gamma distribution whose marginals are univariate, bivariate or trivariate gamma distributions. This property is used to derive maximum likelihood estimators of the polarization degree using two, three or four images. The proposed
estimators provide better performance that the estimators of
moments. These results are illustrated by estimations conducted on synthetic and real images
Intensity and phase noise correlations in a dual-frequency VECSEL operating at telecom wavelength
The amplitude and phase noises of a dual-frequency vertical-external-cavity
surface-emitting laser (DF-VECSEL) operating at telecom wavelength are
theoretically and experimentally investigated in detail. In particular, the
spectral behavior of the correlation between the intensity noises of the two
modes of the DF-VECSEL is measured. Moreover, the correlation between the phase
noise of the radio-frequency (RF) beatnote generated by optical mixing of the
two laser modes with the intensity noises of the two modes is investigated. All
these spectral behaviors of noise correlations are analyzed for two different
values of the nonlinear coupling between the laser modes. We find that to
describe the spectral behavior of noise correlations between the laser modes,
it is of utmost importance to have a precise knowledge about the spectral
behavior of the pump noise, which is the dominant source of noise in the
frequency range of our interest (10 kHz to 35 MHz). Moreover, it is found that
the noise correlation also depends on how the spatially separated laser modes
of the DF-VECSEL intercept the noise from a multi-mode fiber-coupled laser
diode used for pumping both the laser modes. To this aim, a specific experiment
is reported, which aims at measuring the correlations between different spatial
regions of the pump beam. The experimental results are in excellent agreement
with a theoretical model based on modified rate equations
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