136 research outputs found
Generalized Langevin equations for a driven tracer in dense soft colloids: construction and applications
We describe a tracer in a bath of soft Brownian colloids by a particle
coupled to the density field of the other bath particles. From the Dean
equation, we derive an exact equation for the evolution of the whole system,
and show that the density field evolution can be linearized in the limit of a
dense bath. This linearized Dean equation with a tracer taken apart is
validated by the reproduction of previous results on the mean-field liquid
structure and transport properties. Then, the tracer is submitted to an
external force and we compute the density profile around it, its mobility and
its diffusion coefficient. Our results exhibit effects such as bias enhanced
diffusion that are very similar to those observed in the opposite limit of a
hard core lattice gas, indicating the robustness of these effects. Our
predictions are successfully tested against molecular dynamics simulations.Comment: 21 pages, 7 figure
Nonlinear modification of the laser noise power spectrum induced by a frequency-shifted optical feedback
In this article, we study the non-linear coupling between the stationary
(i.e. the beating modulation signal) and transient (i.e. the laser quantum
noise) dynamics of a laser subjected to frequency shifted optical feedback. We
show how the noise power spectrum and more specifically the relaxation
oscillation frequency of the laser are modified under different optical
feedback condition. Specifically we study the influence of (i) the amount of
light returning to the laser cavity and (ii) the initial detuning between the
frequency shift and intrinsic relaxation frequency. The present work shows how
the relaxation frequency is related to the strength of the beating signal and
the shape of the noise power spectrum gives an image of the Transfer Modulation
Function (i.e. of the amplification gain) of the nonlinear-laser dynamics.The
theoretical predictions, confirmed by numerical resolutions, are in good
agreements with the experimental data.Comment: in Physical Review, American Physical Society (APS), 201
Coherent microscopy by laser optical feedback imaging (LOFI) technique
The application of the non conventional imaging technique LOFI (Laser Optical
Feedback Imaging) to coherent microscopy is presented. This simple and
efficient technique using frequency-shifted optical feedback needs the sample
to be scanned in order to obtain an image. The effects on magnitude and phase
signals such as vignetting and field curvature occasioned by the scanning with
galvanometric mirrors are discussed. A simple monitoring method based on phase
images is proposed to find the optimal position of the scanner. Finally, some
experimental results illustrating this technique are presented
Deep and optically resolved imaging through scattering media by space-reversed propagation
We propose a novel technique of microscopy to overcome the effects of both
scattering and limitation of the accessible depth due to the objective working
distance. By combining Laser Optical Feedback Imaging (LOFI) with Acoustic
Photon Taging (APT) and Synthetic Aperture (SA) refocusing we demonstrate an
ultimate shot noise sensitivity at low power (required to preserve the tissues)
and a high resolution beyond the microscope working distance. More precisely,
with a laser power of 10mW, we obtain images with a micrometric resolution over
~8 transport mean free paths, corresponding to 1.3 times the microscope working
distance. Various applications such as biomedical diagnosis, research and
development of new drugs and therapies can benefit from our imaging setup
Sensitivity of synthetic aperture laser optical feedback imaging
In this paper we compare the sensitivity of two imaging configurations both
based on Laser Optical Feedback Imaging (LOFI). The first one is direct
imaging, which uses conventional optical focalisation on target and the second
one is made by Synthetic Aperture (SA) Laser, which uses numerical
focalisation. We show that SA configuration allows to obtain good resolutions
with high working distance and that the drawback of SA imagery is that it has a
worse photometric balance in comparison to conventional microscope. This
drawback is partially compensated by the important sensitivity of LOFI. Another
interest of SA relies on the capacity of getting a 3D information in a single
x-y scan
Self-aligned setup for laser optical feedback imaging insensitive to parasitic optical feedback
International audienceIn this paper we propose a new optical architecture for the laser optical feedback imaging (LOFI) technique which makes it possible to avoid the adverse effect of the optical parasitic backscattering introduced by all the optical interfaces located between the laser source and the studied object. This proposed setup need no specific or complex alignment, that why we can consider the proposed setup as self aligned. We describe the principle used to avoid the parasitic backscattering contributions which deteriorate dramatically amplitude and phase information contained in the LOFI images. Finally, we give successful demonstration of amplitude and phase images obtained with this self aligned setup in presence of a parasitic reflection
Control of the differential interference contrast in reinjected bimode laser
International audienceWe have demonstrated, both theoretically and experimentally, that it is possible to control (i.e., to enhance or cancel) the contrast of the interference pattern appearing in the intensity images obtained with a laser optical feedback imaging (LOFI) setup using a bimode laser. The laser is composed of two coupled orthogonally polarized states that interact (i.e., interfere) through the cross saturation laser dynamics. We created the contrast control by choosing the frequency shift (i.e., the beating frequency) between the feedback electric fields and the intracavity electric fields. We have shown that the interference contrast of the output power modulation of the laser total intensity is independent from the frequency shift and is always maximal. On the other hand, the interference contrast of each polarization state is frequency dependent. We obtained the maximal contrast when the frequency shift was equal to one of the resonance frequencies of the bimode dynamics, and was very low (and almost cancels) for an intermediate frequency located at the intersection of the two resonance curves
Heterodyne beatings between frequency-shifted feedback lasers
International audienceFrequency-shifted feedback (FSF) lasers are potential candidates for long distance telemetry due to the appearance of beatings in the noise spectrum at the output of a homodyne interferometer: the frequencies of these beatings vary linearly with the path delay. In this Letter we demonstrate that these beatings also occur in the heterodyne mixing of two identical, but distinct, FSF lasers. This phenomenon is explained by the passive cavity model and is exploited to characterize the time-spectrum properties of FSF lasers. Consequences on telemetry with FSF lasers are presented
Comparative study of autodyne and heterodyne laser interferometry for imaging
International audienceFor given laser output power, object under investigation and photodiode noise level, we have theoretically compared the signal to noise ratio (SNR) of a heterodyne scanning imager based on a Michelson interferometer and of an autodyne setup based on the Laser Optical Feedback Imaging (LOFI) technique. In both cases, the image is obtained point by point. In the heterodyne configuration, the beating between the reference beam and the signal beam is realized outside the laser cavity (i.e. directly on the detector), while in the autodyne configuration, the wave beating takes place inside the laser cavity and therefore is indirectly detected. In the autodyne configuration, where the laser relaxation oscillations play a leading role, we have compared 1D scans obtained by numerical simulations with different lasers dynamical parameters. Finally we have determined the best laser for LOFI applications and the experimental conditions for which the LOFI detection setup (autodyne interferometer) is competitive comparing to a heterodyne interferometer. © 2010 Optical Society of Americ
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