Created by sebastien.popoff on 29/10/2016

Tutorials Spatial Light Modulators

Setting up a DMD: Diffraction effects

I recently acquired a Digital Micromirror Device (DMD) and when I started setting up the experiment, I faced a problem I did not anticipate which is closely related to blazed gratings. Due to the fact that the surface of a DMD is not flat, diffraction orders are shifted compared to the optical axis. This shift depends on the pixel pitch, the wavelength, and the incident angle. A close look at this diffraction phenomenon is important to configure an experimental setup properly. It is even relevant to consider this effect before choosing the appropriate DMD model to buy.

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Created by sebastien.popoff on 27/10/2014

Tutorials Multimode fibers

Modes of step-index multimode fibers

 

Scattering media were the first type of "complex media" for which wavefront shaping techniques were applied. Quickly, applications were developed for multimode fibers as well. One can consider multimode fiber as a complex media; because of its inherent modal dispersion (different modes travel at different speeds) and also because of the possible coupling between modes, the output field of the fiber does not resemble its input one. Wavefront shaping in multimode fibers has had a fast development because of its applications in biomedical endoscopic imaging and for telecommunications, where the exploitation of the spatial modes in multimode fibers offers a promising way to increase data rates compared to single-mode fibers. 

I present here quickly the expression of the modes of a step-index multimode fiber and the so-called linearly polarized modes, that are convenient for manipulation using shaping techniques.

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Created by sebastien.popoff on 18/11/2013

Talks Wavefront shaping

Coherent control of the total transmission of light through disordered media

Sébastien Popoff

FiO, Orlando, FL, USA,
October 2013

We demonstrate order of magnitude coherent control of total transmission of light through random media by shaping the wave front of the input light. To understand how the finite illumination area on a wide slab affects the maximum values of total transmission, we develop a model based on random matrix theory that reveals the role of long-range correlations. Its predictions are confirmed by numerical simulations and provide physical insight into the experimental results.
Presentation of the article [S.M. Popoff et al., Phys. Rev. Lett. 104, (2014)]

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Created by sebastien.popoff on 28/04/2013

Highlights

From the bimodal distribution to the quarter circle law

[A. Goetschy and A. D. Stone, Phys. Rev. Lett., 1304.5562, (2013)]

Almost thirty years ago, theoreticians predicted that the distribution of the transmission values of a multiple scattering sample should follow a 'bimodal distribution'.  Physically, that means that, in the diffusive regime, there is a large number of strongly reflected channels - the closed channels - and a small number of channels of transmission close to one - the open channels. The existence of these open channels regardless of the thickness of the medium is of big interest for researchers, especially for imaging or communication applications. Nevertheless, such channels have not yet been directly observed. A investigation on those channels requires a measurement of the entire transmission matrix of a lossless scattering medium. For practical reasons (open geometry, limited numerical aperture, noise...) one usually has access to a subpart of the total transmission matrix. In recent experimental measures of the transmission matrix in optics [S.M. Popoff et al., Phys. Rev. Lett., 104, 100601, (2010)] the distribution of the transmission values follows a 'quarter circle law', characteristic of totally uncorrelated systems. This means that the fraction of the transmission matrix measured shows no effect of the correlations at the origin of the bimodal distribution due to the loss of information. In this paper, A. Goetschy and D. Stone theoretically study the effect of the loss of information or the imperfect control on the statistics of the transmission matrix of a scattering system.

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