Created by sebastien.popoff on 21/09/2018

Tutorials Spatial Light Modulators

How to calibrate linearly aligned nematic liquid crystal based SLMs

 

I previously posted a tutorial presenting a technique to calibrate spatial light modulators (SLMs). The approach was based on measuring the interference between two paths that have been reflected off two different regions of the SLM. This technique is always valid but requires aligning a mask, using a lens, and capturing and processing images of interference patterns. Nowadays, most phase-only SLMs based on liquid crystals use linearly aligned nematic crystals. Unlike twisted nematic liquid crystals, they allow phase-only modulation on one polarization while not affecting the orthogonal polarization. This feature can be used to simplify the calibration setup to characterize the SLM with a common path interferometer, not requiring a precise alignment [1]. Furthermore, it only needs a photo-detector, compared to a digital camera in the previously presented approach. This is convenient to measure the inevitable phase fluctuations of an SLM, usually around a 100 to 400 Hz frequency. In this document, we briefly describe the principle of the characterization scheme as presented in [2] and show typical results of the calibration and phase fluctuations.

Written by Paul Balondrade and Sébastien M. Popoff

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Created by sebastien.popoff on 26/12/2016

Talks Wavefront shaping

Controlling Light in Complex Media

Sebastien M. Popoff

LOM Master Seminar

Friday, November 26 2016

Abstract: Seminar talk about the control of light in scattering media for focusing and imaging applications.

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Created by sebastien.popoff on 04/05/2013

Tutorials Spatial Light Modulators

How to characterize and calibrate a phase-only SLM

For most applications in complex media, spatial light modulators are used for their ability to control the phase of a laser beam. Whereas deformable mirrors are insensitive to the input polarization, liquid crystal based SLMs need to work with a given input polarization or sometimes a precise combination of input and output polarizations. It is then necessary for LC SLMs to carefully characterize the modulation to find the setup conditions where amplitude variations are minimal and for which the phase range is at least 2π. In any case, for a given wavelength, it is necessary to know the relation between the value given to a pixel on the SLM and the relative phase shift associated. I present here a typical way to characterize the complex modulation of an SLM.

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Created by sebastien.popoff on 24/04/2013

Highlights

A pioneering experiment: Focusing through scattering media using wavefront shaping

[I.M Vellekoop and A.P. Mosk, Opt. Lett., 15, 2309, 2007]

In 2007 I.M. Vellekoop and A.P. Mosk published their work on the first demonstration of focusing light through a highly scattering medium. Most techniques to image or focus through scattering media relied on selecting only the part of the light that has not been scattered - the ballistic light. The ballistic signals decay exponentially with the thickness of the medium, limiting drastically the depth at which light can be focused. The idea developed by the authors is to use the scattered waves, that are randomly mixed, to focus light through the medium. A scattering sample illuminated by a coherent wave gives rise to a so-called speckle pattern, that results from the interference of the scattered waves. Using a spatial light modulator (SLM), the authors are able to control independently the phase of the different parts of the incident beam. Each segment gives an output seemingly random complex field. By testing different values of the phase for each segment, they are able to put in phase all the contributions, giving rise to a very bright focus spot.

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