Created by sebastien.popoff on 09/12/2015

Tutorials Spatial Light Modulators

How to control a liquid crystal SLM with Python

Most liquid crystal Spatial Light Modulators (SLMs) and some digital micromirror devices (DMDs) are controlled via an analog (VGA) or digital (HDMI/DVI) monitor standard communication protocol. In other words, you plug it to your computer and it is recognized as a monitor display. There is usually no useful tool or API provided with the device to dynamically control the SLM. I previously introduced a way to control an SLM using Matlab/Octave, now that I switched to Python, I present here a way to do this using Python.

DOI

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Created by sebastien.popoff on 24/10/2014

Tutorials Spatial Light Modulators

How to use a binary amplitude Deformable Miror Device (DMD) as a phase modulator: The "superpixel" method

 

I previously presented a technique based on the Lee hologram that allows to use a binary amplitude modulator (like a DLP chip you find in standard projectors) to perform a phase modulation (or amplitude and phase modulation). Recently, a new technique was introduced in [S.A. Goorden et al., Opt. Express (2014)] that allows an accurate complex modulation with less loss in term of spatial resolution. This post is more a highlight on this paper than a proper tutorial. In a nutshell, while the Lee hologram only takes advantage of one dimension to encode the amplitude and phase in fringes, this technique exploits both dimensions of the pixel array using superpixels.

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Created by sebastien.popoff on 20/06/2013

Tutorials Spatial Light Modulators

How to use a binary amplitude Deformable Mirror Device (DMD) as a phase modulator: The Lee hologram method

 

 

The optical field measured at the output of a complex medium (multimode waveguide, multimode cavity, scattering medium...) is the result of the interference of the many paths taken by the light. Most of the applications of wavefront shaping techniques in these media rely on taking advantage of these interference effects to force the medium to perform a desired function. For this reason, the phases of the controlled segments of the input wavefront are usually the most important degrees of freedom to control. Common phase-only Spatial Light Modulators (SLMs) have a limited refresh rate (~100 Hz) due to the liquid crystal technology. This limits the applications in media with a low decorrelation time (like biological tissues) or for experiments for which a long optimization process is needed. Although a new class of phase-only SLMs based on deformable mirrors allows high-speed modulations (>30 kHz), these devices are currently very expensive compared to liquid crystal technology and have a limited resolution (~1000 pixels maximum). On the other hand, fast binary-amplitude modulators with high resolutions are widespread and affordable since the technology is the same one as used in commercial digital projectors. In a recent publication, [D.B. Conkey et al., Opt. Express (2012)] introduced a method to use such a binary-amplitude Deformable Mirror Device (DMD) for phase modulation. This technique allows a fast phase modulation (up to 23 kHz) at the cost of a loss of resolution.

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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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