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Multimode fiber based endoscope using wavefront shaping with a digital micromirror device (DMD)

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Optical microscopy is usually limited to shallow depth (a few hundred microns) in living specimen due to light scattering in biological tissues, that leads to loss of resolution and contrast at increasing depth. Hence, endoscopy appears as an alternative to attain high resolution imaging at depth. However, this technique is intrinsically invasive, as inserting endoscopes may cause tissue damage. To minimize their invasiveness, endoscope probes based on a single multimode fiber (MMF), which diameter is only 125µm (approximately the thickness of a hair), have been proposed. When light propagates in an MMF, incident light is projected into a large number of components (or modes) that undergo unpredictable phase delays, so that the output pattern is usually speckle-like, and cannot be used for imaging. However, by controlling the wavefront of light at the input of MMF, a focus can be obtained at the MMF output and scanned to construct an image. This technique is named “wavefront shaping”.

We are developing a fluorescence microendoscope based on a single MMF using a digital micromirror device for wavefront shaping. Our approach consists in measuring the relation between the input and output fields, the ‘transmission matrix’, then using this information to adjust the phase of all the input modes in order to obtain a focused spot at the output fiber facet, which is scanned across the field of view. Phase modulators include liquid-crystal spatial light modulators and, more recently, digital micromirror devices (DMDs), which have the advantage of faster switching rates. Although DMDs are binary (on/off) amplitude modulators, they can be used for phase modulation by a holographic approach.

The proposed project will entail finalizing the optical setup and the scripts (in Python) for instrument control and transmission matrix computation. Then the setup will be validated on test samples (fluorescent beads, fixed cells…). We are looking for a student with a background in physics and optics and an interest for instrument development. Knowledge in Python programming would be an asset.

Contact :

Irène Wang – tél. 04 76 51 47 29 – irene.wang@univ-grenoble-alpes.fr
Emmanuel Bossy – tél. 04 76 51 47 81 – emmanuel.bossy@univ-grenoble-alpes.fr