Photoacoustic and fluorescent microscopy through a multimode fibre using optical wavefront shaping
Résumé
Many applications for microendoscopy, such as brain imaging, requires the development of thin probes to minimise damage during insertion in tissue. The minimally-invasive endoscope technique presented in this talk relies on the use of a thin multimode fibre to guide light and perform dual photoacoustic and fluorescence imaging. To that end, optical wavefront shaping is carried by means of a spatial light modulator (SLM), which enables to create and raster-scan a diffraction-limited focused spot at the tip of the multimode fibre. The multimode fibre is first calibrated by measuring the transmission matrix linking the input field to the output field. Then, by inverting such matrix, we can determine which input pattern to apply on the SLM to obtain a focused spot at any given location of the fibre output. The pulsed laser source (5 ns pulse, 532 nm wavelength) focused at the distal tip of the fibre raster scans the sample which absorbs the light and generates either a photoacoustic signal, which is detected by a commercial fibre-optic hydrophone, and/or a fluorescent signal, which retro-propagates along the multimode fibre and is acquired by a photomultiplier tube. The fibre-optic hydrophone is set next to the multimode fibre, making the total imaging tip no larger than 250 �m. Two-dimensional imaging of in vitro red blood cells, which absorb light and produce a photoacoustic signal, and fluorescent beads, which absorb light and emit a fluorescent signal, is demonstrated.