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Nonlinear optical endoscopy with micro-structured photonic crystal fibers

Alberto Lombardini 1
1 MOSAIC - MOSAIC
FRESNEL - Institut FRESNEL
Abstract : Nonlinear optical microscopy (NLOM) is a very powerful tool for live tissue imaging. It does not require staining of the samples and offers high spatial resolution and molecular specificity. Thanks to the combination of different nonlinear contrast mechanisms, such as two-photon fluorescence (TPEF), second harmonic generation (SHG) and coherent anti Stokes Raman scattering (CARS), NLOM is now emerging as a serious candidate for future in-vivo label-free optical histology. However, the translation of nonlinear imaging into clinics depends very much on the development of miniature and portable endoscope probes. One main issue is represented by the transmission of the excitation ultra-short pulses with optical fibers. Pulse propagation in classical silica fibers is affected by a number of problems, such as temporal and spectral broadening, or generation of a strong background at the CARS wavelength. In this thesis, we propose the use of a novel type of photonic crystal fiber, the Kagomé lattice hollow core fiber, for the delivery of ultra-short pulses in nonlinear endoscopy. These fibers allow undistorted pulse delivery, over a broad transmission window, with minimum background signal generated in the fiber, thanks to the propagation in a hollow-core. We solved the problem of spatial resolution by means of a silica micro-bead inserted in the Kagomé fiber large core. We have developed a miniature imaging system, based on a piezo-electric tube scanner, an achromatic micro-lenses assembly and a custom design Kagomé double-clad fiber. With this system we were able to image biological tissues, in endoscope modality, activating different contrasts such as TPEF, SHG and CARS, a result which finds no equal in current literature. The integration in a portable probe (4.2 mm in diameter) shows the potential of this system for future in-vivo multimodal endoscopy.
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Alberto Lombardini. Nonlinear optical endoscopy with micro-structured photonic crystal fibers. Optics [physics.optics]. Aix-Marseille Université, 2016. English. ⟨tel-01577793v2⟩

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