Highly Nonlinear Multimode Tellurite Fibers: From Glass Synthesis to Practical Applications in Multiphoton Imaging
Résumé
Herein, a tellurite multimode optical fiber with a rectangular step-index core to combine easy coupling of the pump power with the exceptionally cubic nonlinearity of such type of glass is developed. An alkali-free composition (TeO$_2$–ZnO–La$_2$O$_3$) is prepared to produce a preform with the use of both the stack-and-draw and the direct-bonding methods. The linear and nonlinear optical responses of the fiber are characterized and are then used to generate a homogeneous supercontinuum (SC) in the near-infrared domain. The spectral broadening, which is obtained in this work in a short fiber sample, is used for imaging mouse kidney cells, labeled with three different fluorochromes, by means of two-photon absorption (2PA) and three-photon absorption (3PA). Despite the multimode nature of the output beam, clear images of tubules, actin, and nucleus are collected with a spatial resolution down to 1.2 μm. Although image acquisition at better resolution has already been reported in the literature using relatively long spans of graded-index silica multimode fibers, here, the combination of high third-order susceptibility of the glass and the large Raman gain in short segments of tellurite fiber is exploited to obtain nonlinear imaging with adequate spatial resolution
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