Scanning fiber-optic nonlinear endomicroscopy with miniature aspherical compound lens and multimode fiber collector

Abstract

A flexible scanning fiber-optic endomicroscope using a miniature compound lens and a multimode-fiber (MMF) collector was developed for two-photon fluorescence (TPF) and second-harmonic generation (SHG) imaging. The compound lens consisted of a pair of aspherical lenses and exhibited reduced chromatic aberration compared with gradient-index lenses, thus increasing the TPF/SHG collection efficiency. The introduction of a short MMF collector at the distal end of the double-clad fiber of the endomicroscope further mitigated the adverse influence of chromatic aberration of the lens and enhanced the TPF/SHG collection efficiency. Both ray-tracing simulations and experiments on TPF imaging of fluorescent beads and SHG imaging of rattail tendon demonstrated approximately nine (approximately four) times improved collection efficiency for TPF (SHG) with the new endomicroscope design that utilized a compound lens and an MMF collector.

Fig. 1

(Color online) (a) Schematic of the fiber-optic scanning endomicroscope with a miniature aspherical compound lens and a multimode-fiber collector. (b) Photo of the distal end optics. (c) Photo of the assembled distal end of the endomicroscope with all the optical components and a piezoelectric transtecer (PZT) resonant fiber scanner encased in a hypodermic tube.

Fig. 2

(Color online) (a) Representative optical ray plots for excitation light and nonlinear optical signals when using the GRIN lens and the aspherical compound lens. (b) Chromatic longitudinal focal shift from the DCF tip for the nonlinear optical signals with 810 nm excitation when using the GRIN lens and aspherical compound lens. (c) Calculated collection efficiency of TPF/SHG signals when using the GRIN lens, the compound lens, and the compound lens plus the MMF collector.

Fig. 3

(Color online) Typical TPF images of a monolayer of 4.86 µm fluorescein beads [(a), (b)] and SHG images of formalin-fixed rattail tendon [(c), (d)] acquired by a scanning all-fiber-optic endomicroscope with the GRIN lens–DCF configuration [(a), (c)] and the compound lens–DCF plus the MMF collector [(b), (d)]. The averaged fluorescence intensity of the beads in (b) is about nine times as that in (a). The averaged SHG intensity of the fiber bundles indicated by the arrows or ellipses in (d) is about four times as that in (c). All the images were acquired under the same experimental conditions, and the images were shown without renormalization.