Double-clad fiber with a tapered end for confocal endomicroscopy

Abstract

We present a double-clad fiber coupler (DCFC) for use in confocal endomicroscopy to reduce speckle contrast, increase signal collection while preserving optical sectioning. The DCFC is made by incorporating a double-clad tapered fiber (DCTF) to a fused-tapered DCFC for achromatic transmission (from 1265 nm to 1325 nm) of > 95% illumination light trough the single mode (SM) core and collection of > 40% diffuse light through inner cladding modes. Its potential for confocal endomicroscopy is demonstrated in a spectrally-encoded imaging setup which shows a 3 times reduction in speckle contrast as well as 5.5 × increase in signal collection compared to imaging with a SM fiber.

Keywords: (030.6140) Speckle; (060.2340) Fiber optics components; (110.2990) Image formation theory; (170.1790) Confocal microscopy; (170.2150) Endoscopic imaging.

Fig. 1

Confocal imaging with a DCF. (a) Light propagation schematic. Illumination light originates from the core of the DCF defined by its MFD (taken at the 1/e² intensity point) and numerical aperture (illumination half-angle β). It is collimated with a lens of focal length f_coll and fills an objective lens (of focal length f_obj, aperture 2a_obj, illumination cone half-angle α). Light collection is performed through the inner cladding of the DCF (of diameter d). Light propagates along the z-axis and r_f, r_p and r_s define the radial coordinates at the fiber, objective lens pupil and sample planes, respectively. (b) Optical sectioning (u_1/2) for a perfect plane reflector as a function of the ratio d/MFD for different values of the pupil filling factor A. c) Excitation efficiency (η) for a perfect plane reflector at focus (u = 0) as a function of the ratio d/MFD for different values of A.

Fig. 2

Double-clad tapered fiber (DCTF). (a) Schematic of the DCFC with a tapered end, the gray area representing a high index gel drop to diminish back reflections. (b) Schematic of the tapered end. (c) MFD (solid red line) and inner cladding diameter d (dotted black line) as a function of the taper ratio.

Fig. 3

Characterization of the down tapered DCFC. (a) Spectral response of the MM inner cladding transmission. (b) Spectral response of the SM core transmission. (c) Cross section of the original DCF. (d) Cross section of the DCTF. (e) Side view of the angle cleaved DCTF. Scale bar: 50 μm. Error bars represent 1 standard deviation.

Fig. 4

Spectrally encoded confocal microscopy setup used to demonstrate imaging with the tapered-fiber DCFC. SE is achieved using a polygon-based wavelength-swept laser fiber coupled to an acquisition triggering mechanism and to an imaging arm through a circulator (C) spliced to the tapered-fiber DCFC. The imaging arm consists of a collimating lens, a galvanometer mounted mirror (G), two telecentric telescopes, a transmission grating (GR) and an objective lens. InGaAs photo-detectors (PD) collect coherent and partially coherent light. PC : polarization controllers.

Fig. 5

Simultaneously acquired images of a 7 day old mouse embryo fixed in a solution of 4% of paraformaldehyde. (a) Average of 20 images of the SM signal (scale bar of 50 μm). (b) Average of 20 images of the MM signal (scale bar of 50 μm). (c) Zoom of the SM image. (d) Zoom of the MM image. Intensity scale of the SM and MM images were normalized independently.