Figure 3.

Measurements of light collection efficiency using 2-photon generated fluorescent point sources. (A) Schematic representation of the two-photon microscope used to measure the collection field of optical fibers in quasi-transparent fluorescent medium. The inset shows a magnification of the fiber facet surroundings. (B) Section y = 0 of the collection field of 0.22/50 μm, 0.39/200 μm, and 0.50/200 μm optical fibers, as indicated, in a 30 μM PBS:fluorescein solution, obtained through the fiber PMT as shown in (A). Isolines at 10%, 20%, 40%, 60%, and 80% of the maximum number of photons are shown (in black, blue, green, yellow, and red, respectively). (C) Analytical calculations of collection efficiency diagrams for the three fibers in (B) immersed in a transparent homogeneous medium (n = 1.335) assuming a gaussian source with lateral FWHM r_{x, z} = 3 μm, axial FWHM r_y = 32 μm. Isolines at 10%, 20%, 40%, 60%, and 80% of the maximum number of photons are shown (in black, blue, green, yellow, and red, respectively). (D) Comparison of normalized experimentally-measured (blue curve) and analytically-calculated (orange curve) axial collection efficiency profiles (x = 0, y = 0) for the same fibers in (B). Normalization is done with respect to the average of the data points within the firsts 80 μm. The horizontal dashed lines represent z₀. The width of the blue curves for the 0.39 NA/200 μm and 0.50 NA/200 μm fibers represents mean ∓ standard deviation over four different fibers.