Specially designed liquid-core optical fiber can convey light with demand wavelength and analytes in the same pathway according to the application of the fiber. Water is one of the most useful and common core fluid due to its non- toxicity and other practical optical properties, and which makes water core optical fiber advantageous for biomedical sensing, optical imaging, non-linear optics and optical transmission. Since the numerical aperture of the liquid core fiber is not able to approach to an infinite small value, thus it is necessary to use expensive solid state lens to couple light into or out of the liquid core fiber. In this paper, we have demonstrated a unique method to fabricate water-core lensed fibers by filling light water and heavy water respectively into hollow Teflon-Amorphous Fluoroplastics fibers, and to control the focal length and the spot size by pumping water into or out of the fiber end. By simulation, the focal length over the range of f = 3.87-1.33 mm has been demonstrated using the distilled water, and f = 4.95-1.38 mm using the heavy water. To further reduce the focal length, we have limited the lens aperture by the fiber core and have demonstrated a focal length over f = 0.34-0.27 mm in response to the change of the heavy water lens volume from 0.68 to 1.54 nL. Further simulation shows that the focused spot size can be reduced to 2-6 μm by adjusting the refractive index and fiber geometry. Compared to other optical focusing methods, such lensed fiber and the tuning of its focal length are far easier to make at a much lower cost.
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