Abstract
A method for adjusting the working distance and spot size of a fiber probe while suppressing or enhancing the back-coupling to the lead-in fiber is presented. As the optical fiber probe, a lensed optical fiber (LOF) was made by splicing a short piece of coreless silica fiber (CSF) on a single-mode fiber and forming a lens at the end of the CSF. By controlling the length of the CSF and the radius of lens curvature, the optical properties of the LOF were adjusted. The evolution of the beam in the LOF was analyzed by using the Gaussian ABCD matrix method. To confirm the idea experimentally, 17 LOF samples were fabricated and analyzed theoretically and also experimentally. The results show that it is feasible in designing the LOF to be more suitable for specific or dedicated applications. Applications in physical sensing and biomedical imaging fields are expected.
Highlights
Optical fiber is a cost-effective flexible medium to deliver light with low loss
The lensed optical fiber (LOF) is a piece of optical fiber with an integrated lens at one end, so that it can collimate or focus the beam coming through a lead-in fiber
It isdistributions necessary toofadjust the back-coupling efficiency, the working core mode is approximated as having a Gaussian shape, the overlap integral is made with distance, and/or the spot size of the beam at the working distance of a LOF probe, dependingtwo on Gaussian beams having different spot sizes, well as different radii of wavefront curvatures
Summary
Optical fiber is a cost-effective flexible medium to deliver light with low loss. In addition to light transmission, many researchers have investigated its applications in various fields ranging from physical sensing to biomedicine [1,2,3,4,5,6,7]. In the case of a “Michelson-type” system, such as optical coherence tomography (OCT), a separated reference arm is used and the LOF acts only as a focuser for the sample arm probe [2]. The major parameters affecting the back-coupling efficiency of a LOF are the length of the beam expansion region and the radius of the lens curvature These parameters influence the working distance and the beam diameter of the LOF probe. Since the fundamental mode of a single-mode fiber (SMF) has a Gaussian-like intensity profile, we can use the ABCD method modified for the Gaussian beam with quite high accuracy, but without suffering from heavy computing time Based on this Gaussian ABCD method, the fabrication parameters are simulated so that the probe has a different back-coupling efficiency while maintaining the same working distance. Of single-mode fiber (SMF), spot at each interesting half of the mode-field diameter of single-mode fiber (SMF), w: beam spot at each interesting position, n: refractive index of the medium
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