Coupling efficiency of ultra-small gradient-index fiber probe

In this paper, the coupling efficiency of ultra-small GRIN fiber probe is studied for its focusing performance. Based on the light transmission characteristics of a Gaussian beam and the principle of optical imaging, with the analytical methods, the theoretical formula is deduced for the coupling efficiency of ultra-small GRIN fiber probe. Experiments were set-up and conducted for verification. Per the experimental results, for an ultra-small GRIN fiber probe with the focal length of 0.4 mm, the coupling efficiency measured at the focusing position was 57%, and above 44% within the 0-0.6 mm range. However, for a single-mode fiber, the coupling efficiency dropped to 17% when the distance increased to 0.2 mm. Thus, the ultra-small GRIN fiber probe boasts a superior focusing performance and coupling efficiency. This paper provides a theoretical basis for the application and research on ultra-small GRIN fiber probe.