Benchmarking the light coupling efficiencies under varied-NA combinations between fibers and lenses

Deploying fiber waveguides to collect light signals is one of the core functionalities behind numerous optical systems. Greater light-coupling efficiency is always being demanded for high-end waveguide-based devices and instrumentation. To provide practical element-selection advice for efficient coupling usages, we benchmark the free-space-to-fiber coupling coefficients under the case of varied numerical aperture (NA) combinations between fibers and lenses. Two common scenarios, i.e., normal incidences and wide-angle excitations, are investigated at a wavelength of 650 nm, 1000 nm, and 1550 nm, respectively. The systemic characterizations reveal that self-made high-NA fiber produces larger coupling efficiency in both schemes. Particularly for the broad incoming angular arrangement, the experimental three-wavelength values of high-NA fiber are all above 3% within an angle interval of 0-65°. The latter empirical model also confirms its remarkable performance, where the calculated lineshapes fit the measurements. All in all, our solid data suggests the NA of the fiber should be given high priority to create a high-coupling optical system.