High-coupling tapered hyperbolic fiber microlens and taper asymmetry effect

This paper proposes a new scheme of a tapered hyperbolic-end fiber (THEF) microlens for efficient coupling between laser diode and single-mode fiber. The THEFs were fabricated by symmetrically tapering the fiber during the etching process and hyperbolically lensing the tip during the fusing process. The hyperbolic microlenses of the THEFs have demonstrated up to 82% coupling efficiency for a laser with an aspect ratio of 1:1.5. The influence of tapering asymmetry on the coupling has also been investigated experimentally and theoretically. The axially symmetrical tapered microlenses of the THEFs showed far-field profiles well approximated by a Gaussian profile, while the asymmetric taper deviated significantly from a Gaussian profile. The coupling efficiency for both symmetrical and asymmetrical microlenses were calculated based on the Fresnel diffraction theory and the results were in good agreement with the measurements. This study makes it possible to fabricate the hyperbolic microlenses using unique etching and fusion techniques that result in a more than 2 dB improvement in coupling efficiency when compared to the currently available hemispherical microlenses.