# Lensed Chemically Etched Optical Fibers for Free-Space Coupling of Multicore Fibers

## TL;DR

This paper presents a new design for lensed optical fibers that efficiently couple with multicore fibers, showing low losses and high tolerance.

## Contribution

The novel design of lensed, chemically etched optical fibers enables efficient and robust free-space coupling with multicore fibers.

## Key findings

- LEOFs achieved average coupling losses of 0.76 dB under direct contact and 0.40 dB at optimal distance.
- FIFO multicore couplers showed average insertion losses of 3.23–3.30 dB in TX and 3.20–3.27 dB in RX at 1550 nm.
- Core-resolved losses as low as 1.09 dB were observed for well-aligned channels.

## Abstract

The present study introduces a novel design for lensed, chemically etched optical fibers (LEOFs) designed for efficient coupling with multicore fibers (MCFs). Experimental characterization at a wavelength of 1550 nm yielded an average coupling loss of approximately 0.76 dB under direct physical contact and 0.40 dB when the fiber end was positioned at an optimal working distance. Moreover, it was experimentally demonstrated that LEOFs exhibit high tolerance to longitudinal displacement and minimal wavelength-dependent variation in coupling efficiency. Based on this approach, fiber-in–fiber-out (FIFO) multicore couplers were fabricated using bundles of LEOFs that had been aligned to MCF cores. Bidirectional measurements yielded average insertion losses of 3.23–3.30 dB in TX and 3.20–3.27 dB in RX transmission directions at 1550 nm, with core-resolved losses as low as 1.09 dB for well-aligned channels. The results confirm the viability of LEOF-based multicore free-space coupling, with further improvements expected from enhanced fabrication accuracy.