Gradient-index optical fiber lens for efficient fiber-to-chip coupling

Area of Science:

• Photonics and Optical Engineering
• Materials Science

Background:

• Miniaturized optical components are crucial for advanced photonic integrated circuits.
• Gradient-index (GRIN) lenses offer unique light-manipulating properties.
• Fabricating GRIN lenses on optical fiber tips presents significant challenges.

Purpose of the Study:

• To propose and fabricate a novel gradient-index optical fiber lens.
• To demonstrate its light-focusing capabilities and coupling efficiency.
• To enable efficient light coupling into silicon-on-insulator (SOI) chips.

Main Methods:

• Focused ion beam (FIB) milling for precise fabrication on a single-mode fiber tip.
• Second-order effective medium theory for designing the refractive index profile.
• Three-dimensional finite-difference time-domain (FDTD) simulations for optical analysis.
• Confocal optical measurements for experimental validation.

Main Results:

• Fabricated gradient-index fiber lenses with a parabolic effective refractive index distribution.
• Achieved focusing of a 10.4 μm mode field diameter into spot sizes of 3-5 μm.
• Demonstrated light coupling into SOI chips with efficiency comparable to commercial lensed fibers.
• Localization of focused spots 4-6 μm from the fiber tip.

Conclusions:

• The FIB-milled gradient-index fiber lens is a viable and effective solution for optical coupling.
• The design and fabrication method enable precise control over light focusing at the fiber tip.
• This technology holds promise for enhancing performance in photonic integrated circuits and optical communication systems.