Analysis and evaluation of graded-index fiber lenses

TLDR

The study investigates graded‑index multimode fibers as fiber‑lenses. The authors evaluate coupling efficiency between two single‑mode fibers with fused graded‑index fiber‑lenses, using a ray‑matrix model of the complex beam parameter to predict efficiency versus transverse and axial displacements and compare to measurements. Measured coupling losses were 0.1–1.6 dB, beam waists expanded to 39.5 μm, and fiber separations exceeding 3.0 mm were achievable with low loss, especially using multiple‑pitch (>3.0 mm) VAD lenses.

Abstract

Graded-index multimode fibers are investigated for use as fiber-lenses. The performance of the fiber-lenses is based on the coupling efficiency between two single-mode fibers with fiber-lenses fused to their end faces. Coupling losses ranged from 0.1 to 1.6 dB for the various fiber-lenses. Expanded beam waists up to 39.5 μm were measured and a fiber separation of > 3.0 mm was possible while retaining the excess loss within the above range. Low loss coupling was observed with multiple-pitch ( > 3.0 mm long) VAD lenses. A theoretical analysis considers a ray matrix transformation of the complex beam parameter and predicts the coupling efficiency as a function of the relative transverse and axial displacements of the two fibers. A comparison to the corresponding measurements is made.