Analysis and evaluation of graded-index fiber-lenses

Graded-index multimode fibers fabricated by both the MCVD and the VAD process 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 efficiencies ranged from 0.1 dB to 1.6 dB for the various fiber lenses. Expanded beam waists up to 39.5micron 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 displacement of the two fibers. A comparison to the corresponding measurements is made. Finally several possible applications of fiber lenses are discussed. These applications include: expanded beam single mode connectors, laser redundancy device (with polarization maintaining fiber), laser/fiber coupling, fiber optic rotary joints, silicon chip coupling (e.g., the mechanical relay), lens arrays, and fiber bundles.