Analysis and evaluation of graded-index fiber lenses

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.