Launch Light Design for Coupling Loss Measurement of Step-Index Multimode Fiber Connections

Measurement of coupling loss using step-index multimode fiber connections that are used in multi-gigabit optical communication systems must be accurate and precise to meet rigorous system specifications. We propose a launch light design for performing coupling loss measurement with reproducibility for step-index plastic cladding silica fibers. In the proposed design, the modal power distribution of the launch light is represented by the encircled angular flux (EAF). Specifically, the lower-order mode and the higher-order mode sides of the EAF boundary conditions were systematically calculated. The EAF boundary conditions were designed using our developed flowchart, which is composed of an optical model design, a performance review, and a model redesign based on the equilibrium mode distribution obtained from round-robin tests. The validity of the EAF boundary conditions was confirmed by optical design computing simulation and experiment. Under practical conditions wherein the misalignment range of a fiber connection is 40 μm or less in the lateral direction offset and 100 μm or less in the axial direction offset, the EAF boundary conditions of the launch light were determined such that the connection loss was 2.0 dB or less and the variation in loss due to modal power distribution variations was ±0.20 dB or less. The use of the launch light that meets the determined EAF boundary conditions allows for sufficient measurement reproducibility in verifying that the connection losses meet the system requirements. The alignment tolerance thus achieved is suitable for practical purposes.