20 February 2009 Mode field perturbations and numerical aperture broadening due to angular misalignment in multimode fiber coupling
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Large core multimode fibers (MMF) in the range of 100-1000 μm core diameter are commonly used with infrared lasers in medical, bio-photonics and other fiber optics applications. Angular misalignment of the laser source to the multimode fiber can lead to unusual angular patterns at the exit of the fiber. The angular content of the launch beam can result in under-filling, non-uniform angular filling, or over-filling of the MMF modes. Typically, the beam condition optics at the distal end of the device has a limited impact on these angular modes. The result is often loss of output power at the distal end or an uncontrolled angular and/or spatial distribution of light. We have investigated angular misalignment perturbations of various fiber and coupling optics combinations in a medical laser therapy device. We have quantified the resulting far field perturbations, as well as the resulting broadening of the fiber output numerical aperture (NA). Angular misalignment may cause the development of so called "donut modes" with highly inhomogeneous far field mode distribution, as well as a substantial NA broadening effect which may impact therapy. We have shown that in order to avoid these perturbations, tight tolerancing of fiber coupling opto-mechanics as well as a thorough alignment procedure is required.
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Andreas Rose, Andreas Rose, Kyle Johnston, Kyle Johnston, Karl-Friedrich Klein, Karl-Friedrich Klein, Brian Catanzaro, Brian Catanzaro, Luis De Taboada, Luis De Taboada, } "Mode field perturbations and numerical aperture broadening due to angular misalignment in multimode fiber coupling", Proc. SPIE 7173, Optical Fibers and Sensors for Medical Diagnostics and Treatment Applications IX, 71730Y (20 February 2009); doi: 10.1117/12.824212; https://doi.org/10.1117/12.824212

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