High-performance lensless in-line filters

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Abstract
Lensless in-line bandpass filters with low insertion losses and high backreflection losses are achieved with expanded-core fibers. It has been experimentally shown that using a large modal-field diameter of expanded-core fibers can resolve the problem of the trade-off between the insertion loss and backreflection. The reduction in the outer diameter, usually occurring when the modal-field diameter is enlarged to an extent, was suppressed at an optimized heating condition.