Reversely tailoring antiresonant hollow-core fibers with elliptical-shaped tubes for enhanced in-line bandpass filters.

We report a simple and effective approach to reversely tailoring the tubes of an antiresonant hollow-core fiber (AR-HCF). The AR-HCFs with elliptical-shaped shrinkage tubes, implemented by a traditional flame brushing technique, were characterized for what we believe to be the first time. We have thus achieved a 70 nm wide bandpass filter with a minimum extra loss below 0.1 dB and the out-of-band suppression up to 28 dB in the 800 nm wide spectral range via the combination of down-tapering and reverse-tapering of AR-HCFs. The elliptical-shaped tubes can reduce the deformation-induced confinement loss when tailoring the thickness of HCF. The ability to tailor the shape and thickness of tubes of AR-HCF in a submicron-scale using a traditional flame taper rig will benefit the fabrication of in-fiber HCF devices such as the low-loss HCF bandpass filter with high out-of-band suppression.