  Dispersion engineering is essential to the performance of most modern optical
systems including fiber-optic devices. Even though the chromatic dispersion of
a meter-scale single-mode fiber used for endoscopic applications is negligible,
optical lenses located on the fiber end face for optical focusing and imaging
suffer from strong chromatic aberration. Here we present the design and
nanoprinting of a 3D achromatic diffractive metalens on the end face of a
single-mode fiber, capable of performing achromatic and
polarization-insensitive focusing across the entire near-infrared
telecommunication wavelength band ranging from 1.25 to 1.65 um. This represents
the whole single-mode domain of commercially used fibers. The unlocked height
degree of freedom in a 3D nanopillar meta-atom largely increases the upper
bound of the time-bandwidth product of an achromatic metalens up to 21.34,
leading to a wide group delay modulation range spanning from -8 to 14 fs.
Furthermore, we demonstrate the use of our compact and flexible achromatic
metafiber for fiber-optic confocal imaging, capable of creating in-focus sharp
images under broadband light illumination. These results may unleash the full
potential of fiber meta-optics for widespread applications including
hyperspectral endoscopic imaging, femtosecond laser-assisted treatment, deep
tissue imaging, wavelength-multiplexing fiber-optic communications, fiber
sensing, and fiber lasers.
