  Direct imaging instruments have the spatial resolution to resolve exoplanets
from their host star. This enables direct characterization of the exoplanets
atmosphere, but most direct imaging instruments do not have spectrographs with
high enough resolving power for detailed atmospheric characterization. We
investigate the use of a single-mode diffraction-limited integral-field unit
that is compact and easy to integrate into current and future direct imaging
instruments for exoplanet characterization. This achieved by making use of
recent progress in photonic manufacturing to create a single-mode fiber-fed
image reformatter. The fiber-link is created with 3D printed lenses on top of a
single-mode multi-core fiber that feeds an ultrafast laser inscribed photonic
chip that reformats the fiber into a pseudo-slit. We then couple it to a
first-order spectrograph with a triple stacked volume phase holographic grating
for a high efficiency over a large bandwidth. The prototype system has had a
successful first-light observing run at the 4.2 meter William Herschel
Telescope. The measured on-sky resolving power is between 2500 and 3000,
depending on the wavelength. With our observations we show that single-mode
integral-field spectroscopy is a viable option for current and future exoplanet
imaging instruments.
