  The Keck Planet Imager and Characterizer (KPIC) is an instrument at the Keck
II telescope that enables high-resolution spectroscopy of directly imaged
exoplanets and substellar companions. KPIC uses single-mode fibers to couple
the adaptive optics system to Keck's near-infrared spectrometer (NIRSPEC).
However, KPIC's sensitivity at small separations is limited by the leakage of
stellar light into the fiber. Speckle nulling uses a deformable mirror to
destructively interfere starlight with itself, a technique typically used to
reduce stellar signal on a focal-plane imaging detector. We present the first
on-sky demonstration of speckle nulling through an optical fiber with KPIC,
using NIRSPEC to collect exposures that measure speckle phase for
quasi-real-time wavefront control while also serving as science data. We repeat
iterations of measurement and correction, each using at least 5 exposures. We
show a decrease in the on-sky leaked starlight by a factor of 2.6 to 2.8 in the
targeted spectral order, at a spatial separation of 2.0 {\lambda}/D in K-band.
This corresponds to an estimated factor of 2.6 to 2.8 decrease in the required
exposure time to reach a given SNR, relative to conventional KPIC observations.
The performance of speckle nulling is limited by instability in the speckle
phase: when the loop is opened, the null-depth degrades by a factor of 2 on the
timescale of a single phase measurement, which would limit the suppression that
can be achieved. Future work includes exploring gradient-descent methods, which
may be faster and thereby able to achieve deeper nulls. In the meantime, the
speckle nulling algorithm demonstrated in this work can be used to decrease
stellar leakage and improve the signal-to-noise of science observations.
