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The Keck Planet Imager and Characterizer (KPIC) is an instrument at the KeckII telescope that enables high-resolution spectroscopy of directly imagedexoplanets and substellar companions. KPIC uses single-mode fibers to couplethe adaptive optics system to Keck's near-infrared spectrometer (NIRSPEC).However, KPIC's sensitivity at small separations is limited by the leakage ofstellar light into the fiber. Speckle nulling uses a deformable mirror todestructively interfere starlight with itself, a technique typically used toreduce stellar signal on a focal-plane imaging detector. We present the firston-sky demonstration of speckle nulling through an optical fiber with KPIC,using NIRSPEC to collect exposures that measure speckle phase forquasi-real-time wavefront control while also serving as science data. We repeatiterations of measurement and correction, each using at least 5 exposures. Weshow a decrease in the on-sky leaked starlight by a factor of 2.6 to 2.8 in thetargeted 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 requiredexposure time to reach a given SNR, relative to conventional KPIC observations.The performance of speckle nulling is limited by instability in the specklephase: when the loop is opened, the null-depth degrades by a factor of 2 on thetimescale of a single phase measurement, which would limit the suppression thatcan be achieved. Future work includes exploring gradient-descent methods, whichmay be faster and thereby able to achieve deeper nulls. In the meantime, thespeckle nulling algorithm demonstrated in this work can be used to decreasestellar leakage and improve the signal-to-noise of science observations.

On-sky speckle nulling through a single-mode fiber with the Keck Planet Imager and Characterizer