  The BICEP3 Polarimeter is a small aperture, refracting telescope, dedicated
to the observation of the Cosmic Microwave Background (CMB) at 95GHz. It is
designed to target degree angular scale polarization patterns, in particular
the very-much-sought-after primordial B-mode signal, which is a unique
signature of cosmic inflation. The polarized signal from the sky is
reconstructed by differencing co-localized, orthogonally polarized
superconducting Transition Edge Sensor (TES) bolometers. In this work, we
present absolute measurements of the polarization response of the detectors for
more than $\sim 800$ functioning detector pairs of the BICEP3 experiment, out
of a total of $\sim 1000$. We use a specifically designed Rotating Polarized
Source (RPS) to measure the polarization response at multiple source and
telescope boresight rotation angles, to fully map the response over 360
degrees. We present here polarization properties extracted from on-site
calibration data taken in January 2022. A similar calibration campaign was
performed in 2018, but we found that our constraint was dominated by
systematics on the level of $\sim0.5^\circ$. After a number of improvements to
the calibration set-up, we are now able to report a significantly lower level
of systematic contamination. In the future, such precise measurements will be
used to constrain physics beyond the standard cosmological model, namely cosmic
birefringence.
