  Recently, 12442 system of Fe-based superconductors has attracted considerable
attention owing to its unique double-FeAs-layer structure. A steep increase in
the in-plane upper critical field with cooling has been observed near the
superconducting transition temperature, $T_c$, in KCa$_2$Fe$_4$As$_4$F$_2$
single crystals. Herein, we report a high-field investigation on upper critical
field of this material over a wide temperature range, and both out-of-plane
($H\|c$, $H_{c2}^{c}$) and in-plane ($H\|ab$, $H_{c2}^{ab}$) directions have
been measured. A sublinear temperature-dependent behavior is observed for the
out-of-plane $H_{c2}^{c}$, whereas strong convex curvature with cooling is
observed for the in-plane $H_{c2}^{ab}$. Such behaviors could not be described
by the conventional Werthamer--Helfand--Hohenberg (WHH) model. The data
analysis based on the WHH model by considering the spin aspects reveals a large
Maki parameter $\alpha=9$, indicating that the in-plane upper critical field is
affected by a very strong Pauli paramagnetic effect.
