  We study telecom-wavelength spectra of a Rydberg state in an atomic vapor
with a three-photon excitation scheme. Two lasers of 780 nm and 776 nm are used
to pump Rubidium-85 atoms in a vapor cell to the $5D_{\mathrm{5/2}}$ state,
from which a probe beam of 1292 nm in the O-band telecommunication wavelength
drives a transition to the $21F_{\mathrm{7/2}}$ Rydberg state. We investigate
the probe spectra over the power of pump lasers. The simulation based on a
4-level theoretical model captures the main features of the experimental
results. This spectroscopic study paves the way for future experiments of
making a direct link between fiber optics and radio transmission via Rydberg
atoms.
