  Combination of the electromagnetically-induced-transparency (EIT) effect and
Rydberg-state atoms has attracted great attention recently due to its potential
application in the photon-photon interaction or qubit operation. In this work,
we studied the Rydberg-EIT spectra with room-temperature $^{87}$Rb atoms.
Spectroscopic data under various experimental parameters all showed that the
contrast of EIT transparency as a function of the probe intensity is initially
enhanced, reaches a maximum value and then decays gradually. The contrast of
spectral profile at the optimum probe field intensity is enhanced by $2-4$
times as compared with that at weak intensity. Moreover, the signal-to-noise
ratio of the spectrum can potentially be improved by 1 or 2 order of magnitude.
We provided a theoretical model to explain this behavior and clarified its
underlying mechanism. Our work overcomes the obstacle of weak signal in the
Rydberg-EIT spectrum caused by an apparent relaxation rate of the Rydberg
polariton and weak coupling transition strength, and provides the useful
knowledge for the Rydberg-EIT study.
