  Infrared optical measurement has a wide range of applications in industry and
science, but infrared light detectors suffer from high costs and inferior
performance than visible light detectors. Four-wave mixing (FWM) process allows
detection in the infrared range by detecting correlated visible light. We
experimentally investigate the stimulated FWM process in a hot $^{85}$Rb atomic
vapor cell, in which a weak infrared signal laser at $1530~$nm induces the FWM
process and is amplified and converted into a strong FWM light at $780~$nm, the
latter can be detected more easily. We find the optimized single- and
two-photon detunings by studying the dependence of the frequency of input laser
on the generated FWM light. What's more, the power gain increases rapidly as
the signal intensity decreases, which is consistent with our theoretical
analysis. As a result, the power gain can reach up to 500 at a signal laser
power of $0.1~\mu$W and the number of detected photons increased by a factor of
250. Finally, we experimentally prove that our amplification process can work
in a broad band in the frequency domain by exploring the response rate of our
stimulated FWM process.
