  Monitoring and stabilizing the power ratio of laser pairs is significant to
high-precision atom interferometers, especially as the compact electro-optic
modulated all-fiber laser system prevails. In this Letter, we demonstrate a
novel method to in-situ probe the relative power of laser pairs and to
stabilize the power ratio of two Raman lasers using a high-dispersion virtually
imaged phased array (VIPA) etalon. Sub-microsecond resolution on probing laser
power transformation during atom interferometer sequence is achieved and the
power ratio of two Raman lasers (PRTR) is tightly locked with high bandwidth
despite of environmental disturbances, showing an Allan deviation of
$4.39\times 10^{-5}$ at 1000 s averaging time. This method provides a novel way
to stabilize the PRTR and diagnose the multi-frequency laser systems for atom
interferometers and could find potential application in broad quantum sensing
scenarios.
