  In this paper we report the long-term optical observation of the faint soft
X-ray transient SAX J1810.8-2609 from OGLE and MOA. We have focused on the 2007
outburst, and also did the cross-correlate between its optical light curves and
the quasi-simultaneous X-ray observations from swift. Both the optical and
X-ray light curves of 2007 outburst show multi-peak features.
Quasi-simultaneous optical/X-ray luminosity shows that both the X-ray
reprocessing and viscously thermal emission can explain the observed optical
flux. There is a slightly X-ray delay of 0.6+-0.3 days during the first peak,
while the X-ray emission lags the optical emission by ~2 days during the
rebrightening stage, which suggests that X-ray reprocessing emission
contributes significantly to the optical flux in the first peak, but the
viscously-heated-disk origin dominates the optical flux during rebrightening.
It implies variation of the physical environment of the outer disk, even the
source stayed in low-hard state during the whole outburst. The ~2 day X-ray lag
indicates a small accretion disk of the system, and the optical counterpart was
not detected by OGLE and MOA during quiescence, which constrained it to be
fainter than MI = 7.5 mag. There is a suspected short-time optical flare
detected at MJD = 52583.5 without X-ray counterpart detected, this single flux
increase may imply a magnetic loop reconnection in the outer disk as proposed
by Zurita et al (2003). The observations cover all stages of the outburst,
however, due to the low sensitivity of RXTE/ASM, we cannot conclude whether it
is an optical precursor at the initial rise of the outburst.
