  We present the results of a systematical analysis of the intrinsic optical
afterglow light curves for a complete sample of gamma-ray bursts (GRBs)
observed in the period from Feb. 1997 to Aug. 2005. These light curves are
generally well-sampled, with at least four detections in the $R$ band. The
redshifts of all the bursts in the sample are available. We derive the
intrinsic $R$ band afterglow lightcurves (luminosity versus time within the
cosmic proper rest frame) for these GRBs, and discover a fact that they
essentially follow two universal tracks after 2 hours since the GRB triggers.
The optical luminosities at 1 day show a clear bimodal distribution, peaking at
1.4*10^{46} ergs~s^{-1} for the luminous group and 5.3*10^{44} ergs~s^{-1} for
the dim group. About 75% of the GRBs are in the luminous group, and the other
25% belong to the dim group. While the luminous group has a wide range of
redshift distribution, the bursts in the dim group all appear at a redshift
lower than 1.1.

Astrophysics