  We obtained and analyzed more than 100 hours of multicolour optical time
series of the blazar CTA 102 during its 2012 and 2016 outbursts. The object
reached almost 11-th mag at the end of 2016, which is perhaps the brightest
blazar state ever observed! During both outbursts, CTA 102 showed significant
and rapid variability on intra-night time scales, reaching up to 0.2 mag for 30
min on some occasions. The "rms-flux" relation, built for all datasets, shows a
large scatter and no apparent saturation on the magnitude scale. The ensemble
structure function of the light curves can be fitted well with a straight line
of a slope of $\sim$0.4. The time lags between the different optical bands
appear to be consistent with zero, taking into account our time resolution. We
discuss different variability scenarios and favor the changing Doppler factor
of the emitting blobs as the most plausible one to account for the observed
intra-night variability.
