  This paper is concerned with the estimation of the volatility process in a
stochastic volatility model of the following form: $dX_t=a_tdt+\sigma_tdW_t$,
where $X$ denotes the log-price and $\sigma$ is a c\`adl\`ag semi-martingale.
In the spirit of a series of recent works on the estimation of the cumulated
volatility, we here focus on the instantaneous volatility for which we study
estimators built as finite differences of the \textit{power variations} of the
log-price. We provide central limit theorems with an optimal rate depending on
the local behavior of $\sigma$. In particular, these theorems yield some
confidence intervals for $\sigma_t$.
