  We describe a novel tool for the quantum characterization of optical devices.
The experimental setup involves a stable reference state that undergoes an
unknown quantum transformation and is then revealed by balanced homodyne
detection. Through tomographic analysis on the homodyne data we are able to
characterize the signal and to estimate parameters of the interaction, such as
the loss of an optical component, or the gain of an amplifier. We present
experimental results for coherent signals, with application to the estimation
of losses introduced by simple optical components, and show how these results
can be extended to the characterization of more general optical devices.
