  A new measuring technique dedicated to bubble velocity and size measurements
in complex bubbly flows such as those occurring in bubble columns is proposed.
This sensor combines the phase detection capability of a conical optical fiber,
with velocity measurements from the Doppler signal induced by an interface
approaching the extremity of a single-mode fiber. The analysis of the probe
functioning and of its response in controlled situations, have shown that the
Doppler probe provides the translation velocity of bubbles projected along the
probe axis. A reliable signal processing routine has been developed that
exploits the Doppler signal arising at the gas-to-liquid transition: the
resulting uncertainty on velocity is at most 14\%. Such a Doppler probe
provides statistics on velocity and on size of gas inclusions, as well as local
variables including void fraction, gas volumetric flux, number density and its
flux. That sensor has been successfully exploited in an air-tap water bubble
column 0.4m in diameter for global gas hold-up from 2.5 to 30\%. In the
heterogeneous regime, the transverse profiles of the mean bubble velocity
scaled by the value on the axis happen to be self-similar in the quasi fully
developed region of the column. A fit is proposed for these profiles. In
addition, on the axis, the standard deviation of bubble velocity scaled by the
mean velocity increases with Vsg in the homogeneous regime, and it remains
stable, close to 0.55, in the heterogeneous regime.
