  Forty years have passed since the discovery of pulsars, yet the physical
mechanism of their coherent radio emission is a mystery. Recent observational
and theoretical studies strongly suggest that the radiation outcoming from the
pulsar magnetosphere consists mainly of extraordinary waves polarized
perpendicular to the planes of pulsar dipolar magnetic field. However, the
fundamental question whether these waves are excited by maser or coherent
curvature radiation, remains open. High quality single pulse polarimetry is
required to distinguish between these two possible mechanisms. Here we showcase
such {\it decisive} strong single pulses from 10 pulsars observed with the
GMRT, showing extremely high linear polarization with the position angle
following locally the mean position angle traverse. These pulses, which are
relatively free from depolarization, must consist of exclusively single
polarization mode. We associate this mode with the extraordinary wave excited
by the coherent curvature radiation. This crucial observational signature
enables us to argue, for the first time, in favor of the coherent curvature
emission mechanism, excluding the maser mechanism.
