  When a soliton propagates in a discrete lattice it excites small-amplitude
linear waves in its wake. In a dc current-biased Josephson-junction (JJ) array
these manifest as electromagnetic (EM) waves excited by a (magnetic field
induced) chain of propagating magnetic vortices. When the vortex velocity and
the phase velocity of one of the excited EM waves match, phase-locking occurs.
This produces resonant steps in the current-voltage characteristics where
amplification of EM radiation occurs. We report the first observation of
phase-locking-induced amplification of EM radiation at 77K and above in JJ
arrays made of high temperature superconductors.
