  Controlling and reversing the effects of loss are major challenges in optical
systems. For lasers losses need to be overcome by a sufficient amount of gain
to reach the lasing threshold. We show how to turn losses into gain by steering
the parameters of a system to the vicinity of an exceptional point (EP), which
occurs when the eigenvalues and the corresponding eigenstates of a system
coalesce. In our system of coupled microresonators, EPs are manifested as the
loss-induced suppression and revival of lasing. Below a critical value, adding
loss annihilates an existing Raman laser. Beyond this critical threshold,
lasing recovers despite the increasing loss, in stark contrast to what would be
expected from conventional laser theory. Our results exemplify the
counterintuitive features of EPs and present an innovative method for reversing
the effect of loss.
