  We consider a simple model for a black-hole laser: a Bose-Einstein condensate
with uniform speed of sound and partially uniform flow, establishing two
horizons, a black-hole and a white-hole horizon. Waves confined between the
horizons are amplified similar to radiation in a laser cavity. Black-hole
lasing appears as an instability with discrete sets of modes given
approximately by a round-trip condition. We found that, in addition to the
regular Hawking radiation, trans-Planckian radiation does tunnel out of the
black-hole laser.
