  Stimulated emission of radiation has not been directly observed in
astrophysical situations up to this time. Here we demonstrate that photons from
pulsar B1641-45 stimulate pulses of excess 1720 MHz line emission in an
interstellar OH cloud. As this stimulated emission is driven by the pulsar, it
varies on a few millisecond timescale, orders of magnitude shorter than the
quickest OH maser variations previously detected.
  Our 1612 MHz spectra are inverted copies of the 1720 MHz spectra. This
"conjugate line" phenomenon enables us to constrain the properties of the
interstellar OH line-producing gas.
  We also show that pulsar signals suffer significantly deeper OH absorption
than do other background sources; confirming earlier tentative findings that OH
clouds are clumpier on small scales than neutral hydrogen clouds.
