  Emission at 4.6 um was observed from an N2O filled hollow core fiber laser.
8-ns pump pulses at 1.517 um excited a vibrational overtone resulting in lasing
on an R and P branch fundamental transition from the upper pump state. At
optimum gas pressure of 80 Torr a photon conversion efficiency of 9% and a
slope efficiency of 3% was observed from a mirrorless laser. The laser
threshold occurred at an absorbed pump energy of 150 nJ in a 45-cm long fiber
with 85 {\mu}m core diameter. The observed dependence of the laser output on
gas pressure is shown to be a result of line broadening and relaxation rates.
