  For the first time, lasing at NV$^{-}$ centers in an optically pumped diamond
sample is achieved. A nanosecond train of 150-ps 532-nm laser pulses was used
to pump the sample. The lasing pulses have central wavelength at 720 nm with a
spectrum width of 20 nm, 1-ns duration and total energy around 10 nJ. In a
pump-probe scheme, we investigate lasing conditions and gain saturation due to
NV$^{-}$ ionization and NV$^{0}$ concentration growth under high-power laser
pulse pumping of diamond crystal.
