  Laser-threshold magetometry using the negatively charged nitrogen-vacancy
(NV-) centre in diamond as a gain medium has been proposed as a technique to
dramatically enhance the sensitivity of room-temperature magnetometry. We
experimentally explore a diamond-loaded open tunable fibre-cavity system as a
potential contender for the realization of lasing with NV- centres. We observe
amplification of the transmission of a cavity-resonant seed laser at 721 nm
when the cavity is pumped at 532 nm, and attribute this to stimulated emission.
Changes in the intensity of spontaneously emitted photons accompany the
amplification, and a qualitative model including stimulated emission and
ionisation dynamics of the NV- centre captures the dynamics in the experiment
very well. These results highlight important considerations in the realization
of an NV- laser in diamond.
