  In order to better understand the underlying fundamental physical processes
in nitrogen-vacancy centers in diamond, and to realize improvements in the use
of this system for technological applications, it is imperative to gain new
insight into the origins of the apparent inhomogeneous broadening. In this work
we make use of a novel type of spectroscopy developed specifically for this
task. The pump-probe spectroscopy closely follows Doppler-free spectroscopy
used in atomic vapor. We show that the origin of inhomogeneous broadening comes
from local magnetic field variations in the diamond lattice.
