  We investigate optically detected magnetic resonance signals from an ensemble
of nitrogen-vacancy centers in diamond. The signals are measured for different
light powers and microwave powers, and the contrast and linewidth of the
magnetic-resonance signals are extracted. For a wide range of experimental
settings of the microwave and light powers, the linewidth decreases with
increasing light power, and more than a factor of two "light narrowing" is
observed.Furthermore, we identify that spin-spin interaction between
nitrogen-vacancy centers and substitutional nitrogen atoms in the diamond leads
to changes in the lineshape and the linewidth of the optically detected
magnetic-resonance signals. Finally, the importance of the light-narrowing
effect for optimizing the sensitivity of magnetic field measurements is
discussed.
