  We employ the technique of optical detection of magnetic resonance to study
dipolar interaction in diamond between nitrogen-vacancy color centers of
different crystallographic orientations and substitutional nitrogen defects. We
demonstrate optical measurements of resonant spin flips-flips (second Larmor
line), and flip-flops between different spin ensembles in diamond. In addition,
the strain coupling between the nitrogen-vacancy color centers and bulk
acoustic modes is studied using optical detection. Our findings may help
optimizing cross polarization protocols, which, in turn, may allow improving
the sensitivity of diamond-based detectors.
