  Band-resolved frequency modulation spectroscopy is a common method to measure
weak signals of radiative ensembles. When the optical depth of the medium is
large, the signal drops exponentially and the technique becomes ineffective. In
this situation, we show that a signal can be recovered when a larger modulation
index is applied. Noticeably, this signal can be dominated by the natural
linewidth of the resonance, regardless of the presence of inhomogeneous line
broadening. We implement this technique on a cesium vapor, and then explore its
main spectroscopic features. This work opens the road towards measurement of
cooperative emission effects in bulk atomic ensemble.
