i am trying to learn laser locking in Rb87 saturation absorption spectroscopy setup, while reading few papers i cam across the term "subnatural linewidth", I understand linewidth to be the FWHM of a spectrum output, what does subnatural means?
When you measure a free running laser you see its natural linewidth.
If you stabilize this laser onto, say, another laser with much smaller linewidth (called also "coherence cloning"), you can reduce the linewidth below the natural linewidth, to a subnatural linewidth.