  We present an experimental technique enabling mechanical-noise free,
cavity-enhanced frequency measurements of an atomic transition and its
hyperfine structure. We employ the 532nm frequency doubled output from a Nd:YAG
laser and an iodine vapour cell. The cell is placed in a traveling-wave
Fabry-Perot interferometer (FPI) with counter-propagating pump and probe beams.
The FPI is locked using the Pound-Drever-Hall (PDH) technique. Mechanical noise
is rejected by differencing pump and probe signals. In addition, this
differenced error signal gives a sensitive measure of differential
non-linearity within the FPI.
