Dual Wavelength Cavity Ringdown Spectroscopy For High Precision Methane Isotope Ratio Measurements

We demonstrate a frequency stabilized cavity ringdown spectrometer capable of measuring simultaneous isotopes of methane (\chem{^{12}CH_4}, \chem{^{13}CH_4}, \chem{^{12}CH_3D}) of enriched samples to high precision ($\delta$D$\textless$0.03$\%$ and $\delta$13C$\textless$0.01$\%$). The spectrometer employs coupling of two orthogonally polarized CW lasers into a ringdown cavity for simultaneous spectral measurements over the full wavelength range of 1.45-1.65$\mu$m. In addition, we discuss the necessity of modeling methane lineshapes with the Galatry profile to achieve the highest precision