Birefringence effect of high reflectivity cavity mirrors and its influence on cavity enhanced spectroscopy

In laser absorption spectroscopy, in order to improve gas detection sensitivity, optical cavity with high finesse is used to prolong the interaction path between the laser and the absorber. However, the birefringence of high reflectivity cavity mirrors generates two polarization eigenstates, and owing to the different phase shifts along the two directions, the cavity mode will be split. In this work, we first measure the cavity enhanced signal under birefringence and observe the mode split. And a model to mimic cavity enhanced spectroscopy under birefringent effect is presented, which can accurately fit the different polarization ratios at transmission. Finally, we propose a cavity ring-down signal model considering different coupling efficiencies of the two polarization directions of the cavity. Comparing with the conventional exponential model, the standard deviation of residual maximum suppression is as high as 9 times. And this analysis is helpful in improving the signal-to-noise ratio and uncertainty of cavity ring-down signal and increasing the accuracy of concentration inversion.