Effects of linear birefringence and polarization-dependent loss of supermirrors in cavity ring-down spectroscopy.

In cavity ring-down spectroscopy (CRDS), residual or stress-induced birefringence (10(-7)-10(-6) rad) of supermirrors will lift the polarization degeneracy of TEM(00) modes and generate two new polarization eigenstates in the cavity with small resonant frequency splitting (approximately 0.1 kHz); the new eigenstates are nearly linearly polarized. When both modes are excited simultaneously, the intracavity polarization state will evolve as the energy decays in the cavity. Without polarization analysis, such mode beating would not be observable. However, real supermirrors have a linear polarization-dependent loss (dichroism) that leads to a change in the loss rate as the polarization state evolves and thus to deviation from the expected single-exponential decay. We develop a model for the evolution of the intracavity polarization state and intensity for a cavity with both birefringence and polarization-dependent loss in the mirrors. We demonstrate, experimentally, that these parameters (both magnitudes and directions) can be extracted from a series of measurements of the cavity decay and depolarization of the transmitted light.