Role of spurious reflections in ring-down spectroscopy

Spurious coherent reflections from optical elements that re-enter an exit port of a two-mirror ring-down cavity can significantly change the effective reflectivity of the cavity mirrors, thus altering the cavity decay time. For a 25-cm-long Fabry-Perot cavity with a decay constant of 40 mus , we find that a specular reflection of only 10(-4) of the transmitted ring-down power that is mode matched back toward the cavity could change the decay time by as much as +/-0.4 mus , depending on the phase of the returning reflection. The perturbation of the decay time is proportional to the electric field, so a decrease in the spurious reflected power of 100 times will result in a perturbation that is only 10 times smaller. We demonstrate the effect with a cw system by purposely introducing a spurious reflection.