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 μs , we find that a specular reflection of only 10<sup>-4</sup> 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 μs , 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.
© 2002 Optical Society of America
(120.6200) Instrumentation, measurement, and metrology : Spectrometers and spectroscopic instrumentation
(140.4780) Lasers and laser optics : Optical resonators
(300.1030) Spectroscopy : Absorption
R. W. Fox and L. Hollberg, "Role of spurious reflections in ring-down spectroscopy," Opt. Lett. 27, 1833-1835 (2002)