Phase-shift cavity ring-down spectroscopy on a microsphere resonator by Rayleigh backscattering

Abstract

The optical ring-down time in a silica microsphere resonator is measured using phase-shift cavity ring-down spectroscopy. When detecting Rayleigh backscattered light that is coupled into the input waveguide it is found that the shift of the modulation phase angle, $\ensuremath{\Delta}\ensuremath{\phi}$, is approximately linearly dependent on the amplitude modulation frequency. The microresonator therefore behaves as if it was a delay line. A model involving two different time scales for the buildup of the Rayleigh backscattered light and the decay of the whispering gallery modes is used to explain the observations.