A precise measurement of quality factor for Planar Microtoroid Resonators

The quality factor(Q) parameter is the most important parameter for optical micro cavities. In this paper, we present the Planar Microtoroid Resonators fabricated in our laboratory. Then, measuring methods common used are introduced. After that, a precise measurement of quality factor is demonstrated in which the photon lifetime was directly measured by cavity ringdown. This is done by repeatedly scanning the laser into resonance with a mode that was critically coupled to the taper. As the laser scanned into resonance, the power transferred into the cavity increased until the maximal power of the resonant mode was attained. At this moment, an external modulator is used to cut-off the pumping laser. As the resonant power outpouring, a cavity ringdown spectrum is observed and recorded. The cavity lifetime can be obtained through analyzing the spectrum. Finally, the quality factor is estimated from the cavity lifetime which is consistent with the widely used measurement of the frequency line shape.