Figure 3: Near-IR characterization of a BaF2 microresonator.
From: Mid-infrared ultra-high-Q resonators based on fluoride crystalline materials
(a) Linewidth measurement (blue line) of an under-coupled resonance with calibration sidebands (resulting from phase modulation of the laser) at 1 MHz and Lorentzian fit (red line). We extract a typical full-width at half maximum of 
resulting in an optical factor of Q∼4.4 × 109. (b) Cavity-ringdown measurements. We observed the transmission spectrum of two resonances while scanning at the same speed. Only the ultra-high-Q mode features a ringdown signal (right resonance). (c) Analysis of ringdown structure. We extract successive amplitudes of maxima (red circles) and of minima (green circles) to fit the evolution of the relative amplitude ringdown (d). Theoretical fit of the ringdown relative amplitude (red line). The measured amplitude decay of 
results in a linewidth of 
and a Q factor of 6.4 × 109.
