Ring resonators are fascinating both for their geometric simplicity and for their ability to e ciently con ne optical energy within small mode volumes. In the past couple of decades, optical ring resonators have been employed in many applications, including cavity quantum electrodynamics (QED) (Lefevre-Seguin and Haroche 1997, Vernooy et al. 1998), solid-state microlasers (Baer 1987, Liu et al. 2004), nonlinear optics (Raman (Lin and Campillo 1997), hyper-Raman (Klug et al. 2005), Brillouin scattering (Culverhouse et al. 1991), and four-wave mixing (Klug et al. 2005)), electro-optic modulators (Cohen et al. 2001), optical signal processing (such as lters (Little et al. 2004), dispersion compensation (Madsen e t a l. 1999), a nd s low-light gener ation (Heebner e t a l. 2 002)), opt ical f requency combs (Del’Haye et a l. 2007), optical gyroscopes (Armenise et a l. 2001), opto-mechanics (including strain (Huston and Eversole 1993), radiation pressure (Kippenberg et al. 2005), and ultrasound sensing (Ashkenazi et al. 2004)), and frequency stabilization (Sakai et al. 1991).