Robust Fiber-Integrated High-Q Microsphere for Practical Sensing Applications

Whispering gallery modes (WGM) resonators (K. J. Vahala, 2003; A. B. Matsko et al., 2006) have been extensively studied in a large variety of geometrical shapes and in a wide range of promising studies and applications (A. Chiasera et al., 2010), both fundamental like CQED and practical like low threshold lasers and sensors. The Silica spherical microresonators, made from commercial optical fibers, could support WGM with ultra-high quality factors (Q) (about 107∼9). With the merits of very small model volume and good compatibility with fiber-integrated optics, they are promising for a number of passive and active devices as filters, lasers and modulators (A. Chiasera et al., 2010). In WGMs, the electromagnetic wave is strongly confined within the microcavity in the manner of "totally internally reflection" (TIR). Thus, there is minimal reflection optical losses at the cavity interface. With the negligible silica absorption loss, the resonant modes can reach the ultra-high Q. Benefitting from the high-Q (M. L. Gorodetsky et al., 1996), the WGMs behave as extreme-narrow linewidth resonant dips. Therefore, a small shift of the resonant dip can be detected with high resolution, meaning that the WGMs are potential in the sensing researches (F. Vollmer et al., 2008; Y. Sun et al., 2008; M. Sumetsk et al., 2007; F. Xu et al., 2008; I. M. White et al., 2008). Intuitively (as shown in Fig. 1), photons can travel around many trips inside the high-Q microcavity, and interact with the detected matters around the microcavity many times (Fig. 1-a). By contrast, in the traditional fiber sensors each photon can interact with the detected matters only once (Fig. 1-b). Therefore, the microcavity-based sensors can have more superior performance than the conventional fiber sensors, demonstrating a higher sensitivity or resolution.