Intracavity chemical absorption sensing using microsphere whispering-gallery modes

Summary form only given. Laser light is injected into whispering-gallery modes (WGMs) of a fused-silica microsphere using a tapered-fiber coupler. As the laser frequency scans through a WGM resonance, the fiber output shows a resonance dip because of the intrinsic loss of the WGM. The high Q of a WGM makes the dip sharp; the depth of the dip is sensitive to a change in the intrinsic loss. Such a change is provided by interaction of the evanescent part of the mode with an absorber surrounding the microsphere, and thereby intracavity absorption sensing is accomplished.