Abstract
The optical loss of whispering gallery modes of resonantly excited microresonator spheres is determined by optical lifetime measurements. The phase-shift cavity ring-down technique is used to extract ring-down times and optical loss from the difference in amplitude modulation phase between the light entering the microresonator and light scattered from the microresonator. In addition, the phase lag of the light exiting the waveguide, which was used to couple light into the resonator, was measured. The intensity and phase measurements were fully described by a model that assumed interference of the cavity modes with the light propagating in the waveguide.
Highlights
Optical microresonators, of different geometries, support ultra-low-loss whispering-gallery modes (WGM) when evanescently excited from adjacent waveguiding structures
Inexpensive and simple phase shift cavity-ring-down spectroscopy (CRD) method to whispering gallery modes of a microresonator sphere we take a first step to a chemical sensor, which is suited to absorption detection of very small amounts of analyte
Complementary information is available from measurements of light scattered by the microresonator and from light that is allowed to interfere with incoming light - here the light transmitted by the waveguide, which was used to couple light into the resonator
Summary
Of different geometries, support ultra-low-loss whispering-gallery modes (WGM) when evanescently excited from adjacent waveguiding structures. The high quality factor (Q) and small mode volume associated with WGMs makes these microresonators suitable for numerous optical devices, such as microlasers [8,9] and add-drop elements [10], as well as chemical sensors [11,12] The latter application is of particular interest in this work. The variation in the phase-shift with modulation frequency can be related to the optical lifetime [20] This phaseshift ring-down approach has been extensively applied to fiber-optic loops [21] and mirror cavities [22]. Inexpensive and simple phase shift CRD method to whispering gallery modes of a microresonator sphere we take a first step to a chemical sensor, which is suited to absorption detection of very small amounts of analyte. As will be shown below, the model is readily transferable to the mirrored cavities typical for phaseshift CRD setups
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