Design of asymmetrically tapered fibers for multimode input to enable dissipative sensing enhancement in microresonators

Abstract Asymmetric fiber tapers, with nonadiabatic downtapers and adiabatic uptapers, allow multimode input to a whispering-gallery microresonator for the enhancement of dissipative sensing through interference-enabled effective strong overcoupling. The sensitivity enhancement is realized by monitoring absorption-induced changes in the throughput dip depth, relative to changes in the linewidth, of the whispering-gallery mode (WGM) with two fiber modes input. Herein, we describe a method of designing asymmetrically tapered fibers that permits the degree of nonadiabaticity to be controlled. That is, the design can determine how strongly various higher-order fiber modes get excited, relative to the fundamental mode. In this way, an asymmetric taper can be optimized to minimize excitation of fiber modes that do not couple into the microresonator's WGM but only provide undesired loss. The success of our design method is confirmed by agreement between calculated and measured modeinterference beat lengths, by the cleanliness of the beat signal indicating that only two fiber modes are beating, and by an increase in the enhancement factor for methane sensing.