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4 March 2019 Enhanced absorption sensing using non-adiabatic tapered fiber coupling to a whispering-gallery microresonator
Sreekul Raj Rajagopal, Limu Ke, A. T. Rosenberger
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Proceedings Volume 10904, Laser Resonators, Microresonators, and Beam Control XXI; 109041D (2019) https://doi.org/10.1117/12.2508024
Event: SPIE LASE, 2019, San Francisco, California, United States
Abstract
Adiabatically tapered fibers are often used to excite whispering gallery modes (WGMs) of microresonators used as chemical sensors. Recently it was demonstrated that using a non-adiabatic tapered fiber can enhance refractive index sensing. The incoming light is distributed between fundamental and higher-order fiber modes, whereas only the fundamental mode is detected because the uptaper is adiabatic. The interference effect between these fiber modes when exciting a WGM leads to the sensitivity enhancement. We have shown theoretically that even greater enhancement is possible for absorption sensing. For a given WGM, the predicted enhancement can be calculated by measuring the throughput power when the two fiber modes are in and out of phase at the input. Enhancement can be confirmed by sending the light in the reverse direction through the asymmetrically tapered fiber so that only one fiber mode is incident on the microresonator. Using a carefully designed asymmetrically tapered fiber, we have demonstrated this enhancement in experiments using a hollow bottle resonator (HBR) with an internal analyte. Absorption in the analyte causes a change in the WGM throughput fractional dip depth; these changes were studied with varying analyte concentration for forward and reverse propagation to evaluate the absorption sensitivity. For both liquid and gaseous analytes, our measured sensitivity enhancements are not inconsistent with the predicted enhancements of at least a factor of 100.
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Sreekul Raj Rajagopal, Limu Ke, and A. T. Rosenberger "Enhanced absorption sensing using non-adiabatic tapered fiber coupling to a whispering-gallery microresonator", Proc. SPIE 10904, Laser Resonators, Microresonators, and Beam Control XXI, 109041D (4 March 2019); https://doi.org/10.1117/12.2508024
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KEYWORDS
Microresonators
Absorption
Fiber couplers
Resonators
Sensors
Refractive index
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