We demonstrate an ultrasensitive refractive-index (RI) sensor utilizing the polarimetric interference of a rectangular silica microfiber. The measured sensitivity is as high as 18, 987 nm/RIU (refractive-index unit) around the RI of 1.33, which is 1 order of magnitude higher than that of the previously reported microfiber devices. Theoretical analysis reveals that such high sensitivity not only is originated from the RI-induced birefringence variation but also relies on the unique birefringence dispersion property for the rectangular microfiber. We predict that the sensitivity can be enhanced significantly when the group birefringence approaches zero.
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