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We demonstrate the inscription of fiber Bragg gratings by 193 nm ArF excimer laser in microfibers drawn from the standard single mode telecommunication fiber. Fiber Bragg gratings are directly inscribed in a series of microfibers with diameter ranged from tens of μm to 3.3 μm without hydrogen loading or other treatment to photosensitize the microfibers. Four reflection peaks are observed where three correspond to high order mode resonances. The resonance wavelength depends on the fiber diameter and it sharply blueshifts as the diameter is decreased below 10 μm. The gratings are characterized for their response to ambient refractive index. The higher order mode resonance exhibits higher sensitivity to refractive index.

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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.

Nucleic acid detection with label-free biosensors circumvents the need for costly fluorophore functionalization steps associated with conventional assays by utilizing optical fiber transducers. In spite of their technological prowess, however, these biosensors' sensitivity is limited by the design/configuration of their transducers. Therefore, it is imperative to integrate novel optical fiber transducers with existing label-free approaches to overcome those limitations. Herein, we present a high sensitivity label-free fiber optic biosensor that employs polarimetric interference of a high-birefringence (Hi-Bi) microfiber to specifically detect DNA molecules. A slight target DNA concentration change is converted into an optical wavelength shift of polarimetric interference generated by the microfiber Sagnac interferometer. The sensor provides a log-linear response to target ssDNA concentrations range from 100 pM to 1 μM and a minimum detectable concentration of 75 pM.