We propose and demonstrate a Fabry-Perot (F-P) optical fiber tip sensor for high-resolution refractive-index measurement fabricated by using 157-nm laser micromachining, for the first time to our knowledge. The sensor head consists of a short air F-P cavity near the tip of a single-mode fiber and the fiber tip. The external refractive index is determined according to the maximum fringe contrast of the interference fringes in the reflective spectrum of the sensor. Such a sensor can provide temperature-independent measurement of practically any refractive index larger than that of air and offers a refractive-index resolution of ~4 x 10(-5) in its linear operating range. The experimental data agree well with the theoretical results.

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A graphene coated microfiber Bragg grating (GMFBG) for gas sensing is reported in this Letter. Taking advantage of the surface field enhancement and gas absorption of a GMFBG, we demonstrate an ultrasensitive approach to detect the concentration of chemical gas. The obtained sensitivities are 0.2 and 0.5 ppm for NH3 and xylene gas, respectively, which are tens of times higher than that of a GMFBG without graphene for tiny gas concentration change detection. Experimental results indicate that the GMFBG-based NH3 gas sensor has fast response due to its highly compact structure. Such a miniature fiber-optic element may find applications in high sensitivity gas sensing and trace analysis.

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