Abstract
Bi-core optical fiber structures are studied for applications in sensing. In this paper, an analysis is performed on the spectral characteristics of light propagating in these fibers with central launching core illumination from a standard single mode fiber. Reflective and transmissive configurations are addressed. The characteristics of a reflective bi-core fiber structure for measurement of strain, temperature and absolute value of torsion are investigated and highlights for further research are presented.

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Abstract
A sensor based on 2 hollow core microspheres is proposed. Each microsphere was produced separately through fusion splicing and then joined. The resultant structure is a Fabry-Perot interferometer with multiple interferences that can be approximated to a 4-wave interferometer. Strain characterization was attained for a maximum of 1350 mu epsilon, achieving a linear response with a sensitivity of 3.39 +/- 0.04 pm/mu epsilon. The fabrication technique, fast and with no chemical hazards, as opposed to other fabrication techniques, makes the proposed sensor a compelling solution for strain measurements in hash environments.

Abstract
New miniaturized sensors for biological and medical applications must be adapted to the measuring environments and they should provide a high measurement resolution to sense small changes. The Vernier effect is an effective way of magnifying the sensitivity of a device, allowing for higher resolution sensing. We applied this concept to the development of a small-size optical fiber Fabry?Perot interferometer probe that presents more than 60-fold higher sensitivity to temperature than the normal Fabry?Perot interferometer without the Vernier effect. This enables the sensor to reach higher temperature resolutions. The silica Fabry?Perot interferometer is created by focused ion beam milling of the end of a tapered multimode fiber. Multiple Fabry?Perot interferometers with shifted frequencies are generated in the cavity due to the presence of multiple modes. The reflection spectrum shows two main components in the Fast Fourier transform that give rise to the Vernier effect. The superposition of these components presents an enhancement of sensitivity to temperature. The same effect is also obtained by monitoring the reflection spectrum node without any filtering. A temperature sensitivity of -654 pm/°C was obtained between 30 °C and 120 °C, with an experimental resolution of 0.14 °C. Stability measurements are also reported.