We present an optical fiber refractometer based on a Fabry-Perot interferometer defined by two fiber Bragg gratings and an intracavity long-period grating that makes the light confined in the resonator interact with the surrounding medium. The external refractive index is monitored by the resonant frequencies of the Fabry-Perot interferometer, which can be measured either in transmission or in reflection. In this first experiment, wavelength shifts measured with a resolution of 0.1 pm have allowed one to establish a refractive index detection limit of 2.1x10(-5).

We measure the bulk and light-induced absorption as well as the dark- and photoconductivity in doped and undoped photorefractive Bi12TiO20 crystals and compare the results obtained using different techniques. From these data we compute the quantum efficiency for the photogeneration of charge carriers and the carriers mobility–lifetime product, and characterize the suitability of these crystals for holographic recording. A two-center model theory is shown to adequately describe absorption and photoconduction in these materials.

A high sensitivity refractive index sensor based on the combination of mechanically induced long period gratings (LPG) and fiber tapers was developed for real-time fuel quality analysis. The sensor was built in a Mach-Zehnder configuration by employing a pair of in-series gratings. In order to enhance sensor sensitivity, the region between both LPGs was tapered down from 125 to 10 μm. The system was tested by measuring water concentration in ethanol and ethanol concentration in commercial gasoline. The tapered sensor has shown an average sensitivity of 930 nm/RIU, 18 times higher than the non-tapered version. The resolution limit of the system using spectral interrogation was estimated to be 0.06% of ethanol dissolved in gasoline. For the purpose of real-time monitoring, an interrogation system based on white light interferometry (WLI) and virtual instrumentation was employed to evaluate ethanol evaporation in water, avoiding the use of spectral analysis. The WLI system, using phase tracking techniques, enabled us to record the evolution of the ethanol concentration in water with a resolution of 0.005% (v/v).

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