Different strategies for designing optical couplers, optimized to enhance the pump absorption in the rare-earth-doped core of microstructured fiber lasers, are illustrated. Three kinds/configurations of optical couplers have been designed and compared as examples of the different design strategies which can be followed. Their effectiveness to enhance the performance of an ytterbium-doped, double cladding, microstructured optical fiber laser has been accurately simulated. They consist of a suitable cascade of multiple long-period gratings (MLPGs) inscribed in the fiber core region. The characteristics of the MLPG couplers have been simulated via a homemade computer code based on both rate equations and an extended coupled mode theory. The proposed MLPG couplers seem particularly useful in the case of low rare-earth concentration but, even for a middle-high ytterbium concentration, as N(Yb)=5×10(25) ions/m(3), the slope efficiency S can be increased up to 20%, depending on the fiber length.
Evanescent field optical sensors are accurately designed for hydrocarbon monitoring in water. Various kinds of waveguide sensors are optimized by considering a polydimethylsiloxane polymeric overlay as sensor region. The simulation results suggest that the selection of a suitable waveguide cross section can enhance the sensor performance. In particular, the hollow waveguide sensor exhibits very intriguing performance, the absorbance being quite linear with respect to the contaminant concentration. For the toluene pollution the absorbance exhibits a slope S A TE = 2.52 × 10 −2 ppm −1 for a waveguide reference length L = 1.18 mm. In order to simultaneously detect different pollutants in water such as toluene, benzene, chlorobenzene and ethilbenzene, an array of four miniaturized hollow waveguide sensors is designed.
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