Davide Passaro

Toward A Highly Specific DNA Biosensor: PNA-Modified Suspended-Core Photonic Crystal Fibers

The feasibility of a biosensor for DNA detection based on suspended-core photonic crystal fibers is investigated. The possibility of functionalization of the hole surface, which allows DNA strand binding, is demonstrated, along with the selective detection of DNA through hybridization of immobilized peptide nucleic acid probes with their full-complementary and mismatched DNA segments.

All-Silica Hollow-Core Microstructured Bragg Fibers for Biosensor Application

The possibility to exploit all-silica hollow-core microstructured Bragg fibers to realize a biosensor useful to detect the DNA hybridization process has been investigated. A Bragg fiber recently fabricated has been considered for the analysis performed by means of a full-vector modal solver based on the finite-element method. Since the DNA molecules necessary for the biosensor realization are in aqueous solution, it has been taken into account a microstructured fiber with water-filled holes. The dispersion curve and the confinement loss spectrum have been calculated in order to understand how a biofilm layer on the inner surface of the fiber holes can modify the fundamental mode properties. The numerical analysis results have successfully demonstrated the DNA bio-sensor feasibility in hollow-core Bragg fibers.

Single-Mode Regime in Large-Mode-Area Rare-Earth-Doped Rod-Type PCFs

In this paper, large-mode-area, double-cladding, rare-earth-doped photonic crystal fibers are investigated in order to understand how the refractive index distribution and the mode competition given by the amplification can assure single-mode propagation. Fibers with different core diameters, i.e., 35, 60, and 100 mum, are considered. The analysis of the mode effective index, overlap, effective area, gain, and power evolution along the doped fiber provides clear guidelines on the fiber physical characteristics to be matched in the fabrication process to obtain a truly or effectively single-mode output beam.

Cut-off analysis of 19-cell Yb-doped double-cladding rod-type photonic crystal fibers

Yb-doped double-cladding large mode area rod-type photonic crystal fibers are a key component for power scaling in fiber laser systems. Recently, designs with 19-cell core defect, that is with 19 missing air-holes in the center of the photonic crystal cladding, have been proposed, with reported core diameter up to 100 μm. In this paper an analysis of the cut-off wavelength of the first high-order mode in such low-NA fibers is reported, accounting for different approaches for the definition of the cladding effective index. Results have shown that taking into account the finite fiber cross-section and considering the first cladding mode of the actual fiber is mandatory to obtain a correct estimate of the cut-off wavelength.

Resilient long-distance sensor system using a multiwavelength Raman laser

This paper shows a long-distance remote sensing system using a multiwavelength Raman laser. The sensor network is based on a 50 km long standard single-mode fiber (SMF) and is composed of a simple cavity based on a loop mirror and four fiber Bragg gratings (FBGs) arranged in a star configuration. FBGs are used for both the sensing function and the selection of the lasing wavelengths. The system is designed to be inherently resilient to fiber failures. The multiwavelength laser has been characterized for temperature measurements showing a good stability performance.

Single-mode analysis of Yb-doped double-cladding distributed spectral filtering photonic crystal fibers

Hybrid large mode area Ytterbium-doped double-cladding photonic crystal fibers with anti-symmetric high refractive index inclusions provide efficient amplified spontaneous emission spectral filtering. Their performances have been analyzed by numerical simulations and experimental measurements. In particular, the fiber single-mode behaviour has been studied, by taking into account the fundamental and the first higher-order mode. Two approaches, the core down-doping and the reduction of the air-hole diameter in the inner cladding, have been successfully applied to reduce the higher-order mode content, regardless of the bending of the doped fiber, without significantly affecting its spectral filtering properties.

Suppression of Higher-Order Modes by Segmented Core Doping in Rod-Type Photonic Crystal Fibers

A large mode area Yb-doped rod-type photonic crystal fiber design with a low refractive index ring in the core is proposed to provide an improved suppression of the first higher-order mode compared to the case of uniform core doping, in a way which is more robust against fluctuations in the refractive index value. After applying a scalar step-index model for a first parameter optimization of the proposed design, a full-vector modal solver based on the finite element method has been exploited to analyze the guided mode overlap and effective area for the most promising fibers identified. Finally, a spatial and spectral amplifier model has been considered to study the gain competition among the fundamental and the first higher-order mode guided in the Yb-doped rod-type fibers. Results have demonstrated the effectiveness of the low refractive index ring in suppressing the higher-order mode, thus providing an effectively single-mode behavior for the rod-type fibers.

Guiding Properties of Silica/Air Hollow-Core Bragg Fibers

The guiding properties of realistic silica/air hollow-core Bragg fibers have been investigated by calculating the dispersion curves, the confinement loss spectrum, and the field distribution of the guided modes through a full-vector modal solver based on the finite-element method. In particular, the silica bridge influence on the fundamental mode has been analyzed by comparing the properties of an ideal structure, without the silica nanosupports, and of two realistic fibers, with squared off and rounded air-holes. Simulation results have demonstrated the presence of anticrossing points in the dispersion curves, associated to the transition of the fundamental mode into a surface mode. It has been shown that surface modes are responsible for the sharp loss peaks, also experimentally measured, which pollute the loss spectrum of the fundamental mode and of the higher order modes. Then, the influence on the guiding properties of each geometric characteristic in the hollow-core Bragg fiber cross-section has been deeply investigated, thus showing which parameter it is better to change in order to properly modify the loss values or its spectral behavior. Moreover, in order to improve the loss properties of hollow-core Bragg fibers, the number of silica and air layers in the fiber cladding has been increased, and the layer thickness has been modified. Results have shown that the first change is more effective for the loss reduction, while the second is useful for a spectral shift. Finally, among the different possible applications, the feasibility of a DNA biosensor based on a hollow-core Bragg fiber has been demonstrated.

Confinement loss spectral behavior in hollow-core Bragg fibers

The influence of each cross-section geometric parameter on hollow-core Bragg fiber guiding properties has been numerically investigated. Fabricated fibers have been modeled, giving insight into the spectral behavior of the confinement loss. It has been verified that, by changing the amount of silica and air in the fiber cladding, it is possible to change the reflection conditions undergone by the field within the core, thus shifting the confinement loss spectrum.

Air-suspended solid-core fibers for sensing

Microstructured optical fibers have the potential to provide improved performance relative to more traditional spectroscopic fiber sensors. In fact the manipulation of the geometry of the fiber cross section can allow to maximize the interaction of light and sample. Recently, solid air-suspended core fibers have been appointed as the most promising design for evanescent field sensing. In this kind of device, sensing is carried out through the interaction between evanescent tails of index-guided modes and sample, which fills cladding holes. Suspended core fibers are made by three silica webs joining in the fiber center and forming the core. This design can provide an evanescent field power fraction greater than any other structure previously proposed, together with a wide transmission band. In this paper, the electromagnetic field behaviour of the guided modes of a range of suspended-core fibers is investigated, using a full-vectorial finite element based modal solver. The impact of different design parameters and materials on guidance, the amount of power in the cladding and the possibility of obtaining effective single-mode guidance are also investigated.