Enrico Coscelli

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.

Dispersion Engineering of Highly Nonlinear Chalcogenide Suspended-Core Fibers

Chalcogenide optical fibers are currently undergoing intensive investigation with the aim of exploiting the excellent glass transmission and nonlinear characteristics in the near- and mid-infrared for several applications. Further enhancement of these properties can be obtained, for a particular application, with optical fibers specifically designed that are capable of providing low effective area together with a properly tailored dispersion, matching the characteristics of the laser sources used to excite nonlinear effects. Suspended-core photonic crystal fibers are ideal candidates for nonlinear applications, providing small-core waveguides with large index contrast and tunable dispersion. In this paper, the dispersion properties of As2S3 suspended-core fibers are numerically analyzed, taking into account, for the first time, all the structural parameters, including the size and the number of the glass bridges. The results show that a proper design of the cladding struts can be exploited to significantly change the fiber properties, altering the maximum value of the dispersion parameter and shifting the zero-dispersion wavelengths over a range of 400 nm.

Thermal Effects on the Single-Mode Regime of Distributed Modal Filtering Rod Fiber

Power scaling of fiber laser systems requires the development of innovative active fibers, capable of providing high pump absorption, ultralarge effective area, high-order mode suppression, and resilience to thermal effects. Thermally induced refractive index change has been recently appointed as one major limitation to the achievable power, causing degradation of the modal properties and preventing to obtain stable diffraction-limited output beam. In this paper, the effects of thermally induced refractive index change on the guiding properties of a double-cladding distributed modal filtering rod-type photonic crystal fiber, which exploits resonant coupling with high-index elements to suppress high-order modes, are thoroughly investigated. A computationally efficient model has been developed to calculate the refractive index change due to the thermo-optical effect, and it has been integrated into a full-vector modal solver based on the finite-element method to obtain the guided modes, considering different heating conditions. Results have shown that the single-mode regime of the distributed modal filtering fiber is less sensitive to thermal effects with respect to index-guiding fibers with the same effective area. In fact, as the pump power is increased, their single-mode regime is preserved, being only blue-shifted in wavelength.

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.

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.

Thermally resilient Tm-doped large mode area photonic crystal fiber with symmetry-free cladding

The design of a Tm-doped photonic crystal fiber with ∼ 80 μm core diameter and robust single-mode guiding is proposed. State-of-art modal discrimination is obtained through the suppression of the inner cladding C(6ν) symmetry, which fosters the delocalization of the LP(11)-like mode. The effects of thermally-induced refractive index change are investigated by means of a computationally-efficient thermal model, and the possibility to obtain wide-band single-mode propagation and effective area exceeding 2500 μm(2) under a heat load of over 300 W/m is demonstrated.

Hybrid Ytterbium-doped large-mode-area photonic crystal fiber amplifier for long wavelengths.

A large-mode-area Ytterbium-doped photonic crystal fiber amplifier with build-in gain shaping is presented. The fiber cladding consists of a hexagonal lattice of air holes, where three rows are replaced with circular high-index inclusions. Seven missing air holes define the large-mode-area core. Light confinement is achieved by combined index and bandgap guiding, which allows for single-mode operation and gain shaping through distributed spectral filtering of amplified spontaneous emission. The fiber properties are ideal for amplification in the long wavelength regime of the Ytterbium gain spectrum above 1100 nm, and red shifting of the maximum gain to 1130 nm is demonstrated.

Single-Mode Propagation in Yb-Doped Large Mode Area Fibers With Reduced Cladding Symmetry

Thermal effects are currently appointed as the main bottleneck to power scaling of fiber lasers. In this letter, the single-mode properties of Yb-doped rod-type photonic crystal fibers with reduced symmetry in the inner cladding under severe heat load have been analyzed. Three fibers with different cross-sectional geometry have been compared by calculating the singlemode regime and effective area with a full-vector modal solver based on the finite-element method, properly combined with a thermal model to include the effects of fiber heating on the guiding properties. Results have demonstrated that, with an optimized cross-sectional design, it is possible to obtain singlemode propagation for a coupled pump power up to 200 W, while keeping the effective area ~2400 μm2.

Analysis of the Modal Content Into Large-Mode-Area Photonic Crystal Fibers Under Heat Load

Yb-doped double-cladding photonic crystal fibers are one of the key enabling factors for the development of high-power fiber lasers, due to their capability to provide a very large-mode-area together with the effective suppression of high-order modes, while allowing strong pump absorption and efficient conversion. Thermal effects are currently considered as the main bottleneck for future power scaling; since beyond a certain average power, they allow guidance of high order modes and energy transfer to them, causing a sudden degradation of the beam quality. In this paper, the effects of the heat load on the modes of double cladding fibers are thoroughly analyzed with a full-vector modal solver based on the finite-element method with integrated steady-state heat equation solver. Fibers with different inner cladding designs are compared to provide a deeper understanding of the mechanisms beyond the mode reconfinement and coupling. The influence of the fiber design on the robustness of the single-mode regime with respect to fiber heating has been demonstrated, providing a clear picture of the complex interaction between modes. On the basis of simulation results it has been possible to group fiber modes into three families characterized by peculiar reaction to heating.

Single-Mode Design Guidelines for 19-Cell Double-Cladding Photonic Crystal Fibers

Yb-doped double-cladding photonic crystal fibers have become key components for power scaling in fiber laser systems, by providing many advantages, especially an ultra large effective area. The single-mode regime, which is a mandatory requirement for high quality laser beams, can be obtained in such large core active fibers only through a careful design. In this paper the cut-off properties of 19-cell photonic crystal fibers have been thoroughly investigated with the avoided-crossing approach, in order to find guidelines for the design of single-mode fibers. The air-hole diameter and the core refractive index have been changed, as well as the number of air-hole rings in the fiber inner cladding. Simulation results have shown that, regardless of the air-hole ring number, the guided-mode cut-off properties are strongly influenced by the main design parameters, especially by the core refractive index. In particular, a wider single-mode wavelength range can be obtained in 19-cell fibers with small air-holes and low core refractive index. Moreover, double-cladding photonic crystal fibers with larger inner-cladding provide better guided-mode cut-off properties, which can have positive effects on the amplification process in practical applications.