Umberto Minoni

Ge-Doped Microstructured Multicore Fiber for Customizable Supercontinuum Generation

Supercontinuum generation in a multicore fiber in which several uncoupled cores were doped with dissimilar concentrations of germanium was studied experimentally. Germanium doping provided control over the separation between the zero-dispersion wavelength and the 1064-nm wavelength of a Q-switched Nd:YAG pump laser. Supercontinua generated independently in each core of the same piece of fiber displayed clear and repeatable differences due to the influence of germanium doping on refractive index and four-wave mixing. The spectral evolution of the subnanosecond pump pulses injected into the different cores was accurately reproduced by numerical simulations.

Second harmonic generation in multimode graded-index fibers: spatial beam cleaning and multiple harmonic sideband generation

We study experimentally and numerically the spectral and spatial dynamics of second harmonic generation in an all-optically poled multimode graded-index fiber. In contrast with poled single-mode fibers, in a multimode graded-index fiber a pump can generate a series of sharp sidebands around its second harmonic (SH) that originate from the sub-millimetric periodic evolution of the intensity at the fundamental frequency. The mutual interaction between the fundamental and its SH may also strongly affect the spatial distribution of guided light for both colors: when increasing the pump power, both fundamental and SH output beams evolve from disordered multimode speckles into two bell-shaped beams.

On the application of optical forward-scattering to bacterial identification in an automated clinical analysis perspective.

The Optical Forward Scattering (OFS) technique can be used to identify pathogens by direct observation of bacteria colonies growing on a culture plate. The identification is based on the acquisition of scattering images from isolated colonies and their subsequent comparison with reference images acquired from known bacteria. The technique has been mainly studied for the identification of pathogens in the food-safety field. This paper focuses on the possibility of extending the applicability of the technique to the field of clinical laboratory automation. This scenario requires that the paradigm of image acquisition at fixed colony-dimension, well established in the food-safety applications, should be substituted by an acquisition at fixed incubation time. As a consequence, the scatterometer must be adjustable in real-time for adapting to the actual features of the bacterial colony. The paper describes an OFS system prototype qualified by the possibility to tune both the laser beam diameter and the acquisition camera field of view. Preliminary experiments on bacteria cultures from pathogens causing infections of the urinary tract show that the proposed approach is promising for the development of an automated bacteria identification station. The new OFS approach also involves an alternative method for building a reference image database for subsequent image analysis.

Efficiency of dispersive wave generation in dual concentric core microstructured fiber

We describe the generation of powerful dispersive waves that are observed when pumping a dual concentric core microstructured fiber by means of a sub-nanosecond laser emitting at the wavelength of~1064 nm. The presence of three zeros in the dispersion curve, their spectral separation from the pump wavelength, and the complex dynamics of solitons originated by the pump pulse break-up, all contribute to boost the amplitude of the dispersive wave on the long-wavelength side of the pump. The measured conversion efficiency towards the dispersive wave at 1548 nm is as high as 50%. Our experimental analysis of the output spectra is completed by the acquisition of the time delays of the different spectral components. Numerical simulations and an analytical perturbative analysis identify the central wavelength of the red-shifted pump solitons and the dispersion profile of the fiber as the key parameters for determining the efficiency of the dispersive wave generation process.

Dispersive Wave Emission in Dual Concentric Core Fiber: The Role of Soliton–Soliton Collisions

Soliton-soliton collisions have a crucial role in enhancing the spectrum of dispersive waves in optical fibers and collisions among in-phase solitons lead to a dramatic enhancement of the dispersive wave power, as well as to its significant spectral reshaping. We obtained a simple analytical model to estimate the spectral position, width, and amplitude of the dispersive waves induced by a collision of two in-phase solitons. We tested our theory in the case of a dual concentric core microstructured fiber.

Second harmonic generation and beam cleaning in optically poled multimode graded-index fibers

Recent experimental results have confirmed that nonlinear effects in multimode graded-index (MM-GRIN) fibers lead to a wealth of observable spatiotemporal effects. In spite of the large number of modes supported by the fiber, by increasing the input power the transverse profile evolves in a clean bell-shaped beam [1, 2]. Next, the input laser line experiences a strong spectral broadening and Geometric Parametric Instability (GPl) peaks are observed [3].

Soliton-Soliton Collision Enhanced Dispersive Wave Emission in Microstructured Fiber

We developed an analytical approach to estimate the influence of soliton-soliton collision on the growth rate of dispersive waves in optical fibers. Our theoretical model shows an excellent agreement with numerical results.

Optical continuum generation seeded by stimulated Raman scattering

An optical continuum was generated in microstructured optical fibre using an incoherent pump generated by stimulated Raman scattering in a step index fibre with normal dispersion. Accelerated spectral broadening to longer wavelengths was observed.

Flexible measuring machine based on a double interferometer

In this work a prototype of a flexible electro-optical measuring machine suitable for dimensional quality control of inner and outer diameters of mechanical workpieces, is presented. Two measuring probes are displaced by means of motorized slides over a range of +/- 100 mm enabling the measurement of diameters from 0 up to 200 mm. The position of the probes is gauged by a double interferometer designed for the application. The characterization of the prototype has shown that repeatability and accuracy are better than +/- 0.16 micrometers .