Exact design solutions for photodiode transimpedance amplifiers based on FET input OP-AMPs

We calculate the transfer function of an optical receiver composed by a photodiode and a FET input operational amplifier as a current to voltage converter. According to the theory of low-pass filters, the receiver bandwidth and quality factor are here analytically evaluated, proposing design solutions and their related sensitivity to eventual parameter fluctuations. We exemplify the combined action of photodetection and filtering comparing the analytical expression with full numerical SPICE simulations

Nonlinear sculpturing of optical pulses in fibre systems

We present a general method for the design of fibre-based nonlinear pulse shaping. By combining direct numerical simulations with machine-learning strategies, we efficiently identify the optimal working parameters for achieving a given pulse target

Observation of geometric parametric instability induced by the periodic spatial self-imaging of multimode waves

Spatio-temporal mode coupling in highly multimode physical systems permits new routes for exploring complex instabilities and forming coherent wave structures. We present here the first experimental demonstration of multiple geometric parametric instability sidebands, generated in the frequency domain through resonant space-time coupling, owing to the natural periodic spatial self-imaging of a multimode quasi-continuous-wave beam in a standard graded-index multimode fiber. The input beam was launched in the fiber by means of an amplified microchip laser emitting sub-nanosecond pulses at 1064 nm. The experimentally observed frequency spacing among sidebands agrees well with analytical predictions and numerical simulations. The first order peaks are located at the considerably large detuning of 123.5 THz from the pump. These results open the remarkable possibility to convert a near-infrared laser directly into a broad spectral range spanning visible and infrared wavelengths, by means of a single resonant parametric nonlinear effect occurring in the normal dispersion regime. As a further evidence of our strong space-time coupling regime, we observed the striking effect that all of the different sideband peaks were carried by a well-defined and stable bell-shaped spatial profile.Comment: 5 pages, 4 figure

Nonlinear beam self-imaging and self-focusing dynamics in a GRIN multimode optical fiber: theory and experiments

Beam self-imaging in nonlinear graded-index multimode optical fibers is of interest for many applications, such as implementing a fast saturable absorber mechanism in fiber lasers via multimode interference. We obtain an exact solution for the nonlinear evolution of first and second order moments of a laser beam carried by a graded-index multimode fiber, predicting that the spatial self-imaging period does not vary with power. Whereas the amplitude of the oscillation of the beam width is power-dependent. We have experimentally studied the longitudinal evolution of beam self-imaging by means of femtosecond laser pulse propagation in both the anomalous and the normal dispersion regime of a standard telecom graded-index multimode optical fiber. Light scattering out of the fiber core via visible fluorescence emission and harmonic wave generation permits us to directly confirm that the self-imaging period is invariant with power. Spatial shift and splitting of the self-imaging process under the action of self-focusing are also emphasized

Nonlinear optics in multimode fibers

We overview the emerging field of nonlinear optics in multimode optical fibers, which enable new methods for the ultrafast light-activated control of temporal, spatial and spectral degrees of freedom of intense pulsed light beams

Nonlinear polarization dynamics of Kerr beam self-cleaning in a GRIN multimode optical fiber

We experimentally study polarization dynamics of Kerr beam self-cleaning in a graded-index multimode optical fiber. We show that spatial beam cleaning is accompanied by nonlinear polarization rotation, and a substantial increase of the degree of linear polarization.Comment: 5 pages, 6 figure

Nonlinear dynamics of spatio-temporal waves in multimode fibres

Nonlinear multimode fibers provide an intriguing test-bed for exploring complex spatio-temporal beam dynamics. We overview recent experimental observations of Kerr beam self-cleaning, parametric sideband series and supercontinuum generation in passive and active multimode optical fibers

Crisis Behavior in Autism Spectrum Disorders: A Self-Organized Criticality Approach

The Autism Spectrum Disorder (ASD) represents a set of life-long disorders. In particular, subjects with ASD can display momentary behaviors of acute agitation and aggressiveness called crisis behaviors. These events are problematic for the subject and care providers but little is known about their occurrence, namely, possible relations among intensity, frequency, and duration. A group of ASD subjects (n=33) has been observed for 12 months reporting data on each crisis (n=1137 crises). Statistical analysis did not find significant results, while the relation between crisis duration and frequency showed a good fit to a "power law" curve, suggesting the application of Self-Organized Criticality (SOC) model. The SOC is used to describe natural phenomena as earthquakes, bank failures of rivers, mass extinctions, and other systems where a type of "catastrophic events" is necessary to maintain a critical equilibrium. In a sense, subjects at risk of crisis behavior seem to fit the same model as seismic zones at risk of earthquakes. The employment of the same strategies, as those successfully developed for known SOC systems, could lead to important insights for ASD management. Moreover, the SOC model offers possible interpretations of crisis behavior dynamics suggesting that they are unpredictable and, in a sense, necessary

Multiphoton-absorption-excited up-conversion luminescence in optical fibers

We experimentally demonstrate a previously unforeseen nonlinear effect in optical fibers: up-conversion luminescence generation excited by multiphoton absorption of femtosecond infrared pulses. We directly estimate the average number of photons involved in the up-conversion process, by varying the wavelength of the pump source. We highlight the role of nonbridging oxygen hole centers and oxygen-deficient center defects and directly compare the intensity of side-scattered luminescence with numerical simulations of pulse propagation

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.Comment: 11 pages, 12 figure