We study the nonlinear propagation of femtosecond pulses in the anomalous dispersion region of microstructured fibers, where soliton fission mechanisms play an important role. The experiment shows that the output spectrum contains, besides the infrared supercontinuum, a narrow-band 430-nm peak, carrying about one fourth of the input energy. By combining simulation and experiments, we explore the generation mechanism of the visible peak and describe its properties. The simulation demonstrates that the blue peak is generated only when the input pulse is so strongly compressed that the short-wavelength tail of the spectrum includes the wavelength predicted for the dispersive wave. In agreement with simulation, intensity-autocorrelation measurements show that the duration of the blue pulse is in the picosecond time range, and that, by increasing the input intensity, satellite pulses of lower intensity are generated.

We present a detailed investigation of the different processes responsible for the optical nonlinearities of silicon nanocrystals at 1550 nm. Through z-scan measurements, the bound-electronic and excited carrier contributions to the nonlinear refraction were measured in presence of two-photon absorption. A study of the nonlinear response at different excitation powers has permitted to determine the change in the refractive index per unit of photo-excited carrier density sigma(r) and the value of the real bound-electronic nonlinear refraction n(2be) as a function of the nanocrystals size. Moreover at high excitation power, a saturation of the nonlinear absorption was observed due to band-filling effects.

In this paper, we determine the optical nonlinear coefficient of hydrogenated amorphous silicon (a-Si:H) waveguides. Up to date, the data reported in the scientific literature for similar structures show a very large variability and the final assessment of their nonlinear performance is still an open issue. We performed a complete and careful characterization of more than 50 waveguides. A nonlinear coefficient of 790 + j20 W−1 m−1 was found, confirming that a-Si:H is a good candidate for nonlinear silicon photonic devices. Nevertheless, free-carrier-dynamics exhibits a recombination time in the nanosecond range, which can hinder their exploitation in ultrafast applications requiring high-power optical beams.

We describe an experiment in which a train of femtosecond pulses is coupled into a photonic crystal fiber (PCF) by means of an offset pumping technique that can selectively excite either the mode LP(01) or LP(11) or LP(21). The PCF presents a wide range of wavelengths in which the fundamental mode experiences normal dispersion, whereas LP(11) and LP(21) propagate in the anomalous dispersion regime, generating a supercontinuum based on the soliton fission mechanism. We find that the existence of a cut-off wavelength for the higher-order modes makes the spectral broadening asymmetrical. This latter effect is particularly dramatic in the case of the LP(21) mode, in which, by using a pump wavelength slightly below cut-off, the spectral broadening occurs only on the blue side of the pump wavelength. Our experimental results are successfully compared to numerical solutions of the nonlinear Schrödinger equation.

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Abstract:We study the effect of Two-Photon Absorption (TPA) nonlinear losses on Gaussian pulses, with power that exceeds the critical power for self-focusing, propagating in bulk kerr media. Experiments performed in fused silica and silicon highlight a spontaneous reshaping of the input pulse into a pulsed Bessel beam. A filament is formed in which sub-diffractive propagation is sustained by the Bessel-nature of the pulse.References and links 1. R. Boyd, Nonlinear Optics (Academic Press, 1992). 2. G. G. Luther, J. V. Moloney, and A. C. Newell, "Self-focusing threshold in normally dispersive media," Opt. Lett. 19, 862-864 (1994)

We demonstrate the validity of the Shackled-frequency-resolved-optical-gating technique for the complete characterization, both in space and in time, of ultrashort optical pulses that present strong angular dispersion. Combining a simple imaging grating with a Hartmann-Shack sensor and standard frequency-resolvedoptical-gating detection at a single spatial position, we are able to retrieve the full spatiotemporal structure of a tilted pulse.

We propose an experimental technique that allows for a complete characterization of the amplitude and phase of optical pulses in space and time. By the combination of a spatially resolved spectral measurement in the near and far fields and a frequency-resolved optical gating measurement, the electric field of the pulse is obtained through a fast, error-reduction algorithm.Ultrashort laser pulses are widely used in many laboratories and are routinely adopted for many applications. In past decades, various techniques were developed to measure the electric field of optical pulses as a function of time or frequency. The different approaches belong to two main categories: spectrographic and interferometric techniques. The most known examples of these approaches are frequencyresolved optical gating (FROG) [1] and spectral interferometry for direct electric-field reconstruction (SPIDER), respectively [2]. The first is based on the measurement of the temporally resolved spectrum (spectrogram). An iterative inversion algorithm is applied to the measured spectrogram in order to retrieve the electric field. The second approach consists of the measurement of the interference between a pair of spectrally sheared replicas of the input pulse. A direct inversion of the measured interferogram yields the electric field of the pulse.These techniques have an analog in the spatial coordinate, and are often applied by assuming that temporal and spatial features of pulses are independent. This assumption is no longer valid for situations involving beam focusing [3], pulse shaping using zero-dispersion line [4] and compression [5], or nonlinear interactions (see, e.g., [6,7]) leading to space-time coupling effects. In the past few years some techniques have been proposed to characterize the amplitude and phase profile of pulses both in space and time. Among these, the most interesting are variants of FROG and SPIDER techniques with extension to the spatial dimension characterization. Indeed, in [8] a combination of FROG and digital holography is proposed to characterize the complete (3D) electric field of a train of laser pulses possessing at least one point in space where all the frequencies are present. This method is based on the use of a tunable filter or of a series of bandpass filters. A variation of this technique is proposed in [9], which has the advantage of being a single-shot measurement. The same authors recently proposed a technique that is based on crossed-beam spectral interferometry [10]. By exploiting the coherence between a reference pulse and the one under investigation, this allows for a characterization of the field both in time and space with high spectral resolution, but requires a highresolution scan along the spatial dimension.On the other hand, the extension to the spatial dimension of the SPIDER technique has been proposed in [11][12][13]. Thanks to a direct inversion algorithm, the technique is capable of a fast reconstruction of the electric field as a function of one transverse spatial coordinate and time...