Luca Tartara

Dispersive wave generation by solitons in microstructured optical fibers.

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.

Bound electronic and free carrier nonlinearities in Silicon nanocrystals at 1550nm

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.

Enhancement of stability and efficiency of a nonlinear mirror mode-locked Nd:YVO(4) oscillator by an active Q-switch.

The stability and the peak power of a nonlinear-mirror mode-locked Nd:YVO(4) laser are significantly increased by the insertion of an acousto-optic modulator inside the laser cavity. The repetition rate for reliable operation can be varied in the range 35 kHz - 50 kHz. The laser generates the most intense and stable mode-locked pulses of width 9 ps lying underneath a Q-switched envelope of width 110 ns with a Q-switch modulation frequency of 38 kHz. For 10 W of pump power, a 224 times enhancement of peak power over that of cw mode-locking is obtained under reliable Q-switched and mode-locked operation.

Spatiotemporal amplitude and phase retrieval of space-time coupled ultrashort pulses using the Shackled-FROG technique

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-resolved-optical-gating detection at a single spatial position, we are able to retrieve the full spatiotemporal structure of a tilted pulse.

Phase-matched nonlinear interactions in a holey fiber induced by infrared super-continuum generation

By coupling 110fs pulses at lambda=1.55µm into a 45cm long holey fiber, we observe the generation of a broadband continuum, followed by the appearance of a sharp blue line at 430nm. Blue light is generated in a high-order mode by a third-harmonic generation process that is phase-matched with the super-continuum component at 1.29µm. Such an interpretation is supported by measurements performed by using 1.29µm pulses and also by simulation. In addition we find that the choice of an input polarization tilted with respect to the fiber-birefringence axis can lead to the generation of new frequencies through a four-wave mixing process that exploits non-diagonal components of the third-order susceptibility.

Wavelength shifting of optical pulses through cascaded second-order processes in a lithium–niobate channel waveguide

We investigate the efficiency of wavelength shifting (generation of a pulse at the wavelength λp−Δλ from a signal at λp+Δλ under the action of a pump at λp) through a cascaded second-order process in a lithium–niobate channel waveguide. With a 58-mm-long Ti-diffused channel waveguide, λp=1.1 μm (the wavelength of phase matching for the first step of the second-harmonic generation), Δλ of several nanometers and 20 ps pulse duration, wavelength conversion with unit efficiency is obtained with a pump pulse energy of the order of 102 pJ.

Complete retrieval of the field of ultrashort optical pulses using the angle-frequency spectrum

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.

Nonlinear characterization of hydrogenated amorphous silicon waveguides and analysis of carrier dynamics

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.

Full modal analysis of the Brillouin gain spectrum of an optical fiber

We present a numerical study of stimulated Brillouin scattering in optical fibers based on a full modal analysis of the acoustic and optical properties. The computation of each acoustic mode supported by the fiber structure allows us a deep and detailed investigation of the characteristics of the Brillouin gain spectrum. We focus our attention on optical fibers acting as acoustic antiwaveguides where the biggest contribution to the Brillouin response often comes from very high-order modes but it is sometimes overlooked because of computational issues. Our analysis clearly highlights their role and their dependence on the physical and geometrical structure of the fiber.

Frequency Shifting of Femtosecond Pulses by Reflection at Solitons

The frequency shifting of an ultrashort optical pulse through reflection at a fundamental soliton is experimentally investigated in a microstructured optical fiber. The frequency up- and down-conversion is observed with efficiencies up to 70%. Through employing femtosecond pulses, relevant features of the interaction are highlighted, and the role of either the leading or the trailing edge of the soliton is proven.