Group-velocity dispersion

Résumé

In optics, group-velocity dispersion (GVD) is a characteristic of a dispersive medium, used most often to determine how the medium affects the duration of an optical pulse traveling through it. Formally, GVD is defined as the derivative of the inverse of group velocity of light in a material with respect to angular frequency, : \text{GVD}(\omega_0) \equiv \frac{\partial}{\partial \omega} \left( \frac{1}{v_g(\omega)} \right)_{\omega=\omega_0}, where \omega and \omega_0 are angular frequencies, and the group velocity v_g(\omega) is defined as v_g(\omega) \equiv \partial \omega / \partial k. The units of group-velocity dispersion are [time]2/[distance], often expressed in fs2/mm. Equivalently, group-velocity dispersion can be defined in terms of the medium-dependent wave vector k(\omega) according to : \text{GVD}(\omega_0) \equiv \left( \frac{\partial^2 k}{\partial \omega^2}\right)_{\omega=\omega_0}

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https://en.wikipedia.org/wiki/Group-velocity_dispersion

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Peculiarities of second harmonic generation with chirped femtosecond pulses at high conversion efficiency

Pierre-Marc Jean Marie Dansette

Frequency doubling of an infrared laser radiation in non-linear optical crystals is a widely used technique to obtain light in the visible range. The second harmonic generation process is influenced by several well-known parameters. In this article we study the effect of group delay dispersion on the second harmonic generation process for femtosecond pulses. We show, both through simulation and experiments, that for certain parameters even a small amount of chirp can have a detrimental effect on the conversion efficiency as well as the second harmonic beam quality. We also check the effect of higher order dispersion. By properly accounting for those effects the crystal length and focusing conditions can be optimized to reach high conversion efficiency, while maintaining low sensitivity to chirp variations and good beam quality.

ELSEVIER2020

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Kerr nonlinearity of Thulium-doped fiber near 2 um

Adrien Billat, Camille Sophie Brès, Steevy Joyce Cordette, Svyatoslav Kharitonov, Ludovic Zulliger

The nonlinear coefficient and group velocity dispersion of a thulium-doped fiber near 2μm are evaluated via four-wave mixing. Nonlinearity of thulium-doped fiber can be used for the design of doped-fiber lasers in this spectral region.

2015

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Ultrafast electron diffraction is a powerful technique to investigate out-of-equilibrium atomic dynamics in solids with high temporal resolution. When diffraction is performed in reflection geometry, the main limitation is the mismatch in group velocity between the overlapping pump light and the electron probe pulses, which affects the overall temporal resolution of the experiment. A solution already available in the literature involved pulse front tilt of the pump beam at the sample, providing a sub-picosecond time resolution. However, in the reported optical scheme, the tilted pulse is characterized by a temporal chirp of about 1 ps at 1mm away from the centre of the beam, which limits the investigation of surface dynamics in large crystals. In this paper, we propose an optimal tilting scheme designed for a radio-frequency-compressed ultrafast electron diffraction setup working in reflection geometry with 30 keV electron pulses containing up to 10(5) electrons/pulse. To characterize our scheme, we performed optical cross-correlation measurements, obtaining an average temporal width of the tilted pulse lower than 250 fs. The calibration of the electron-laser temporal overlap was obtained by monitoring the spatial profile of the electron beam when interacting with the plasma optically induced at the apex of a copper needle (plasma lensing effect). Finally, we report the first time-resolved results obtained on graphite, where the electron-phonon coupling dynamics is observed, showing an overall temporal resolution in the sub-500 fs regime. The successful implementation of this configuration opens the way to directly probe structural dynamics of low-dimensional systems in the sub-picosecond regime, with pulsed electrons. (C) 2017 Author(s).

Amer Inst Physics2017

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