Chirp

Résumé

vignette|Un chirp linéaire d'amplitude constante. Un chirp (mot d'origine anglaise signifiant « gazouillis ») est par définition un signal pseudo-périodique c modulé en fréquence autour d'une fréquence porteuse et également modulé en amplitude par une enveloppe a dont les variations sont lentes par rapport aux oscillations de la phase \phi : c(t) = a(t)e^{i\phi(t)} La partie réelle de ce signal est tout simplement : Re(c(t)) = a(t)\cos{\phi(t)} Cas particulier du chirp linéaire On considère souvent le cas particulier du chirp à rampe de fréquence linéaire et à enveloppe constante A qui est tellement courant d'utilisation que l'on a tendance à ramener la notion de chirp à ce seul cas particulier : c(t) = Ae^{i(a.t+b).t+c} Dans les applications radar ou sonar le chirp linéaire est souvent le signal utilisé pour réaliser la compression d'impulsion. L'impulsion étant de durée fini

Source officielle

https://fr.wikipedia.org/wiki/Chirp

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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

Chargement

Both Parallel Imaging (PI) and Compressed Sensing (CS) can significantly accelerate the MRI acquisition process. Moreover, in the context of CS, it has been recently shown that the reconstruction quality can be drastically enhanced by a “Spread Spectrum (SS) technique” using quadratic phase profiles in MRI, which amounts to the modulation of the image probed by a linear chirp. Combination of CS and PI for higher accelerations is of great interest. In this work, we propose Parallel Spread Spectrum MR Imaging (PSSI) as a generalization of SS technique for parallel MRI. Extensive numerical simulations on the Shepp-Logan phantom together with some illustrations on real brain images demonstrate that the SS can be used as a complement of parallel imaging for higher accelerations, whilst applying this technique in parallel MRI can also drastically enhance the reconstruction quality and stability rather than CS using basic Fourier measurements.

2010

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Vibration monitoring based on optical sensing of mechanical nonlinearities in glass suspended waveguides

Yves Bellouard, Enrico Casamenti, Pieter Vlugter, Tao Yang

Vibration monitoring plays a key role in numerous applications, including machinerypredictive maintenance, shock detection, space applications, packaging-integrity monitoring andmining. Here, we investigate mechanical nonlinearities inherently present in suspended glasswaveguides as a means for optically retrieving key vibration pattern information. The principle isto use optical phase changes in a coherent light signal travelling through the suspended glasswaveguide to measure both optical path elongation and stress build-up caused by a given vibrationstate. Due to the intrinsic non-linear mechanical properties of double-clamped beams, we showthat this information not only offers a means for detecting excessive vibrations but also allows foridentifying specific vibration patterns, such as positive or negative chirp, without the need forany additional signal processing. In addition, the manufacturing process based on femtosecondlaser exposure and chemical etching makes this sensing principle not only simple, compact androbust to harsh environments but also scalable to a broad frequency range.

2021

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