In optics, an ultrashort pulse, also known as an ultrafast event, is an electromagnetic pulse whose time duration is of the order of a picosecond (10−12 second) or less. Such pulses have a broadband optical spectrum, and can be created by mode-locked oscillators. Amplification of ultrashort pulses almost always requires the technique of chirped pulse amplification, in order to avoid damage to the gain medium of the amplifier.
They are characterized by a high peak intensity (or more correctly, irradiance) that usually leads to nonlinear interactions in various materials, including air. These processes are studied in the field of nonlinear optics.
In the specialized literature, "ultrashort" refers to the femtosecond (fs) and picosecond (ps) range, although such pulses no longer hold the record for the shortest pulses artificially generated. Indeed, x-ray pulses with durations on the attosecond time scale have been reported.
The 1999 Nobel Prize in Chemistry was awarded to Ahmed H. Zewail, for the use of ultrashort pulses to observe chemical reactions at the timescales on which they occur, opening up the field of femtochemistry.
There is no standard definition of ultrashort pulse. Usually the attribute 'ultrashort' applies to pulses with a duration of a few tens of femtoseconds, but in a larger sense any pulse which lasts less than a few picoseconds can be considered ultrashort. The distinction between "Ultrashort" and "Ultrafast" is necessary as the speed at which the pulse propagates is a function of the index of refraction of the medium through which it travels, whereas "Ultrashort" refers to the temporal width of the pulse wavepacket.
A common example is a chirped Gaussian pulse, a wave whose field amplitude follows a Gaussian envelope and whose instantaneous phase has a frequency sweep.
The real electric field corresponding to an ultrashort pulse is oscillating at an angular frequency ω0 corresponding to the central wavelength of the pulse. To facilitate calculations, a complex field E(t) is defined.
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vignette|Un chirp linéaire d'amplitude constante. Un chirp (mot d'origine anglaise signifiant « gazouillis ») est par définition un signal pseudo-périodique modulé en fréquence autour d'une fréquence porteuse et également modulé en amplitude par une enveloppe dont les variations sont lentes par rapport aux oscillations de la phase : La partie réelle de ce signal est tout simplement : On considère souvent le cas particulier du chirp à rampe de fréquence linéaire et à enveloppe constante qui est tellement courant d'utilisation que l'on a tendance à ramener la notion de chirp à ce seul cas particulier : Dans les applications radar ou sonar le chirp linéaire est souvent le signal utilisé pour réaliser la compression d'impulsion.
L'automodulation de phase (self-phase modulation, souvent abrégé en SPM en anglais) est un effet optique non linéaire d'interaction lumière-matière (gaz, solide, liquide). Une impulsion ultra-courte, lorsqu'elle voyage dans un milieu matériel, induit une variation de l'indice de réfraction de ce milieu par effet Kerr. Cette variation induit à son tour un décalage de phase dans l'impulsion, ce qui conduit à une modification du spectre en fréquence de l'impulsion.
A bandwidth-limited pulse (also known as Fourier-transform-limited pulse, or more commonly, transform-limited pulse) is a pulse of a wave that has the minimum possible duration for a given spectral bandwidth. Bandwidth-limited pulses have a constant phase across all frequencies making up the pulse. Optical pulses of this type can be generated by mode-locked lasers. Any waveform can be disassembled into its spectral components by Fourier analysis or Fourier transformation.