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Optical pulses are flashes of light, which are in most cases generated by lasers (→ laser pulses) and delivered in the form of laser beams. Due to the enormously high optical frequencies, optical pulses can be extremely short (ultrashort) when their optical bandwidth spans a significant fraction of the mean frequency. For example, a Gaussian pulse with a center frequency of 300 THz (corresponding to a wavelength of 1 μm) can easily have a bandwidth of 30 THz, and this already corresponds to a pulse duration of ∼15 fs (femtoseconds) or 0.000000000000015 seconds if the pulses are transform-limited. The shortest optical pulses generated directly in lasers have durations around 5 fs, corresponding only to a few optical cycles (→ few-cycle pulses). Pulse compression techniques get down to very few femtoseconds, and high harmonic generation even allows the generation of attosecond pulses. On the other hand, many commercially important laser sources (particularly Q-switched lasers) generate nanosecond pulses, which also have many applications.
Due to the short pulse durations and the potential for strong focusing, optical pulses can be used for generating extremely high optical intensities even with moderate pulse energies. For example, a 10-fs pulse with only 10 mJ energy has a peak power of the order of 1 TW = 1000 GW, corresponding to the combined power of roughly 1000 large nuclear power stations. This power may be focused to spots with only a few micrometers diameter. Therefore, amplified ultrashort pulses are very important for high intensity physics, studying phenomena like multi-photon ionization, high harmonic generation, or the generation of even shorter pulses with attosecond durations.
Depending on the required pulse duration, pulse energy, and pulse repetition rate, different methods of pulse generation and pulse characterization are used, in total covering extremely wide parameter regimes. Ultrashort pulses (with picosecond or femtosecond durations) are usually generated with mode-locked lasers, whereas Q-switched lasers typically allow for nanosecond pulse durations.
Pulse propagation in media has many interesting aspects. The peak of a pulse in a transparent medium propagates with the group velocity, not the phase velocity. Dispersion can cause temporal broadening (or sometimes compression) of pulses. For high peak intensities, optical nonlinearities can strongly affect the pulse propagation; often they lead to pulse broadening, but strong nonlinear pulse compression is also possible.
There are various methods for measuring the achieved pulse duration or for pulse characterization in other respects. Particularly for measuring the duration of ultrashort pulses, purely optical techniques are very important, since electronics are too slow for such purposes.
See also: ultrashort pulses, pulse generation, pulse characterization, double pulses, pulse propagation modeling, pulsed lasers, pulse duration, pulse energy, pulse repetition rate, carrier-envelope offset, spectral phase, ultrafast lasers, mode-locked lasers, Q-switched lasers
