Femtosecond and transform limited ultrashort laser pulses

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The main structure element for generating femtosecond light pulses are lasers. Within two decades after the laser development  the duration of the shortest pulse fall by six orders of magnitude from the nanosecond to the femtosecond regime. Nowadays femtosecond pulses in the range of 10 fs and below can be generated directly from compact and reliable laser oscillators. With the help of some simple comparisons, the incredibly fast femtosecond time scale can be put into perspective. For example, on a logarithmic time scale, one minute is approximately halfway between 10 fs and the age of the universe. Taking the speed of light in vacuum into account, a 10 fs light pulse can be considered as a 3 µm thick slice of light whereas a light pulse of one-second span approximately the distance between earth and moon. It is also useful to realize that the fastest molecular vibrations in nature have an oscillation time of about 10 fs. It is the unique attributes of these light pulses that open up new frontiers both in basic research and for applications. The ultrashort pulse duration, for example, allows freezing the motion of electrons and molecules by making use of pump-probe techniques that work similar to strobe light techniques.

A bandwidth-limited pulse (also called  transform limited pulse) is a pulse of a wave that has the minimum possible duration for a given spectral bandwidth. Optical pulses of this type can be generated mode-locked lasers. Transform-limited pulses have a constant phase across all frequencies making up the pulse. The length of a pulse thereby is determined by its complex spectral components, which include not just their relative intensities, but also the relative positions of these spectral components. For different pulse shapes, the time-bandwidth product is different.

A transform limited laser pulse can only be kept together if the dispersion of the medium the wave is travelling through is zero; otherwise, dispersion management is needed to revert the effects of unwanted spectral phase changes. For example, when an ultrashort laser pulse passes through a block of glass, the glass medium broadens the pulse due to group velocity dispersion.

Ultrafast lasers have been of increasing interest in material processing applications due to their capability of precise micromachining of a large variety of materials: metals, semiconductors, polymers, dielectrics, biological materials, etc.

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