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Ultrafast Lasers

Definition: lasers emitting ultrashort pulses

Alternative terms: femtosecond lasers, picosecond lasers

More specific terms: picosecond lasers, femtosecond lasers, mode-locked lasers, mode-locked fiber lasers, mode-locked diode lasers, titanium–sapphire lasers

German: Ultrakurzpulslaser

Categories: laser devices and laser physicslaser devices and laser physics, light pulseslight pulses


Cite the article using its DOI: https://doi.org/10.61835/rx4

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The term ultrafast lasers is used for different kinds of lasers and laser systems:

  • There are mode-locked lasers emitting ultrashort pulses, i.e. light pulses with durations of femtoseconds or picoseconds: mostly below 100 ps, often even well below 100 fs. These are nearly always mode-locked lasers, although e.g. gain switching can also provide ultrashort pulses in the picosecond domain. Typical pulse repetition rates of ultrafast lasers are of the order of 100 MHz, but it is also possible to have only a few megahertz or many gigahertz.
  • In some cases, one employs cavity dumping to a mode-locked laser for obtaining pulse trains with higher pulse energy at a lower pulse repetition rate (e.g. 1 MHz instead of 100 MHz).
  • The term is also used for ultrafast laser systems comprising an ultrafast laser and some kind of pulse amplifier. In some cases, the high repetition rate pulse train of a mode-locked laser is just amplified to a higher power level, raising the pulse energy as much as the average power. In other cases, the pulse repetition rate is substantially or even dramatically reduced with a pulse picker. The pulse energy obtainable with amplification can then be much higher; in extreme cases, it is multiple joules, and the peak power can be extremely high (in the terawatt or even petawatt region).

A more precise but less common term is actually ultrashort pulse lasers; such lasers utilize ultrafast processes and emit light with very fast changes of optical power, but are strictly speaking not ultrafast themselves.

Types of Ultrafast Lasers

The most important types of ultrafast lasers (without amplifiers) are briefly listed in the following:

See the article on ultrafast amplifiers for common type of amplifiers, including both laser amplifiers and parametric amplifiers.

Physical Phenomena

The following phenomena of ultrafast optics and ultrafast laser physics are most relevant in ultrashort pulse lasers:

The research area of ultrafast lasers and their applications is called ultrafast laser physics and ultrafast optics. It deals with all kinds of effects occurring in these lasers, but also with phenomena which can be investigated using ultrashort laser pulses. Examples of such application areas are high-intensity physics (→ high harmonic generation), frequency metrology, laser spectroscopy, and terahertz science.

Developments in the Field of Ultrashort Pulse Generation

The field of ultrashort pulse generation has had roughly three decades to develop and can thus be considered relatively mature. Some of the most important developments which are more or less finished are listed in the following:

Further developments can be expected in the near future:

  • The choice of solid-state gain media is still growing. New laser crystal materials with interesting properties have been developed, which promise superior performance or even entirely new achievements. For example, new ytterbium-doped laser gain media such as sesquioxides and tungstates could prove even better than Yb:YAG for thin-disk lasers with even higher powers in ultrashort pulses, or for shorter pulses at high power levels. On the other hand, ultrabroadband gain media such as Cr2+:ZnSe should be suitable for the generation of pulses with 20 fs duration or less in the spectral region around 2.7 μm, even though this expectation has not been fulfilled, without the reason being very clear so far. (Excessive nonlinearity is perhaps an explanation.)
  • Mode-locked fiber lasers [18] have been showing impressive advances in performance for several years. It is to be expected that this development will continue, also for commercial lasers, although some fundamental limitations arise from fiber nonlinearities. Most promising is the potential for cost reduction in cases where performance requirements are moderate and production quantities are high. See also the article on fiber lasers versus bulk lasers.
  • Ultrafast amplifier devices for lower repetition rates (mainly diode-pumped regenerative amplifiers) will become more and more important for laser material processing, e.g. in the form of laser micromachining.
  • Passively mode-locked VECSELs (see above) surely have the potential for significant further advances in performance, particularly in the area of multi-gigahertz repetition rates combined with multi-watt output powers and/or sub-picosecond pulse durations. Furthermore, the application of wafer-scale technologies may allow mode-locked VECSELs to be fabricated at very low cost, making possible new application fields with stringent cost limits.

Concerning applications, it is to be expected that many more ideas will be generated. Note that certain parameter regions have only recently be accessible with laser sources, so that those working on the application side can start thinking about using such sources, some of which should soon become commercially available. It appears realistic to expect that ultrafast technology will gain further importance and permit new exciting developments.

Applications of Ultrafast Lasers

The output of an ultrafast laser has various remarkable properties which are of interest for a wide range of applications in fundamental research. There is also a wide range of industrial applications, which have become more attractive with the advent of compact, powerful and cost-effective mode-locked lasers.

The applications include very diverse areas, and exploit different aspects of ultrashort pulses; some examples:

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[18]M. E. Fermann, “Ultrafast fiber oscillators”, in Ultrafast Lasers: Technology and Applications (eds. M. E. Fermann, A. Galvanauskas, G. Sucha), Marcel Dekker, New York (2003), Chapter 3, pp. 89–154
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Dr. R. Paschotta

This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics AG. How about a tailored training course from this distinguished expert at your location? Contact RP Photonics to find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, training) and software could become very valuable for your business!

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