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Encyclopedia of Laser Physics and Technology

© Dr. Rüdiger Paschotta

Last update: 2008-05-09

Definition: lasers emitting ultrashort pulses

The term ultrafast lasers is often used for mode-locked lasers emitting ultrashort pulses, i.e., pulses with durations of femtoseconds or picoseconds. A more precise term is actually ultrashort pulse lasers. These are nearly always mode-locked lasers, although e.g. gain switching can also provide ultrashort pulses.

Types of Ultrafast Lasers

The most important types of ultrafast lasers are briefly listed in the following:

• Titanium-sapphire lasers, often Kerr lens mode-locked, can generate the shortest pulses with durations down to approximately 5 femtoseconds. Their average output power of typically hundreds of milliwatts, combined with a pulse repetition rate of e.g. 80 MHz and a pulse duration of some tens of femtoseconds or shorter, leads to an enormous peak power. Unfortunately, Ti:sapphire lasers require pump light from some green laser, which makes them more complex and expensive.
• There are various kinds of diode-pumped lasers, based e.g. on ytterbium-doped or chromium-doped laser crystals. They are often passively mode-locked with a SESAM. While the pulse durations are not as short as those possible with Ti:sapphire lasers, there are diode-pumped ultrafast lasers covering wide parameter regions in terms of pulse duration, pulse repetition rate, and average power (see below).
• Fiber lasers can also be passively mode-locked, e.g. using nonlinear polarization rotation or a SESAM. They are more limited than bulk lasers in terms of average power and particularly peak power, but may conveniently be combined with a fiber amplifier. The article on mode-locked fiber lasers gives more details.
• Mode-locked diode lasers can either be monolithic devices or external cavity diode lasers, and may be actively, passively or hybrid mode-locked. Typically, mode-locked diode lasers operate at rather high (multi-GHz) pulse repetition rates with moderate pulse energy.
• An ultrafast laser oscillator can be part of an ultrafast laser system which may also contain an amplifier (e.g. a fiber amplifier) in order to increase the peak power and average output power.

Physical Phenomena

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

• The Kerr effect leads to self-phase modulation, which is a refractive index change which instantly follows the pulse intensity. It also allows for Kerr lens mode locking. Related nonlinearities such as Raman scattering and self-steepening occur when the nonlinearity has a finite response time. Chromatic dispersion has a great influence on the effect of such nonlinearities on the pulse formation.
• Saturable absorbers used for passive mode locking introduce optical losses which are reduced for high optical intensities. Even for so-called slow absorbers, the change in losses usually recovers within picoseconds after the passage of a pulse.

The research area of ultrafast lasers and their applications is called ultrafast laser physics. It deals with all kinds of effects occurring in these lasers, but also with phenomena which can be investigated using ultrashort laser pulses. Examples for such application areas are high intensity physics (→ high harmonic generation), frequency metrology, laser spectroscopy, and terahertz science. There is also a wide range of industrial applications, which becomes more attractive with the advent of compact, powerful and cost-effective mode-locked lasers, and includes diverse areas as femtosecond material processing (particularly micromachining, waveguide writing), medical treatments (e.g. in ophthalmology), laser microscopy and tomography, metrology (e.g. with frequency combs), characterization of high-speed electronics with electro-optic sampling, terahertz spectroscopy via optical sampling, and optical fiber communications.

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:

• While dye lasers dominated the field at earlier times, these have been nearly completely replaced with long-lived, powerful and efficient diode-pumped solid-state lasers. Basically only for some special spectral regions, dye lasers are still used. Their competitors are synchronously pumped optical parametric oscillators.
• An important technological development is that of semiconductor saturable absorber mirrors (SESAMs, [10]) as very versatile devices mainly for passive mode locking. While such devices have been used already in the early 1990s, their benefits have been greatly increased later on by improved SESAM designs, a wider range of used semiconductor materials, improved fabrication techniques, and last not least by a now much more solid understanding of their optimum use – particularly in extreme parameter regions. The latter has been achieved only in the years around 2000. Today, SESAMs can be used in very wide parameter regions concerning pulse duration, laser wavelength and power level.
• The pulse duration achievable with solid-state lasers (without external pulse compression) has come down to the region around 5.5 femtoseconds, corresponding to about two optical oscillation cycles (→ few-cycle pulses) [5,6]. This is done with Kerr-lens mode-locked titanium-sapphire lasers. The resulting optical spectra are extremely wide, with ultrabroad bandwidths of the order of an octave (→ octave-spanning spectra), even though the full width at half maximum is usually somewhat smaller.
• Further shortening of ultrashort pulses is possible with pulse compression techniques. Advanced compression setups allow for pulse durations below 3 femtoseconds (although the concept of pulse duration becomes somewhat nontrivial in this parameter regime). The technique of high harmonic generation even allows the generation of attosecond pulses.
• Diode-pumped solid-state lasers, particularly those based on ytterbium, have been developed for extremely high average output powers of up to 80 watts [9,11] and for pulse energies of multiple microjoules [12]. This progress was based on thin disk laser heads, an improved understanding of Q-switching instabilities, damage issues of saturable absorbers, resonator design, and most importantly on a solid understanding of the complicated interplay of all these aspects.
• Both Nd:YVO₄ lasers and erbium-doped bulk lasers with miniature resonators have been developed for the generation of pulse trains with extremely high pulse repetition rates of tens of gigahertz or even well above 100 GHz. This required mainly optimized resonator designs and an improved understanding of Q-switching instabilities. Such lasers are emitting picosecond pulses with moderate average output powers.
• After many years where mode-locked semiconductor lasers were limited to rather low output powers, novel optically pumped passively mode-locked vertical external cavity surface-emitting lasers (VECSELs) have been demonstrated to be suitable for a combination of high (multi-GHz) pulse repetition rates with multi-watt average output powers.
• Nonlinear frequency conversion (e.g. with optical parametric oscillators) has been proven to work well even at extremely high power levels. In some respects, the setups can even become simpler than those of lower-power devices. Access to a wider range of wavelengths is crucial for some applications, such as laser projection displays (→ RGB sources).

Further developments can be expected for the near future:

• The choice of solid-state gain media is still growing. New laser crystal materials with interesting properties are developed, which allow superior performance or even entirely new achievements. For example, new ytterbium-doped gain media such as sesquioxides or 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 hasn't been fulfilled within several years, without the reason being very clear so far. (Excessive nonlinearity is perhaps an explanation.)
• Even without improved gain media, it should be possible to push passively mode-locked thin-disk lasers to even higher power levels beyond 100 W.
• Mode-locked fiber lasers [14] are showing impressive advances in performance for several years already. It has to be expected that this development will go further, making mode-locked fiber lasers fierce competitors for some of their bulk laser counterparts (see also: fiber lasers versus bulk lasers).
• Amplifier devices for lower repetition rates (mainly diode-pumped regenerative amplifiers) will become important for material processing, e.g. for micromachining.
• Being around only for a few years, passively mode-locked VECSELs (see above) surely have a potential for significant further advances of performance, particularly in the area of multi-GHz 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 has 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 people on the application side can start thinking about using such sources, which partly should soon become commercially available. It appears realistic to expect that ultrafast technology will further gain importance and will be the basis for new exciting developments.

Bibliography

See also: ultrafast laser physics, pulse generation, mode locking, passive mode locking, mode-locked lasers, Kerr lens mode locking, titanium-sapphire lasers, femtosecond lasers, picosecond lasers, ultrashort pulses, optical sampling, laser applications

Categories: lasers, pulses

This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. Contact this distinguished expert in laser technology, nonlinear optics and fiber optics, and find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, or staff training) could become very valuable for your business!