Encyclopedia … combined with a great Buyer's Guide!

Ultrashort Pulses

Definition: optical pulses with durations of picoseconds or less

More general term: light pulses

German: ultrakurze Pulse

Category: light pulses


Cite the article using its DOI: https://doi.org/10.61835/41l

Get citation code: Endnote (RIS) BibTex plain textHTML

Light pulses as generated in mode-locked lasers can be extremely short, especially with passive mode locking. There is no universally accepted definition of “ultrashort”, but the term usually is applied to pulses whose pulse duration is at most a few tens of picoseconds, and often even in the range of femtoseconds.

Ultrashort pulses (or the lasers that produce them) are sometimes called “ultrafast” – even though these pulses are no faster (have no higher velocity) than longer pulses. However, they do have short rise and fall times, and they make it possible to investigate ultrafast processes (→ ultrafast optics). They can also be used for fast optical data transmission, where “fast” means a high data rate, not a high velocity.

Ultrashort Pulse Calculations

Average power:
Repetition rate:
Pulse energy:calc
TBP:(e.g. 0.44 for Gaussian)
Bandwidth (Hz):calc
Bandwidth (nm):calc
Peak power:calc(assuming Gaussian shape)
Beam radius:(1/e2 value)
Peak intensity:calc(on the beam axis)
Fluence:calc(on the beam axis)

Enter input values with units, where appropriate. After you have modified some values, click a “calc” button to recalculate the field left of it.

Generation of Ultrashort Pulses

Ultrashort light pulses are usually generated with passively mode-locked lasers, but sometimes also with optical parametric amplifiers (possibly using a supercontinuum as input) or with free electron lasers. It is in principle also possible to start with longer pulses and apply some method of pulse compression, but in most cases the input pulses for such compression are already ultrashort.

The article on ultrafast lasers lists some important areas of ultrashort pulse generation, including the generation of few-cycle pulses, where the pulse duration is only a small multiple of an optical cycle (few-cycle pulses).

Optical Bandwidth

Intrinsically, ultrashort pulses have a broad optical bandwidth. Even if they are instantaneous frequency is nearly constant throughout the pulse duration, the optical spectrum has a width which is at least of the order of the inverse pulse duration. This is essentially because e.g. for resonantly exciting some medium, a change of optical frequency matters only if it is large enough to cause a significant phase change within the pulse duration [2]. That is the case only if the frequency change is of the order of the inverse pulse duration.

In many cases, the instantaneous frequency of a pulse is approximately constant, and the time–bandwidth product is somewhat below unity. For example, an unchirped Gaussian pulse with a 1 ps pulse duration (full width at half maximum) has an optical bandwidth of ≈0.44 THz. Extremely short pulses (few-cycle pulses) can even have octave-spanning optical spectra, and are thus very far from being monochromatic.

Spatial Properties and Propagation

Concerning their spatial properties, ultrashort pulses are usually generated in the form of laser beams. Essentially, they can be focused to very small spots just as it is possible with stationary beams. However, various limitations come into play particularly in the regime of few-cycle pulses. For example, the broad optical bandwidth of such pulses leads to problems with the chromatic dispersion of lens materials, which leads to chromatic aberrations of the focusing optics unless special correction techniques are employed. This can lead to complicated spatio-temporal effects, which may effectively make the focused pulses longer than the pulses before focusing. Possible measures against such distortions include the use of reflective or diffractive (instead of refractive) optics as well as the careful compensation of various types of aberrations, e.g. using suitable lens combinations.

The propagation of ultrashort pulses in media gives rise to a range of interesting phenomena, particularly when optical nonlinearities are involved. This can be investigated with pulse propagation modeling. Relevant physical effects in such models can be chromatic dispersion, the Kerr effect, Raman scattering, and gain saturation, to name just some important examples. While in many cases purely one-dimensional models (ignoring much of the spatial aspects) can be used, full 3D models are needed for certain situations.

Shaping Ultrashort Pulses

When ultrashort pulses are generated in a mode-locked laser, instead laser they can be subject to various pulse shaping phenomena. For example, a saturable absorber causes high losses to the beginning and sometimes also the end of the pulse, and can thus modify the duration and shape of a pulse. The effects of optical nonlinearities and chromatic dispersion, however, are often much stronger than those of saturable absorbers.

Outside the laser, ultrashort pulses can further be manipulated with various kinds of pulse shapers.

Characterization of Ultrashort Pulses

There are various methods for pulse characterization. While some only allow the measurement of fundamental pulse parameters such as the pulse duration, others can be used for “complete” characterization in the sense that the whole time-dependent electric field and the spectral phase can be obtained. The results can be visualized in various ways, e.g. with graphs of time- or frequency-dependent functions, or with spectrograms.

Most frequently, one measures pulse durations using autocorrelators.


[1]R. Paschotta, tutorial “Modeling of Pulse Amplification
[2]Spotlight article of 2007-10-11: "Understanding Fourier Spectra"

(Suggest additional literature!)

See also: light pulses, pulse propagation modeling, pulse characterization, pulse duration, parabolic pulses, sech2-shaped pulses, pulse shapers, mode locking, mode-locked lasers, ultrafast lasers, femtosecond lasers, ultrafast laser physics, software news 2016-07-22

Questions and Comments from Users

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

Please do not enter personal data here; we would otherwise delete it soon. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.

Spam check:

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.


Share this with your friends and colleagues, e.g. via social media:

These sharing buttons are implemented in a privacy-friendly way!