定义：
脉宽为皮秒或者更短的光脉冲

锁模激光器，特别是被动锁模的锁模激光器，产生的光脉冲可以是非常短的。对于“超短”的范围并没有严格的定义，但一般来说超短脉冲的脉宽最多也就数十皮秒，大多数情况下，其脉宽为飞秒量级。 
需要注意的是与超短脉冲经常出现的一个概念为“超快”（ultrafast），事实上超短脉冲并没有比更长的脉冲具有更快的速度，只是超短脉冲通常可以用来研究超快过程（参阅超快光学），也可以用于高速光学数据传输。后者的高速指的是高传输速率，而不是高传输速度。 
超短脉冲通常通过被动锁模激光器产生，但有时也利用光参量放大器（可能是使用超连续进行输入）或自由电子激光器。有时也可以利用更长的脉冲进行脉冲压缩得到超短脉冲。超快激光器一文列出了超短脉冲产生的方法和特征，其中包括仅有几个光学周期的脉冲产生。 
超短脉冲通常是以激光束的形式产生的。本质上来说，它们可以被集中到很小的光斑，但是多种限制因素在超短脉冲尤其是几个周期的脉冲时产生很大的影响。例如，这种脉冲的超宽的光学带宽会导致透镜材料的色散产生很大问题，从而导致了聚焦时的色差问题，这种问题需要进行特殊校正技术进行解决。这还会导致复杂的时空问题，使得聚焦的脉冲脉宽比聚焦前的脉宽大。针对这种扭曲的措施包括使用反射或衍射（而不是折射）的光学设计以及各种像差的仔细补偿，如使用合适的镜头组合。 
超短脉冲在介质中的传播会引起一系列有趣的现象，在涉及到光学非线性时尤为明显。这些现象可以用模拟脉冲传播的方法进行研究。相关的物理效应包括色散，克尔效应，拉曼散射，增益饱和等等。 
有多种方法用于脉冲基本参数的测量来进行脉冲的表征，不仅仅包括基本的参数如脉冲脉宽，还有是“完整”表征如整个时域上的电场和谱相位。这些结果可以以各种方式，例如时间上或频域上的函数以及光谱图。

Definition: optical pulses with durations of picoseconds or less

More general term: light pulses

Light pulses as generated in mode-locked lasers can be extremely short, particularly for passive mode locking. There is no commonly accepted definition of “ultrashort”, but usually this label applies to pulses if their pulse duration is at most a few tens of picoseconds, and often even in the range of femtoseconds.

Ultrashort pulses (or lasers generating them) are sometimes called “ultrafast” – although these pulses are not faster (do not have a higher velocity) than longer pulses. They do, however, 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; in that case, “fast” means a high data rate, not actually a high velocity.

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 [1]. 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.

Bibliography