12. Short and Ultrashort Laser Pulses

In this contribution some basic properties of femtosecond laser pulses are summarized. In Sect. 12.1 we start with the linear properties of ultrashort light pulses. Nonlinear optical effects that would alter the frequency spectrum of an ultrashort pulse are not considered. However, due to the large bandwidth, the linear dispersion is responsible for dramatic effects. For example, a 10 fs laser pulse at a center wavelength of 800 nm propagating through 4 mm of BK7 glass will be temporally broadened to 50 fs. In order to describe and manage such dispersion effects a mathematical description of an ultrashort laser pulse is given first before we continue with methods how to change the temporal shape via the frequency domain. The chapter ends with a paragraph on the powerful technique of pulse shaping, which can be used to create complex-shaped ultrashort laser pulses with respect to phase, amplitude and polarization state.

In Sect. 12.2 the generation of femtosecond laser pulses via mode locking is described in simple physical terms. As femtosecond laser pulses can be generated directly from a wide variety of lasers with wavelengths ranging from the ultraviolet to the infrared no attempt is made to cover the different technical approaches.

In Sect. 12.3 we deal with the measurement of ultrashort pulses. Traditionally a short event has been characterized with the aid of an even shorter event. This is not an option for ultrashort light pulses. The characterization of ultrashort pulses with respect to amplitude and phase is therefore based on optical correlation techniques that make use of the short pulse itself. Methods operating in the time–frequency domain are especially useful.