Generation of Short Laser Pulses

For laser pulses, on the other hand, the time duration of the pulse is not necessarily limited by the duration of the pump pulse but may be much shorter. Before we present different techniques to achieve ultrashort laser pulses we will discuss the relations between the relevant parameters of a laser which determine the time profile of a laser pulse. 

In active laser media pumped by a pulsed source (e.g., flashlamps, electron pulses, or pulsed lasers) the population inversion necessary for oscillation threshold can be maintained only over a time interval aT that depends on duration and power of the pump pulse. A schematic time diagram of pump pulse, population inversion, and laser output is shown in Fig.11.1. As soon as threshold is reached, the laser emission starts. If the pump power is still increasing, the gain becomes high and the laser power rises faster than the inversion, until the increasing induced emission reduces the inversion to the threshold value. 

The behavior of the laser output strongly depends on the time scale of relaxation processes. If they are slow, the induced emission drives the population inversion even below threshold and the laser output discontinues until the pump has reproduced sufficient population inversion to reach threshold, where induced emission starts again. The laser output consists in such cases of more or less irregular spikes , with typical pulsewidths of a few ys, which appear during the whole time interval AT while the pump power is above threshold (Fig.11.2). The flashlamp-pumped ruby laser is a typical example of such a spiking laser. 

If the relaxation processes are sufficiently fast, they rapidly damp the oscillatory fluctuations of the population inversion and a steady-state inversion is reached which equals the threshold inversion (see Sect,5.7). 

In these cases the laser output follows the pump pulse (see Sect.5.9) and ceases when the pump power drops below threshold. 

Because of the short radiative lifetimes of most electronically excited free atoms or molecules, the output power of many gas lasers does not show spikes, but follows quasistationarily the time dependence of the pump pulse. Since the relaxation rate of excited dye molecules is in the sub-nanosecond range, dye lasers also generally have no spiking behavior and Fig.11.1 applies to flashlamp-pumped arid laser-pumped dye lasers. 

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In Table 6.1 the different techniques for the generation of short laser pulses and their typical parameters are compared. It shows that pulse widths below I ps can be achieved with the CPM technique and with Kerr lens mode-locking. In the next section we will discuss how short laser pulses can be further compressed by nonlinear effects in optical fibres. 

The lasers we have considered so far have a solid material as the active medium. Generally, flash-lamp pumping is employed and short pulses are obtained at a repetition rate of typically 10 Hz. High peak powers are obtained in the Q-switched mode (MW-GW). Gaseous media can also be used for the generation of short laser pulses. We shall here consider the nitrogen laser, the excimer laser and the copper vapour laser. The relevant parts of... 

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