Time Profiles of Pulsed Lasers

In active laser media pumped by a pulsed source (e.g., flashlamps, electron pulses, or pulsed laser) 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 a pump pulse, the population inversion, and the laser output is shown in Fig. 11.1. As soon as threshold is reached, laser emission starts. If the pump power is still increasing, the gain becomes high and the laser power rises 

The time profile of the laser pulse is not only determined by the amplification per round trip G(t) (Sect.5.2) but also by the relaxation times Tj, Tjj of the upper and lower laser levels. If these times are short compared to the rise time of the pump pulse, quasi-stationary laser emission is reached, where the inversion AN(t) and the output power PL(t) have a smooth time profile, determined by the balance between pump power Pp(t) which creates the inversion, and laser output power P Ct) which decreases it. Such a time behaviour, which is depicted in Fig. 11.1a, can be found, for instance, in many pulsed gas lasers such as the excimer lasers (Sect.5.7). 

In some pulsed lasers (e.g., the N2 laser) the lower laser level has a longer effective lifetime than the upper level [11.14]. The increasing laser power Pl(1) decreases the population inversion by stimulated emission. Since the lower level is not sufficiently fast depopulated it forms the bottle neck for maintaining threshold inversion. The laser pulse itself limits its duration and it ends before the pump pulse ceases (self terminating laser) (Fig. 11.1b). 

For time-resolved laser spectroscopy pulsed dye lasers are of particular relevance due to the continuously tunable wavelength. They can be pumped by flashlamps (T ss 1/xs to 1ms), by other pulsed lasers, for example, by copper-vapor lasers (T si 50ns), excimer lasers (T s 15ns), nitrogen lasers (T = 2rl0ns) or frequency-doubled Nd-YAG-lasers (T = SrlSns). Because of the short relaxation times rj, (=il0 s) no spiking occurs and the situation of Fig. 11.1a is realized (Sect.5.7). The dye-laser pulses have durations 

If the relaxation times r/, are long compared to the rise time of the pump pulse, a large inversion A A may build up before the induced emis- 

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