Self-terminating laser

Population inversion is often achieved by a multi-level atomic or molecular system in which the excitation process, called pumping, is accomplished by electrical means, by optical methods, or by chemical reactions. In some cases, the population inversion can be sustained to produce a continuous wave (CW) output beam that is continuous with respect to time. In other cases, the lasing action is self terminating, so that the laser is operated in a pulsed mode to produce a repetitive pulse train or a single-shot action. ... 

Fig. 6.1 Time profiles of the pump power Pp(t), the inversion density AN(t), and laser output power Pl( )- (a) for sufficiently short lifetime t/ of the lower laser level and (b) for a self-terminating laser with...

Fluorescence and stimulated emission on transitions (p = 0 -> p" > 0) lead to a fast rise of the population densities A (p") of high vibrational levels in the Sq state. This would result in a self-termination of the laser oscillation if these levels were not depopulated quickly enough by collisions. The relaxation of N(v") toward the thermal equilibrium population No(v") can again be probed by a weak visible probe laser. Polarization spectroscopy (Sect. 2.4) with femtosecond pulses allows one to separately determine the decay times tvib of population redistribution and the dephasing times [814]. 

Self-terminating laser Laser in which the population inversion terminates naturally when the lower state becomes sufficiently populated from the upper state to destroy the inversion. 

In some pulsed lasers (for example, the N2 laser) the lower laser level has a longer effective lifetime than the upper level [11.14]. The increasing laser power Pl(0 decreases the population inversion by stimulated emission. Since the lower level is not sufficiently quickly depopulated, it forms a bottleneck 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). 

In some active media of pulsed lasers the lower laser level has a longer lifetime than the upper level. After a time T the increasing induced emission populates the lower level faster than it is depopulated by spontaneous emission and the inversion drops below threshold. In such "self-terminating" lasers the duration of the laser pulse is limited by the slow depopulation rate of the lower level and the laser emission may stop even before the pulsed pump drops below the initial threshold. One example of this laser type is the N2 laser [11,4] which is pumped by a fast transverse gas discharge ( 100 ns duration) and delivers output pulses of a few ns (1-10 ns) with typical peak powers of 0.1-1 MW. 

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