When a laser is disturbed during operation, e.g. by fluctuations of the pump power, its output power does not immediately return to its steady state. Many lasers (e.g. solid-state lasers and most laser diodes) operate in the so-called class B regime, with the upper-state lifetime often being much longer than the cavity damping time. In that regime, changes in pump power lead to so-called relaxation oscillations. These are usually damped, eventually leading back to the steady state. Particularly pronounced oscillatory behavior with relatively low oscillation frequencies (often in the kilohertz regime) occurs in doped insulator solid-state lasers, whereas semiconductor lasers normally exhibit strongly damped relaxation oscillations with very high frequencies in the gigahertz region. Other lasers, e.g. many gas lasers, operating in the class A regime with an upper-state lifetime below the cavity damping time, do not exhibit relaxation oscillations, but only an exponential relaxation to the steady state.
As Figure 1 shows, class B lasers can exhibit strong spiking e.g. when the pump power is suddenly turned on. After the emission of a few spikes (pulses), the laser power exhibits damped relaxation oscillations. The oscillation frequency is similar to the inverse period of the spikes.
Calculations of relaxation phenomena can be based on the dynamic equations as presented in the article on laser dynamics, which can (for small fluctuations, not for spiking) be linearized around the steady state. In the following, the main results of such an analysis for class-B lasers are given. The frequency of the relaxation oscillations is determined by the intracavity power Pint, the resonator losses ρ, the round-trip time TR of the resonator, and the saturation energy Esat and the upper-state lifetime τg of the gain medium:
The cavity damping time corresponds to TR / ρ, and the first term in the radicand is larger than the second one in the mentioned class B regime.
For solid-state lasers (with τg >> TR), the second term of the radicand is negligible (except for operation close to threshold), so that the equation simplifies to
The equations are valid for both four-level and three-level gain media. However, only for four-level gain media can the former equation be transformed into
The damping time of the oscillations can be calculated from
For operation just above the laser threshold, the relaxation oscillations are slow, and their damping time is about twice the upper-state lifetime of the gain medium. For higher powers, the oscillations can faster, and the damping time gets shorter. For four-level lasers, the damping time is inversely proportional to the pump parameter r.
Note that a saturable absorber in the laser resonator, which may be used for passive mode locking, can strongly reduce the damping ; the oscillations can even become undamped, so that the steady state becomes unstable. This leads to the phenomenon of Q-switching instabilities and Q-switched mode locking.
The characterization of the laser dynamics can deliver useful information on the laser parameters such as the resonator losses or the gain saturation energy, thus also the laser cross sections.
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