Laser dynamics refers to the evolution of certain quantities of lasers over time, such as optical power and gain.

The dynamic behavior of the laser is determined by the interaction between the optical field in the cavity and the gain medium. Generally speaking, the laser power will vary with the difference between the gain and the resonant cavity, and the rate of change of the gain is determined by the process of stimulated emission and spontaneous emission (it may also be determined by the quenching effect and the energy transfer process).

Some specific approximations are used. For example, the laser gain is not too high. In a continuous light laser, the relationship between the laser power P and the gain coefficient g in the cavity satisfies the following coupling differential equation:

Where T_R is the time required for one round trip in the cavity, l is the cavity loss, g_ss is the small signal gain (at a given pump intensity), τ_g is the gain relaxation time (usually close to the upper energy state lifetime), and E_sat is the saturated absorption energy of the gain medium.

In continuous wave lasers, the most concerned dynamics are the switching behavior of the laser (usually including the formation of output power spikes) and the working state when there is a disturbance in the working process (usually a relaxation oscillation). In these respects, different types of lasers have very different behaviors.

For example, doped insulator lasers are prone to spikes and relaxation oscillations, but laser diodes are not. In a Q-switched laser, the dynamic behavior is very important, where the energy stored in the gain medium will change greatly when the pulse is emitted. Q-switched fiber lasers usually have very high gains, and there are some other dynamic phenomena. It usually causes the pulse to have some substructures in the time domain, which can not be explained by the above equation.

A similar equation can also be used for passive mode-locked lasers; then the first equation needs to add an additional term to describe the loss of the saturable absorber. The result of this effect is that the attenuation of the relaxation oscillation is reduced. The relaxation oscillation process does not even attenuate, so the steady-state solution becomes no longer stable, and the laser has some instability of Q-switched mode-locking or other types of Q-switching.