TABLE OF CONTENTS In addition to active parameter modulation discussed in the previous chapter there is another effective method of influencing the laser dynamics. This method is based on the use of elements with the optical properties, which change depending on intensity of the light illuminating them. Such elements can be responsible for a dependence of the cavity Q-factor on he laser output or, equivalently, they execute what is called 'passive Q-switching'. The decrease of losses with increasing power reduces the laser stability threshold and enhanced pulsations. On the contrary, the increase of losses with increasing power leads to more stable laser behaviour. For example, the oscillatory transient can be transformed to aperiodic one.
Sometimes, the nonlinearity of the media filling the laser cavity has greater influence on the laser dynamics than the time dependence of the parameters. This has been used to explain, for example, the undamped spiking of a neodymium glass laser, which depends on the spectral composition of pumping. In semiconductor lasers thermal and mechanical instabilities of parameters have little influence on the laser dynamics, so that material nonlinearities are the only possible cause of the frequently observed fast pulsations.
Automatic control of cavity losses was first used to remove spikes in a ruby laser [500 503]. An electrooptic Kerr cell in combination with a polarizer served as the control element. The control voltage applied to the cell electrodes was the amplified signal from a photocell that intercepted part of the laser...
