TABLE OF CONTENTS The peak output power of a pulsed laser increases, the more the initial inversion exceeds the threshold level. In free running operation the deviations from the threshold level do not go beyond several per cent and the peak power of a solid-state laser is limited to tens of kilowatts. Considerably more powerful pulses, called giant pulses, are obtained when lasing is delayed for a time needed for a high inversion level to be reached [580]. The goal can be achieved by Q-switching of the laser cavity. In one method the laser threshold is held high, while the population difference increases, and the threshold is then reduced rapidly to the minimum possible level once the required inversion is reached and an output pulse is desired. An alternative method uses powerful pumping to ensure a sufficiently fast growth of inversion. But the latter did not find application.
Methods of the cavity Q-switching are divided into active and passive groups. Active methods use modulation devices that change the cavity losses by a given law or in accordance with an external control signal. Passive modulating elements are those controlled directly by the radiation field in the laser cavity.
Active modulation devices are also divided into two groups: opto-mechanical and electrooptical. The simplest modulator of the first type is a punched disk of an opaque material [581]. Rotated around an axis parallel to the cavity axis, the disk blocks and...
