A shot of the laser system begins in the oscillator section, which produces the initial weak pulse of light that is then admitted to the chain of power amplifiers. The oscillator section consists of the master oscillator and of the preamplifier. The master oscillator generates a sequence of several nearly identical pulses.
One of these pulses, occurring before the intensity in the train reaches its maximum, is chosen by an electro-optical preselector (not represented in the picture) and is led via a clever optical arrangement into the preamplifier to gain energy. Next, this pulse is used to activate the laser-triggered spark gap which serves as a fast electrical switch admitting the voltage signal to the Pockels cells PC1 and PC2. Upon receiving this triggering electrical signal, the Pockels cells become transparent for the next pulse in the train, which is consequently selected for the laser shot. The energy of this pulse is then increased in the preamplifier to about 10 millijoules. The electric signal generated by the spark gap also triggers the Pockels cell PC3 serving as a gate isolating the laser chain from the “noise” — so called amplified spontaneous emission or ASE — produced by the preamplifier. In addition, this signal triggers the Pockels cells PC4 and PC5 further in the laser chain (see the detailed Asterix IV/PALS scheme).
The length of the laser pulse supplied by the oscillator section is determined by the pressure of the active medium in the oscillator and by the power fed into a device called acousto-optical mode coupler that is an integral oscillator’s part. The length of the pulse (~800ps) is reduced as the pulse propagates through the laser chain and the early pulse saturates the active medium, inhibiting amplification of the late pulse. In a similar fashion the pulse is shortened also in the saturable absorber (see the detailed Asterix IV/PALS scheme), where its leading edge is discriminated until the dye becomes bleached. The pulse length is thus reduced on its way through the system by a factor of ~2, resulting in the pulse duration about 350 ps.
The short pulses in the oscillator are produced using the technique of mode locking. This is based on appropriate fixing the phases of all longitudinal modes that may exist in the laser resonator. The frequencies at which the laser may oscillate correspond to wavelengths, an integer multiple of which equals twice the resonator length.