CAVITY   DUMPING

     Imagine a laser in which there was no output coupler but rather, two high reflectors in the cavity. Although there would be no usable output beam, the intracavity power would be very high, much higher than in a laser that has an output coupler (which represents a loss in the cavity). Now, if one of those mirrors was suddenly removed, a massive output pulse would result. All of the energy circulating in the cavity would

Figure 1.1. Cavity dumping using an EO cell.

exit in a single pulse with a pulse length equal to the round-trip time for radiation in the cavity. This is the premise for cavity dumping.

       Theoretically, a mechanism to remove a mirror instantly could be used as a cavity dumper, but such an arrangement would not be practical given the large time it would take to move the mirror physically. Practically, an EO modulator may be used as a cavity dumper by changing the polarization of intracavity radiation, which is then deflected from an intracavity polarizing beamsplitter as shown in Figure 1.1. With the Pockels cell unenergized, intracavity light passes through it without a phase shift. Light inside the cavity is polarized such that it passes through a polarizing beamsplitter without loss, as shown in Figure 1.1(a). When the cell is energized, it effects a polarization change of 90 degrees, and light of this polarization is reflected from the beamsplitter in a single massive pulse. The losses imposed by the cell and beamsplitter are almost 100%, more than enough to cause lasing to cease. To repeat the process, the EO cell is unenergized, lasing begins again, and an intense intracavity light builds rapidly, ready for the next pulse to be generated. Cavity dumping techniques will work with any laser, including solid-state and gas lasers, to produce fast pulses, up to about 20 times more powerful than CW laser power. Such a technique can be done repetitively to produce a train of pulses.