In a regenerative amplifier it is necessary to trap a laser pulse in a cavity for up to a microsecond and then send it into the rest of the laser system. The time the laser pulse spends in the cavity determines the number of round trips. The pulse length, although often significantly shorter than the cavity round trip time, can in some cases approach it. The round trip time also determines the size of the cavity. In some systems, particularly those that have a gain medium with a long energy storage time, it is necessary to switch a pockels cell to V_λ/2, hold it there for 1μs and then switch it off again. The transitions may have to be fast, e.g. ~1ns, and well synchronised, i.e. better than 1ns accuracy.
It is hard to make pulse generators that can achieve this performance and until recently only hard valve devices could offer this. Alternative systems with more than one pockels cell could do the job but degrade the cavity because of the many surfaces involved.
Using a single crystal double pockels cell and a pair of avalanche pulsers we can build a suitable alternative. A single crystal 5mm in diameter with three electrodes is used as a series pair of pockels cells.
The centre electroe is grounded and the two ends of the crystal as switched to the V_λ/2 voltage at relevant but completely independently determined times.
The pulsers which drive each end of the crystal only have to charge up each end of the cell quickly and then hold the charge there for longer than the required cavity opening time. The centre ground electrode helps to isolate the pulser systems from each other.
A schematic of the system is shown. Pulser 1 opens the cavity and then at some time later pulser 2 closes the cavity. As the two events are independent it should be possible to monitor the laser pulse build up in the cavity and switch it out when it has reached the required amplitude, subject to the total delay being less than about 1μs. We have yet to build such a system.
The pockels effect has several factors contributing to it which operate over different time scales. It is found that the voltage required to achieve λ/2 switching falls gradually over about 1μs. The pulsers have discharge circuits which, to first order, match this behaviour so that the transmission of a pockels cell and polarisers is maintained over the full 1μs.
This system has distinct advantages over others, namely:-

The transmission characteristics of such a system have been obtained with a cw laser diode and a sampling system. Results are shown in the to the left. The measurements indicate that rise and fall times of ~1ns are achievable for pulse durations from zero to >1μs.
In laser systems that use a gain medium with a short storage time the pockels cell can be driven by two short pulses separated by the time the pulse is trapped in the cavity. The three electrode cell system offers the possibility of separate control of the two events with no problems of cross talk between the pulse sources and yet only as many optical surfaces as a single pockels cell.