Pockels Cell & Q-Switch Driving

The crystal presents a small capacitance to the driver, typically a few to tens of picofarads. Switching it means moving kilovolts across that capacitance in nanoseconds, which takes amperes of peak current (Figure 1), at a set instant relative to the pump and the cavity. The driver therefore has to deliver a fast high-voltage edge into a capacitive load with low jitter, because in a Q-switched cavity the timing of the switch sets the timing and the energy of the emitted pulse.

Jitter is the second constraint, and it is where cavity dumping is most demanding. Dumping requires the switch to fire at a precise moment in the intracavity build-up, and any shot-to-shot variation in that moment translates into variation in output energy and timing. For systems that synchronize the laser to an external event, an amplifier chain, a probe pulse, a machine clock, low jitter between the trigger and the HV edge is what makes the laser usable as a timed source rather than a free-running one.

Repeatability ties the two together. A driver that produces a slightly different edge or delay on each shot makes every optical pulse slightly different, which matters in precision machining, ranging and scientific work. Overshoot or ringing after the edge modulates the retardation of the cell, so the settled level after the transition matters as much as the speed of the transition.

The BNC Approach

The DEI PVX pulse generators are built for this load: a small capacitance that needs a fast kilovolt transition placed precisely in time. The half-bridge (totem-pole) output drives the crystal actively in both directions, so the edge that opens the switch and the edge that closes it are equally fast, with over/undershoot under 5 percent. The output switches between two user-supplied DC supplies, VHigh and VLow: the cell rests at one level, for example the quarter-wave voltage that holds a Q-switched cavity closed, and switches to the other for the width of the gate. The TTL gate sets pulse width and repetition rate; the supplies set amplitude.

Shot-to-shot jitter is under 200 ps on the PVX-4141 and under 1 ns on the PVX-4130 and PVX-4151, which suits these drivers to synchronized Q-switching. Front-panel voltage and current monitors (1000:1 into 1 MΩ and 10 A/V into 50 Ω on the PVX-4141) let the laser engineer observe the edge into the real crystal, check the switching level, and confirm that the top has settled before the optical pulse is generated.

For cells with a lower switching voltage at a high repetition rate, the PVX-4151, the current production version of the PVX-4150, delivers 1,500 V with rise and fall under 25 ns (under 20 ns typical) and jitter under 1 ns, from single shot to 240 kHz continuous at 1,500 V and 5 MHz in burst. Where the cell can be driven differentially, the PVM-4210 module puts +950 V and −950 V on the two electrodes at once, a 1,900 V differential swing, with rise and fall of 25 ns or less, built-in high-voltage supplies, and single shot to more than 20 kHz continuous. The choice follows from the half-wave or quarter-wave voltage of the crystal, its capacitance with the cable, the cavity timing and the repetition rate. A 20 to 25 ns edge suits Q-switching and the dumping of cavities with a round trip of about that length or longer; dumping a cavity with a round trip of a few nanoseconds needs a faster driver than the models listed here.

Talk to a BNC applications engineer at info@berkeleynucleonics.com or 800-234-7858. Bring your crystal capacitance, half-wave voltage, cavity timing, and repetition rate, and we will match a DEI pulse generator to the switching precision your laser requires.