Selecting the Optimum Pockels Cell for Q-Switching

For Q-Switching applications, the device must be able to withstand extremely high intra-cavity optical power densities. These are usually many times higher than those indicated by the peak output power which is measured outside the cavity because the circulating optical field is enhanced by the cavity Q. In many cases, only those devices based on KD*P will withstand such high intensities, although lithium niobate has been shown to be effective in moderately high power applications around 1µm wavelength.

Next is the choice of longitudinal or transverse field device. This is often dictated by the required aperture of the Pockels cell. Large apertures are more easily obtained in longitudinal devices as the half wave voltage is effectively independent of crystal dimensions. For a transverse field device, the half wave voltage is determined by amongst other things, the ratio of length to aperture (higher being more favourable). The apparent benefits of lower switching voltage are however often outweighed by other factors. In particular, most of the transverse field devices require multiple crystal designs to counter effects of birefringence and sometimes walk-off as well which occur in devices where the beam does not propagate along the optic axis. For most applications, longitudinal cells offer the simplest solution and are thus often preferred.

Another consideration is safety. Will the Pockels cell be in an exposed position where the user may have access to the device? If so, then great care must be exercised in the choice of interconnect used on the cell. For exposed positions we recommend the use of the EM5XX range where high voltage BNC type connectors are employed. For those units which are embedded within an enclosed sction of the laser system, or where other engineering safety controls are in place, a more open style of interconnect may be used such as the simple or stud pin terminals used on the EM508M and EM510M devices (see left). This also allows a more compact packaging and can additionally present lower capacitance to the Pockels cell driver.