In a 2P microscope, pockels cells are employed for fast control of the laser beam intensity. I use it for both switching off the laser beam during turnarounds of the resonant scanner, between two frames if they are not immediately one after the other, and to adjust the beam intensity when scanning in z for multi-plane imaging. In total, the pockels cell is quite essential for me. Alternatively, people use mechanical shields to blank the beam during the turnaround, or slow motorized rotating λ/2-plates to adjust the laser intensity on a timescale of seconds.
Recently, I found out how a defect pockels cell can look like. For comparison, the first video shows a properly working pockels cell, although the refractive index-matching liquid inside might be a little bit low. The air-liquid interface can be clearly seen at some points.
In the second video, the crystal inside the pockels cell is clearly broken and therefore visible. This could be clearly seen immediately when looking at the laser beam, which was strongly diffracted after passing the pockels cell.
This defect occured most likely when the cell driver remained switched on for an extended period of time, with the offset voltage being set to a rather high value. So this happened due to the permanent voltage applied to the crystal, and not due to the pulsed laser intensity.