The ideal pulse shape for many applications is a perfect square… here’s why the edges are not perfectly square:
The PCO-7125 “rise time” and “fall time” specification is <17 ns typical as measured from 10% to 90% of the full waveform. That means the rise time and fall time is measured between two points on the curve, one at 10% of maximum current and the other at 90% of maximum current. The trace below has to have an 11 ns rise time and 14 ns fall time. Any added inductance to the output such as strip line or long leads on the diode package would increase this rise and fall time. This will also affect the minimum pulse width and the shape of the pulse.