PPE tells you more about the laser than just how much energy is in each pulse. Generally speaking, a laser with a PPE of under 1mJ indicates inferior fiber; the pulse energy is managed to reduce the liklihood of fiber damage caused by the laser being absorbed by fiber impurities. How do you know what the PPE of the laser is if it’s not listed in the specification sheet? Divide the maximum power by the lowest Pulse Repetition Frequency (PRF) for a Q-switched laser (optimum pulse repetition frequency (PRFo) for MOPA), and you get the approximate PPE, i.e., 20W / 20,000 = 0.001J = 1mJ.
Another advantage MOPA has over a Q-switched fiber laser is the ability to go below the optimized pulse repetition frequency (PRFo). PRFo is the frequency where both pulse energy and power are at their maximum. Below PRFo, power is reduced approximately linearily, and pulse energy is usually capped at the maximum (fun fact, in diode-pumped open-air lasers, pulse energy increases below PRFo). Since we consider power to be “heat”, we’re effectively engraving with a lower heat affected zone (HAZ), which means better cuts–typically we want high PPE to engrave, and power (heat) to mark/anneal. Combine this with the ability to modify the pulse width, or the time each pulse interacts with the material, and we have complete control over the application.