How do FROG-based devices, like the GRENOUILE and UPM, compare to SPIDER?
Experimentally, SPIDER is really a FROG, but with a Michelson interferometer added to one arm and a pulse-stretcher added to the other. So it’s simply a FROG, but with two very complicated devices added to it. Due to the Michelson interferometer, SPIDER is interferometric and so is very alignment-sensitive. Worse, in all, it has multiple interdependent and sensitive alignment knobs, which can take a day (or longer!) to align and which must be constantly realigned as the beam walks, the pulse changes, or the room temperature drifts. GRENOUILLE, on the other hand, is non-interferometric, has no sensitive alignment knobs, and remains aligned forever.
