Okay, so SPM (self-phase modulation) is an effect that happens when an ultrashort pulse travels through a medium and it leads to a change in that pulse's frequency spectrum. It is explained that it occurs because an ultrashort pulse somehow induces a varying change of refractive index and this is a part that I don't fully understand. I'd like an explanation of how this varying change of refractive index happens and why it happens. Also, by saying that pulse's frequency spectrum changes... does this have something to do with the change of spectral density? That is (or at least how I understand it) we get a change in amounts of specific frequency components? Also, it is said (I think) that an ultrashort pulse has a wide frequency spectrum but it is very narrow in time domain. Does this mean that with ultrashort pulse we mean that we have a lot of different frequencies/wavelengths squeezed in a very small amount of time? I'd love a reply from a person who could give me a solid reply because, as you can see, I am a bit unsure about everything I learned/know . . .