This had made me realize there are other fundamental things I am not understanding. Maybe the understanding of a fs pulse is related to the understanding of a photon.
Let's say a photon is a wave packet. So the single "photon" is made up of plane waves of a wide band of wavelengths. But when we usually talk about photons, we assign them a definite wavelength (momentum by p = h/l) and energy (frequency by E = hf). So it could be the same for a fs pulse, that it is just a different type of wave packet than a photon and not built of photons? This is not precluding that it could decay to several photons through dispersion or matter interaction.
It comes down to what is the most fundamental thing. We say there are standing waves in the laser resonator cavity. But actually it's photons which are related in phase whose electric fields interfere destructively at the nodes. But since, fundamentally, we are counting photons, you should see noise at the nodes related to the finite counting of photons. This noise is real, as I was brought to understand when I was reading about noise in LIGO. Every emag exchange of energy or momentum, even in something like a capacitor, is quantized and mediated by something like a photon. (Using a loose definition of photon as an emag wave packet, and not necessarily at visible frequencies or whatever).
Something else to unpack is if the momentum is precisely defined in a photon, the position must be imprecise. Although if you consider some real photons emitted from an electronic transition, there is a fair bit of uncertainty in the momentum. Photons from a laser going through a pinhole at 2 meters distance have much more position uncertainty than photons selected from a particular distant star, does this affect the uncertainty in wavelength? Probably not because h/4pi is such a tiny number.
Ok, I've run out of time right now.
