Four Level Systems

[Music] all right welcome back to another session of introduction to photonics so what we've been discussing lately is light generation and amplification we've been specifically looking at how photons interact with matter through which you could have absorption of those photons and subsequently you could have emission of photons either as spontaneous emission or stimulated

emission and we took this example of an optical amplifier which is part of an optical fiber communication system so as we know now OBM doped silica fibers are used in these optical amplifiers and a beam drove silica essentially consists of three energy levels so you are able to go from the ground state the valence

orbital of this atom erbium atom to a higher energy level with the absorption of 980 nanometer photon and then we said the excited state is relatively short-lived so and then the next energy level is relatively close when I say relatively close what I mean by that is that energy difference is corresponding to the phonon energy of

the glass okay phonon energy is the vibrational energy that you have in the glass so when you have two energy levels that are very close to each other that's within the phonon energy of the gap of the glass you lose that energy through just non radiative relaxation okay so that's what's happening in the erbium it

goes to that higher energy level and then goes down to one step below where that energy level is relatively long-lived so this energy level is in the order of it's got a lifetime the order of 10 millisecond so whatever you pump to the higher energy level goes and stays or accumulates in this higher in in this