Lasers part- 02

[Music] so welcome back to introduction to photonics in the last lecture we started discussing about lasers so we said okay lasers nothing but an amplifier with feedback and we considered this simple case of gain medium on either side of the gain medium we had mirrors providing the feedback and we said okay once you

pump the gain medium with an external energy source then you excite these atoms in the gain medium which can provide spontaneous emission and those spontaneously emitted photons when they get bounced by the mirror reflected by the mirror they come around and stimulate the transitions inside the gain medium and then you have a buildup

of stimulated emission photons and then we said okay it's not like all these stimulated photons survive within the cavity we basically said that first of all you get laser action when you are able to overcome the loss in the cavity with the cavity gain so that was one condition and then the other condition

was that we said if you know therefore this waves to be consistent inside the cavity you have to essentially satisfy the constructive interference condition and you know that essentially meant that only certain frequencies can survive inside the cavity so even though you may have other frequencies that are possible because of the cavity that may be

supported by the cavity only certain frequencies whose gain is equal to or greater than the loss inside the cavity will survive within the cavity right so we talked about that concept and then then we said okay it's probably better that we look at the saturation of the signal photons or the saturation of the gain inside the