Lasers Part 1

[Music] welcome to another session of introduction to photonics so we will start with a slightly different topic today so we've been discussing optical amplifiers in the last lecture and even the last lecture I promised that we will get on to looking at fundamental principles of lasers we didn't get an opportunity to do that in the last

lecture but let's let's get started with that so when we talk about a laser what are the things that come to our mind what do we associate a laser with so the properties of the laser that coherence right both in terms of spatial as well as temporal coherence so that that actually leads to higher activity very

narrow spectral with emission and so on so how do we go about building a laser and then what are the characteristics of the laser is what we will look at now this part we already discussed briefly previously right so laser is gonna consists of laser is going to consists of a gain medium right which you know it

provides gain as long as you can provide this external excitation which we call as the pump right the pump is required to take this medium above thermal equilibrium conditions and then we knew we know that it's going to excite the atoms in the gain medium to a higher energy level from which you can achieve

stimulated emission and we said we will make a laser by providing feedback so we put a mirror here and another mirror here right let's call this mirror with reflectivities r1 let's call this mirror with reflectivities r2 so that those provide the feedback so when you have these you know excited ions initially providing spontaneous emission and then

with feedback from either ends you are actually able to provide able to get stimulated emission and then we get laser output right that laser output could be from just one end or it could be from this end also right it depends on what is the relative reflectivity of the two mirrors okay typically what we