Spatial & Temporal Coherence

[Music] okay welcome to another session of intro to photonics so the last couple of lectures we've been looking at some more examples of wave optics from specific properties in wave optics including multiple slit interference and then the last lecture we just introduced the concept of coherence right so we looked at what temporal coherence is is all

about so we towards the end of the last lecture we were examining certain examples for you know sources and we said okay sunlight is a source so light source right but when you look at the the spectral width of sunlight and especially we're just looking at picking up the visible region and that's quite

broad so the result that the coherence time or the coherence length is quite short right as opposed to helium neon laser which has a single a so-called single frequency operation but you know the line width can be in the order of megahertz in which case we found that the coherence length is rather high so

you know what are the uses of something like this if you are building an interferometer right in general you would say okay what do you want longer or shorter coherence length huh shorter why would why would we need shorter coherence length that is actually one good example of that I will come back to

that but in general you want to have longer coherence length so that you don't have to match the path lengths so accurately right so you know for example with the LED we are saying the coherence length is 20 microns so that means both the paths have to be matched to 20 micron accuracy

so that you can have interference right and that is very very hard to do whereas you know in the if you are trying to build an interferometer with a 300 meter coherence length you could roughly put the two mirrors you know at some certain distance and chances are that you will get good interference right so if you