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How Sensitive are Lasers to Back Reflections of their Beam?

kecked

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that makes no sense to me but rotator plates are usually 1-2mm thick. they come in 1/4 and 1/2 wave. I really think your wasting your time.
 



gazer101

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I agree, will put it on my long-term project list just for fun but its not really practical

Are you sure they are only 1-2mm? The ones I saw online looked way thicker 🤔
 

CurtisOliver

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What type of rotator are you on about gazer? Kecked is talking about waveplates. This is a classic case of overengineering imo. Yes, back reflection can cause issues on the odd occasion. But for most part it isn't a problem for most lasers we are dealing with. AR coatings or a even a brewster window like mentioned is more than enough in advanced setups.

Cheapest way of guaranteeing no back reflection:

1612386187944.png
 

gazer101

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What type of rotator are you on about gazer? Kecked is talking about waveplates. This is a classic case of overengineering imo. Yes, back reflection can cause issues on the odd occasion. But for most part it isn't a problem for most lasers we are dealing with. AR coatings or a even a brewster window like mentioned is more than enough in advanced setups.

Cheapest way of guaranteeing no back reflection:

View attachment 71603
Ah I was thinking of something totally different, are you sure the above setup would hypothetically work when the beam is reflected perfectly (same angle) back into the aperture (I don't understand how the waveplate acts as an isolator in this case)?

Thanks
 

CurtisOliver

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Waveplates change the polarisation direction. PBS's are dependant on polarisation.

If you look carefully at the diagram above you see that the waveplate switches the polarisation direction which is then reflected the other way by the PBS. Replace the purposeful mirror with your backreflection and you now limit all of that light re-entering the emitter. Its still excessive but does what you want it to.

1612388040468.png

Half Waveplates will switch the one linear polarisation to the other fully. Quarter will convert it into circular polarisation.
In your case you have two passes through a quarter.
1/4+1/4 = 1/2. So you get the same outcome but reversed.
 

gazer101

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Oh ok, so the waveplate basically does what this does then right?: 1612390029598.png
 

kecked

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Yes waveplates. Curtis has it right. Looks like an interferometer. You measuring paramagnetism or such?
maybe magnetic susceptibility? Like raman...my favorite. Never mind I didn’t read the caption.
 




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