Faraday Rotator: Optics for Amplifying Laser Light

[SchroedingersLion]

Hi, another optics question:

In the amplifier of a Laser one can have amplified spontaneous emission (ASE) which is an undesired effect. One of my sheets (appended) suggests one can take countermeasures with so called Faraday rotators.
But the way I understand it, these optical isolators will just stop light from coming back - so not only the ASE light but also the ordinary laser light. But isn't the whole principle of an amplifier to send the light back and forth through the medium in order to increase the intensity via stimulated emission?
If I just wanted to cut off ASE light between two amplifiers, I could simply use an ordinary polarizer, couldn't I?

Regards

 

[SchroedingersLion]

I still have this question:
The slide seems to be based on this page of a book:
https://books.google.de/books?id=3R...e&q=laser amplifier static decoupling&f=false


But even there, I don't see how such a rotator would help vs ASE...

 

[Tom.G]

I don't see how such a rotator would help vs ASE...

It seems to apply only to polarized light. Similiar action as the reflection of a circularly polarized beam being reflected with the opposite handedness.

from your reference, https://books.google.de/books?id=3R...e&q=laser amplifier static decoupling&f=false

Pg. 11, Sec 1.5.1.1 Static Decoupling. "...a Faraday polarizer can be used to stop any return of linearly polarized light."..."Owing to the pseudo-vector nature of a magnetic field, the polarization rotation direction is reversed for a beam propagating in the reverse direction."[/I]

Cheers,
Tom

 

[SchroedingersLion]

Thanks for the reply!

So my current interpretation is this: Assume we have two spatially separated, otherwise identical, gain media. Between them is a faraday rotator.
The first polarizer of the rotator only allows light with a certain polarization to pass - the signal. Most of the ASE gets blocked. The signal enters the second medium for further amplification. The part of the signal that is scattered back by the second medium towards the rotator gets blocked by the rotator (since the rotation of the polarization plane is reversed for the opposite propagation direction). Also, all spontaneous emission created in the second medium towards the first medium can't pass the rotator (since 99% won't have the correct polarization). In total, only my signal goes from medium to medium and gets amplified.
Do you agree?

 

[Tom.G]

The article is saying is that any reflection back toward the first amplifier is blocked. The last sentence of the section says: Therefore the linear polarization of a reflected beam is rotated 90° and can be stopped by an analyzer.

Also, all spontaneous emission created in the second medium towards the first medium can't pass the rotator (since 99% won't have the correct polarization).

That could apply only if the ASE from the second stage is polarized, but ASE does not have phase coherence.

Pg. 12, Sec 1.5.1.2 Dynamic Decoupling seems to cover that scenario using a Saturable Absorber; a material that changes it's absorption inversely with the applied power. The backward-traveling ASE from the subsequent stage, being low power, will be preferentially blocked.

Don't know why both a Faraday rotator and a Saturable Absorber would be needed at low power, maybe they aren't. This has gotten deeper into the subject than my laser optics knowledge goes. (I'm learning along with you on this!) Hopefully someone with a deeper understanding can jump in and follow up with you. If there aren't any other replies in a few days and you need to follow this more deeply, post here again and I will try to recruit some more assistance for you.

Cheers,
Tom

 

[SchroedingersLion]

Tom, thanks again!

As I already had my (luckily successful) oral examination, I will not have the time to spend more work on this. But in case someone is knowledgeable on this, I would still be happy to read the answer. I might ask the lecturer if I see her sitting in her office next time I walk by the room and, in this case, will let you know.

Regards,
SL