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What happens if laser gain media is given constant velocity?

julianthedragon

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Here with a (potentially) dumb idea that somebody with a stronger physics/lasers background can hopefully pick apart.

So this much I understand about lasers: there is A) a lasing medium, be it solid-state, dye, gas, semiconductor, etc. and B) a pump source such as electricity, light from another laser, a chemical reaction, etc. In this case, let's take the laser to be solid-state and optically pumped by another laser.
The way lasing has been described to me is that light passes through the lasing medium, exciting its electrons to a higher energy state. Then the electrons fall back to their base energy state, emitting photons. These photons then stimulate emission of more photons traveling at the same wavelength and direction and so on until the laser is indeed lasing.

Here is my idea. You have a wheel of solid lasing medium that is spinning very quickly at a constant velocity, and you aim the pump source (the existing laser) tangent to the wheel but still touching. Could the velocity of the moving lasing medium at the part the source reaches have an impact on the wavelength of emission? Would it be possible at all to have the medium emit light with this kind of set up?

Thanks for humoring me
 
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CurtisOliver

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Here with a (potentially) dumb idea that somebody with a stronger physics/lasers background can hopefully pick apart.

So this much I understand about lasers: there is A) a lasing medium, be it solid-state, dye, gas, semiconductor, etc. and B) a pump source such as electricity, light from another laser, a chemical reaction, etc. In this case, let's take the laser to be solid-state and optically pumped by another laser.
The way lasing has been described to me is that light passes through the lasing medium, exciting its electrons to a higher energy state. Then the electrons fall back to their base energy state, emitting photons. These photons then stimulate emission of more photons traveling at the same wavelength and direction and so on until the laser is indeed lasing.

Here is my idea. You have a wheel of solid lasing medium that is spinning very quickly at a constant velocity, and you aim the pump source (the existing laser) tangent to the wheel but still touching. Could the velocity of the moving lasing medium at the part the source reaches have an impact on the wavelength of emission? Would it be possible at all to have the medium emit light with this kind of set up?

Thanks for humoring me
Well unless someone can think of something I haven’t considered then I would suggest that it wouldn’t make a difference at all. At the end of the day, the medium whether static or in motion would apply the same rules. The pump exciting the electrons to the upper state and then dropping down to it’s transition states. Those energy levels are unlikely to change with physical motion. All that’s happening is you are lasing in a new section of the medium at any given time. When a solid crystal is damaged you can offset the intracavity beam to avoid the damaged area. In dye lasers, you circulate the liquid medium at high velocity to avoid triplet states. Funny enough I started working on a solid state dye a while back (on hold for the moment), that essentially does exactly what you are proposing. I’m spinning a solid ‘wheel’ medium to avoid degrading the dye. As far as I'm aware I’m not going to be experiencing a new area of laser physics by doing that.
The only factor I can consider that may have some impact is whether you spun the medium at a high enough velocity to alter its physical properties therefore altering the optical properties slightly.
 
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julianthedragon

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All that’s happening is you are lasing in a new section of the medium at any given time. When a solid crystal is damaged you can offset the intracavity beam to avoid the damaged area. In dye lasers, you circulate the liquid medium at high velocity to avoid triplet states. Funny enough I started working on a solid state dye a while back (on hold for the moment), that essentially does exactly what you are proposing. I’m spinning a solid ‘wheel’ medium to avoid degrading the dye.
Super interesting, definitely learned a few new things I wasn't intending to from your reply and what a neat coincidence that you were working on something similar.

I'd imagine Doppler effects would mess with the pump and output wavelengths. Probably want to have the beam go through the disk on a cord that gave Brewster angle interfaces,.:)
I was imagining a Doppler type of effect too but I wasn't sure if there was a problem with that logic. My intuition tells me if the logic does check out, the wheel would have to be spinning pretty fast to have any noticeable effect though.

We can use a formula to calculate how much velocity we would need to see a certain change in wavelength.

From Wikipedia, we have:
Screen Shot 2022-08-21 at 9.07.20 PM.png
where f is new frequency, f(0) is original frequency, c is the speed of light, v(s) is source velocity, and v(r) is velocity of the viewer.

Since frequency = c/wavelength,
we can replace f and f(0) with λ and λ(0) and replace the part in parentheses with its reciprocal (c and unit conversions on both sides cancel), so we have:

Screen Shot 2022-08-21 at 9.24.49 PM.png
We can also assume v(r) = 0.

Already we can see we have to reach a decent chunk of the speed of light to change the wavelength significantly.

Let's say we're going for a change from 405nm to 406nm.
Then we have:
Screen Shot 2022-08-21 at 9.35.18 PM.png
Therefore the lasing medium would have to be moving at 1/405 the speed of light or 740,228.291 m/s to produce that 1nm of change, lol.
 

CurtisOliver

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This is why questions like this aren’t dumb as they lead to interesting discussions. Doppler shift is a cool phenomenon. So technically you can but it takes incredible rpm to achieve any effect. Most mediums would of structurally failed by that point. If miraculously a medium does survive it would of definitely of warped.
 

julianthedragon

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Would it ever be possible for the lasing medium to be a beam (not necessarily laser beam) of particles moving at the speed of light itself? Imagine aiming the source laser orthogonal to the medium for zero Doppler effect, and increasing the fraction of c by bringing the beams closer to parallel, picking up more of c as the angle of intersection closes in / the projection of source beam onto medium beam velocity increases.

In this thought experiment, and using the Wikipedia equation (assuming it doesn’t break down when v(s) is close to c), the limit of the Doppler effect here would be close to c, which leads to the result that you could decrease the wavelength as much as you want in the negative direction while in the positive direction the most you can do is double the wavelength.
When v = c, λ = 2λ(0)
When v = -c, λ = 0

Also, in general the Doppler effect only seems to work when the velocity of the source (in this case lasing medium, not to be confused with the pump source), is slower than the wave (speed of light) which is obviously guaranteed
 

CurtisOliver

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Would it ever be possible for the lasing medium to be a beam (not necessarily laser beam) of particles moving at the speed of light itself? Imagine aiming the source laser orthogonal to the medium for zero Doppler effect, and increasing the fraction of c by bringing the beams closer to parallel, picking up more of c as the angle of intersection closes in / the projection of source beam onto medium beam velocity increases.
Research into free electron lasers. I think you’ll be pleasantly surprised.

As for having a beam of non photons travelling at the speed of light. Electrons have mass therefore cannot be accelerated to light speed (c vacuum). They can get close but can not reach it unless the medium allows it. Cherenkov radiation occurs when electrons exceed the speed of light within the medium which in the case of a nuclear reactor is water. But electrons cannot exceed the speed of light in a vacuum.
 

kecked

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The velocity of the photons does not change due to the rotation of anything. There is a velocity change on entry and exit. photons are not dragged around the medium. To a photon that wheel isn’t moving at all. You can’t move fast enough. Second the surfaces would have to be perfect so the lens effect from the radius of the wheel doesn’t ruin your beam by wiggling. I don’t think you can manufacture such a system easily and the bent surface would require optics to undo the divergence on exit. Now maybe a tape of some kind where the beam passes through a flat section could would in a solid state dye thing. Had often wondered if you could flow the dye in a flat tube and do the same. Think that’s called a flow cell dye but I have never seen one.

its hard to wrap brain around the quantum world vs classical world. The Doppler we seein light from the stars has a different origin than Doppler from sound. It also takes distances the size of the universe.

so here is a different thought. Can you compress the media with a wave to change the density and hence velocity. Yes you can. Think that’s how an AOM works.
 

Eidetical

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I was envisioning a sort of "Doppler broadening" effect like in gas lasers when I made my comment. I'm glad there are physicists here to shine correct light on such things.
 




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