Minimizing spatial hole burning

[Niles]

Hi

Say I am looking at a linear laser cavity consisting of a gain medium filling the *entire* cavity, please refer to the attached picture. The cavity will have standing waves in it, which will result in spatial hole burning of the gain medium at the antinodes of the optical field.

Say the intracavity field starts out by being P-polarized at point A. Then it propagates to point B, where I put a device that changes the polarization to S. Then it propagates back to point where, where a device makes it P-polarized again, etc..

Assuming the gain-medium is birefringent, this should in principle minimize spatial hole burning, right? Because the optical path length of P- and S-polarized beams are different, so the standing-wave pattern would also change as well?


Niles.

 

[tommw]

Interesting thought.. Do you consider the points A and B to be inside the cavity as well? (By the way, I don't see any picture attached.)
Any thoughts about how such a "device" would look like?
Normally you'd use a Halfwaveplate to rotate polarization. This will, however, not work in your case. But a quartewaveplate could do the trick.
The latter has been proposed by Siegman, named twisted-mode-technique, see Applied Optics, Vol. 4, Issue 1, pp. 142-143 (1965) It works in the inverse way as you suggested: The polarization within the gain medium will be circular.

I also think that with your suggestion you will implicitely create a ring-cavity (there won't be any standing waves at all because p- and s-polarizations won't interfere).

 

[Niles]

I forgot the attachment, thanks to berkeman for letting me know. In principle the gain medium should fill the whole cavity.


Niles.

 

[Niles]

Thanks. I haven't thought much about how I would do it in practice, I just thoight of the theory. But you say quarter waveplates? I don't see how that could work. So first time it passes through and becomes circular, then changes direction when reflection of the mirror, and then it gets a 90 degree phase shift again, which is a total of 360 degrees. So we just end up with the same as initially?

 

[PJC01]

Hello Niles,

Thank you for your question about minimizing spatial hole burning in a linear laser cavity. You are correct that using a combination of P- and S-polarized beams can help reduce spatial hole burning in the gain medium.

Spatial hole burning occurs when there is a standing wave pattern in the laser cavity, resulting in areas of high and low intensity. The high intensity areas can cause a non-uniform distribution of excited atoms in the gain medium, leading to a loss of efficiency and potential damage to the medium.

By changing the polarization of the intracavity field at different points, as you described, the standing wave pattern will also change. This can help distribute the intensity more evenly throughout the gain medium, reducing the effects of spatial hole burning.

However, it is important to note that this method may not completely eliminate spatial hole burning. Other factors such as the birefringence of the gain medium and the alignment of the cavity components can also play a role. It may be necessary to also consider other techniques, such as using a non-linear crystal or adjusting the cavity length, to further minimize spatial hole burning.

I hope this helps answer your question. Keep exploring ways to optimize laser cavity design and minimize spatial hole burning for more efficient and stable laser operation.