Nd:YVO4 Crystals with non-parallel end faces. Advantages?

[brianpe]

I have a couple of Nd:YVO4 laser crystals I think are taken out of Coherent Verde lasers. The end faces are not plane parallel. The Verde's are pumped with large multimode fibers so the pump beam has random polarization. Because Nd:YVO4 is birefringent, the angled end faces result in two slightly different beam angles for each polarization direction of the pump. So it seems they are doing this to reject the extraordinary polarization as the resonator is only aligned to one polarization.

I think I get the mechanics of this. But I'm wondering what are the reasons for it. While absorption is not as high for the non-dominant pump polarization it still exists and can contribute to lasing. Is there a thermal or beam quality advantage to doing this? The Verde's are also frequency doubled. Does this have any downstream improvement in the beam that's fed into the doubling crystal?

Thanks for any ideas.

[VDX]

... could it be something like Brewster angle?

Viktor

[brianpe]

... could it be something like Brewster angle?

Viktor

No, this is something else. Brewster's angle generally has both ends at the same angle so the crystal faces end up parallel. Also, it's usually a pretty large angle and obviously Brewster-cut. In this case the angle is very small -- it's almost not noticeable to the naked eye. But it makes a huge difference when aligning a cavity. Here's a close up of one such crystal:



The left side looks parallel but the right side is cut at what, maybe 1º angle? And this doesn't seem to be a defect; both Verde crystals have this.

[VDX]

...yes, definitely not Brewster.

Then maybe a "trick" for controlling Fresnell-interferencies or something similar by small lateral displacements?

Viktor

[kecked]

Don’t they end pump from both ends so this could be why they are different to couple the lensing on input so the output beam matches from both pumps in phase.

[Farbe]

I would guess that they do that just to suppress unwanted etalon effects.
Are these crystals AR coated?
They might have uncoated faces (to be able to run with higher power densitys). Uncoated would mean much more reflection (11%) therefore much more etalon behavior.
However even if the faces are AR coated, reflection by the crystal faces will cause unwanted interference.

[brianpe]

That's one thick etalon! But yes, I can see how this can help. I don't see this in many other crystals so if it's an effect, it's pretty small (or allows them looser tolerance in manufacturing. IE, crystal alignment is not important).

The pump idea also got me to thinking maybe it's much simpler: this will change the exit angle of any stray pump radiation that makes it through the crystal, helping to prevent damage to the pump on the other side.

I'm having some custom crystals made by CASIX....and hoping I didn't make a mistake by giving them parallel end faces.

[Farbe]

The pump idea also got me to thinking maybe it's much simpler: this will change the exit angle of any stray pump radiation that makes it through the crystal, helping to prevent damage to the pump on the other side.

That shouldn't work.
If the beam that passes the crystal gets diverted, the input beam into the angled faces also gets diverted by the same amount. So to match the pump beam incident on the angled face, it also needs to be angled slightly.
This should result in both the remaining passing pump energy and the "fresh" pump beam incident on the angled face, being collinear.