Why gain medium of a solid state laser are polished from sides facing mirrors and the rest is unpolished?

Question

First of all aren't know if its the right place to ask this question or not. I was wondering the reason behind this "setup" of polishing only the sides facing the resonator mirrors? And is it applied on all types of gain medium crystals?

Crystals examples

Answer 1

First, it is important to understand that polishing is an additional processing step in the manufacturing process and thus for time/money reasons optical elements are only polished on surfaces which will provide benefit for the final application. So let’s examine which surfaces should be polished and why for a solid state laser. For simplicity I will assume the solid state laser is of the Fabry-Perot geometry (the type OP describes) although the considerations are largely the same for other geometries (e.g. for a ring laser).

The sides facing the resonator mirrors are often polished to improve (or allow) lasing. For lasing to occur the round trip gain must be positive. The gain medium (the laser crystal) is the only element that supplies gain while loss can come from many mechanisms. Losses may come from surface defects or scattering in any of the intracavity optical surfaces that light passes though (end mirrors, gain crystal surfaces, etc.). So the polish on intracavity optical surfaces is often very good.

However these same considerations do not hold true for sides not facing the resonator mirrors. So there needs to be a good reason to polish those sides. I have several dozen cylindrical Ruby laser rods, and while every single one has polished ends, roughly half have polished exteriors while the others do not.

So why would you want the other sides to be polished? I can think of a few reasons

1. Improved efficiency of lamp pumping. If the solid state laser is pumped though the side of the crystal, polishing the sides will improve the efficiency by which pump light can be transferred to the gain medium. This would likely be the case for a lamp-pumped Nd:YAG laser or a lamp-pumped Ruby laser.
2. Improved thermal contact. For many solid state lasers output power is in one way or another limited by thermal considerations. Polished edges/sides of the gain medium crystal will allow the crystal to make better thermal contact to a heat sink.

There may be other reasons I am not aware of to polish the outside of the crystal as well.

But if none of these reasons apply, the crystal will probably not be polished on its exterior. For example, common green laser pointers which use Nd:YVO4 are end-pumped by 808 nm laser diodes and thus not typically polished on their circumference. I would say that most of the laser crystals I have collected are not polished on their outside surfaces.

Finally, it is relevant that polishing a cylindrical surface is much more difficult than polishing a flat one, so even if there is a possibility to improve performance, the polishing still may not be done. I assume this is why some of my Ruby laser rods are polished and some are not.