The Laser Project: A ridiculously overbuilt green DPSS "portable" laser

[brianpe]

Ok, so for the last couple of years I've been working on building a portable DPSS laser. It's a Z-fold cavity design powered by two 40W FAP modules. All electronics are battery operated by an integrated LiPo pack. At full tilt there is enough battery to run it for about 30 minutes. It's portable in that it is entirely self-contained and requires no external power or cooling. It does not fit in your pocket.

I may have gotten a little carried away.

The final laser weighs about 50 pounds. It has handles, so you can "hand hold" it....for a few minutes before your arm strength gives out. But hey, it does work. Does it work well? Er, no, it has a lot of issues I need to resolve. The electronics work great. The physics....not so much. Resolving those problems will be the next chapter, but I had to get this thing finished off. You can only have so many loose electronics and optics in your garage before something gets damaged, and I was starting to have trouble keeping all the optics clean. Now at least everything's packaged in a dust free environment while I work out where I went wrong with the physics.

So what's wrong? Lots. The biggest problem is efficiency. I am only squeezing about a watt of green out of it, with an input current of almost 40 amps. It also mode hops as the current increases, it has a couple of weak phantom beams, and it thermal lenses and the power decreases past about 50 amps.

Here are some glamour shots:

Homemade DPSS laser

• brianpe
• Jun 12, 2022
• 10

A DPSS laser I built from scratch. It's huge, heavy, and I'm glad it's done. Only it's not...

 

[CurtisOliver]

It might not be the most efficient system, but it’s a dpss you built. So good job!
I also had a bit of a laugh when you said it’s portable. When it’s bigger and looks heavier than my 4W head.

 

[liveforphysics]

Have you tried slowly rotating the fibers pumping the crystals to find an angle they couple power best into the crystals?

 

[brianpe]

Have you tried slowly rotating the fibers pumping the crystals to find an angle they couple power best into the crystals?

I did try inserting a 1/2 wave plate and rotating that -- there was a small (5%) improvement depending on orientation. I haven't tried rotating the whole fiber. These FAPs have random polarization so they will lose some efficiency here for sure. The Verdi's use them into vanadate too so I am assuming the efficiency loss can't be that bad or Coherent wouldn't use them. I could be wrong though.

There could also be a mode matching problem where the pump beam is too large for the cavity mode in the crystal. I think this is called the mode filling factor and there is a recommended value where the pump beam is slightly larger than the mode volume. My factor is 1.4 (pump beam is 450u, mode area is 320u). But when I decrease the pump diameter I get less power. I don't know what the ideal value is.

I think the biggest IR->green conversion issue is the energy density in the KTP. The cavity mode there is 250u and that may not be tight enough. I have recalculated new mirrors that can get this down to about 100u, but I want to learn to do the math here to prove that out before I spend $$$ on new mirrors. There's also the possibility that I am getting destructive interference on the green conversion. I am double passing green through the crystal so I can extract all of it instead of wasting 50% back into the laser crystal. Those two green beams could be interfering.

So yeah, lots to ponder and experiment with.