Replacing 808nm diode in 100mW 532nm DPSS (Viasho VA-I-N-532)

[weartronics]

Hi PL, today (the day before I have to present my NYE show), I discovered that the 808nm diode in my 100mW 523nm DPSS (Viasho VA-I-N-532) has died, outputting only a few mW. I'm not sure of the cause, because the laser head has always been protected from static, and it only has a few hours running time since it was bought new from Viasho (several years ago).

What power, emitter size, and divergence angles do you guess the original diode has? It's on a C-mount marked "214918", I can see it has some FAC lens installed, and the maximum drive current is preset at 1A by Viasho.

I am lucky enough to have a spare 2W 808nm C-mount diode on-hand, from another project. It also has some FAC lens installed, but I expect the emitter size and divergence angles are different from the original diode. I installed this mismatched diode without any alignment, and at this stage it appears to have brought the laser back to life in time for the NYE show.

I have a few questions:

* I'm surprised that I get reasonable output power with this mismatched diode and without any alignment. What (if anything) can I do to improve the efficiency? Is there some guide for which optics to move in what order, and what effects to measure to judge the alignment in this type of laser?

* What is this secondary beam that they blanked out with what looks like a razor blade?!?! This secondary beam appears to contain about half the 532nm power, but it never leaves the laser head. It was present with the original diode, and it's still present with the new diode. I am suspicious that I was sold a dud with some resonant modes (not TEM00) that they just blocked from leaving the head, but I don't have enough experience to know that for sure. Hopefully there is some better explanation. Have you ever seen it before?

[shrad]

hi weartronics

a friend at laserfreak already reported some work on a viasho unit with similar design as yours (but without the waste beam)

DO NOT MOVE THE OPTICS

he did, and it was proved that this design absolutely SUCKS when you need to align properly, due to poor alloy stability and bad screw quality

I suggest you only adjust diode position, keep the beam path as it is till it reaches the KTP, and then maybe try to adjust the KTP angle for it to leave only one beam

I also suspect your pump diode is a bit far from the lenses, as it seems at least 20% of the pump power is lost up and down the diode mounts... maybe approaching it will catch these wasted percents

if you do that, maybe the pump collimation will not hit the crystals as it did before and there, you might have to tweak the YVO4 to last lens distance, using an IR viewing card at low power (in order not to fume the card ^^) to check for focus before the crystal

hope this helps, as these units are quite difficult to work on... one point is your previous diode probably had the pump beam wasted a bit like here, as I had the same issue in the crystalaser units I have tweaked (and they are high end scientific ones) and it seems normal

[weartronics]

Hi Shrad, thanks for your advice. One point I should add is that the photo is actually taken with the original diode before I removed it. With the new diode installed, it looks about the same.

[shrad]

OK, that's a good thing, it means your diode is close to the previous specs

what you can do then is try to redress the KTP orientation to have one beam exiting, but I cannot tell you it will work, it would need a closer look at the optics to tell

[weartronics]

DO NOT MOVE THE OPTICS

Now that I have a full year until the next show, I decided to move the optics!

I noticed that the screws holding the KTP in its mount are very loose (they will become completely free with some vibration). They cannot be tightened because it changes the alignment and the 532nm output disappears. There is no spring biasing the screws, nor any thread binding material, they are just loose...

Also, the KTP mount doesn't have any method to adjust the angle, so I have no idea how it is adjusted in the factory. I know it wasn't done using a positioning arm, because the optics are mostly screwed in place, not glued in place. I loosened the mount so that I can wiggle the crystal with a toothpick (maybe this is how it is adjusted in the factory, and why the screws are so loose)...

Two beams are always present. Wiggling the crystal can change the shape of the beams from circular to rectangular. Both beams always take the same shape, and the displacement of the beams is always pretty constant. Sadly, after loosening the mount I am unable to secure the KTP in any position which produces a circular beam anymore.

The second beam was very deliberately blanked out with the razor blade glued to the collimator. I guess if the manufacturer can't easily eliminate this spurious beam, I doubt that I can. But I'm still confused about its origin - if you have any other ideas, please let me know.

[shrad]

some non parallel mirror effects between KTP and YVO4? a KTP with non parallel facets?

I'd go for an extra mode popping up due to alignment problems

maybe you could replace the screws with better ones with retaining rings?

[mccarrot]

I loosened the mount so that I can wiggle the crystal with a toothpick (maybe this is how it is adjusted in the factory, and why the screws are so loose)...

No, they use chopsticks.

[weartronics]

some non parallel mirror effects between KTP and YVO4? a KTP with non parallel facets? I'd go for an extra mode popping up due to alignment problems

You're right, adjusting the angle of the YVO4 can move the beams around! I think it's not an extra mode, I think it's just lasing on two different axes, probably some non-parallel crystal facets as you suggest (I don't know which crystal though). It's difficult to superimpose the beams, because none of the optics have adjustments beyond first loosening the screws, pushing them around, then tightening the screws again. Even if I found a good position by pushing them around, the position changes when the screws are tightened. I wish I could work out how they were aligned at the factory.

[shrad]

how about positioning the KTP with a third hand, and using UV curing glue to maintain the thing once aligned?