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"Joules" the 60J ND:Glass laser

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Steve Conner

Sat Nov 28 2009, 10:33AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Maybe the laser was run for a long time with the shutter closed, and the reflected beam damaged the optics. That would explain the burnt shutter, low output and poor spot size.

Z28Fistergod

Sat Nov 28 2009, 03:17PM

Registered Member #2040 Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180

Steve McConner wrote ...

Maybe the laser was run for a long time with the shutter closed, and the reflected beam damaged the optics. That would explain the burnt shutter, low output and poor spot size.

Maybe, but I wouldn't expect a reflected beam with relatively little power to be able to damage the optics. The beam intensity within the resonator is probably 2 - 10 times as powerful as the output, and it doesn't damage the optics under normal operation. Then again I don't know a whole lot about dielectric mirrors, would it matter if the beam passed through the OC backwards?

Sat Nov 28 2009, 06:39PM

Registered Member #2372 Joined:
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Posts: 62

It is somewhat interesting that the shutter is outside of the cavity. most of the lasers I have seen have the shutter inside the cavity so all they have to do is kill the oscillation instead of taking on the actual laser output.

...

Sat Nov 28 2009, 08:20PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

There are advantages to having the shutter outside the cavity, it wouldn't surprise me if this laser has enough gain to lase without the rear mirror (although placing the shutter on the output end would solve that problem).

As to the optics, as long as they clean they will work fine. Is it possible to see the optical surfaces of the mirrors? Make sure that cap is discharged when trying this of course!

Can you try shooting the laser at a piece of paper with black toner on it? If you don't have a laser printer an inkjet should work, but it works best to just make a photocopy of a black piece of paper. This gives a good high contrast way to see where the laser is making power vs where it isn't, looking at the image you are showing it looks like most of the power is around the edges, which would mean the laser is ether lasing in circular TEM01, or that there is a burn on one of the mirrors.

If you are up for it, its not necessarily hard to align a laser like this, you could remove the OC mirror and inspect it carefully. I would recommend doing a dry run of the alignment procedure (try one of the ones on Sam's laser faq, basically you shine a low power alignment laser such as a HeNe laser through the laser, and position it so that its reflection shines strait back into the laser, and then tweak the OC so that its reflection also reflects into the laser. The tricky part is figuring out which reflection is the surface you care about, so its good to take careful nuts when the laser is actually aligned!)

You can get a suitable replacement optic for less than $50 so its not the end of the world if one is damaged.

Z28Fistergod

Sat Nov 28 2009, 08:51PM

Registered Member #2040 Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180

I think part of it is that the beam path inside the resonator is completely sealed from outide air/dust. Putting the shutter inside would make it harder to seal it off. I think it was kind of an afterthought anyway, or maybe an option. The shutter is only held on by two thumb screws and has a seperate connector. I didn't incorporate it because the motor is pretty weak (probably why it got shot!) and it adds complexity.

The deisgn makes it easy to access the mirrors, and like I said earlier the front is a little scratched up, but not to the point that I think it would cause too many problems. I'm going to maintain my thoery that it was replaced with an incorrect part. I'm just debating on what level of reflectance I should get for the replacement. I can't seem to find a good source of information for this. I've read anything from 50% to 90% for Nd doped lasers, so I don't know, maybe I should get a 70%. Maybe a 60% since glass is supposed to have a higher gain the YAG.

Edit: A couple pictures of the toner. The second one was shot through the back of the paper.

1259445906 2040 FT79911 Burn From Behind

Daedronus

Tue Dec 01 2009, 06:36PM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
Location:
Posts: 370

The mismatch between the burn size and the laser rod is quite wired.
The whole thing acts as if there is a beam expander after the optical coupler.

You could try to ask about it on the alt.lasers newsgroups.

And, you can also get new mirrors, a pair should be 60$ shipped, from the Chinese sellers. I would try with 2-3m radius HR and plano OC, around 40% reflecting.

IntraWinding

Tue Dec 01 2009, 11:52PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

Check Sam's Laser FAQ for all things Laser

: ---> Home-Built Pulsed Solid State (PSS) Laser

All the practical optical cavities here produce a diverging beam (the Plano-Plano cavity is unstable)

But a good quality beam will appear to originate from a diffraction limited spot so you can correct the divergence (within defraction limits) with a suitable converging lens. Ultimately the the beam divergence is limited by beam diameter. The larger the diameter the smaller the obtainable divergence.

Daedronus

Wed Dec 02 2009, 09:16AM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
Location:
Posts: 370

Plano Plano configurations are on the border between stable and a-stable. In practical terms they work well but they are extremely sensitive to misalignment.

My current yag plaything uses plano plano mirrors and more or less every time I take it out to play I have to fine tune it a little for maximum output.

The burn pattern is closely matched to the laser rod size.

Z28Fistergod

Wed Dec 02 2009, 08:56PM

Registered Member #2040 Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180

Daedronus wrote ...

And, you can also get new mirrors, a pair should be 60$ shipped, from the Chinese sellers. I would try with 2-3m radius HR and plano OC, around 40% reflecting.

I've read 40% is a good value for a Q-switched Nd based laser, but I've never seen anything close to that for a non-Q-switched. What makes you say 40% would be good?

I've seen the $32 pair of mirrors from the Chinese sellers on ebay, but I'm weary of a precision optical component that can be sold for $16 including shipping from halfway around the world and still generate a profit, especially if I want to put it in a high powered laser like this one. Then again I don't want to spend the couple hundred or so it would cost from a company like Newport. I'm not a gambling man but it might be worth it to try the cheapies.

Daedronus

Wed Dec 02 2009, 10:23PM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
Location:
Posts: 370

"Gain coefficients for pulsed systems are on the order of (0.3â€“0.5) cmâˆ’1, and the output
mirror reflectivity ranges from 0.6 to 0.8. Pulsed systems that are also Q-switched
have the highest gain coefficients, typically 1.5â€“2.5 cmâˆ’1, and the output coupler
reflectivity is between 0.4 and 0.6."

from
Solid State Laser Engineering 6th ed - W. Koechner (Springer, 2006)

I was recalling something else but it looks like you are right.

The Chinese optics should be fine. I think the only difference is the damage threshold for the coatings, but in non q-switch mode I don't think we are near that.

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