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Old 05-04-2012, 02:32 PM #1
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Default Damaged optics in a Nd:YAG

Among other lasers, I work with Frequency-doubled pulsed YAG in my lab. One of our laser cavities is acting up and I wanted to share the debugging process as it might be informative to anybody building/repairing/restoring a similar laser.

The laser is an old Quantel "Brilliant" and part of a two-oscillator "Twins" system. The laser operates at 10 Hz, producing 5ns, 5mm diameter, 150mJ @532nm pulses (after frequency doubling, 1064nm at perhaps 300mJ).
- The cavity consists of a single Nd:Yag rod pumped by a single flashlamp.
- The rod faces polished at a slight angle (Brewster's angle, I believe)
- It has a guassian output coupler
- The back mirror is slightly concave (i don't know the focal length, maybe 1000mm).
- The Q-switch is an electro-optic Pockels cell.

The problem:
I was running a PIV experiment and I started noticed that the laser energy was starting to fade (my images were getting progressively darker) - at first I suspected the doubling crystal alignment was slipping. After half a day, the energy had significantly dropped, so I decided to check under the hood.

What I checked:
Using an Ophir power meter, the laser power measured 0.5W (@ 10 Hz that's ~50mJ per pulse). This is much too low! The other laser measured a normal 1.4W.

I took some laser burns before (1064) and after (532) the frequency doubler with both lasers. The second laser produced a normal looking burn, while the trouble laser produced a splotchy looking spot at both locations --- that means the frequency doubling optics (and beam combining optics) are OKAY.


Therefore the trouble must be in the cavity.

The laser is old, and there were a good number of flashes on the flashlamp. With the brilliant system, pulse energy is controlled only by Q-switch delay; therefore, every lamp flash is at full energy. I suspected that the lamp might be dying.

I opened the pump chamber and replaced the flashlamp with a new one. The laser rod, UV filter, and reflectors did not show any signs of green beasties growing, so I was hopeful my problem would be solved! It was not.

1064 burns looked the same as before and only a slight increase in 532nm laser output (~60mJ) (Picture 1064nm burn spots attached)

The next thing to check is the back mirror, Pockels cell, 1/4 waveplate, and the polarizer. Unfortunately, the laser is in a cramped "woodshed" built around a wind tunnel. It is very dusty in the area, so I won't venture into Q-switch cavity and disassemble the Q-switch until I move the laser head to an optics bench in a cleaner environment. However, I did peek in the cavity and the back mirror, waveplate, and polarizer look okay (no obvious damage). I tried to use a small mirror to inspect the Pockels cell (P.C.), but the cavity is much too cramped to get a proper look without pulling the P.C. out completely.

I talked with an engineer at Quantel and the suspicion is damage on the P.C. or back mirror (I think the back mirror is okay)

I will update when I find out more!
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Old 05-04-2012, 02:58 PM #2
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Default Re: Damaged optics in a Nd:YAG

Thank you for sharing your experience! Information like this is great when one of us gets something similar on the used market and need to fix it to a somewhat working condition.

Welcome to the forum!
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Old 05-01-2013, 06:19 PM #3
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Default Re: Damaged optics in a Nd:YAG

Okay, a very belated update on the Twins laser (I'm writing my dissertation so this has been near the bottom of my priority list )

A laser technician from Quantel happened to be in the area and we disassembled the laser head to diagnose the problem. Ultimately, the problem was dirt in the cavity that caused the pockels cell to fail. I guess the moral of the story is to keep your optics clean, especially when operating a laser with very high fluence!

As I mentioned, we operate these q-switched lasers in a (very very dirty) laboratory environment. Ultimately, we traded the twins in to Quantel in exchange for a new "evergreen" laser system with a smaller, hermetically sealed head. I'm happy because it's sold as maintenance free, which means I have one less thing to worry about when setting up new PIV experiments. Also, it has higher pulse energy (200mJ!) which is always a good thing . The evergreen lasers from quantel are made with PIV in mind, so they sacrifice things like beam quality for more power, compact construction, and reliability.

I will get pictures of the pockels cell with damaged faces so you can see what they look like. We now keep the laser head in a positive pressure chamber to minimize the muck from the outside lab space from polluting the laser head itself. I will include a picture of this as well, perhaps it can provide inspiration for DIYer's and aftermarketers who want to keep their lasers operating happily into old age
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Old 05-02-2013, 02:18 AM #4
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Default Re: Damaged optics in a Nd:YAG

I'd definitelty love to see some images of the P.C. assembly!
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Old 05-02-2013, 09:56 AM #5
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Default Re: Damaged optics in a Nd:YAG

Thanks for sharing your laser experience
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Old 05-16-2013, 06:39 PM #6
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Default Re: Damaged optics in a Nd:YAG

I have some pictures of the laser in the postive pressure chamber. It's just an 80/20 frame with plexi panels. There is a blower fan installed on top which blows air into the box. The fan has been fitted with a piece of air filter (cut from a typical furnace air filter) which keeps the inside of the box a good bit cleaner than the lab space around it!

This experiment requires modulating the polarization of the laser pulse, so there is a cut out on top of the box so I can stick my hand in and electrocute... errr... I mean "adjust" the extracavity pockel cell. This is not the pockel cell that was responsible for the damage of the old laser. I don't have pictures of the damaged PC yet, but I should be able to get them.

Anyways, if you've got high energy pulse lasers (especially if they're open cavity, or DIY ssy1, etc...), I recommend keeping the optics emaculately clean. A postive pressure box is a good idea because it keeps the laser much cleaner, and it enhances saftey by keeping any high voltage parts in, while also keeping things from finding their way into the beam path.
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