Homebuilt ruby laser

[Laserbuilder]

This is a compilation of my posts from LPF, where I had told the story of building this laser. The LPF thread is located here https://laserpointerforums.com/f50/l...ser-99986.html

Long time ago I have found a quite big (8*120 mm) ruby crystal from a pulsed holography setup and a couple of big 5 kJ flashlamps. So, I immediately decided to try to build a ruby laser.

At the first attempt I tried to put the lamp and the ruby crystal into a cylindrical glass reflector. I knew that the lamp didn't match the crystal by size and the performance would be very low. The reflector with the mounts, lamp and the crystal formed the "laser core" as I call it. The laser core I'v mounted on a metal base and attached the adjustable mirror mounts on the opposite sides. At first I tried to use a full reflection prism as a HR and a random OC mirror. I took it to the spectrophotometer and measured 45% transparancy on 694 nm wavelength for it.

The power supply was a very crude and unsafe pile of electrical circuitry including a 1000 uF capacitor bank, microwave oven transformer and a single rectifying diode to charge the caps, an induction coil with an ignition transformer connected in series with the flashlamp. The flash energy could reach 4.5 kJ, but I saw no lasing at all.

When the first version of the laser failed, I rebuilt the laser into the second version. I purchased two 2 kJ lamps that ideally matched the crystal. I had to remake the mounts for both the crystal and lamps. The resonator mirrors I left the same. I connected both lamps in series, which allowed me to use them with the existing power supply. Then I've built around the lamps a box from white bathroom tiles, thinking that they will be good as a diffusive light reflector. And, after aligning the mirrors I got first flashes of red coherent light! The laser was working. At maximum pumping energies and with focused beam it struck sparks from pieces of iron.

though the laser was already operational, it was quite bad. The lasing threshold was big (at least 2 kJ of pumping energy), the white tiles quickly were becoming grey from very intence light, the crystal overheated quickly -- each new shot decreased output energy until seizing.

And then, accidentally I had found a shiny sheet of chromium plated steel. Of course, I wrapped it around the lamps instead of those tiles. The lasing threshold fell to 1.5 kJ, maximum output increased to, possibly, 2-3 J of laser light. Metal sparked quite spectecularly, when hit with the focused beam. The output energy became more stable and decreased only with the crystal overheating. Waiting 5 minutes between the shots for the crystal to cool down made the energy stable)))

Than I decided to replace the full reflection prism, which worked as a HR with a conventional flat HR mirror for 694 nm, which I found in the rubbish bin in the optics lab with a pile of other mirrors. It was in a quite bad state, with holes burnt in the reflective coating. I glued it to the place and aligned the resonator. It made great effect! Th lasing threshold dropped to 900 J and maximum output increase at least by 2 times! I estimated the output energy to be at least 4-5 J with 4.5 kJ of pumping. Shooting an unfocused beam in the rubber causes a 5 cm flame out of it, focused beam drills a tiny hole through 0.8 mm thick metal sheet in 2-3 shots with loads of sparks and shoots through a coin (1.2 mm thick) in 6 shots.

[Eidetic]

Wonderful to see the sparks fly!

[Jem]

Fantastic work. Love it!

[buffo]

This is the laser I wanted to build for myself when I was a kid... So awesome to see you've actually done it!

I suspect you are at the low end in terms of what is possible with a ruby rod of that size. The ideal reflector is an ellipse with the rod at one focus and the flashlamp(s) at the other focus. Have you ever tried that arrangement? From what I'm told, you can get a significant increase in pumping efficiency with this design. (Perhaps 2 to 3 times better coupling)

What is the max voltage on your main capacitor bank feeding the flashlamps?

Adam

[Laserbuilder]

The ideal reflector is an ellipse with the rod at one focus and the flashlamp(s) at the other focus. Have you ever tried that arrangement?

No, I haven't as I didn't have a possibility to do that. Also it is said that the close arrangement with the rod and lamps very close each to other works equally well.

What is the max voltage on your main capacitor bank feeding the flashlamps

It is 3 kV at the moment. The capacitance is 600 uF which corresponds about 2.5 kJ of stored energy.

[sbk]

This is the laser I wanted to build for myself when I was a kid... So awesome to see you've actually done it!

Hehe, exactly the same for me, I was trying to build a ruby laser with some Lego, flashlights, color filters and lenses when I was 8 years old

Congrats for this awesome build, Laserbuilder ! And thanks for sharing

[buffo]

it is said that the close arrangement with the rod and lamps very close each to other works equally well.

I think a close arrangement works better than many other non-symmetrical arrangements, yes, but you're still going to lose most of the pumping energy from the top, bottom, and back side of the lamps. With the elliptical reflector, a great deal more of the pumping energy (from the entire circumference of the lamp - not just one side) makes it into the rod. Given how big that rod is, with a more efficient pump you should be able to get a *lot* more power output.

I have a book I keep here at my desk: "The Story of the Laser", by John Carroll. It's one of the first books I ever read about lasers - back when I was a kid. It was published in 1964, so a good bit of the information is now out of date. However, near the end of the book there is an account of a teenager who built his own ruby laser from scratch as part of a science fair project. He used 4 flashlamps clustered around the rod (which was less than 1/2 the size of yours) and he did not have much of a reflector at all. (The entire cavity was simply wrapped in aluminum foil, which also served as the trigger electrode for the lamps.)

The book doesn't give many details about his design, but he did have 4000V supply feeding his capacitor bank. (Size in Farads is not known.) However, given there were 4 flashlamps, I think it's safe to assume he had at least 2000 joules of pumping energy, if not a great deal more than that. But based on the damage he could do to a razor blade (merely deep pits in a single shot), his power output was probably close to yours (call it 5 to 10 joules at most).

All that being said, later in the same chapter they discuss the elliptical reflector idea and mention that it can reduce the pumping threshold energy by a factor of 10. You mentioned that your rod began lasing at around 900 joules. This book suggests that with the right reflector it might begin lasing at around 150 joules!

So if you ever find the time to re-visit the cavity design, I would recommend you give the elliptical reflector concept some serious consideration.

Adam

[clickamouse]

But will it burn tape?

[Draco]

I have wanted to try that as well, I built a 1kJ power source but right now I don't have a laser rod, was thinking either YAG or ruby, maybe I will revisit that when I have some spare money for that since I don't have a practical use for it at this time

[buffo]

Probably easier to find a YAG rod (all those surplus rangefinder units, right?), but you just know you'd rather have the ruby...

There was a point where I seriously considered buying a raw synthetic ruby boule and then paying someone to do all the finishing work on it, but it turns out that folks who know how to use an optical bench, test flats, diamond saws, etc, and are likewise skilled at hand-lapping optical crystals don't exactly work cheaply! Buying a finished ruby rod is definitely the way to go.

But even if you do all that, in the end you've got a pulsed laser that is laughably inefficient and has a very low pulse repetition rate. It's cool because of it's history, and it might even be able to put a pinhole in a razor blade, but at the end of the day it's a bunch of heavy, lethal equipment that will just clutter up your laser room. At least that's what I've been telling myself all these years.

That's also why I'm so envious of Laserbuilder. He did what so many of us always dreamed of doing. He basically said, "to hell with the consequences, I'm doing this", and now he's got a working ruby laser and you and I don't.

But if I had that huge capacitor bank in my house, my wife would scream bloody murder... (And that would be before she ever realized how dangerous they are.) Plus she still hasn't completely forgiven me for burning a hole in the couch with that old Medical Argon I was experimenting with years ago. Can you imagine the damage I could cause with a working Ruby laser?

"Here, hold my beer and watch this!"

Laserbuilder: Rock on, dude. We're all living vicariously through your posts!

Adam