Lambda Ruby Laser Head

[Laserbuilder]

Hi to all, I have some experience in building and tweaking the ruby laser. Yes, the full-reflection prism works as a HR mirror, but you will lose at least a half of possible output energy, because the beam will become TEM10 single mode. I know from my experience that OC is needed with at least 45-50% of reflection\transmission for 694 nm. It may depend on the amount of Cr3+ dopant in the ruby crystal. Pale pink rubies need more reflection of the OC because the amplification ratio is lower than in deep red crystals. Anyway, 50% OC will give you reliable lasing with the prism or a proper HR mirror at the HR side.

The Ruby Rod and Flash tube are approx 95 mm in length. (Active Area).

What is the diameter of both? Mine was 8*120 mm and lasing started from around 1-1.2 kJ of pumping energy.

And yes, you'll have to deal with high voltage capacitors, be very careful!

Also look through Jarrod's laser worl site, he is very experienced in building ruby lasers from scratch, he also tried a very well polished copper coin as a HR and it worked.

Details on my ruby laser project are here. https://laserpointerforums.com/f50/laserbuilder-s-homebuilt-ruby-laser-99986.html

 

[ultimatekaiser]

yeah you really dont need much reflectivity at all as long as the back side is totally reflecting or as dang close as possible. maybe only 20% or so. most of the ruby lasers i've worked with had either a prism HR and a 20 or 40% range OC, or the other we handmade had a dielectric HR, and a 43% OC. though we also used a glass microscope slide (unmeasured but flat glass) and it worked, though threshold was much higher due to the lower reflectance.

I tend to find the problem is more just making sure you have enough input energy. Ruby doesn't like to lase unlike YAG crystals. it's only a 3 stage laser, and therefore is quite stubborn to pump on top of the fact that it decays on its own wavelength as well-so if you don't saturate it you will get nothing as it will re-absorb it's own light emission. It also doesn't cool very well. YAGs are far more thermally conductive and dissipate heat faster...Ruby suffers in that area, so don't over pump it. too frequent flashing can heat the rod till it develops microfractures, or even shatters quite fast. Be patient and allow ample cooling time between flashes if it isn't water cooled with deionized water. convection cooled rubies are quite slow to recover. Mine has a built in delay of several seconds by design to let the caps charge to help with this issue, but even then it still has a temp lockout built in for safety sake.

 

[Laserbuilder]

Ruby doesn't like to lase unlike YAG crystals. it's only a 3 stage laser, and therefore is quite stubborn to pump on top

Yes, you are right. For more efficient pumping the light pulse from the flashlamp should be "shaped" properly, usually you need a pulse with the shape close to rectangular. You can achieve it by putting an big inductor coil (the amount of uHn in it should be adjusted, typical value is 100 to 200 uHn) in a series with the flashlamp, or putting smaller inductors between the capacitors, like in common LC filters. The multi-stage C-L-C-L-C-etc bank will give the best perfomance of the laser, because the pumping process will be more efficeient.

The point is to make the flash duration equal to the time of life of the metastable state of the Cr3+ ions in the ruby and support the intensity of pumping light constant during this time.

 

[ultimatekaiser]

Yes, you are right. For more efficient pumping the light pulse from the flashlamp should be "shaped" properly, usually you need a pulse with the shape close to rectangular. You can achieve it by putting an big inductor coil (the amount of uHn in it should be adjusted, typical value is 100 to 200 uHn) in a series with the flashlamp, or putting smaller inductors between the capacitors, like in common LC filters. The multi-stage C-L-C-L-C-etc bank will give the best perfomance of the laser, because the pumping process will be more efficeient.

The point is to make the flash duration equal to the time of life of the metastable state of the Cr3+ ions in the ruby and support the intensity of pumping light constant during this time.

yes. up to about 3ms if you're doing the whole ruby florescence lifetime. Yag is a few hundred us if I remember correctly. though you can shorten it if you have precise enough electronics in order to preserve life of the lamps/energy requirements. I'm not positive of the electronics in mine, i haven't critically analyzed them. there's alot crammed into a tiny space in there, as mine is made to be portable. one of the only portable ones eve made. it's quite a unique specimen. I really should finish fixing it up again. I botched my alignment and need to get it going again.

 

[Mosc007]

I have a question about the Ruby Head from the Lambda laser I hope someone can help me with.

Inside the head is a piece of rectangular glass about 1mm thick. It is between the Flash Lamp and the Ruby Rod. But there are no gaskets or anything to hold it in place. It's just sitting loose in a machined out groove.

Is this just a protection sheet when the head is opened ?... Or is it needed when running the laser ?

It doesn't seem right that it just sits loose in there.

 

[ultimatekaiser]

Edit: nevermind. There usually is a plate of material in between the lamps and the rod in larger lasers to help protect the rod in the event of a lamp bursting, as they're incredibly expensive to replace. They also control water flow in many cases, but not sure in your case. one of my old lasers has samarium doped glass in between the rod and lamps to filter out the UV from the lamps as it just results in waste heat in the particular rod that was used in it. I'm assuming this is what you're talking about.

 

[Mosc007]

Edit: nevermind. There usually is a plate of material in between the lamps and the rod in larger lasers to help protect the rod in the event of a lamp bursting, as they're incredibly expensive to replace. They also control water flow in many cases, but not sure in your case. one of my old lasers has samarium doped glass in between the rod and lamps to filter out the UV from the lamps as it just results in waste heat in the particular rod that was used in it. I'm assuming this is what you're talking about.

It may be to control water flow. I have only traced the water flow a bit. I will have a better look at the ports and see how the water flows accross the Lamp and Rod.

I would have expected it to be held in place. Not just sitting loose in the groove. From what I can see it doesn't appear to have any coatings on it.

 

[ultimatekaiser]

usually in the case of the lamps it wouldn't have a coating, if it was made to be a filter, it'd be doped into the glass, but as alot of lampmakers now do that directly to the lamp glass if necessary it's not as widely common. and if it's loose in there, it's not a big deal if the water flow is in the correct direction as the water pressure will hold it in place.

 

[Mosc007]

usually in the case of the lamps it wouldn't have a coating, if it was made to be a filter, it'd be doped into the glass, but as alot of lampmakers now do that directly to the lamp glass if necessary it's not as widely common. and if it's loose in there, it's not a big deal if the water flow is in the correct direction as the water pressure will hold it in place.

I don't know which Ports are In and Out for the cooling water. But I would assume water would normally flow into the bottom of the block and out of the top block to make sure no Air is trapped inside.

I have my Reservior, Pump, Radiator and Fans ready. I will just use Distilled water in it.


I will have a very slow cycle time as I am going to run the whole thing from 12 Volts. An inverter to charge the Caps to 1200 Volts and another inverter for the Trigger transformer supply. It will probably be a 30 second cycle time. Plenty of time to cool it down again.

 

[Laserbuilder]

By the way, check if your ruby crystal has any coatings on its faces. These lasers (for "unqualified" laser users) may have mirror coatings sputtered directly on the faces of the crystal to make the whole unit simpler, cheaper and more reliable.

 

[Mosc007]

By the way, check if your ruby crystal has any coatings on its faces. These lasers (for "unqualified" laser users) may have mirror coatings sputtered directly on the faces of the crystal to make the whole unit simpler, cheaper and more reliable.

I did check when I first looked at it. The Rod does have an AR coating on the ends. But the ends are not silvered. Maybe at the power this head runs at Silver is not an option.

Another interesting thing about the rod. It's not Round. Its only about 75% Round then has 3 flat faces on it. In the manual you line the center flat face up with the flash tube. It must increase the effiency of the pumping.

 

[ultimatekaiser]

I did check when I first looked at it. The Rod does have an AR coating on the ends. But the ends are not silvered. Maybe at the power this head runs at Silver is not an option.

Another interesting thing about the rod. It's not Round. Its only about 75% Round then has 3 flat faces on it. In the manual you line the center flat face up with the flash tube. It must increase the effiency of the pumping.

They may have truncated the flashing side and polished it is what it sounds like. and in theory, yeah it might I suppose...

 

[Mosc007]

They may have truncated the flashing side and polished it is what it sounds like. and in theory, yeah it might I suppose...

Yes, The 3 Flats are highly polished.

 

[Mosc007]

Slowly getting the parts together for the build. I expect probably a couple of months before its ready to fire.

I received and assembled the Capacitor bank. I went for 4 of 470uF 1200 VDC Polypropylene capacitors. Very low ESR and a very High current rating.

The calcs say with 1880 uF charged up to 1200 Volts that will give me 1353 Joules of stored energy. If I can dump it into the flash tube fast enough that will give me a peak output of over 1,000,000 Watts.

The Capacitors are a lot larger than I expected. They are 100 mm in Diameter and 150 mm High. The assembled bank is heavy, about 6 Kg.

Images of the bank.

 

[Laserbuilder]

Don't forget an at least 100-150 uHn choke in series with the lamp, otherwise it will be smashed into pieces

 

[Mosc007]

Don't forget an at least 100-150 uHn choke in series with the lamp, otherwise it will be smashed into pieces

Yep. I have only done a rough calc and it came out to be 64 uH. I need to measure the tube and calc the resistance so I can then calc the Inductor more accurately. It's a 4 inch tube and I guess it will be about 2 ohms when firing.