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Registered Member #2040
Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180
**Two new videos here, and many more pictures further down A video of the laser cutting a paper clip in half. A video showing the laser punching a hole through a metal cup **I just realized that I forgot to name this creation, so she gets the first name that comes to mind and that is "Joules," **Picture of interface board added **Videos:
I have finally got my 60J ND:Glass laser working. It's taken a couple months, I can't really remember, maybe 6? This is actually the second time it's been working. The first time I had it working I was using a BX-24 micro controller from Netmedia, which worked OK until it stopped downloading new programs. So instead of buying another and risking the new one going bad too, I took a different route and made a circuit to interface between my computer and the control electronics.
The laser head is a Coherent Everpulse 11. It‘s water cooled, and rated for a maximum of 60J output at 1ms. It has dual flash lamps, and the laser rod is about 14 inches long with a diameter of 7mm if I remember correctly. It’s able to be configured to fire both lamps in series or parallel. The power supply that is supposed to be used with this laser had two 3KV 185uF capacitors, presumably to fire the flash lamps in parallel. I had a lot of trouble trying to find anything close to that, so I decided to fire the lamps in series. The storage capacitor is a 15KV 100uF Maxwell, which is charged to a maximum of about 5150V. To charge the capacitor I use three MOTs in series, which are rectified by a full bridge of 48 6A 1KV diodes. Two of the MOT primaries are wired in series to reduce current draw, while the other is connected regularly. This configuration will charge the storage cap to maximum charge in less than a second. The series discharge inductor is a portion of a roll of 14AWG THHN bought from Home Depot. It used to be 1mH, but performance was anemic so I’ve taken quite a few turns off; I believe it’s about 300uH now. The discharge connections to the laser head are HN coaxial fittings. I tried to keep this scheme but I kept getting arc-overs in the connectors. I’m sure it works OK when the cables are only carrying 3KV, but at 5KV it was too unreliable. For now I have 14 AWG THHN inside of ¼†plastic tubing with ring terminals connecting the laser head. It seems reliable so far, so I probably wont change it.
The main purpose of the circuit is to provide an interface between the computer and the triggering circuit, the charging relay, and the AD7819 8 bit ADC that monitors the storage capacitor’s charge. I found that the ADCs are very sensitive to voltage transients caused by the HV trigger circuit, so I included a few protection measures in the final circuit. To protect the ADC I used ISO742xC isolator chips from TI, and relays that disconnect the reference line and the supply rails. When the laser is fired the supply rails and reference voltage are disconnected, and the whole ADC circuit floats for about half a second, supported by a large filter cap located on board. In this way, it’s virtually impossible for those nasty spikes to make their way back to the ADC.
Since I was already using the isolator chips, I went ahead and used them on every I/O line. There is also an 8 bit buffer that is used to drive the relays. To interface to the computer I used the parallel port because it’s so easy to do so. The only issue with the parallel port is that it only has 5 input lines, but I needed 8 for the ADC data and one additional line for the ADC BUSY signal. To get around this I multiplexed the 8 data lines onto 4 of the inputs and connected the BUSY line directly to the remaining input.
The control circuit was my first attempt at using SMDs, and it turned out great. Not that I wanted to, but the ISO chips only come in surface mount so I had to. Sometimes I will try to get by using photo paper with the toner transfer etching method, but I didn’t even try with this board. I went straight to the press and peel because of the fine traces and pads. Altogether it was a good experience, although there is a learning curve to soldering. I ended up intentionally solder bridging all adjacent pins together and then using a solder sucker to remove the excess, which worked great for me.
The trigger circuit is a 2uF capacitor that is discharged through an ignition coil by means of an IGBT. The capacitor is charged to about 340V by doubling regular 120V mains. The trigger transformer that came with the laser head was arcing internally, so it was replaced by an ignition transformer I had on hand. Unfortunately, it’s so large that I can’t put the cover back on the laser head. If anyone knows of a specific model car that uses a smaller transformer, please share.
The program I use to control the laser was written in VB; the free express version to be exact. The parallel port .dll was downloaded from , it works well. The program will charge and maintain the storage capacitor at a preset level, and automatically recharge after the laser is fired. Alternatively it can be charged just once when the user clicks the “charge once†button. This is what I use 99% of the time. It can also be set to fire automatically, at intervals set by the user. Nothing too fancy, just functional.
The flash lamp terminals are exposed to the cooling water, so a mixed bed de-ionizing filter is used to keep the storage capacitor from discharging through the water. The flash lamps are also triggered through the water. The trigger transformer is connected directly to the reflector. I was initially worried about the de-ionized water insulating the flash lamps from the trigger transformer, so I included a valve to bypass the filter so I would have a way to regulate the resistance. It turns out that I didn’t need it as I’ve never used it. The pump is a regular HVAC condensate pump. The laser head has quick connect double shut off male fittings and I was lucky enough to find the same type to use for the cooling lines and the connections at the power supply. The cooling water never gets more than warm so no heat exchanger is needed, which is good since the DI water will eat away at the metal. So far I haven’t had any trouble with the water eating away at the brass fittings.
Well I think that’s it. I have included a couple pictures; the whole is kind of messy since it is a working prototype. I will link to some movies once I get them uploaded. Questions and comments are encouraged.
The original controller
The interface board. Three more relays, filter caps and about 30 jumpers are on the bottom.
*** Update 12 Dec 09 *** Pictures posted below and videos posted above
I set up a focusing lens today and had a little fun with the laser. The whole setup is modeled after a CO2 cutting setup. It has a nozzle assembly which gets compressed air blown through it to protect the lens. I'm not sure how an industial pulsed laser like this one usually protects the lens, but I would think it is similar to the CO2. The nozzle is made from paper at the moment so it isn't going to last too long, but it should work until I figure out something better.
I thought the patterns produced by the exploding metal were pretty cool, so I got a little picture happy and posted some of the best ones below. The target is a stainless steel outlet cover 0.8mm thick.
One of the pictures shows a distinct grid of IR dots (displayed as purple) around a hole that was shot through the outlet cover. I don't know what caused this; I think it could be something internal to the camera. If anyone knows please chime in.
Besides the outlet cover, I also tried a few other things listed below. Penny - 1 shot to penetrate License Plate - 1 Outlet Cover - 2 Quarter - 3 1/8" aluminum - 3
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
Wow, that is actually a very nice setup you have there!
Beats the controller used to fire my yag, It was just a backplane connector with pins strageticly removed to allow insertion of the 2 connectors from the power supply/head placed on a breadboard with pots for the charge voltage/simmer current and the interlocks. For repetitive firing I hooked it up to a signal generator.
I am really looking forward to seeing your head in action, this was taken at ~2J (haven't had time to measure this head), I can only imagine what 60 would be like
Registered Member #2040
Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180
The videos have been posted. You should take a look and compare it to your 2J laser, you can verify that I'm actually getting what I think I am if you could be so kind. I'm not sure just how to measure the energy since everything that absorbs the 1060nm just explodes. & I guess if you can compare the violence of your explosions to the violence of mine we might be able to get an idea.
Edit: I'm thinking I should try to use a dark 1060nm absorbing liquid or a black plastic under water and measure the temperature rise.
Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
Could you put the beam through a lens to expand the spot to a large enough area that you can get it absorbed without vaporising anything and then measure temperature rise?
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
What is the input energy you are using for those shots? I would put them well under 10J, but it is hard to say just by looking at them.
Have you tried tweaking the alignment of the resonator mirrors? Its possible that they got knocked off slightly.
If you have any lenses of reasonable quality (they don't need to be anti reflection coated, but I don't think a dirty plastic lens will hold up to the pulse), try focusing the beam down on a razor blade (or other thin steel) and see how large of a hole you can burn through it. 1J can just barely make it through, with 2-4J you can see light though the hole, and when you get into the really high energies you should be able to make it through several blades.
Also, you might have better luck photographing the effect if you set up a camera for a long exposure (or nighttime/firework mode), use a dark room, trigger the camera and then trigger the laser. You will need to either stop the camera down really low (if you have a camera with such controls), or use a filter to keep it from washing out.
Registered Member #2040
Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180
... wrote ...
What is the input energy you are using for those shots? I would put them well under 10J, but it is hard to say just by looking at them.
EEK...I was afraid of that. I did do a little tweaking with the mirrors and it seems like I picked up some power, but I definately didn't gain by a multiple of 6 or more. I'm beginning to wonder if that power supply I mentioned before, the one with two 185uF capacitors at 3KV, was capable of driving this laser to it's maximum output? With those two capacitors the power supply has a maximum energy of 1665J, which means the laser needs an efficiency of atleast 3.6% to get 60J output. Is 3.6% outrageous for an ND:Glass laser? I know it is for a YAG, but the Glass lasers are more efficient. To answer your question, the storage capacitor was charged to 5163V for 1333J.
Edit: When I said that I tweaked the mirrors, what I did was turn each adjustor screw until the output seemed to be maximized, and then moved on to the next one. Doing it in this way will ensure the mirrors are parallel to each other, but not neccesarily perpenicular to the laser rod. I need to figure out how to align the mirrors perpendicular to the rod to get maximum output.
IntraWinding wrote ...
Could you put the beam through a lens to expand the spot to a large enough area that you can get it absorbed without vaporising anything and then measure temperature rise?
That sounds like a good idea. I tried a few different substances today, without much luck. One of the best (most amusing) was Mrs. Butterworths waffle syrup. I wanted to see if it woud absorb the laser, so I put some in a small container with my old standby piece of ABS in the bottom. Well it turns out the syrup doesn't absorb very well. When the ABS in the bottom of the dish exploded it sent syrup flying everywhere. Not very productive but it was pretty funny.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
I like my laser which is a Ho:YAG so it lases at 2100nm, and is absorbed by water, which lets me do things like burn holes in icecubes.
The key to making an absorber is to get something which is only slightly absorbing at 1064nm, so that the pulse is absorbed through the length of the absorber as opposed to vaporizing off the top layer. If you a video camera which is sensitive to IR (most are to some degree) you can use an IR led (or in a pinch, the output from a tv remote control) to see if materials are opaque at 1064nm. I would try coffee, and diluting it until you don't generate any steam when you fire the laser. With 10 shots you should be able to get a noticeable temperature change, at least enough to give you a general idea of how much power you have.
Your 1.3kJ might be able to hit the 60j output, although the laser design can have a huge effect on efficiency, so you can't just say that since I have 1.3kj and these lasers are 2% efficient I must be getting 25j out.
Registered Member #2040
Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180
I understand that there will be some variance with design and individual models, I was just wondering if it was possible with that power supply.
I have to say that something must be wrong. The laser came with a shutter, as I'm sure most do, made of aluminum. On the backside of it there is a circle of melted/scorched metal where the laser had been fired with the shutter closed. I tried this at around 1700 Joules input and got nothing. I guess I have my work cut out for me. Any suggestions?
Edit: The only thing I can think of is that there is a problem with the optics. I tried a technique called "walking the beam" (or something like that) that I found in Sam's laser FAQ. The idea is that the mirrors are adjusted individually for max power, which means they are parallel to each other, and then both mirrors are adjusted in the same direction. The idea is that by adjusting them both in the same direction they will remain parallel to each other and the only angle that is changing is the angle between the rod and mirrors. If they are moving more towards perpendicular to the rod, there should be a power increase. I tried this and power decreased on every adjustment axis I tried, so they must be pretty close.
I also checked and cleaned the optics with no luck there either. So I'm really at a loss. The inside of the OC looked to be slightly scratched, I don't think it's much but then again I'm not a laser. Do the mirrors deteriorate over time? This laser is 24 years old. At the moment, all I can think to do is replace the mirrors, but then I have to guess as to what the OC transmittance should be.
Nice laser head! Does it use a rod or slab nd:glas piece? I heard some of them use rods some slabs.
Regarding the power, I'm willing to bet it's not well aligned. For now it's probably best if you only tweak only one of the mirrors so you don't go out of aliment with the rod any further. And, don't align for max apparent power, it's not going to work well. Get something like black rubber, eventually expand the beam a little, and align the laser for the most uniform burn pattern. When is perfectly aligned the burn pattern should be perfectly uniform.
And, do you happen know the actual discharge length for the main cap?
And, here are some pictures from my yag head, 200J electrical, 100x7mm nd:yag rod.
The pulse length for my head right now is about 30ms, measured at the main cap.
Registered Member #2040
Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180
Daedronus wrote ...
Nice laser head! Does it use a rod or slab nd:glas piece? I heard some of them use rods some slabs.
Regarding the power, I'm willing to bet it's not well aligned. For now it's probably best if you only tweak only one of the mirrors so you don't go out of aliment with the rod any further. And, don't align for max apparent power, it's not going to work well. Get something like black rubber, eventually expand the beam a little, and align the laser for the most uniform burn pattern. When is perfectly aligned the burn pattern should be perfectly uniform.
And, do you happen know the actual discharge length for the main cap? ...
This particular laser uses a rod. Some use slabs, like NIF.
I'm not sure what you mean by discharge length of the cap, the flashlamp arc length maybe? If that's what you mean then the answer is about 14 inches per flashlamp, and they are wired in series, so about 28" altogether.
I'll keep your sugestion in mind for when I get the proper lens.
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"Joules" the 60J ND:Glass laser
& 
I guess if you can compare the violence of your explosions to the violence of mine we might be able to get an idea.
