LIBS Spectrometer - Build
I have wanted to have a Laser Induced Breakdown Spectroscopy (LIBS) Spectrometer for a while. These things are cool, because they can tell you the atomic makeup of a sample. One very nice specimen is the ChemCAM on the Curiosity Rover.
After reviewing that units and finding out that they used Ocean Optics spectrometers, I looked on eBay and found that they are commonly available. Spectrometers from a variety of manufacturers can be had, but without any experience it's hard to make sure the quality is ok. So I got lucky on a unit with several thermo-electrically temperature controlled channels (and the seller even sent me a halogen light source of which there was no mention in the auction). Unfortunately the box was completely banged up and one of the handles of the spectrometer case was broken off. Other parts were bent. I feared the worst. Luckily, the spectrometers themselves seemed to have sustained no damage at all!!:
So far so good. Of course, I still need a laser, so the spectrometer got shelved for the moment.
Laser try 1:
The laser needed is a q switched unit so that there is a strong short pulse capable of creating a plasma. The most common such laser is the Nd:YAG. There are many such units available on eBay, and it seems that the laser tattoo removal machines would also be a good candidate.
I found a unit that seemed good and purchased it. It came without a power supply, but the control circuit was attached. So all that was needed was an external high voltage capacitor, high voltage power source and a switch. Sam Goldwasser was very helpful in setting that up.
Here, you can the laser on the right hand side, two high voltage capacitors, an EMCO high voltage power supply and an arduino shield that I am just using for the switch that is soldered to it.
The flash tube fires just fine, but no laser pulse comes out. I did a lot of checking with Sam's help - removing parts, aligning them with a He-Ne laser, bigger capacitance, higher voltage, photo sensor on the flash lamp to determine how fast the caps discharge - but have not been able to figure out what is wrong. It's my first YAG, so I am really short on both experience and equipment to troubleshoot this thing. Back to the drawing board.....
Laser try 2:
Another unit from eBay, but this time it has a mechanism to align the mirrors:
Result: Same as previous laser, got the flash lamp to fire, but no lasing...
Even tried a more serious discharge capacitor:
But it did not help.
Laser try 3:
Sam Goldwasser was very gracious and lent me a YAG laser of his, complete with power supply. That finally worked, in the sense that I got a q-switched output. There is a very audible 'crack' when the laser hits something and a bit of the surface vaporizes. Now we are talking!
In the background, the case with the analog dial and knob is the power supply, and in the front you can see the YAG laser pointed into a microscope objective. In between, you can see some ZAP-IT paper, with some large circles where the unfocused YAG hit it and small circles where it was focused.
Some tests show that this unit can blast corrosion of steel samples and anodization from aluminum samples. So far so good.
Next, we have to be able to collect the plasma light into the spectrometer somehow. So I came up with a contraption that may work:
There is a lab jack onto which I mounted the Laser, the focusing optics and a rail. The rail holds a lens and an SMA connector. I can slide the SMA connector and the lens along the rail to adjust the focus, and thus the area where the spectrometer collects light. The blue box on the left is a halogen light source, which lets me pump light into the SMA cable that connects to the connector on the rail. That illuminates the spot onto which the lens/SMA connector focus. Once that is done, the SMA cable can be connected to the spectrometer. In Fact, the fiber optic cable has so many members that I can leave the halogen source and the spectrometer all plugged in at once..
I thought that maybe I could just turn on acquisition on the spectrometer and then hit the 'fire' button on the power supply real quick. But that did not work, either because of the LabView software for the OceanOptics unit, or because the integration time would let stray light down out the signal. I played with the idea of building a light proof box, but the proper way of doing this is some sort of synchronization between the spectrometer and the power supply. Also, I did not like the way LabView worked together with the spectrometer, so I installed Windows XP, first in a virtual machine under linux, and then on an old laptop. That seemed required to get the OceanOptics OOIBase32 software to work. It did work in both the virtual and the real XP machine...
But first I looked at the power supply. There is a DIN plug at the back of it and it looks like that might be for an external trigger of some sort. Sam could not remember what that was for and how it worked, so, I took the unit apart took some pictures and traced the DIN plug. The hope was that Sam might remember with a partial schematic....
That also explains what the extra switch on the front plate does that Sam did not remember about - it toggled between and external and the front plate trigger button.
Sam did find a schematic for the power supply eventually (impressive, because the date puts it as more than 10 years old):
Ok, I don't want to switch this with an external button, but with a 5V signal. Also, I prefer to have optical isolation in this case. So what I came up with is this:
To test this, I put the opto into the power supply in an empty space on the existing circuit board. I disconnected the existing wires from the DIN connector and put them on the output side of the opto. Then, I pulled some wires out and tried to fire the YAG from a 6V battery. This worked the first time - Yay!
Now I can fire the laser with a signal from the spectrometer - or so I thought...
It turns out that the spectrometer has a fairly rich set of connectivity options available in the form of a DB15 HD connector:
'Strobe Single Fire' sounds just right!
Unfortunately, looking at that with a oscilloscope reveals that they really mean one strobe per acquisition period. And it also looks like I can't turn off acquisition with the GUI. All that I can do is suppress output to the GUI so it looks like acquisition has stopped...
Ok, lets look at the external hardware trigger. In this case, it looks like this is just what I need. But I have to hook up an external button that triggers the external hardware trigger at the same time as the external trigger for the power supply. Oh well, since the spectrometer provides 5V as well as the trigger input, we can set this up fairly easily
Ok, as a proof of concept (and not having a push button on hand) I completed the whole setup with flying wires and a small breadboard:
We have signal:
The two targets are a steel magnetic base (onto which I had painted a black dot at the very top edge with a sharpie. subsequently I hit the sharpie and the paint below enough to hit bare metal) and an 6061 aluminum sheet. The impacts on the aluminum sheet are hard to see, but there are two dots towards the top of the sheet that are the relevant impacts. Thre are 3-4 more dots in the upper left, but they are 'shoot-through' from a sheet of mica that I had placed in front of the aluminum in a subsequent experiment. Here is what the mica looked like afterwards. You can see several white dots towards the top:
I was worried that I would not be able to collect enough light, but it looks like only one channel (with a smaller slit size) is not over-saturated.
I built a box to house the trigger button and to connect between the laser power supply and the spectrometer
Here a picture taken during assembly process. The wires are still missing.
There was a bit of a noise problem after putting all this together that caused the spectrometer trigger to go off spontaneously.
A scope trace on that signal shows the problem:
So I modified the schematic slightly by adding a 10 nF capacitor on the trigger line:
That quieted the noise down sufficiently:
Since the trigger is an edge trigger, the rise time is important. When adding capacitance to this signal it is important to make sure that the rise time does not get affected too much:
Looking good and no more false trips.