Laser Hacks

[Geraldine Ferraiz]

Becauseof this, [Julian] had to break out the big guns. After experimenting with the power level and pulse duration of the 2940 nm laser, he found the settings necessary to break down the varnish while stopping short of cooking the paint it was covering. After hitting it with a few pulses, he could then come in with a cotton swab and wipe the residue away. It was still slow going, but it turns out most things are in the art conservation world.

Hidden states are a fascinating aspect of matter, as these can not normally be reached via natural processes (i.e. non-ergodic), but we can establish them using laser photoexcitation. Although these hidden states are generally very unstable and will often decay within a nanosecond, there is evidence for more persistent states in e.g. vanadates. As for practical uses of these states, electronics and related fields are often mentioned. This is also the focus in the press release by the Ecole Polytechnique Federale de Lausanne (EPFL) when reporting on establishing hidden states in magnetite (Fe3O4), with the study published in PNAS (Arxiv preprint link).

[B. Truc] and colleagues used two laser frequencies to either make the magnetite more conductive (800 nm) or a better insulator (400 nm). The transition takes on the order of 50 picoseconds, allowing for fairly rapid switching between these metastable states. Naturally, turning this into practical applications will require a lot more work, especially considering the need for femtosecond pulsed lasers to control the process, which makes it significantly more cumbersome than semiconductor technology. Its main use at this point in time will remain a fascinating demonstration of these hidden states of matter.

In most gas lasers, the stimulated emission action takes place within a closed optical cavity, typically formed by a glass tube whose ends are sealed with mirrors, one of which is partially silvered. The gas in the tube is stimulated, by an electrical discharge in the case of a helium-neon laser, and the stimulated photons bounce back and forth between the mirrors until some finally blast out through the partial mirror to form a coherent, monochromatic laser beam. By contrast, an open-cavity laser has a gas-discharge tube sealed with the fully silvered mirror on one end and a Brewster window on the other, which is a very flat piece of glass set at a steep angle to the long axis of the tube and transparent to p-polarized light. A second mirror is positioned opposite the Brewster window and aligned to create a resonant optical cavity external to the tube.

To switch mirrors easily, [Les] crafted a rotating turret mount for six different mirrors. The turret fits in a standard optical bench mirror mount, which lets him precisely align the mirror in two dimensions. He also built a quick alignment jig, as well as a safety enclosure to protect the delicate laser tube. The tube is connected to a high-voltage supply and after a little tweaking the open cavity starts to lase. [Les] could extend the cavity to almost half a meter, although even a waft of smoke was enough obstruction to kill the lasing at that length.

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Luckily, china is offering some really cheap DIY kits. These have no where near the power and capabilities that I would like to have, but the nice thing about DIY projects is that you can change them. You can hack, modify and extend them and you will gain a lot of hands-on experience that would miss by buying an off the shelf product.

So, I started looking for a >2W laser that uses an opensource controller (Arduino) and has some decent stepper motors (Nema17) and is easily hackable. I found a banggood.com kit and decided to pull the trigger...

The kit itself was delivered with within 3 weeks, which is not bad for a free shipping product from Shanghai. All parts were packaged pretty well, but this is in no-way a review of the kit, so let's start building!

Although there are some instructions, I find these cheap Chinese kits to assemble themselves if you follow some common sense. So I downloaded some high-res images of the final product and started assembly. The size of the screws was the biggest challenge for me, but in the end everything was assembled pretty robuust and I was ready to start focussing on the software.

I am no Windows person. I used to be, but almost 10 years ago I switched to a Mac. This was an issue. The controller that is used in the laser cutter is Arduino based. The Arduino uses an FTDI chip to do a USB to Serial conversion. This is pretty well supported on the major 3 operating systems (Windows, *nix, OSX). Somehow, this cutter was not detected by my Mac, which was strange, because I use to do Arduino projects all the time... But I didn't feel like tinkering with this (yet) and I fired up an old Windows laptop and installed the software.

As we can see, by default the software is doing line (vector) drawings pretty good! But for the middle image I had to use different software. These kind of thick lines are not supported in the editing part of the software, and overall, the software was pretty poorly written. Sometimes the laser would just stop moving, but kept on punching a hole in my workpiece (upper left image). This was not acceptable!

On the bright side, it looked like my axes where installed correctly. There is no deformation in the image, so the hardware is working correctly.

As you can clearly see, there are some artefacts in this big piece. Some extra circles were drawn and there were some moments where the laser stopped. (see burn marks). Also, the outline of the image is drawn, but I want the image to be filled in completely.

Normally, a person would stop here. This is what the kit can do, let's start engraving stuff and impress the world... well, that's nice, but if you're a geek like me the project is just starting... let's change the controller software, drive the motors myself and see what this thing is really capable of!

I think it's wrong to sell counterfeits as the original thing. Certainly when the project is a open source/open hardware collaboration that supports makers, hobbyists and enthusiasts. Even though the controller was advertised as Arduino based, it is not compatible with the Arduino IDE, so I really wanted to swap the board for the real thing.

In this picture it is really clear to see that the USB2Serial chip is different on the chinese board. Understandable, a FTDI is not a cheap chip, and to press BOM costs, they probably changed it. The downside is that my Mac did not recognise the chip, so I swapped the board and I could use my good old Mac again :-)

The next problem was the firmware... a empty arduino will not drive stepper motors and fire up a laser, so I needed some control software...

When searching for control software, you will find that a CNC machine has almost the same type of controls as an laser cutter. In theory, it is just a X/Y frame that can possible have a Z axis to control the cutting dept. Luckily, CNC controls have a standard called g-code. This is a (serial) interface that will specify some abstract commands (turn spindle on/off, go to position (x,y), draw line, draw circle, etc).

It was pretty obvious that by supporting this standard, I could interface with a lot of existing software, so that became my highest priority.

I found this really cool project that does exactly what I want: use an Arduino to convert g-code to electrical signals for driving stepper motors and turning on/off pins that could drive cooling/spindles/lasers/etc.

So I quickly downloaded GRBL and started to look for support for the control board used in my laser cutter...

Off coarse, my laser cutter was not supported out-of-the-box. Which, I don't really mind. I'm a professional software engineer, so getting the machine to work should be a piece of cake for me...

I created my own header-file to support the stepper motor drivers and the laser.

After a couple of hours of reading wiki's, looking at datasheets and hacking code, my cheap Chinese cutter was fully supported by the GRBL project. I have all the ingredients now to do some real work, let's define a workflow.

Now that my lasercutter can be driven used g-code, I need a certain workflow.

Having seen the extremely high prices of commercial software, I really wanted to embrace open source, and create a full open source workflow.

As a start, I will use the open source graphics editor GiMP to convert any photo to grayscale.

After this step, I'm using Inkscape to brush up the image, create paths and generate g-code by using 2 extensions. One for raster-engraving (images) and one for vector-engraving (lines).

And finally, using a open source tool named Universal G-Code Sender, I can send the g-code to my laser cutter, align the laser on a start point and see how long a job takes to finish.

Running all this software on a Ubuntu laptop makes the full software stack open source. It sounds like a bit of a hassle, and I'm pretty sure that there are better alternatives, but this was a quick solution... and it works really well!

Wow! This was a result that I didn't expected! The lines are pretty sharp and the circle is perfectly round. Also, the laser is now carving almost a millimeter into wood, which was far beyond my expectation of a 2500mW laser.

The 2 small artifacts in the bottom of the image are my own fault, but you can clearly see that the quality and sharpness is way better. Some edges were still a bit shaky, so I dropped the speed of the laser and decreased the power of the leaser.

Finally I wanted to test a photo-realistic image. Here are my results:

But I won't stop here! This was a very nice introduction into g-code and driving stepper motors. But I started off this article by telling that I wanted a high power laser, not a toy. So I still have some plans to build on this foundation. I want to extend the frame, move the laser to the back by adding mirrors, and change this 2.5W laser into a 200W+ CO2 laser. This will enable me to engrave and cut materials that I can use for enclosures or artwork...

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