Topic: Analog laser shunt modulation

[dtewksbury]

Hi everyone. I have been busy buying red and violet lasers off eBay. As a consequence, I have found that the LM317 laser supplies are fine for driving the lasers, but when it comes to blanking and ramps they are of course useless.

So, I am designing an analog shunt modulator for these supplies that has a fast response. I have simulated the circuit and it seems to have a delay of about 1 microsecond, and it is very linear with an expected input of 0-5 V (of course). You can adjust the maximum power and blanking points. The circuit is very simple and I will post it if anyone is interested (once I have determined that it doesn't destroy the laser diodes).

Anyway, I have tested it's blanking with my laser driver software and it is awesome (as far as blanking response anyway), I have attached an image of Quisp being scanned with a 650nm 200mW diode I bought from eBay.

I have to apologize about the laser spill, I had to focus the laser on the wall and that made the spot a little too large for the scanner mirrors, I will have to do something about the optics with these cheap lasers to get the spot smaller on the mirrors (maybe a beam expander will work), and this is with a glass collimating lens.

I will post some images with gradients as well when I get a chance.

So here it is, a $49 300mW laser diode (running at 200mW, measured with a coherent power meter). Plus about $10 worth of parts for the modulator.

[Fanny Pack]

Good work.  I used to use Norm's driver and it is a shunt type.  It worked fine but was a bit bulky and needed a big heatsink since the driver was always allowing max current to flow.  I ended up going with the tiny die4drive units and just being careful not to touch my diode cases while they were in use. 

[dtewksbury]

I have never heard of the Die4Drive units, but they look good. I am also curious about the ZVS unit that is mentioned on the site. I have smoked quite a few FETs while trying to build ZVS power supplies. Its all a learning process.

[drlava]

Here is a high speed shunt driver schematic I posted back in 2008:
http://www.photonlexicon.com/forums/showthread.php?t=5428

Since that time, I've found the FlexMod more flexible, precise, and easier to use.

[dtewksbury]

I didn't realize there were so many available. I did try to search for them before I decided to design one, but that's what seems to happen a lot. Anyway it is very simple and only uses two transistors, it is very linear as well, with a fast response time.

I am not sure about this linear thing though, the very low intensity end I think would benefit from a non linear response when you get down to near the laser threshold, then a sharp cutt off for blanking, I find it too 'steppy' at the lower end, even though the response is linear.

I have attached an image of the circuit so far.

[James]

Once again, I'd say I am not an expert, but I have heard from others that it's not a good idea to completely drop all current flow through the laser diode for blanking. The idea is to turn it down so low that you can't really see it. I believe if you do it that way, the diode responds to coming on faster and it lasts longer.

Have you ever heard of the lasorb?

James. 

[dtewksbury]

I have heard of lasorb, it was demonstrated to me at ILDA this year.

I am not turning the laser off, there is a pot to set the Blank level (idle current). You can't shunt the diode too low otherwise the capacitor on the output will see a large dv/dt and that will significantly slow the responce. The way this circuit works is to simply steer the current from the laser to the shunt, so the capacitor should only see a very slight voltage change.

I have added a makeshift aperture in the laser path to mask the mirrors better, I have attached some more images, much better now without the spill from the mirror edges.

Notice how the Ramp image shows quite a sudden drop off when it gets near black (near the threshold level of the laser), this is where I would like to add some non linearity in. I have decreased the color depth to 5 bits so you can see the stepping.

The images below don't do any justice to how these laser images actually look, I thing that 650nm does not do well through the lens of my camera.
BTW I am using a Canon 40D with a 17-85 zoom lens, admittedly, this is not the best lens, but it works quit well at 532nm.

[James]

Check out the gears in LaserBoy.

Open LaserBoy and select the three gears, frames 1, 2, & 3 with the space bar. You'll see that each frame is selected when the background behind the yellow frame stat 2D frame (index) changes to dark red.

With only those three frames selected, choose 'o' for output, '1' for ILDA, and '5' for frame set effect.

Then type gears and hit enter. It is the name of the gears frame set effect and a new file will be created called gears.ild.

Open that file into LaserBoy and just for fun, type 'o' for output, '1' for ILDA, '5' for frame set effect and move_t2b for the frame set effect. You don't have to select any frames. This effect uses the whole set.

It looks better of you go into menu 'h' and hit 'n' to add distance span before you call the move_t2b effect.

James. 

[dtewksbury]

OK I will see if I can figure it out. I have to be honest with you and say that I haven't really used LaserBoy yet. It's mainly because I have no hardware to view it's output.

Or does it just output to ILDA for playback in other software/hardware?

I will make the effort, and have a look, I feel guilty now.........

[James]

Yes.

It saves all flavours of ILDA including what I call !3.

It also saves as wave, DXF (directory) and plain ASCII text tables (all lossless vector art formats).

Plus bitmap raster iamges.

James. 

I feel guilty now.........

Good! That's what I was going for... 

[dtewksbury]

It said move_t2b is not a registered LaserBoy frame set effect?

[James]

This list of registered effects should appear on the right side where the palettes usually are.

Effects are "registered" as an array of text strings for lookup and pointers-to-functions for the calls.

What version do you have?

The current is:

http://akrobiz.com/laserboy/code/LaserBoy_2009_11_13.zip

That is a fairly recent effect.

As a matter of fact, move_l2r and move_t2b should be both frame and frame set effects.

Looks like I still need to work on it a bit. It blacks out some lines before it should.

James. 

PS. Did you see this (ild file) animation?

http://www.akrobiz.com/laserboy/free_art/countdown.zip

I did this with the new angle_wipe frame set effect.

[dtewksbury]

2009_10_20, I will download the latest now.

[dtewksbury]

Ah it looks like you have used small square grab boxes like I do for each vertex in my editor.

[James]

If you look at menus 'k' and 'l' you will see I also have a unique system for selecting a consecutive series of vertices; between the egg and the spider.

The [{ ]} keys move them independantly and the 'u' 'i' 'o' keys move them through the lit segments.

Sorry about stealing your thread dude! 

James.