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12-19-2014, 07:53 PM #1
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Targol
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Getting a DVD burner laser to lase, possible with 3V?

Hi guys

I took out a laser diode from a DVD burner. Since I don't have a driver, and don't know a place to buy a LM317 from, I instead hooked it up in series with a resistor to prevent it from burning out.
My problem now is that the laser diode is glowing, but it's not creating a laser beam.

I measured the voltage across the resistor and the diode with a multimeter and got these values.

If I got my math right that means that the diode has a resistance of 28Ω (since the voltage across it is 2.33 times the one across the resistor), which would make the current 75 mA.

So I'm thinking my problem is that I am not getting enough of something, but I'm not sure whether that is current (in which case I could lower the series resistor some more) or maybe the diode needs to have a certain voltage, and that is higher than 3V (in which case lowering the resistor won't help and I need more batteries).

What do you think, am I better off lowering the resistance or increasing the voltage?
What I'm worried about is that 0.075 * 0.075 * 28 = 0.15, and 150mW is about a normal value for a DVD burner laser from what I've seen on Youtube videos. So I'm afraid if I put more power into the diode it'll burn out.

By the way what it's currently doing looks like this:

Last edited by Targol; 12-19-2014 at 08:01 PM.

12-20-2014, 02:10 AM #2
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Cyparagon
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Re: Getting a DVD burner laser to lase, possible with 3V?

A laser diodes does not have electrical characteristics like those of a resistor, so it is not correct to give it a resistance value. The electrical characteristic is like a diode (because it is a diode), but with a higher forward voltage than a typical silicon diode (.7V versus 2-3V for a red laser diode)

75mA should be enough for it to lase, but it might be dead now because you haven't heat sinked it.

Is there a lens in front of the diode? If not, that's your problem. Mount the diode in an appropriate module (aixiz modules are pretty standard) which will include a lens and will have adequate heat sinking for short runs.
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12-20-2014, 11:57 AM #3
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Targol
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Re: Getting a DVD burner laser to lase, possible with 3V?

It has a little heat sink and I'm careful to only run it for 30 seconds or so at a time. It does get a little warm to the touch after that so I stop and wait until it's cooled down again.

I didn't know that lasers need a lens for straight beams, I thought those were for focusing them onto a point, but apparently you do! Seems like the light is coherent, but not in a beam by default like as per Thorlabs.com - Tutorials so I'll get a lens, thanks.

12-20-2014, 03:13 PM #4
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Cyparagon
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Re: Getting a DVD burner laser to lase, possible with 3V?

Technically it's already a laser beam. But all lasers have divergence (widening of a beam as it gets further away from the source) - it's just a law of physics. And divergence is inversely proportional to beam size. Wider beam, lower divergence. Narrower beam, higher divergence. Since the beam size is 2 microns out of the laser diode, the divergence is very, very high. The lens takes advantage of this divergence and uses it as the first half of a beam expander. You place the laser diode in the focal point of a system like so:

after collimation (which is like focusing in reverse), the beam size goes up ~2000 times, so the divergence goes down ~2000 times.
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12-26-2014, 01:33 AM #5
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The Lightning Stalker
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Re: Getting a DVD burner laser to lase, possible with 3V?

The diode will drop more like 2.3-2.5V.
You should really use an ammeter to see the
exact current through the diode. Use short
straight leads to keep inductance down to a
minimum. Also, be careful that the
resistor is not wirewound for the same
reason.

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