Sciencing DVD-RW Laser Diodes

If you’ve played around with laser diodes that you’ve scavenged from old equipment, you know that it can be a hit-or-miss proposition. (And if you haven’t, what are you waiting for?) Besides the real risk of killing the diode on extraction by either overheating it or zapping it with static electricity, there’s always the question of how much current to put into the thing.

[DeepSOIC] decided to answer the latter question — with science! — for a DVD-burner laser that he’s got. His apparatus is both low-tech and absolutely brilliant, and it looks like he’s getting good data. So let’s have a peek.

Laser Detector on 3D Printer Scrap
Laser Detector on 3D Printer Scrap

First up is the detector, which is nothing more than a photodiode, 100k ohm load resistor, and a big capacitor for a power supply. We’d use a coin-cell battery, but given how low the discharge currents are, the cap makes a great rechargeable alternative. The output of the photo diode goes straight into the scope probe.

He then points the photodiode at the laser spot (on a keyboard?) and pulses the laser by charging up a capacitor and discharging it through the laser and a resistor to limit total current. The instantaneous current through the laser diode is also measured on the scope. Plotting both the current drawn and the measured brightness from the photodiode gives him an L/I curve — “lumens” versus current.

laser_curve

Look on the curve for where it stops being a straight line, slightly before the wiggles set in. That’s about the maximum continuous operating current. It’s good practice to de-rate that to 90% just to be on the safe side. Here it looks like the maximum current is 280 mA, so you probably shouldn’t run above 250 mA for a long time. If the diode’s body gets hot, heatsink it.

If you want to know everything about lasers in general, and diode lasers in particular, you can’t beat Sam’s Laser FAQ. We love [DeepSOIC]’s testing rig, though, and would love to see the schematic of his test driver. We’ve used “Sam’s Laser Diode Test Supply 1” for years, and we love it, but a pulsed laser tester would be a cool addition to the lab.

What to do with your junk DVD-ROM laser? Use the other leftover parts to make a CNC engraver? But we don’t need to tell you what to do with lasers. Just don’t look into the beam with your remaining good eye!

15 thoughts on “Sciencing DVD-RW Laser Diodes

  1. I tried driving a laser from a DVD once, and have a question.

    It was a red laser. I removed all lenses in front of the diode, but left it in the mount for some heatsinking. It was pointed at the ceiling. Then driven with a typical variable voltage/current power supply, set to be current limited at all times. (That’s not ideal, but good enough for a quick experiment with an expendable laser.)

    I started at zero current, and slowly increased it. Soon I saw a dim, 8 inch wide spot on the ceiling, which was about 5 feet away. It was not uniform, but had a pattern of tiny “ripples”. As I increased the current, the spot got brighter, and the pattern remained constant.

    When I reached some current point (I forget exactly what, may have been around 150mA) and the spot was quite bright, I noticed the ripple pattern started to change slightly. A few more mA made the change become more dramatic. If I decreased it past that point, the pattern instantly returned to “normal”. I assumed this change meant I was slightly exceeding the current limit of the diode, and didn’t try pushing it much further than that point.

    I’ve since read somewhere that a diode laser requires a minimum “threshold current” to actually lase, and below that acts more like a LED. However, I haven’t been able to find anything suggesting what a typical threshold might be, as a percent of max current rating.

    So now I wonder, was I actually driving it as a LED all that time, and the pattern change was because it was on the verge of lasing? Or was the pattern change due to overcurrent as I originally assumed?

    1. “LED all time?” No, very unlikely. The changes in the “ripple pattern” are mode hops and also the reason for the “wiggly” part of the above curve (current vs. light) above around 300mA. Whereas the threshold current in this curve lies at around 65mA – nearly no light output below. The pattern change could be on the verge of destruction :-)

    2. There is no “typical threshold current” for a DPSS laser diode to start lasing. It depends on the diode (get the hands on the datasheet), the temperature of the diode, the batch of the diode, and … how it has been driven in the past (or how “old/damaged” it is).

    3. My (educated) guess: It was lasing, and you are correct in assuming that pushing it further could damage it. The ‘LED’ part is right at the bottom of the curve, and is dim enough that you wouldn’t really see it on the ceiling.

  2. A minor danger is that you will blind yourself. Don’t care: evolution in action.

    The *real* danger is you will blind someone else: the Dunning-Kruger effect in action.

    Anybody who perpetrates “experiments” (and I use that word loosely) as described above is clearly clueless and lazy – because it is simpler easier and *safer* to RTFDS (read the fine data sheet).

    He’s already *unknowingly* buggered his phone’s camera, as he admits further down the page.

  3. I use simpler method. Connect laser diode to 5 V supply and 470 Ohm resistor. No lasing – go lower with resistance, to 390 Ohms. No lasing, go lower to 330 Ohms, etc … until lasing starts. Typically max. current is about 50% greater than minimum lasing current, so my final resistor will be about 2/3 (67%) of the value required to start lasing. Also I found that constant current is not required with my described setup: all my laser diodes are fine so far. I have only tried 5 mW though. Of course ***if datasheet is available use that***.

  4. When the temp of the laserdiode rises, the output at the same current falls. If the temp gets too high the laserdiode dies. Therefore most LDs have a photodiode built in to monitor the output. So you can build a circuit that lowers the current, when the output falls, if you want to drive the LD at its limit. You can also use the built in photodiode to make your measurement. As better the cooling, as higher you can drive the LD, some people use peltier cooling for LDs to get higher output.But if you drive the LD over its maximum limit, even for a very short time, it is dead. Mostly because the end mirrors gets damaged.

  5. 1. Lumen (lm) is a photometric unit and means how a human perceives the power of light output. This is wavelength-dependent and not very useful for technical applications.

    2. Lumen applies to the total light output and the used photo diode only measures a nondescript cross-section of an unknown beam profile. This yields only a relative result; if at all useful, relative movement of the measurement assembly must be fixed to incredibly tight tolerances to yield any useful result

    3. Instead of measuring the capacitor discharge time, you could just measure the photodiode current directly. All but the very cheapest multimeters have decades (i.e. orders of magnitude) of input current range available – this is sufficient.

    ==> This is how you do it correctly:

    A) Use a thermopile as a sensor. Any small Peltier-type thermoelectric cooling element will do. These are available at various electronics distributors. Part numbers are TEC101703 or APH-031-10-13-S.

    B) The surface of the TEC should be blackened for uniform spectral absorption. Ordinary black soot will do if no high-power pulsed lasers are measured. You can use a copper plate as a surface heat spreader.

    C) Calibrate using a known beam or heat source. A SMD resistor soldered on one edge of the heat spreader is sufficient.

    D) For high power levels with significant temperature rise at the sensor surface, you should do a curve fit to a known first-order model (exponential time function). If you use said SMD resistor, this is no effort at all. (You can use a linear regression after doing a logarithmic transform or just use the respective feature of a spreadsheet calculation program)

    1. (Hit “Report” button on accident) (And thanks to Hubert D for expressing this politely):

      I was a bit mislead by the title “Sciencing […]”..

      For doing a relative measurement only, using a photo diode/transistor like this is fine.

      The “wigglyness” in the output curve is transitions (“hops”) in (longitudinal) lasing modes occuring when the laser significantly heats up. For a laser diode that is supposed to be used in a part of its operating area in which these mode hops do not occur, this might indeed be the easiest way to determine maximum operating current.

      Using a the electrolytic capacitor as supply for the laser diode, you save yourself the hassle of recording a large number of data points for plotting manually if you do not have access to a proper function generator/component tester.

      Again, if you want to do a “proper” calibration to factory-default power output, the thermopile is a very simple and useful setup.

  6. Measuring with a peltier using the seebeck effect works very well, but needsa few seconds to measure, i have built one myself. So it is dangerous for the Laser if you are near maximum power and cooling is insufficient. It is too slow to see the decrease of output power when heating up. This can only be seen if you use a photodiode. A photodiode is ok in this measurement because you do not want to measure the accurate output of the diode but the tendency.

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