Solar Irradiance and comparing incoherent vs coherent "power" + CD Diodes, maybe + burn speed -> mW

A project log for CD/DVD mechanisms and cartesian thinggie[s?]

DVD-laser-etcher, dremmel-router, possibly a 3D printer? Who knows!

esot.ericesot.eric 09/10/2015 at 14:534 Comments

UPDATE (12-10-15):


Take all bits regarding mW vs. mA with quite a bit of speculation!

(new log-entry to follow).



This log has changed quite a bit since its introduction... all additions are at the end, including burn-speeds vs mW ratings, and possibly using the CD laser as well as the DVD for extra oomph.


There has been discussion as to whether a laser diode is particularly effective, considering those in a DVD drive border on around a quarter of a watt...

For comparison: suggests that in 2011 "The most probable value of TSI representative of solar minimum is 1360.8 ± 0.5 W m−2"... so I'm no expert, and didn't read thoroughly enough to know what TSI is, (probably Total Solar Irradiance? At the surface of the earth...?)

Just throwing out some random-math... a typical magnifying-glass is, what... 5cm in diameter... so that's ~20cm^2... and if I recall my dimensional-analysis, one 1cm = .01m, but 1cm^2 is .001m^2... so 20cm is .2m, but 20cm^2 would be .02m^2?. Sure, let's go for it...

So, a magnifying-glass can set fire to paper... with 1360W/m^2 * .02m^2 = 27.2W...?

I'm trying to find some comparison between coherent light vs incoherent, in terms of power-transmitted... 27.2W from incoherent light might be equivalent to something smaller, with coherent light... but I don't really know what to search for.

Regardless, say you've a 100W LED and focus that with a magnifying glass... seems like it'd set fire to something, to me...

So I've managed to burn out several laser-diodes, now... but also managed to find a 20x DVD burner in a computer I apparently forgot about (and here I thought I'd be stuck with 12x!)... so probably 275-300mW? I definitely recall getting some "etching" in a piece of black electrical-tape with my old 16x laser...

I'm almost thinking of getting a high-wattage white LED and shoving it in a dead DVD's optics assembly... to see what'd happen. Seems to me the laser-diodes have roughly the same "viewing"-angle, so the optics-assembly would roughly create a parallel "beam", and then that's easily refocussed to a point with a magnifying glass (as shown in a previous log). Some power is wasted due to incoherence (how much? Seriously, if 250mW coherent can do what it did, certainly 10W of incoherent could match that... right?)


Here's something, and may allude to why my search has been elusive...

It really all depends on the material itself, whether it's crystalline or not, the grain size, grain size dispersion, grain shape, etc.. It's hard to say exactly what will happen in advance, even if you know all these things.
On the other hand, if the material is 'dark', i.e.: a good absorber, at your wavelength, you probably won't see much of a difference between the two light sources. For example, an ideal 'black body' is not sensitive to polarization, collimation, or wavelength."

Ah Hah!

So our black electrical-tape might not care too much, or it might, depending on how good an absorber it really is... And crystalline structure, or lack thereof, might have a significant(?) effect on how sensitive any material is to coherence vs. incoherence, and thus why it's been difficult to find a simple answer...


This site's handy!

Duh, we could power-up BOTH the DVD burner laser AND the CD burner laser!


Also, finally, a listing of expected power-ratings based on DVD speed:

According to Sony’s product brochure for one of their high-power red laser diodes, SLD1236VL, the diode’s output power (CW or continuous power) will be somewhere along this list:

But, again, per a previous log-entry... I think we need to consider laser-diodes like we consider LEDs... remember the first LEDs ran at 20mA and were barely visible in daylight, whereas now 1mA is more than enough for most LEDs to be quite visible (if not bright). So we can't really determine what *current* to supply to the laser-diode based merely on its burn-speed nor its power-output... ... The recommended-maximum--found on some pages--for a 16x laser-diode is 250mA, but how they came up with that number...? Dunno. Certainly not from the mW rating! (and, actually... if the output power is supposed to be 250mW, and the diode converted every bit of its current into light-output-energy... Then shouldn't we be able to use P=VI? 250mA would relate to a 1V forward-voltage, whereas they're sitting somewhere around 1.7V, no? So that might be safely bumped-up a bit... Again, per a recent log-entry, there doesn't seem to be a distinct indicator when you're about to burn out the laser-diode, so maybe assuming Current ~= power is a safe way to go...)


Here's a chart similar to the above, of Sharp's CD-RW lasers vs mW-ratings, and a couple other goodies, from:

CD-RW Laser Diodes from Sharp (writing speed -> optical power-output (pulsed)):

And here's a pseudo-answer to the question, above... re: mA -> mW (optical)

The Sharp datasheet for the CD-RW laser-diodes says at Po = 100mW (assumed *optical* power output), the typical current is 141mA. This makes since, WRT the question above... certainly the output-power isn't going to equal the input-power, they can't be 100% efficient, right?

Quick math: 141mA, 2.1V... P=VI = 296mW (consumed power) for 100mW optical power. Less efficient than I thought.

Another useful tidbit from the datasheet: the 225mW diode appears to be rated for "225mW (CW 120mW)." A brief search suggests "CW" means "Continuous Wave" (as directly-opposed to pulsed)... So, for etching-purposes (or laser-pointers) it seems, where the diode might remain on for long durations, it's wise to underpower that diode somewhat dramatically... (nearly half!). So, again, that seems to corroborate with the mW <-> mA presumptions used on most sites I've seen... erring on the side of preventing burn-out. Here we've a diode which apparently almost consumes 3x the power it outputs, but should only be driven at 1/2 the power it's rated to output... so then, driving it at 1:1 (mA:rated mW) leaves a little bit of a safety-buffer, right? 1/6th x power-buffer...? So, somewhere in there, it seems plausible that it's actually somewhat possible to safely drive, say, a 250mW diode with 500mA, and maybe even slightly higher, as long as it's only pulsed at a 50% duty-cycle... Which might be useful e.g. for raster-scanning.

Of course, again, these values are specific to *this* diode... And, again, this diode is Infra-Red.

And... I'm not certain I parsed this correctly, but this page is a wealth of potentially-legit info and also suggests that 100mW is "plenty fun"... but, also, suggests that 150mA ~= 100-150mW, so that seems to suggest the author was running his diodes at 150mA and causing fires with matches and burning holes in paper with something more like 60mW output-power... SO, that discussed, err on the side of protecting your eyes and whatnot, and use the lowest current necessary for any early testing... It sounds like 150mA should do something.

WARNING: Do not get crazy and start thinking about experimenting with CD-R lasers for fun! You can't *see* 'em, so you won't know if your eyeballs are burning until your retina's already ablaze.


esot.eric wrote 09/21/2015 at 10:43 point

Another diode bit the dust... This time from a 12x burner. According to the chart, here, 12x corresponds to 200mW (assuming the different manufacturers use the same mW vs. speed ratings)... My current-source has a maximum output of ~250mA. It blew out while adjusting the pot, when the dial was somewhere 'round 150mA (guessing by memory of the position of the pot).

So, I'm not sure... 150mA shouldn't've been anywhere near 150mW (output, nor even consumed)... so I dunno. And if it was rated for 200mW, then even 250mA doesn't seem like enough to reach its rating... so, again, I dunno.

I read that one of my drives claimed to be capable of burning slower media at higher speeds... which suggests a *higher* power-output? That shouldn't be it. And then there's the continuous-wave vs. pulsed-wave rating... so maybe... but again it should've been running at ~150mA, which should be much less than 150mW.

@RoGeorge commented, here:

that laser-diodes have a built-in photodetector used to control the actual power-output via feedback loop... There's some good info at that link. Note, very few (if any?) of the drives I've worked with actually have even solder attached (let alone a trace) to the third (photodetector) pin on the laser-diodes. HOWEVER: the drives *also* inherently have quite a few *other* photodetectors in the system, which may well be used for the same purpose.

Frankly, though, I'm kinda surprised this one blew... It makes me wonder if there's something wrong with my circuit... Maybe the pot is noisy as it's rotated (though, it shouldn't ever have been able to output more than 250mA due to the fixed-resistors in series... right?). I should change it to a multi-turn, anyhow, as adjusting between 100mA to the maximum of 250mA occurs in less than a quarter of a turn. Or maybe I touched something on the solder-side that caused a surge...


I finally broke-down and decided to try out the 20x laser-diode, despite the potential circuitry flaws... and have been running some pretty conclusively-functional experiments. Including melting dots into black electrical-tape as well as permanent-marker on a credit-card. (no-go with permanent-marker on copper-clad, nor on paper). But I've been cautious: among other precautions, the current-source hasn't been set higher than 120mA. (Yep! Melting of plastic at 120mA! The 12x laser shoulda handled that!).

This was using the full optics-assembly straight from the drive. Including the focusing lens. From what I've seen, there's *quite a bit* of lost-light in these systems... At the very least, the ~1/2in collimated beam entering the ~1/4in focusing lens. Also, there's a polarizer, which should result in half the light being lost. Then there's the wide-angle output from the diode itself. Unless there's another collimator *right at* the diode, there's probably quite a bit of loss around there, as well. So, it seems to me, that even a 12x burner laser, plausibly even an 8x, should achieve this level of material-burnage if those factors are removed... (just one lens right at the diode).

There's video... I'll post it eventually... and probably move this stuff along with some others to a new log.

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esot.eric wrote 09/21/2015 at 12:15 point

Hah! That DVD chart is allegedly *continuous* power!

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frankstripod wrote 09/10/2015 at 23:44 point

Great info and links Eric!

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esot.eric wrote 12/11/2015 at 02:11 point

Thanks, Frank! BUT I think, maybe, I should add a comment here to negate your"great info" bit... I think in my ramblings I made a horrendous mistake in understanding... Update to follow. 

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