identify absolute max current for LDs, visually?

Hi everyone,

I was thinking about laserdiodes, their last moments before they die from exceeded current/output, and how to test their individual limit without datasheet.

normally, COD would be the "normal" reason for destruction. it is instantly, and I dont expect to do anything about/against it. but then there are the modes, coherence length, and other stuff. after searching quite long, I found this post again:
Blu-Ray laser diode hack #58
saying (about 405nm diodes anyway):

From my research so far, it seems the maximum save level of operation is just before the point where the diodes start mode walking (the modes move and shimmer) This effect is most likely caused by current flow instabilities in the junction and is an obvious sign the diode is being overdriven.

Click to expand...

(thanks Heruursciences!)

so.. anyone able to give me hints?
how to do something like that in real world? interference patterns? double-slit? or even michelson interferometer? or just watching the speckle?

I "converted" my good part of diodes, and never noticed anything besides "bright, brighter, brighter, dead" so far. but then without knowing what to look for, and watching the dmm..?

any hints? I volunteer to test suggestions with the near-death-experience of a 16x dvd diode, followed by the death-experience ;-)

manuel

spectacular specle, easily!

..reading online and especially sam's laserfaq, I found two ways I wanted to try:
double-slit experiment and michelson-interferometer. unfortunately I dont have anything to do 50:50 beamsplitting for the M-Interf., only a bunch of 94:6 splitters (uncoated glass, that is), polarising cubes, and dichro-stuff.
so I tried the double-slit way first. read as an example that two slits with 0.05mm with in 0.25mm distance (official non-metric conversion: several times smaller than a hair). tried aluminum-foil and razerblades. I actually saw tiny interferences, but nothing to work with.
playing around, i projected nice specle on a wall while pointing the focused laser on the diffuse side of the foil. the smaller focused the point, the larger the specles. fortunaly, the foil's "roughness" is fine enough for visible light, but the foil still reflects quite straight into one direction, unlike paper, which widens the "specle-beam" too much.
try it, I made half-inch / 1cm large specles on the wall, in perhaps 1 meter distance. perfectly clear, black between the specles!

this is a nice way to find the focal point too. change the distance, until the specle are largest. very fine tuning is possible with this!

I will play around more with this, as soon as I find the time!

manuel

ooops..

i tried it, and thought about it..
allthough it looks like specle, and is spectacular, and beautiful as it is, it is not specle! its simply the aluminum-foils surface, enlarged and projected! the same as if you would hold the end of a needle (or whatever you wish) in front of an uncollimated LD.
still, you can nicely find the focus with this technique.
oh, obviously the pattern projected with this stays exactly the same, as long as neither laser nor foil are moved..

i will try to project real interference-caused specle now.

manuel