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identify absolute max current for LDs, visually?

Krutz

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hey everyone,

yes, i asked that on cpf already, with no response at all.
reading the thread how many LDs everyone killed so far, i have hope you have suggestions! :)

so.. more or less, every laserdiode would die at a different point, depending on parameters not visible (for normal people). like most LDs would die from COD, catastrophal optical damage, which is burnt/melted end-facets due to too high laserpower (inside the cavity).

is there any chance to see that the LD is at its limit? coherence length, modehopping or similar? how complicated it would be to see isnt important, i would test a LD once only, to guess its limit, and then use it some under that limit.
perhaps its not possible at all, if the parameters of the LD change gradually/linearly/exponentially only, and then simply stops to lase altogether.

again, i volunteer to try your suggestions, and blow some LDs on it.

manuel
 





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Re: identify absolute max current for LDs, visuall

My understanding is that by the time there is any visible change, damage has already occured. But maybe someone else will have some ideas you can try.
 
C

Chris.

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Re: identify absolute max current for LDs, visuall

Something to do with a feedback loop from the third pin, you need a datasheet though.
 
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Re: identify absolute max current for LDs, visuall

The third pin is a NC on every DVD diode I've ever seen. :-?
 
C

Chris.

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Re: identify absolute max current for LDs, visuall

NC?
You’re probably correct, it was something I read on here...

My method is to increase very gradually using a potentiometer, running for 1 minute with each increase... If it's being overdriven it will normally get excessively hot but not noticeably brighter.
 

Krutz

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Re: identify absolute max current for LDs, visuall

optical feedback wouldnt be bad, yes.. but then, without knowing at how much mw it will burn out, its no definite help!

how do normal LDs react to increasing current actually?
until now, i was very sure that the output will increase, until it breaks. but then i saw a plot of a LD driven up to 600mA, which was its end.. the output raised fast, until perhaps 200mA, then stayed the same, to drop a bit just before it went LED. so, do they have a maximum output independent of the input?

manuel
 

Daedal

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Re: identify absolute max current for LDs, visuall

I think you could tell once you start seeing the lasing pattern start to change... just keep a very watchful eye.

The truth is that without having about 100 units or so to kill, no real results can be obtained.

I personally wouldn't suggest pushing the diode to any limit unless you have replacements and don't mind losing the diode.

GL;
DDL
 
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Oct 24, 2006
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Re: identify absolute max current for LDs, visuall

Keep in mind there is no such thing as absolute maximum current. You run it much lower than spec, you increase life significantly. You push it further and further, and you shorten its life. If you push it a lot, you play with chance and at some point the life drops at such a rate that it hits 0 seconds, and its dead.

Even if you aren't pushing it to a point where it CODs in a few hours of use, you could be bringing its life down a hundred or a thousand fold if you choose to give it any more power than its meant to handle.

I know this doesn't really answer your question, but given the circumstances I felt like reitorating it. Sorry if its a bit OT.
 

Krutz

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Re: identify absolute max current for LDs, visuall

thanks pseudo, for your reply. absolutely on topic i would say!

i must admit i didnt really understand the "slow degeneration" yet. as i thought or wished, the plan would be:

old-style:
turn up until 300mA, LD dies at 300mA instantly without warning.

better-style:
turn up and observe, at 280mA the specle-pattern changes, i guess COD would occur at 300mA, and categorize that particular LD to work with 260mA "safely", meaning that slow degeneration will still keep it for 10 hours alive.

COD happens from excessive heating from too high intra-cavity-flux, a hypothetical specle-pattern-change would perhaps come from warming, wavelength-shift, temperature-expanded cavitylength, spatial hole burning, or some other effect, which surely would be linear. i fear that the COD-treshold wouldnt be tied to the other effects, meaning that these effects could be at a similar current for a batch of LDs, but the COD treshold would still be significantly different.

i thought i read sams laserfaq enough to have some answers, but still far away from it.. i wish i had a runtime-monitor how long i already read the faq! :)

manuel
 

Benm

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Re: identify absolute max current for LDs, visuall

For COD, there probably is no way to predict the exact point at which it will happen. COD is usually total, once the facets start to degrade, they absorb even more energy due to that, and it's a runaway reaction.

Degradation of the lasing medium is another thing. This would happen similarly to the way LED's lose brightness over their lifespan. A LD would give less and less power on equal current, down to the point where it is no longer able to lase.

Changes in the speckle pattern are not a useful indicator though. LD's normally have only 1 mode in either transverse direction (TEM-00), but many modes in the longitudinal direction. This causes the speckle pattern to shift during normal operation as the temperature just rises and falls.
 




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