Spice models of laser diodes?

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Well, laser diodes are a lot more complicated that this simple model. Onset of lasing which happens sort of digital at xx percent of max current, non-linear relationship between output and input power, overload behavior, wavelength drift with heat and so on.

Seriously, it's a lot more intricate than that. Just to give you an example from the non-LD world: One of the transistor models of an amplifier I recently ran is about two dozen lines of SPICE entries. Those are the sims where the fans come on hard and the office temp creeps up another 3F or so.

I don't think you'll get around behavioral models here.

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Joerg
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How do you want to protect a laser diode with any of these?

Photodiodes are reverse biased if you need speed while PIN diodes used as controlled RF resistors are forward biased and the current sets the resistance. But that works only if the carrier lifetime is sufficient for the frequencies it has to work at.

For more data regarding LD models I'd get in contact with a few university research labs. It is not a trivial task at all.

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Joerg

Try:

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Yeah, they can take _forever_ to turn on. Not much use for protection diodes in most cases.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs
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Phil Hobbs

Thanks, by now I've seen it. But the model presented ain't much to write home about, IMHO.

OTOH, for the RF guy their are very close to the definition of a free lunch :-)

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Joerg wrote in news: snipped-for-privacy@mid.individual.net:

EDN (who I trust quite deeply on matters of useful working ideas) seem to think that a moderately simple diode model can be enough when making basic driver models. All I'm asking for is translation of that model to a subcircuit file I can use in LTspice. I'm new to spice but it's already clear to me that it is UNWISE to model more detail than is strictly needed. So I think EDN have the right idea. I'm after something that other people can also easily use and benefit from, I'm not after a perfect detailed model I can win prizes with.

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Lostgallifreyan

Phil Hobbs wrote in news:xq6dneoJYJOZK-3XnZ2dnUVZ snipped-for-privacy@supernews.com:

I'm not after protection diodes. I knew I should NOT have digressed into that point. All I'm asking for is a translation to LTspice, for that EDN idea. I typed out the net list to help, but that's all I know how to do right now.

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Lostgallifreyan

Joerg wrote in news: snipped-for-privacy@mid.individual.net:

I read somewhere that it was a better version of an earlier one in a Pspice library. How good does it have to be? No point in modelling more detail than needed as a starting point. We don't even HAVE that much, any of us hobbyists and small scale designers. If experts raise their hands in horror saying there's no point in simple modelling rather than too much detail (with which Intusoft, who really know this stuff, would solidly disagree), then we'll all continue to have nothing to go on except dead laser diodes! Surely someone who knows how might change this?

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Lostgallifreyan

Wow, for you a free lunch must be _really_ free!

Cheers

Phil Hobbs

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You'll have to do that piece by piece. LTSpice's preferred method is graphics entry. For example, instead of writing out the netlist line for G you place a voltage dependent current source via point and click and connect it up in the schematic editor.

If you are new to SPICE I strongly suggest not to start with a project like this but first use some of the supplied "jigs" or example circuits and play around with those. Unless you really understand the program it is very easy to reach wrong sim results.

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Joerg

I know, it's just an interesting point.

I rather suspect you're barking up the wrong tree with a SPICE model of a diode laser...it isn't the terminal voltage you care about, it's the light output, and that depends on a whole lot of optical and thermal things that SPICE is never, ever going to get right. They have widely differing timescales, for one thing, which SPICE is horrible at, and for another thing, small amounts of optical feedback have a _huge_ effect on DL performance, including feeding back to the terminal voltage.

Cheers

Phil Hobbs

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Phil Hobbs

That depends on you. If you want the sims to model all the way to failure modes I'd really be surprised if PSPICE had something in a library that would even come close. My PSPICE license is too old so I wouldn't know. But that would be one tough job.

Again, a laser diode is not a linear device when you look at lasing output. Not at all.

Let them disagree. Will they pay for your dead laser diodes?

All I am saying is that I believe it is impossible to correctly model a laser diode by trying to find electrical equivalents for all its behaviors. You need a behavioral model in addition. All it takes for a LD to die is optical overload inside the cavity. A brief wiggle of a fiber connector, a spike of a few usec ... poof. The EDN model isn't helping you with that.

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Joerg wrote in news: snipped-for-privacy@mid.individual.net:

I suspected as much. :) Actually my guess is that their equivalent circuit needs to be constructed then turned into a subcircuit. There's no doubt it CAN ultimately be written as a single subcircuit file, I don't know why EDN's contributor didn't go all the way.

I always reality-check things. I only intend to use LTspice the way I do my case designs and other physical models in SketchUp. Sure, a posh CAD tool can design me a proper screw thread while SketchUp hasn't a chance of this (at least, not the version 4 I choose to stay with), but the idea is to quickly aid visualisation, not to substitute for reality.

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Lostgallifreyan

Joerg wrote in news: snipped-for-privacy@mid.individual.net:

No, but I'm looking at electrical INput..

No, but they DO take my point when I tell them that SpiceMod shouldn't cost a personal user more than a very few of them. :)

Well, Tim G at Intusoft kindly said if I sent him some model numbers, he'd see what he can do. I sent him 5 data sheets. :) Got to try my luck, no? Besides, they might give a workable average for the kinds of high power red single mode diodes hobbyists are using.

From what I've seen of my dead diodes, they are still electrically very similar to live ones so I'm not concerned with advanced optical modelling.

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Lostgallifreyan

Phil Hobbs wrote in news:FcadnUCm-

5ZRI-3XnZ2dnUVZ snipped-for-privacy@supernews.com:

During the transitions, perhaps, but in steady state operation there seems to be a remarkable electrical similarity between a dead diode and a live one. What matters is the way the diode responds to hard electrical changes on the input, as that's what makes the ringing and damaging overshoots. This is true with NO consideration of optical nature, and just modelling that alone, realistically for real laser diodes, is a lot more than we currently have. And likely not that big an ask, it's just not been done much, it seems.

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Lostgallifreyan

Yes, it should be feasible to place it in a subcircuit. However, since you said you just started out with LTSpice I suggest not to do that (yet).

I use DesignCAD 3D for that. Ten bucks at a liquidator. Mostly because it can read in AutoCAD files.

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Joerg wrote in news: snipped-for-privacy@mid.individual.net:

Ah, but Ghost lets me retain a known working OS config, recalled at will, and an INTENSELY useful side effect of this is infinitely renewable demo periods. :)

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Lostgallifreyan

That's the easy part :-)

BTW, I do not understand why EDN placed the SPICE list as a graphic. Somehow that doesn't leave a very professional taste.

The ones I dealt with last were about $1200 a pop. Lots of them. The client would have been very p....d if I had blown some.

Often you can still use them as LEDs :-)

What killed them?

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:-)

I don't do that. IMHO it's quite borderline from an ethics POV to fool demo SW into a fresh time period. Then I either buy it or move on.

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Ringing? I never had that. Don't drive them through a built-in inductor :-)

LDs are usually current driven. The prudent way is to impose a constant DC current, very well stabilized and equipped with belts, suspenders, cushions, airbags. Then the fast signals are fed in via a current "robbing" shunt circuit to ground. That pretty much makes sure you can't fry it.

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Joerg wrote in news: snipped-for-privacy@mid.individual.net:

Precisely. I hope. If a decent electrical model isn't enough then we all have to think further, but most of us haven't even got that much yet. We'd save a LOT of resources if we did.

I wondered about that too. They let registered users on their site download a text list for free though. I think maybe they just want to lead the horse to enough water to improve the chances it will drink theirs. Anyway, I copied it out by hand and posted it earlier..

I bet it would still hurt them less than the ten diodes I bought for £520 when my weekly income was about £75, when I killed four of them just trying to prove to people the existence of death by retroflection (and not by ESD as was claimed) when the sellers and makers refused to accept what is now generally KNOWN. And no, they did NOT reimburse!

Exactly. That weird fact is why I think it's valid to consider meaningful separation between their full behaviour and their electrical behaviour.

Mostly retroreflection as described above, and also deliberate brinksmanship when trying to find the optimum compromise between a short blazing fun life, and a long boring stable one. :)

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Lostgallifreyan

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