Re: Spice models of laser diodes?

Most of us just use a regular diode to simulate because all one (usually) wants to know is that there definitely won't be any ever so slight spike in diode current because LDs can go poof in microseconds. Simulating the optics part would be a major challenge, I think.

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

Thought that might provoke an expert. :) I agree, same here, that's all I usually need too, though like that EDN manual says, a more detailed model that allows models to give warning of imminent demise is useful. Modelling for ESD is likely daft, better that we just take care and put in TVS's and such, but when it comes to fast modulation, a model definitely helps. Surely you'd have a use for that, no?

While I found that a string of four 1N4148's produced a modelled overshoot almost exactly like what I saw on an oscilloscope months before I considered looking at spice, I got a very different result when trying four 1N4001's so it really does need something better than reaching for a standard diode, in a model OR as a dummy diode in a real circuit (where a optically dead laser diode is best anyway). So it really comes down to trying to get something usefully close! EDN's model seems ideal, aimed at solving this problem for general use, as opposed to the elaborate models in private university publications. EDN's is probably tested too, proofread and verified before publishing.

If anyone can help by making that into asy and sub files for LTspice it will help me a lot, and probably a lot of other people too. Four days of searching have found lots of other people askign questions, and very few answers, and NONE complete and verified. Someone could get well known for solving this so other people can have an easier time of it.

In case anyone wants to try making a model, I typed out that netlist and checked it carefully, so this can save you a bit of time:

Vlas 100 50 1 Dlas 50 200 diode1

F1 0 1 Vlas 1 Rs 1 0 {Sm*sps}

I1 2 0 {Sm*sps*Ith*exp((T/25)-1)} G1 0 2 1 0 1 Rlim 2 0 1Meg Ds 2 3 diode Vm 3 0 0

F2 300 400 Vm 1 Dmon 300 400 diode Cmon 300 400 Cmon

.model diode d .model diode d rs=5

.ENDS

But it obviously needs more than that to build a subcircuit file. I don't know how yet, or even if the 'figure 1' in that PDF has all the required detail to make it possible.

I have designed LD circuitry but I am not an expert. Senor Hobbs would be one. TVS don't work well. Their cutoff isn't terribly well defined and they have too much capacitance for this. Just handle the things with care. I've never killed one with ESD, knock on wood ;-)

"File not found" :-(

Some of the 4000 series behave more like PIN diodes.

It's going to be lots of work. You'll need to get into behavioral models and while people have modeled large chunks of jet engines with that it was a ton of work.

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

Sorry, right file, wrong location. Try this:

I've been finding (and been sent) spice models that are in various stages of completion and complexity. Ignoring the rest for now, I'm focussing on that EDN one because it looks like it's meant to be complete. But it's NOT in a single subcircuit file that can be adapted to LTspice, or anything else. I don't think it needs modelling, just translation.

For modelling, I'll look into that too, as equations are given in that PDF file I mentioned earlier, from Intusoft. In their descriptions of how to use SpiceMod (which I learned costs 600 bucks(!) and is therefore the most expensive few tens of kilobytes I've ever heard of) they show a screen that neatly prioritises the data sheet specs for pluging into the equations. While the details for transisotrs (bar a MOSFET) are not similarly revealed, there ought to be enough there in their diode explanations to model a laser diode, electrically. But again, all I'm asking for here right now is for someone who knows how, to translate that EDN article to a subcircuit ready to plug into LTspice. (I can make my own symbol file).

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

Interesting. Robin Bowden didn't make strong recommendations for them either. Just a resistor/capacitor network, and a zener. I concur, though at the time I was all for varistors too as I'd found a LOT fo them cheaply on eBay at the time.. But they apparently change characteristics rapidly over time fi they see anything like the conditions they have to limit, so I'm not too keen now.

Trying not to get away from the focus of the thread because I really do want direct help with model translation as it's unlikely I can do it alone, but this IS interesting to me.. PIN are those fast photodiodes, right? Often used for detecting very short laser pulses and such? When I modelled my driver using the 1N4005 model I found, I got a lot of ringing. I also saw a correlation with the changing of Vf with hard changes between zero and full drive. I analogised it with the hitting of a hard object as opposed to a softer one that inherently damps the impact. If a PIN diode is to respond fast it seems reasonable to think that it is capable of hard brittle responses, otherwise it could not hope to react in time to register a very short laser pulse. Is this a reasonable way to view it? And if so, why so with rectifiers and not signal diodes? Or LED's which I imagine are similar to laser diodes, being usually GaAs based.

Lostgallifreyan wrote in news:Xns9C57D85EDFFE4zoodlewurdle@216.196.109.145:

Meaning: Diode models with little change in Vf with current at 500 KHz square wave drive were also those which rang the most.

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.

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.

Try:

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

Cheers

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 :-)

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.

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.

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?

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

Cheers

Phil Hobbs

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.

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

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.

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.