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Re: 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.


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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.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: Spice models of laser diodes?

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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.
 
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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.

Re: Spice models of laser diodes?

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

Re: Spice models of laser diodes?
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How do you want to protect a laser diode with any of these?


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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.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: Spice models of laser diodes?

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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.

Re: Spice models of laser diodes?
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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.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: Spice models of laser diodes?

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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.

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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.


Re: Spice models of laser diodes?
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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).


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

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: Spice models of laser diodes?

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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.
:)

Re: Spice models of laser diodes?
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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.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: Spice models of laser diodes?

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That's what I usually do (and I make a poing of buying from small firms or
good individual programmers rather than big firms, with special emphasis on
paying for tools written by coders who value writing for ALL of Win32).

The only reason I don't use the freeware Sketchup version 5 is it won't run
on W9X like the old demo v4 does. I really dislike Windows XP.

Re: Spice models of laser diodes?
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Try:
http://preview.tinyurl.com/mfg8g8

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

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal
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Re: Spice models of laser diodes?
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Thanks, by now I've seen it. But the model presented ain't much to write
home about, IMHO.


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

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: Spice models of laser diodes?

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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?

Re: Spice models of laser diodes?
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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.


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


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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.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: Spice models of laser diodes?

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

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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. :)
 
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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.

Re: Spice models of laser diodes?
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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.


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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.


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Often you can still use them as LEDs :-)

What killed them?

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: Spice models of laser diodes?

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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.

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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..

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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!

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

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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. :)

Re: Spice models of laser diodes?
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Wow, for you a free lunch must be _really_ free!

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
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Re: Spice models of laser diodes?

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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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