Spice models of laser diodes?

LDs' output powers are pretty linear with bias current once you're above threshold, so if Class A bias is OK, you're in pretty good shape. You can run a power feedback loop using the monitor photodiode if you really need to, but you usually won't.

Cheers

Phil Hobbs

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Phil Hobbs
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Wow, I didn't know people were still using that term. Last time I heard it was in talking about tube televisions from the 1960s!

Snivets is where the sweep output tube undergoes oscillations (Barkhausen oscillation) between the plate and screen, when plate voltage is lower than screen (Vp(sat) typ. 30V for these types, at Vg2 = 125V). It's supposed to make an electron-beam-driven resonant cavity, producing UHF oscillations something like a klystron I suppose. The solution is to raise the voltage on the beam former grid to about 30V.

Can I get a modern definition for "snivets"?

Tim

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Reply to
Tim Williams

The higher-voltage ones ARE pin diodes. They can make excellent drift step-recovery diodes and impact avalanche diodes. Need 1000 volts in

100 picoseconds?

John

Reply to
John Larkin

You can build a current drive out of a LM317, I've even used it as an AM modulator. But 500kHz, nope, it ain't going to go quite that far.

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Joerg

Risque if you want to do 500kHz as you mentioned in another post.

In a DC application (well, sort of, low kHz stuff) I did it the same way as fast applications: A top current source that makes sure the DC current never ever goes above xxx milliamps. Can be built around a LM317 if you wish. Then an opamp-controlled current sink from the LD anode to ground (the LD cathode is also on ground). This current sink then "wastes" x amount of the current from the upper source by sinking it into ground, depending on the control signal on the IN+ of the opamp. What you control is in essence how much of the top source current goes through the LD and how much is wasted.

The super-prudent approach is a high voltage and resistors. The sum of the resistors makes sure the LD can never get too much current. Then current-rob like above. Resistors can't go unstable on you :-)

Don't hold your breath. For many people (like me) a LD model would only make sense if the optical side is in it as well and that makes one heck of a complicated model. Otherwise it isn't terribly useful.

I think that's ok. Those guys know all this stuff I suppose.

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Joerg

That's a great explanation (and I appreciate the connection to LDOs) Phil; thanks!

I suppose that purposely adding resistance in series between the output of the LM317 (emitter of its drive transistor) and whatever it is you're trying to (current) regulate ought to help, but that's considered a rather ugly hack when it's no harder to do it the "right" way? :-)

---Joel

Reply to
Joel Koltner

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

Want to bet? >:) I grant that it won't go further, I tried the model at 1 MHz and it degraded to crude sine waves, but 500 KHz is ok. As far as I can tell, the main weakness seems to be a small range of acceptable capacitance on my controlling op-amp's feedback loop so parasitic capacitances alone may, or may NOT, make it work well. And it's very dependent on the diode too which is why I want an LD model. But when I built my first real circuit for this design a few months back the LD showed characteristics similar to the 1N4148 model which are much more favourable to damping high frequency ringing than if it had been a 'hard' diode like a 1N4005 that barely responds with Vf changes to current changes and forces. I think LD's do vary Vf a lot with current so they damp the ringing well enough.

But I'll know more when I've ate the pudding.

Reply to
Lostgallifreyan

"Tim Williams" wrote in news:Ticcm.95224$ snipped-for-privacy@newsfe14.iad:

Seconded. All I can think of is that little sniffle that people do when they're trying to work out whether they've got a cold or not.

Reply to
Lostgallifreyan

Phil Hobbs wrote in news:qe6dnQAVoMdevOzXnZ2dnUVZ snipped-for-privacy@supernews.com:

Good stuff. I thoughyt they were fairly linear too, above threshold (and my design sets the low peak to just sub-threshold too. They can mode hop though, which doesn't help, but if they're going to do that they'll do it anyway and impair the effect of a clean drawn line in graphics.

The PWM idea isn't great for graphics either, but it depends how fast the main cycle is, if it's possible to modulate on a 10 MHz carrier without awkward exotic precautions, it isn't going to show up as a dashed line using any scanner currently available. That makes it an attractive idea for other types of laser too, and also AOM's too, which is cool because proportional PCAOM's cost more. (Not that I've even had the money and availaibility of such to try those).

Reply to
Lostgallifreyan

It can work but as Phil had mentioned the LM317's stability becomes iffy beyond a certain frequency when operated as a current source. It's like taking a small car past 120mph.

Remember, laser diodes die quietly. There is no tchk ... *phut* but there is that painful hit in the bank account.

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Joerg

10MHz is sluggishly slow for a decent laser diode.
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Joerg

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

I'm all for pushing the envelope, there's no hide at stake here. :) It sure looks like it will work. It's 'competing' with systems that can do

100 or 200 KHz at best generally, and none of these devices are needed to go further. They work alongside laser systems incapable of more that 10 K, often as not.

The main point is to see what's possible, and to establish the safe operating area so when I really want to use this thing I can have a very good idea how to make it right, at more reasonable figures. And I'll know it will draw a very clean line, which is what the people most likely to use it would want it to do.

But I still want a good laser diode electrical model.. :)

Reply to
Lostgallifreyan

Phil Hobbs wrote in news:zamdnaOrPvgQguzXnZ2dnUVZ snipped-for-privacy@supernews.com:

I've not used them often enough. Always liked the idea though, and I agree that predictable base current and voltage helps a lot. But unless I'm missing something, they make great switched but lousy proportional controllers, no?

Reply to
Lostgallifreyan

[...]

a

Do you have some examples and pics? That would be interesting, I have use the 1N4007 as a RF PIN diode but never as SRD.

One of my next projects needs the opposite, briefly and automatically disconnecting an RF amp from a several kV pulse path so it doesn't go up in smoke. The kind of stuff where people often say "you can't do that". I love it when a project like this comes along.

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Joerg

You can only try that on a breadboard, not SPICE. And then you find out that it can do 250kHz but after switching to a different manufacturer's LM317 ... *POOF*

I'd rather prefer a nice bottle of Chateau d'Yquem :-)

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Joerg

Phil Hobbs wrote in news:AY-dnczxb411gOzXnZ2dnUVZ snipped-for-privacy@supernews.com:

Once I'd never have considered it because it has other troubles to overcome too, I'd used it solely as a fixed constant current drive in every laser I'd built in the previous 7 years..

One problem is that the series resistor must be on the high side of the load, most designs using sense resistors put it on the low side. Another is that

1.5V is dropped across that resistor, other designs use far less, so are more efficient. And another, as you say, is that to damp the feedback for an ideal response to changes in a supply or load, they fixed conditions that may or may not be troublesome to us tinkerers who do weird things with it.

The large voltage drop on the sense resistor is actually a boon, it's far easier to dupe the LM317 accurately when you have that 1.25V to use. I have modified the idea to drop 1V across the voltage control network leaving just

0.25V on the resistor, saving one watt per amp, pretty cool if you're doing this with an LM338. >:) It sucks for fast mod though, stability and solidity is dire, but it's great for a dimmable low-volt light though, a string of LED's and that trick, and the efficiency approaches that of a decent switched power converter.

The high-side sensing seems at first awkward, but it's not that bad, just feed it to the normal input of a differential amp and do the modulation on the inverting input. A second stage is needed to set input protections and an agreeable polarity for response to signals but it's still just one 8 pin chip.

That internal componentry for response to line.load changes was the thing I thought would make the concept useless, but that's why reaching 500 KHz with respectable square waves is so nice a surprise.

Viva the LM317. If I can have 100 of those to do entertaining stuff with it beats having to agonise over Mouser's lists and paying big tax on many small boxes. I really aim to do things with a minimal range of parts types, it's a kind of thing I have.. :)

Reply to
Lostgallifreyan

considered

so

a

Google "Grehkov diode". He's a Russian guy who discovered the DSRD and impact avalanche effects in cheap power diodes.

Also look for papers and patents by Thomas E. McEwan.

We did one water-cooled DSRD pulser that makes -2KV pulses, about 2 ns wide, at 500 KHz. We forward-bias a secret diode at +48 volts for about 80 ns, to let the current build up to 50 amps or so, then reverse-bias it from a 400 volt supply and wait for it to snap.

Here's the pulser head...

formatting link

with the serious parts bolted to a gold-plated copper block. A water-flow cold plate gets bolted to the bottom.

I can show you innards privately. The HV, high power PIN diode turned out to be unusual.

John

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John Larkin

"Joel Koltner" wrote in news:M3kcm.493202$ snipped-for-privacy@en-nntp-02.dc.easynews.com:

If you always do everything the 'right' way, how are you going to learn anything for yourself?

Reply to
Lostgallifreyan

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

Ithink I posted enough about that to make it clear I know about that pain. I'll try it on cheap diodes and take it from there.

Reply to
Lostgallifreyan

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

Not the diode that bothers me. The laser's fast ability is what makes the idea easier. The question is whether the complexity of other parts of the system is worth it, and whether in practise a fast graphic scanner shows a dashed line. That sets limits on ranges for carrier speed. No need to go faster than is required to avoid that.

Reply to
Lostgallifreyan

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