Re: Spice models of laser diodes?

George Herold wrote in news:379c1689-21a2-456e-9d3b- snipped-for-privacy@2g2000prl.googlegroups.com:

Here's something to consider: I think many posts here are assuming a switching modulator wheras I am after a continuously proportional and DC coupled type. I can hybridise with a PWM control, and eventually will go to that, probably, but for now, what do you do if you want linear control? Never mind that diode light output isn't at all linear, we might as well start with what we CAN control... So do we really use a Darlington? I understand that those are made to switch fast, they're not chosen for a long linear operating region... So that leaves some fast single stage transistor, and a sense resistor, and a controller, likely an op-amp.....

Is this getting through? :) The LM317 in data sheet approved constant current mode IS just that, though not a shunt mod. But it does the opamp, the sense resistor, and the hefty transistor to drive up to 1.5A. It's not as daft as it looks, and it performs better than anyone ever told me it could.

Anyway, assuming I do shunt part of a constant current from the diode, what might be the simplest way to do it? As far as I know, something like Robin Bowden's 'Die4drive' circuit might have a basis for this with its MOSFET, opamp amd sense resistor, but that's not a shunt mod either...

Getting back to the topic, whatever we do, it would REALLY help if we all had access to some reasonable semblance (electrically) of a single mode laser diode in spice. It's long overdue. It would save students and schools and hobbyists a lot of money if it was there. Linear Technology have given us LTspice to use for free, now we need things like this to put it to use.

In other news, I seem to have dropped alt.lasers from the cross-post list. Annoying. :)

Lostgallifreyan wrote in news:Xns9C581C5A32400zoodlewurdle@216.196.109.145:

By which I mean, it should have been there too.

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"Anyway, assuming I do shunt part of a constant current from the diode, what might be the simplest way to do it? As far as I know, something like Robin Bowden's 'Die4drive' circuit might have a basis for this with its MOSFET, opamp amd sense resistor, but that's not a shunt mod either..."

Yeah, there are lots of copies of the H&L laser diode current driver on the web. Start with a simple current source (opamp, pass element (FET) and sense resistor) and then add a second 'tap' into the laser diode. A resistor works just fine. H&L add two taps. One is op-amp based and works up to a few MHz and then a 50 ohm resistor for HF modulation.

George H.

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"Anyway, assuming I do shunt part of a constant current from the diode, what might be the simplest way to do it? As far as I know, something like Robin Bowden's 'Die4drive' circuit might have a basis for this with its MOSFET, opamp amd sense resistor, but that's not a shunt mod either..."

Yeah, there are lots of copies of the H&L laser diode current driver on the web. Start with a simple current source (opamp, pass element (FET) and sense resistor) and then add a second 'tap' into the laser diode. A resistor works just fine. H&L add two taps. One is op-amp based and works up to a few MHz and then a 50 ohm resistor for HF modulation.

George H.

I can pretty much assure you that PSpice doesn't include a laser diode model, at least not that I recall. LEDs and photodiodes was about as far as they went. Laser diodes would require a lot of characterization and experimentation (boom!) to get all the details correct.

Charlie

The LM317 has an inductive output characteristic, which resonates with the bypass cap. The reason for the inductive characteristic is that there's a feedback loop inside. The output of the 317 is an emitter, which is naturally low impedance--it's a good voltage source, and the feedback just makes it a better one. Outside the feedback bandwidth, it's still a low impedance because it's an emitter.

Collectors make good current sources naturally--a bit of feedback makes them stiffer and more accurate, but outside the loop bandwidth they're still current sources. Using an emitter as a current source gives you problems, because the feedback is fighting the natural tendency of the device--the output crosses over from a high impedance at low frequency where feedback dominates, to a low impedance at high frequency where the feedback is unimportant.

It's a bit like a LDO, where a collector is forced to act like a voltage source, by wrapping feedback around it, leading to similar sorts of stability problems.

The usual 317 fixed current source circuit uses a resistor in series with the output lead, and the feedback lead connected to the other end of the resistor. That works great for fixed current applications, where there's nothing to excite the resonance, but if you're using the 317 as a modulator, you're bound to have trouble with the resonant peak.

Cheers

Phil Hobbs

I really like Darlingtons for that job, because they have a lot of transconductance. That makes the current source stiffer, especially at low V_CE where MOSFETs start to crap out. Their capacitance is much lower, which is helpful with massively nonlinear loads like laser diodes. Also, the V_BE of a Darlington is much better controlled and less drifty than the V_GS of your average MOSFET. Their betas are usually around 10,000, which means that they're as accurate as the sense resistor anyway. Quiet, stable, and predictable--just the ticket for diode laser drivers, I think.

Cheers

Phil Hobbs

That works great too, and is superior if you need to turn the lasing on and off without going too far below threshold. The key is that there's almost no swing at the emitters (especially if you drive them differentially), so you don't suffer from the poor HF response of the current source--you can put a choke in series and forget about it.

I'm usually doing something fancy with AC modulation of a CW diode, e.g. modulation-generated carrier, so the swing is even smaller.

Cheers

Phil Hobbs

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

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

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

John

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.

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.

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

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.

"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.

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).

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.