Spice models of laser diodes?

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

Not intentional.. I'm using an LM317. Yes, I know it's not the approved way. It WORKS, far better than expected, and most people who explore weird tricks with an LM317 are mightily impressed with it, from the guy who built a class A headphone amp, or the radio ham who built a transmitter round one...

My physical circuit build, AND the later spice modelling, both indicate that an LM317 for a cheap way to get up to an amp-amd-a-half of DC coupled proportional laser drive at up to 500 KHz is very likely to work well. Maybe the inductance is in the LM317 model, I don't know where else it can be in my simple circuit models. What's crucial is that it is the SAME overshoot I saw in the real circuit so spice is already telling me good things, and I've already improved the driver on the strength of that spice model.

Definitely true. Shunting is my favourite method, and I might return to it with this current indea. But half the fun is trying to see how well I can push this LM317 idea. I've seen circuits that are 'better' that don't seem either much better or worse than mine, which also happens to be very polite at startup, no spikes at that moment at all..

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

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.

Reply to
Lostgallifreyan

Seconded. A nice RF transistor in shunt is the ticket. (Anti-snivet resistors required.)

Cheers

Phil Hobbs

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Dr Philip C D Hobbs
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Reply to
Phil Hobbs

Ohhhhhh..... You're using a voltage driver, and trying to trick it into being a current driver. Bad, bad, bad, bad news.

Due to the rolloff in the loop gain, the output of a voltage regulator appears inductive, which will reliably give you a big noise peak if the output cap is too big, and some ringing if it's too small. As Joerg said, you're way better off using current drive. It isn't difficult, just an op amp and a Darlington. Use the Darlington's collector as the output, and sense the current in its emitter. Adding an outboard current limit is easy then too.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

I tried to use an LM317 as a constant current driver for a regular old LED (a high-power white LED -- needed a few watts; this was >5 years ago when white LEDs were still new and spendy) and it just sat there and oscillated on me.

You do see people suggesting using LM317s (or similar) as constant current sources all the time, though... when can it work? It sounds like you're saying it's almost always a bad idea?

Compared to an LM317 though, isn't the main difference that the LM317 essentially has Power->drive transistor->LED->sense resistor whereas Joerg's approach is Power->LED->drive transistor->sense resistor? ...so you're essentially isolating the current sensing from the load itself, to some degree?

---Joel

Reply to
Joel Koltner

Phil Hobbs wrote in news:xoidnU2- iIbBSu3XnZ2dnUVZ snipped-for-privacy@supernews.com:

I thought if constant current config for an LM317 is good enough for the standard data sheet then it's good enough for me. :) (And if you look at the LaserFAQ closely you'll also see that it's good enough for Winfield Hill, though he wasn't trying to modulate it..) Like I said, part of the fun is in making the LM317 do weird and wonderful things. It's fun to know that wherever you have some, you can do some amazing things normally done with other parts, usually exotic, more costly, with MUCH more complex board layouts, etc..

That fits what I see. I've managed to tame it to something respectful. I think an 'overshoot' that results in a minimum-to-maximum deviation of about 3 mA along the 'flat' top of a 500 KHz square wave at 160 mA isn't bad. People who know a lot more than I do have been content with worse.

That's what I intend to try too, though I'll try a MOSFET rather than a darlington. I take it the darlington is to avoid the gate capacitance of a MOSFET? I can see that it will work because its total Vf will be less than the laser diode's own. (A quirk my own circuit is exploiting, in a different way).

Reply to
Lostgallifreyan

Phil Hobbs wrote in news:58CdnWEqNIuvS-3XnZ2dnUVZ snipped-for-privacy@supernews.com:

¿Qué?
Reply to
Lostgallifreyan

"Joel Koltner" wrote in news:_W4cm.439882$ snipped-for-privacy@en-nntp-04.dc.easynews.com:

Never seen that, though I know it HAS been seen, the guy who built a transmitter round an LM317 did so to explore it. I use an LED lamp fopr my main room light when I'm at the compiter, it uses an LM317 with a voltage reference and a pot to dupe it to work as a dimmer. It's not very efficient but it doesn't flicker either like most power converters do, and it's much cheaper. Solid as a rock.

I tend to trust manufacturers data sheets. It beats assuming they're useless. :)

Reply to
Lostgallifreyan

The high speed LD drivers I've seen used the differential amp topology. The LD was the load in one of the shoulders; the max. current was set at the tail. For the faster switching, when the LD is in the "off" state, it was kept at the bias current little below the threshold of lasing. This type of circuit can easily modulate the LD at the rates of up to several GHz; the limits are set by the RF parasitics.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

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Vladimir Vassilevsky

Vladimir Vassilevsky wrote in news:IMWdnYD_roXLQ-

3XnZ2dnUVZ snipped-for-privacy@giganews.com:

But what would you do if you were asked to do 'analog' proportional DC coupled modulations that tracked a continuously varying signal, instead of high speed switched signalling? And how fast would it go then?

Reply to
Lostgallifreyan

The LDs are VERY nonlinear (light vs current), so I would use voltage to frequency conversion, PWM, delta sigma or some other pulse modulation technique. A lot depends on your requirements, however tracking at the speed of the 1/10 of the pulse rate ballpark should not be a big problem.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

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Reply to
Vladimir Vassilevsky

Vladimir Vassilevsky wrote in news:8_2dnS4nlv14f-3XnZ2dnUVZ snipped-for-privacy@giganews.com:

I like that idea. I've considered it but I think it could be more complex than the simple modulator based on an LM317. I like the real visual linearity that could result from very fast PWM though. It's another idea I want to try once I get this LM317 lark worked out of my system.

Reply to
Lostgallifreyan

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I love laser diode's.. K. Libbrecht and Jan Hall's RSI article circa '93? is the classic current robbing circuit. I'd been trying to modulate a LD current driver at high speed. I read the abstract to the above article and said.. "no way". Then I read it and learned something.

But I'm mostly interested in how the current modualtes the wavlength. Which as Phil H. said has all sorts of different time scales. The 785 nm LD's I use have a relaxation oscillation frequency of 6+GHz!

I have no idea about spice models.

George H.

Reply to
George Herold

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. :)

Reply to
Lostgallifreyan

Lostgallifreyan wrote in news:Xns9C581C5A32400zoodlewurdle@216.196.109.145:

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

*sleeps*
Reply to
Lostgallifreyan

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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.

Reply to
George Herold

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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.

Reply to
George Herold

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

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Dr Philip C D Hobbs
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Reply to
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

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Dr Philip C D Hobbs
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Reply to
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

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Dr Philip C D Hobbs
Principal
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Reply to
Phil Hobbs

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