Re: Spice models of laser diodes?

Progress...?

I'm studying that as best I can but it's fairly heavyweight, can anyone help me understand how to use it?

They mention terms like TT (apparently in microseconds, for 'transit time') but they don't say where they get it from. I don't see it in the two diode data sheets I tried to find it in, only reverse recovery time, and if it were the same thing, surely the notation would be same, and it's not. The deeper I look, the thicker looks the plot.

I'll be patient now and lay off the posting because this looks more complex than I thought, and might take time to answer. Please do though, if you know how. I don't mind doing the work but I need to be shown what must be done, and where to get the data I need to start with. Right now I don't even know if it IS possible to translate from data sheet to spice model, and it looks like it isn't.

Time for the serious boffins to duke it out in this thread, if they're interested. :)

ask over on sci.optics.fiber

Steve

snipped-for-privacy@uakron.edu wrote in news:14d6e586-3ca9-44e0-b3da- snipped-for-privacy@k1g2000yqf.googlegroups.com:

For an electrical model? Fibre is specifically optical... The people who I think can answer this hang out in the groups I posted to anyway.

We bought some 850 nm fiber-type laser diodes that acted as if they had a microseconds-response PIN diode built into them, in series with the actual laser. If you applied current suddenly, the voltage would overshoot almost 2:1, as if there were an inductor in series. As long as the current wasn't cut off for too many nanoseconds, it behaved like a diode. Fine for telecom and maybe for burning CDs, but terrible for sending baseband on/off digital stuff.

John

The telcom guys are using baseband. They use scramblers to ensure that there are enough transitions to do clock recovery. There isn't much low freqency energy.

What sort of data were you sending? Were there long strings of

I seem to remember something about not turning something all the way off. I have forgotten the context and reasoning.

I think the idea was to modulate it from 10% to 100% rather than turn it all the way off in order to avoid transients/problems like you described.

It might have been incandescent lamps, but I don't think so. Reminds me of the time I went into a dark machine room many many years ago. (I was sleeping on the couch, babysitting for an all night run.) All the red lights on the disks were on. Wow were we in trouble! After a few seconds I figured out that it was just the keep-warm current to avoid the turn-on transient. Nothing was visible in normal room light.

Definitely a communications breakdown spreading here but I'm not entirely sure it's electro-optical in nature..

It's for a pure digital logic transmission link,

where the light is supposed to follow the logic input no matter what.

A lot of lasers do goofy things when you just turn them on and off. It's hard to make a clean step of light.

We do sometimes bias them a little below the lase threshold. That improves prop delay and sometimes cleans up the edges. I think the laser vendors buy their wafers in some back alleys in Taiwan; you never really know how the next batch will work.

Sometimes keep-warm current was used to raise the resistance of filaments so the drive transistors wouldn't die at turn-on.

John

Might be an application for an optical switch.

You could also try some kicker circuit but with LDs that is a white-knuckle ride.

Yes. For clean, fast modulation, use a CW laser and a lithium niobate modulator. They start around $1500.

Cool. Some of the Optek gigabit-rated lasers had cold-start turnon time constants in the 10's of microseconds. And the folks at Optek don't know much about their own products.

John

Lostgallifreyan wrote in news:Xns9C512D9705DE7zoodlewurdle@216.196.109.145:

Does anyone here know how to adapt a diode model for a laser diode?

entirely=20

=46or an all but clueless first cut i think you would need a four terminal device, to include the transport of emitted photons.

"JosephKK" wrote in news: snipped-for-privacy@4ax.com:

Yes, I was sent an Intusoft newsletter that contained just such an animal, but it's based on a Hitachi 3mW IR diode, I think. But as a 'dead' diode with facet damage from ESD or other surge usually behaves very similarly to a live fresh one, electrically, AND most high power diodes have no monitor photodiodes anyway, there's nothing wrong with only two terminals. What is then critical, if it's to be driven hard, is to include temperature monitoring in the model. In practise diodes will vary but the setting will be a set-and-forget one based on a single measurement of power at maximum drive, once the thermal compensator works. I did this once (before I ever looked into spice) using an LM317 and some thermistors in the sense resistance network. It sounds crude, but I calculated it in some detail, and it worked well. Apart from that LM317, the entire driver was discrete passive parts. (Wasn't modulated though, have to add stuff then.)