Typical capacitance of a bigger laser diode?

We're working with some butterfly-packaged parts in that sort of power range, peak currents roughly half an amp. Capacitances are awful, 200 to 700 pF, so they are hard to drive fast. We're just now testing a new gaasfet driver circuit, target being a 1 amp pulse around 200 ps wide.

Some laser suppliers are Eagleyard, Innovative Photonics, Lumics.

I don't know of any Spice models. We make our own measurments of capacitances, inductance, diode curves, and optical outputs.

An ohm or two, resistive. The capacitance isn't usually an issue below

Cheers

Phil Hobbs

Hey, 700 pF and 1 ohm is a time constant of ... computes furiously ...

'Scratch, scratch', I have no idea. I've got some slightly used ~100 mW Sanyo's in the lab, I could try and measure something. (or put one in the mail...?) These are out of production so... any data may not be that useful.

The capacitance a zero volts? Or when it's running at some current? You want to modulate it at some high frequency? These have a relaxation oscillation up near 6 GHz, so capcacitance may not tell you all you need to know.

George H.

If you know the "I" spec's, why can't you drive it with a current source? You modulate that source instead.

I mean, most lasers have to be current regulated to some degree anyway, I would think if you have the current source at its max rating of the LD, capacitance wouldn't matter because you wouldn't have much control over that unless you plan on doing some sort of Peak and hold in which case, the peak condition will most likely vary with the diode.

Unless we're talking about something else?

Jamie

There's no such thing as a current (or voltage) source at RF. The junction has some resistance, even if it's just the (small, but certainly not negligible) ideal diode resistance. Reality adds parasitic junction resistance, wire bond and lead inductance, junction capacitance, etc.

At DC (or approximations thereof, i.e. up to frequencies where reactives can be ignored for the circuit), one can build a current source, and simply not care what voltage the load generates.

At RF, everything drives transmission lines and the speed of light applies, so it's impossible to create a true current source. A "current source" driving a TL generates a known voltage, dependent on the line's impedance. When the energy reaches the load, the same voltage (in the line) divides between the line and load impedances: to a first approximation, the line becomes the source driving the load, not the source proper. It takes several cycles of reflections, back and forth along the line, before source and load are in agreement about what voltage and current stabilize at. Therefore, it's much easier to simply match source and load, so that although you need to know the RLC characteristics of each in order to do so (matching networks and whatnot), you can simply transmit power into the thing, and it gets modulated accordingly (in this case).

Tim

Only if it's easy to measure. Otherwise I'll contact some university folks who deal with laser stuff a lot.

Essentially from close to zero current (which is usually at several hundred mA) to full rated continuous current where they typically sit around 2V forward.

Yup, AM. Got to figure how much muscle the driver will need.

That would not be cool but capacitance should not have anything to do with oscillatory behavior.

That's like LEDs, I was almost afraid it would be that high as well. I took a look at abusing LM5113 chips but their pull-up is weak and kind of sluggish. They are great when pulling down but I need both directions.

Helmut Sennewald has contributed models for smaller versions on the LTSpice Yahoo group. Just not for big ones.

The resistive part is fine but unfortunately it'll have to be more in the gigeehoitz range :-(

Current steering is the usual method but it won't be quite that easy in this case. I still need to muscle capacitance around using some sort of "gooser" circuit on top.

It's fast and furious AM I have to do, almost 100% swing.

How about optical modulator?

-- Boris

sk

Do people limit voltage swing when modulating these fellows, i.e. 0 < Voff < Vf?

That would make driving a bit faster.

-- Cheers, James Arthur

Then you'll have to worry about the laser rate equations as well as the capacitance, and you can't readily fix those with a T-coil. The relaxation frequency is often about 1.5 GHz.

What's the model number?

Cheers

Phil Hobbs

Don't know yet, just that it'll be in the >100mW class and IR. If we can get a nanosecond of transition time that'll be ok for the beginning. I could muscle the capacitance with a gooser circuit but that's always a bit scary because laser diode can commit optical suicide in less than the blink of an eye. All it takes is they come out with a less capacitive model and ... poof.

Too many bucks :-)

Oh yeah, otherwise the driver will cause global warming and could get carbon-taxed :-)

You don't even have to go to zero volts.

sk

Hi Joerg, Well I think this may be mostly meaningless for your diode. But I ran a pulse into a Sanyo DL-7140-201S. (this is a 70mW diode with typical I forward of 100mA) Signal terminated into 50 ohms and then 1 kohm into diode. With no diode (and just x10 scope probe) I had a 1/e RC time of about 20ns. (16pf probe so seems reasonable.) With the diode the RC time depended on drive level.

Drive level 1/e RC time (V) (ns) 2.0 150ns 3.0 80ns 5.0 30ns 6.0 25ns

You'll have to subtract the probe C to get a guesstimate... At low voltage it didn't really look like an RC.. it had a linear region... anyway. Not much use for your high power diodes.

As far as relaxation oscillations. Well, just saying the words I'm already starting to hand wave... But my limited understanding is that this sets the limit in terms of modulation frequency. Lots of hits searching for "diode laser" +"relaxation oscillation" I'd post a link, but no idea as to the quality of any of them.

George H.

That looks like surprisingly little capacitance. Many such diodes will run at 2V at full bore though.

Yes, that is the 2nd and much tougher barrier that Phil mentioned. I don't think we'll quite hit that region yet.