Typical capacitance of a bigger laser diode?

1 ns shouldn't be hard with a 100 mW laser diode. You can do that with dirt-cheap cmos drivers, or use a couple of ADN2871's.

The real killer is series inductance, internal to the diode package. We're shooting for 100 ps edges at 1 amp, which puts 10 volts across every nH of stray indictance. With packaged diodes, a gaasfet driver isn't going to do that.

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

What's your minimum on/off time? What's the max on/off time? Current?

This may not be difficult or expensive.

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

I think that's just a current shunt driver. Could be tight here. But the real issue with all things electronic around lasers is obsolescence. Some major telco deal falls through or has run its course and the stuff may become hard to purchase. A warning sign is often when distributor stock drops to the low three digits everywhere.

CMOS drivers aren't very fast unless you use logic level ones and then holding the diode close to the current limit is a white-knuckle ride.

Any idea why the LM511x series is so slow pulling up?

The nightmare are those butterfly packages. Whoever "invented" those should be ...

Can't you make that and the diode capacitance part of a Pi-filter, like a "peaker"?

--
Regards, Joerg

http://www.analogconsultants.com/

Thanks, George, that is great. But only if it's quick and easy. Not sure if this diode works though because (6V - 1.5V) / 1k is only 4.5mA and I am looking at 100x that level.

--
Regards, Joerg

http://www.analogconsultants.com/

I don't think it'll be expensive, already got a discrete solutuion for it. Except that I don't know the capacitance which can put a crimp in there. Times not cast in concrete yet, on will be low single digit nsec and then a few usec off, so very low duty cycle. Current roughly up to 1A.

--
Regards, Joerg

http://www.analogconsultants.com/

Dunno, it doesn't seem scary to me. You could always add a schottky diode and do current steering, to get precise diode current and some speedup for free. That would be cheap. You like cheap!

Gosh, that is pokey. But it's a 100 volt power supply thing.

stuck to a corkboard with a big pin?

A little inductive peaking can help, unless there's too much inductance already.

Buy me some more beer and I'll show you our tricks.

--

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro   acquisition and simulation

Yikes, that's almost exactly what we're doing. I hope it's not for the same customer. Starts with letter "n"

--

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro   acquisition and simulation

om

ling

iode

I ask

ed ~100

t one

be

nt?

t

may

o 70mW

e)

.)

Oh just having my own sort of fun. I've got a few resistors and the diode in a little pomona box. Say, can I ask a 'silly' question. Since the probe capacitance is limiting me, can I hang a 50 ohm resistor on the end and let it drive a coax cable... throw away a bunch of signal.

Pulse in----+---1k---+---50R---->scope | | 50R _ | V

Current steering alone often isn't enough. Some of the current will first have to get the capacitance back up from wherever you had to drop it down to in order to get the current to the required low limit (or off).

One of the engineers at a client noticeably winced when I took wire cutters to a laser diode like this. He had given permission but it still must have looked like de-clawing to him.

Still got that wooden nickel so I'll have to come back anyhow :-)

--
Regards, Joerg

http://www.analogconsultants.com/

Mine starts with the next letter in the alphabet :-)

But there are myriad purposes for all this pulsing. In another totally different case we were tracking etalon positions for the sake of measuring mechanical properties of materials.

For some reason I have more respect for this stuff than for high voltage. Probably because I know someone who was going blind from pulsed radar exposure. Once you feel the effects it's usually too late.

--
Regards, Joerg

http://www.analogconsultants.com/

Sounds like you need to make the diode become part of a resonant tank with in the power circuit. Using the diodes capacitance to contribute to the remaining C required.

I can see that being tricky due to the C changing dynamically. At least I would think that to be the case.

Jamie

ask

It's just a matter of more tricks.

--

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro   acquisition and simulation

Talking about beer, that reminds me, I need to get some more :)

P.S. I don't think I want to know about your tricks while intoxicated! :)

Jamie

ask

Yup. I've got one in there which should make it work :-)

--
Regards, Joerg

http://www.analogconsultants.com/

That's one of the nasties because it can make harmonics and those are not liked in the land. But there is a whole bag of trick to overcome this, it depends on how complex a circuit can be tolerated. The nice thing is that RF parts are so cheap these days, thanks to an insatiable appetite of our youngsters to always be connected.

--
Regards, Joerg

http://www.analogconsultants.com/

You can do that for a big laser diode, 100mW and up. But small ones are more in the tens of ohms.

A common trick to get a cheapo probe is to take a piece of coax with BNC at one end, solder a 950ohms 0402 in series at the tip, and then set the scope to 50ohm input. The coax only needs to be terminated at one end and then it is. Gives a nice clean representation of the input signal divided 10:1.

--
Regards, Joerg

http://www.analogconsultants.com/

Well Ok but when I decide to sit down and lets say build an amplifier, after the cost of getting a few CX1000 series, vacuums caps, high grade door knob caps and lots of ceramics and let us not forget the roller inductor with it's clean copper tubing coils and all.

Of course, you could always slip some of that through your employers accounting system.

This would be for my personal enjoyment. At work, we deal with hot cathode 300kWatt+ custom tubes and the last one we got cost $12k. But that brings a smile because I don't pay for that!

Jamie

One of our product lines is a special coax with a steel wire in the center instead of copper. It actually is more like a alloyed used in thermocouple wire but, it works well for lowering the Q and we sell this to companies that make instrument cables for the most part.

Jamie

from

calling

diode

I ask

below

the

can

Think pulse forming networks from radar systems.

?-)