UHF laser modulation

We've been doing a lot with GaN fets lately. Cool parts. I can imagine a simple servo loop.

How much current do you want to divert? At what frequency? Maybe a MMIC would work.

A distributed amp might do, if cost is no big deal.

Take a look at SY88022AL. It's gorgeous and fairly cheap. And crazy fast. I use it to make clean fast variable-amplitude pulses to drive distributd amps. I couldn't find any other way to do it.

It would dissipate more power than a shunt GaN fet.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Not a bad part, especially for $10. (That one is obsolete, but it has surviving relatives.) Way too low power for this use, unfortunately. I need a good amp of AC.

I'm thinking a high-side series GaN FET driven by a transformer, like this:

IbiasDC (very very quiet)

| | GND GND

with the laser bypass chosen so that the current doesn't drop too far below threshold in the OFF periods, and the Ibias bypass big enough to keep the bias supply running normally without destroying its sub-Poissonian character below 1 MHz.

This approach saves dissipation, which is important for satellite use. (There are terrestrial applications too, but its space that's paying the bills at the moment.)

Cheers

Phil Hobbs

You might consider an RF technique, like a series RC or tuned circuit or something, matched into the laser diode. From a GaN fet running at low voltage maybe.

That could be low power, which probably matters in space.

MMICS waste a lot of power, as do diffamps.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

My hope is to make a little tile with an EPC8004 (52 pF Cin) resonated with a few nanohenries of inductance, and drive it at a Q of around 3. The voltage gain of the FET will be less than 1, so Mr. Miller shouldn't be a big problem.

I just don't have a lot of experience doing 1-ns stuff with GaN FETs.

Cheers

Phil Hobbs