Hi, All,
One of my customers is building a quantum magnetometer using nitrogen >vacancies in a small diamond.
It is a fluorescent measurement with a 520 nm pump laser and fluorescence
in the red. They?re having etalon fringe problems, and one of my
suggestions was to use large amplitude modulation of the bias current at
UHF, as is done in DVD writers, e.g. ><https://www.analog.com/media/en/technical-documentation/data-sheets/MAX9483-MAX9484.pdf>.
Of course, all these DVD writer chips are going away, and this laser needs >more current anyway, around 2-3 amps.
The usual way of modulating lasers quickly involves dumping the bias
current to ground with a differential pair, but in this case that would be
a big power hit.
One approach would be to use a GaN FET switch, with just enough capacitance >across the laser that it doesn?t drop too far below threshold in the off >periods.
Any experience doing something like that?
Thanks
Phil Hobbs
On Tue, 21 Jul 2026 13:35:57 -0000 (UTC), Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
Hi, All,
One of my customers is building a quantum magnetometer using nitrogen
vacancies in a small diamond.
It is a fluorescent measurement with a 520 nm pump laser and fluorescence
in the red. They?re having etalon fringe problems, and one of my
suggestions was to use large amplitude modulation of the bias current at
UHF, as is done in DVD writers, e.g.
<https://www.analog.com/media/en/technical-documentation/data-sheets/MAX9483-MAX9484.pdf>.
Of course, all these DVD writer chips are going away, and this laser needs >> more current anyway, around 2-3 amps.
The usual way of modulating lasers quickly involves dumping the bias
current to ground with a differential pair, but in this case that would be >> a big power hit.
One approach would be to use a GaN FET switch, with just enough capacitance >> across the laser that it doesn?t drop too far below threshold in the off
periods.
Any experience doing something like that?
Thanks
Phil Hobbs
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.
On 2026-07-21 10:03, john larkin wrote:
On Tue, 21 Jul 2026 13:35:57 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
Hi, All,
One of my customers is building a quantum magnetometer using nitrogen
vacancies in a small diamond.
It is a fluorescent measurement with a 520 nm pump laser and fluorescence >>> in the red. They?re having etalon fringe problems, and one of my
suggestions was to use large amplitude modulation of the bias current at >>> UHF, as is done in DVD writers, e.g.
<https://www.analog.com/media/en/technical-documentation/data-sheets/MAX9483-MAX9484.pdf>.
Of course, all these DVD writer chips are going away, and this laser needs >>> more current anyway, around 2-3 amps.
The usual way of modulating lasers quickly involves dumping the bias
current to ground with a differential pair, but in this case that would be >>> a big power hit.
One approach would be to use a GaN FET switch, with just enough capacitance >>> across the laser that it doesn?t drop too far below threshold in the off >>> periods.
Any experience doing something like that?
Thanks
Phil Hobbs
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.
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.
It would dissipate more power than a shunt GaN fet.
I'm thinking a high-side series GaN FET driven by a transformer, like this:
IbiasDC (very very quiet)
0------*--||--*
| |
| GND
*--|
|-----* *--||-- Gate drive
*--| | |
| 3 || 3
| 3 || 3
| | |
*----*--------* GND
| |
| ---
--- \ / --
--- V /
| --- -->
| |
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
On Tue, 21 Jul 2026 10:46:55 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
On 2026-07-21 10:03, john larkin wrote:
On Tue, 21 Jul 2026 13:35:57 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
Hi, All,
One of my customers is building a quantum magnetometer using nitrogen
vacancies in a small diamond.
It is a fluorescent measurement with a 520 nm pump laser and fluorescence >>>> in the red. They?re having etalon fringe problems, and one of my
suggestions was to use large amplitude modulation of the bias current at >>>> UHF, as is done in DVD writers, e.g.
<https://www.analog.com/media/en/technical-documentation/data-sheets/MAX9483-MAX9484.pdf>.
Of course, all these DVD writer chips are going away, and this laser needs >>>> more current anyway, around 2-3 amps.
The usual way of modulating lasers quickly involves dumping the bias
current to ground with a differential pair, but in this case that would be >>>> a big power hit.
One approach would be to use a GaN FET switch, with just enough capacitance
across the laser that it doesn?t drop too far below threshold in the off >>>> periods.
Any experience doing something like that?
Thanks
Phil Hobbs
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.
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.
It would dissipate more power than a shunt GaN fet.
I'm thinking a high-side series GaN FET driven by a transformer, like this: >>
IbiasDC (very very quiet)
0------*--||--*
| |
| GND
*--|
|-----* *--||-- Gate drive
*--| | |
| 3 || 3
| 3 || 3
| | |
*----*--------* GND
| |
| ---
--- \ / --
--- V /
| --- -->
| |
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.
On 2026-07-21 12:04, john larkin wrote:
On Tue, 21 Jul 2026 10:46:55 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
On 2026-07-21 10:03, john larkin wrote:
On Tue, 21 Jul 2026 13:35:57 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
Hi, All,
One of my customers is building a quantum magnetometer using nitrogen >>>>> vacancies in a small diamond.
It is a fluorescent measurement with a 520 nm pump laser and fluorescence >>>>> in the red. They?re having etalon fringe problems, and one of my
suggestions was to use large amplitude modulation of the bias current at >>>>> UHF, as is done in DVD writers, e.g.
<https://www.analog.com/media/en/technical-documentation/data-sheets/MAX9483-MAX9484.pdf>.
Of course, all these DVD writer chips are going away, and this laser needs
more current anyway, around 2-3 amps.
The usual way of modulating lasers quickly involves dumping the bias >>>>> current to ground with a differential pair, but in this case that would be
a big power hit.
One approach would be to use a GaN FET switch, with just enough capacitance
across the laser that it doesn?t drop too far below threshold in the off >>>>> periods.
Any experience doing something like that?
Thanks
Phil Hobbs
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.
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.
It would dissipate more power than a shunt GaN fet.
I'm thinking a high-side series GaN FET driven by a transformer, like this: >>>
IbiasDC (very very quiet)
0------*--||--*
| |
| GND
*--|
|-----* *--||-- Gate drive
*--| | |
| 3 || 3
| 3 || 3
| | |
*----*--------* GND
| |
| ---
--- \ / --
--- V /
| --- -->
| |
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.
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
On 2026-07-21 12:04, john larkin wrote:
On Tue, 21 Jul 2026 10:46:55 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
I'm thinking a high-side series GaN FET driven by a transformer, like
this:
IbiasDC˙ (very very quiet)
0------*--||--*
˙˙˙˙˙˙˙ |˙˙˙˙˙ |
˙˙˙˙˙˙˙ |˙˙˙˙ GND
˙˙˙˙˙˙˙ *--|
˙˙˙˙˙˙˙˙˙˙ |-----*˙˙˙ *--||-- Gate drive
˙˙˙˙˙˙˙ *--|˙˙˙˙ |˙˙˙ |
˙˙˙˙˙˙˙ |˙˙˙˙˙˙˙ 3 || 3
˙˙˙˙˙˙˙ |˙˙˙˙˙˙˙ 3 || 3
˙˙˙˙˙˙˙ |˙˙˙˙˙˙˙ |˙˙˙ |
˙˙ *----*--------*˙˙ GND
˙˙ |˙˙˙ |
˙˙ |˙˙ ---
˙ ---˙ \ / --
˙ ---˙˙ V˙˙ /
˙˙ |˙˙ ---˙ -->
˙˙ |˙˙˙ |
˙ 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.
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.
How could they be having etalon fringe problems with such a well-known technology? They must be pushing the boundaries of their particular
laser, or the diamond has other things going on besides NVs.
someone ><2a59d59e3809f827ce709d3815e3950eef4a6a93af5557a93a7fdfba71460843@example.com> >wrote:
<snip AI crapola>
How could they be having etalon fringe problems with such a well-known
technology? They must be pushing the boundaries of their particular
laser, or the diamond has other things going on besides NVs.
Have you ever built an instrument based on a diode laser? I gather that
you haven?t. Your average lowish power diode will start mode hopping at >around 1 ppm of feedback into the cavity.
That level is difficult to avoid in a compact focused beam system.
Cheers
Phil Hobbs
Cheers
Phil Hobbs
On Sat, 25 Jul 2026 09:46:18 -0000 (UTC), Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
someone
<2a59d59e3809f827ce709d3815e3950eef4a6a93af5557a93a7fdfba71460843@example.com>
wrote:
<snip AI crapola>
How could they be having etalon fringe problems with such a well-known
technology? They must be pushing the boundaries of their particular
laser, or the diamond has other things going on besides NVs.
Have you ever built an instrument based on a diode laser? I gather that
you haven?t. Your average lowish power diode will start mode hopping at
around 1 ppm of feedback into the cavity.
That level is difficult to avoid in a compact focused beam system.
Cheers
Phil Hobbs
Cheers
Phil Hobbs
If you modulate the laser current, does that cause problems? Will it
hop in and out of laser modes? How much modulation did you have in
mind?
On 7/21/26 11:23 AM, Phil Hobbs wrote:
On 2026-07-21 12:04, john larkin wrote:
On Tue, 21 Jul 2026 10:46:55 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
[...]
I'm thinking a high-side series GaN FET driven by a transformer, like >>>> this:
IbiasDC˙ (very very quiet)
0------*--||--*
˙˙˙˙˙˙˙ |˙˙˙˙˙ |
˙˙˙˙˙˙˙ |˙˙˙˙ GND
˙˙˙˙˙˙˙ *--|
˙˙˙˙˙˙˙˙˙˙ |-----*˙˙˙ *--||-- Gate drive
˙˙˙˙˙˙˙ *--|˙˙˙˙ |˙˙˙ |
˙˙˙˙˙˙˙ |˙˙˙˙˙˙˙ 3 || 3
˙˙˙˙˙˙˙ |˙˙˙˙˙˙˙ 3 || 3
˙˙˙˙˙˙˙ |˙˙˙˙˙˙˙ |˙˙˙ |
˙˙ *----*--------*˙˙ GND
˙˙ |˙˙˙ |
˙˙ |˙˙ ---
˙ ---˙ \ / --
˙ ---˙˙ V˙˙ /
˙˙ |˙˙ ---˙ -->
˙˙ |˙˙˙ |
˙ 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.)
It may be better to use a push-pull architecture and have the base of
that at an elevated voltage where the laser diode just barely does not
yet turn on. That way you don't have to muscle through its capacitance
every time.
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.
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.
It'll be a lot of experimenting. GaN aren't the only game in town. LDMOS sometimes come in pairs but it can be hard to find them for such low
power. Assuming you need pulses here and there you'd drive them through
a small RF transformer so it becomes push-pull despite the common source architecture. Like an RF power amp. Instead of GND, the base of the secondary of the transformer which is connected to the laser diode could
be at an elevated DC level so the laser diode remains somewhat pre-biased.
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