Fast, efficient IR LEDs?

Joerg wrote:
John Larkin wrote:
On Sun, 07 Feb 2010 17:27:11 -0500, Phil Hobbs
pcdhSpamMeSenseless_at_electrooptical.net> wrote:

On 2/7/2010 5:10 PM, Joerg wrote:
Phil Hobbs wrote:
On 2/7/2010 4:10 PM, Joerg wrote:
Phil Hobbs wrote:
On 2/7/2010 12:29 PM, JosephKK wrote:
[...]

You may wish to consider a laser diode operating below critical
current.

Thanks, I know that trick. Thing is, I need a 5000:1 output power
range, or thereabouts--i.e. 3 uW - 15 mW. The bandwidth is going
to be
way more than enough at the high end, and the problem is to keep
the
feedback poles from crossing at a frequency where there's
over-unity
gain.

There are other approaches possible that require different
approaches,
but they require more tweaking--e.g. two ranges with two LEDs using
different optical coupling fractions.

Or have an offset in there where the LED (or LD below lasing
threshold
as Joseph suggested) runs at a regulated base power level. BTDT,
but in
my case that was in order to remain above lasing threshold.

This gizmo is an advanced photoreceiver that maintains
shot-noise-limited performance (2 dB above shot noise) from ~10 nA to
100 uA, with an honest 1 MHz bandwidth over (almost) the whole range.
Doing that down near the minimum photocurrent is a real genuine
parlour trick.

Luckily I never had to do that. BW was always tens of MHz but they
gave
me plenty of amplitude to work with. However, up there on that
pedestal
it had to be super low noise because we had to extract modulation.


The ones uses two photodiodes wired in series (!) to get a
sub-Poissonian photocurrent to null out the primary photocurrent.
That's a trick I've never seen before, so I might have invented
it. It
obviously requires some careful feedback to keep the currents in
balance, but the result is a nice linear photoreceiver with almost no
additional input capacitance.

Neat! But now you've spilled the beans and can't patent it :-(

Patents aren't worth much anyhow these days. Seems like most of what
they do is trigger patent trolls who then bog down whole businesses.

I can patent it for the next year, at least in the USA. I might do
that, we'll see.

Two photodiodes in series have the same photocurrent but *half the
shot noise*, so the cancellation current is actually quieter than the
photocurrent, without needing resistive degeneration. (I also manage
to keep all 300-kelvin resistors out of the signal path, which is
key.)

The optical feedback is sort of a poor-man's photomultiplier: most of
the LED light goes to another photodiode, driving an ordinary TIA
which produces the output. It's a really sweet solution overall, with
the one disadvantage that it needs two tweaks.

I assume you mean the balancing of the two PDs in series. Is there no
way to servo that? Maybe by occasionally interrupting the optical
path?

There's a bias feedback loop that looks after that. It doesn't have
to be that accurate since the PDs run at 14V of reverse
bias--keeping the junction of the two PDs reasonably still is all
that's required.

The tweaks are for making sure that the two photocurrents are
reasonably close to begin with, and to govern the poorly specified
efficiency of the LEDs. (IR LEDs have output power specs that are
almost as loose as the V_T spec of your average JFET.)

You should be able to buy them in a couple of months, if all goes
well. (No home should be without one, after all.) ;)

Cheers

Phil Hobbs

Just for the heck of it, I asked Jonathan to measure the low-current
linearity of some visible and IR led's. I know that some LEDs can make
visible light at 1 nA, so it will be interesting to see if there is a
linearity knee somewhere. A red LED driving a silicon PIN diode makes
a visually perfect straight-line graph plotted linearly from 0 to 55
mA.

I theory, LED voltage is the log of current, so at some very low
current there won't be enough voltage across the junction to make a
photon of anywhere near the expected wavelength. It could be that
materials defects will kill things before that point.

I did some googling on LED behavior at low currents and found nothing
useful.


Academics have tried to do single-photon generation with LEDs. Pulsed
at very low current. IIRC one paper was from Syracuse, NY. But I don't
think you get access to this stuff directly on the web, probably needs
some paid access like IEEE Explore. Or good connections to a university.

..and HOW does one determine / detect a single photon? Tap into nerve of
a cat?

Joerg wrote:
John Larkin wrote:
On Sun, 07 Feb 2010 17:27:11 -0500, Phil Hobbs
pcdhSpamMeSenseless_at_electrooptical.net> wrote:

On 2/7/2010 5:10 PM, Joerg wrote:
Phil Hobbs wrote:
On 2/7/2010 4:10 PM, Joerg wrote:
Phil Hobbs wrote:
On 2/7/2010 12:29 PM, JosephKK wrote:
[...]

You may wish to consider a laser diode operating below critical
current.

Thanks, I know that trick. Thing is, I need a 5000:1 output power
range, or thereabouts--i.e. 3 uW - 15 mW. The bandwidth is going
to be
way more than enough at the high end, and the problem is to keep the
feedback poles from crossing at a frequency where there's over-unity
gain.

There are other approaches possible that require different
approaches,
but they require more tweaking--e.g. two ranges with two LEDs using
different optical coupling fractions.

Or have an offset in there where the LED (or LD below lasing
threshold
as Joseph suggested) runs at a regulated base power level. BTDT,
but in
my case that was in order to remain above lasing threshold.

This gizmo is an advanced photoreceiver that maintains
shot-noise-limited performance (2 dB above shot noise) from ~10 nA to
100 uA, with an honest 1 MHz bandwidth over (almost) the whole range.
Doing that down near the minimum photocurrent is a real genuine
parlour trick.

Luckily I never had to do that. BW was always tens of MHz but they gave
me plenty of amplitude to work with. However, up there on that pedestal
it had to be super low noise because we had to extract modulation.


The ones uses two photodiodes wired in series (!) to get a
sub-Poissonian photocurrent to null out the primary photocurrent.
That's a trick I've never seen before, so I might have invented it. It
obviously requires some careful feedback to keep the currents in
balance, but the result is a nice linear photoreceiver with almost no
additional input capacitance.

Neat! But now you've spilled the beans and can't patent it :-(

Patents aren't worth much anyhow these days. Seems like most of what
they do is trigger patent trolls who then bog down whole businesses.

I can patent it for the next year, at least in the USA. I might do
that, we'll see.

Two photodiodes in series have the same photocurrent but *half the
shot noise*, so the cancellation current is actually quieter than the
photocurrent, without needing resistive degeneration. (I also manage
to keep all 300-kelvin resistors out of the signal path, which is
key.)

The optical feedback is sort of a poor-man's photomultiplier: most of
the LED light goes to another photodiode, driving an ordinary TIA
which produces the output. It's a really sweet solution overall, with
the one disadvantage that it needs two tweaks.

I assume you mean the balancing of the two PDs in series. Is there no
way to servo that? Maybe by occasionally interrupting the optical path?

There's a bias feedback loop that looks after that. It doesn't have
to be that accurate since the PDs run at 14V of reverse bias--keeping
the junction of the two PDs reasonably still is all that's required.

The tweaks are for making sure that the two photocurrents are
reasonably close to begin with, and to govern the poorly specified
efficiency of the LEDs. (IR LEDs have output power specs that are
almost as loose as the V_T spec of your average JFET.)

You should be able to buy them in a couple of months, if all goes
well. (No home should be without one, after all.) ;)

Cheers

Phil Hobbs

Just for the heck of it, I asked Jonathan to measure the low-current
linearity of some visible and IR led's. I know that some LEDs can make
visible light at 1 nA, so it will be interesting to see if there is a
linearity knee somewhere. A red LED driving a silicon PIN diode makes
a visually perfect straight-line graph plotted linearly from 0 to 55
mA.

I theory, LED voltage is the log of current, so at some very low
current there won't be enough voltage across the junction to make a
photon of anywhere near the expected wavelength. It could be that
materials defects will kill things before that point.

I did some googling on LED behavior at low currents and found nothing
useful.


Academics have tried to do single-photon generation with LEDs. Pulsed at
very low current. IIRC one paper was from Syracuse, NY. But I don't
think you get access to this stuff directly on the web, probably needs
some paid access like IEEE Explore. Or good connections to a university.

..and HOW does one determine / detect a single photon? Tap into nerve of
a cat?

John Larkin wrote:
On Sun, 07 Feb 2010 17:27:11 -0500, Phil Hobbs
pcdhSpamMeSenseless_at_electrooptical.net> wrote:

On 2/7/2010 5:10 PM, Joerg wrote:
Phil Hobbs wrote:
On 2/7/2010 4:10 PM, Joerg wrote:
Phil Hobbs wrote:
On 2/7/2010 12:29 PM, JosephKK wrote:
[...]

You may wish to consider a laser diode operating below critical
current.

Thanks, I know that trick. Thing is, I need a 5000:1 output power
range, or thereabouts--i.e. 3 uW - 15 mW. The bandwidth is going to be
way more than enough at the high end, and the problem is to keep the
feedback poles from crossing at a frequency where there's over-unity
gain.

There are other approaches possible that require different approaches,
but they require more tweaking--e.g. two ranges with two LEDs using
different optical coupling fractions.

Or have an offset in there where the LED (or LD below lasing threshold
as Joseph suggested) runs at a regulated base power level. BTDT, but in
my case that was in order to remain above lasing threshold.

This gizmo is an advanced photoreceiver that maintains
shot-noise-limited performance (2 dB above shot noise) from ~10 nA to
100 uA, with an honest 1 MHz bandwidth over (almost) the whole range.
Doing that down near the minimum photocurrent is a real genuine
parlour trick.

Luckily I never had to do that. BW was always tens of MHz but they gave
me plenty of amplitude to work with. However, up there on that pedestal
it had to be super low noise because we had to extract modulation.


The ones uses two photodiodes wired in series (!) to get a
sub-Poissonian photocurrent to null out the primary photocurrent.
That's a trick I've never seen before, so I might have invented it. It
obviously requires some careful feedback to keep the currents in
balance, but the result is a nice linear photoreceiver with almost no
additional input capacitance.

Neat! But now you've spilled the beans and can't patent it :-(

Patents aren't worth much anyhow these days. Seems like most of what
they do is trigger patent trolls who then bog down whole businesses.

I can patent it for the next year, at least in the USA. I might do
that, we'll see.

Two photodiodes in series have the same photocurrent but *half the
shot noise*, so the cancellation current is actually quieter than the
photocurrent, without needing resistive degeneration. (I also manage
to keep all 300-kelvin resistors out of the signal path, which is key.)

The optical feedback is sort of a poor-man's photomultiplier: most of
the LED light goes to another photodiode, driving an ordinary TIA
which produces the output. It's a really sweet solution overall, with
the one disadvantage that it needs two tweaks.

I assume you mean the balancing of the two PDs in series. Is there no
way to servo that? Maybe by occasionally interrupting the optical path?

There's a bias feedback loop that looks after that. It doesn't have to
be that accurate since the PDs run at 14V of reverse bias--keeping the
junction of the two PDs reasonably still is all that's required.

The tweaks are for making sure that the two photocurrents are reasonably
close to begin with, and to govern the poorly specified efficiency of
the LEDs. (IR LEDs have output power specs that are almost as loose as
the V_T spec of your average JFET.)

You should be able to buy them in a couple of months, if all goes well.
(No home should be without one, after all.) ;)

Cheers

Phil Hobbs

Just for the heck of it, I asked Jonathan to measure the low-current
linearity of some visible and IR led's. I know that some LEDs can make
visible light at 1 nA, so it will be interesting to see if there is a
linearity knee somewhere. A red LED driving a silicon PIN diode makes
a visually perfect straight-line graph plotted linearly from 0 to 55
mA.

I theory, LED voltage is the log of current, so at some very low
current there won't be enough voltage across the junction to make a
photon of anywhere near the expected wavelength. It could be that
materials defects will kill things before that point.

I did some googling on LED behavior at low currents and found nothing
useful.

John

Please be so kind as to give a few makers and part numbers for
silicon PIN diodes usable that way.
Thanks.

Joerg wrote:

Academics have tried to do single-photon generation with LEDs. Pulsed
at very low current. IIRC one paper was from Syracuse, NY. But I don't
think you get access to this stuff directly on the web, probably needs
some paid access like IEEE Explore. Or good connections to a university.

..and HOW does one determine / detect a single photon? Tap into nerve of
a cat?