Re: Fast, efficient IR LEDs?

John Larkin wrote:
> On Sun, 07 Feb 2010 17:27:11 -0500, Phil Hobbs
> <(E-Mail Removed)> 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.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

"John Larkin" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> 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.

Well, everything is radiating, although at 26meV and no bias, there's damned
little all the way out at 2eV. You're looking at the tail end of two
decades of exponent there.

There's no reason why, for instance, you can't get 2.000eV photons from a
semiconductor with 1.984V across it. You can electrolyze water the same
way -- it does proceed below the reaction voltage, it's just endothermic and
slow as hell. You don't get something for nothing, so there's still current
draw, with an electron plopping through the bandgap for every photon
radiated, it's just falling through a slightly lower voltage.

Question for Phil: does the process of photon production cause a blip in the
diode voltage? This should be detectable as shot noise on the diode's
terminal voltage -and- on a photodiode directly in front of the LED, and
there should be perfect correlation between the two effects (minus quantum
efficiency, so maybe you'll only detect 1 in 5 events at the photodiode).
Is this measurable? I think it should be.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Tim Williams wrote:
> "John Larkin" <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed)...
>> 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.
>
> Well, everything is radiating, although at 26meV and no bias, there's damned
> little all the way out at 2eV. You're looking at the tail end of two
> decades of exponent there.
>
> There's no reason why, for instance, you can't get 2.000eV photons from a
> semiconductor with 1.984V across it. ...


If the Fed Reserve finds out they'll want the tax that's owed on those
16mV :-)


> ... You can electrolyze water the same
> way -- it does proceed below the reaction voltage, it's just endothermic and
> slow as hell. You don't get something for nothing, so there's still current
> draw, with an electron plopping through the bandgap for every photon
> radiated, it's just falling through a slightly lower voltage.
>
> Question for Phil: does the process of photon production cause a blip in the
> diode voltage? This should be detectable as shot noise on the diode's
> terminal voltage -and- on a photodiode directly in front of the LED, and
> there should be perfect correlation between the two effects (minus quantum
> efficiency, so maybe you'll only detect 1 in 5 events at the photodiode).
> Is this measurable? I think it should be.
>

Don't know but generating single photons is done differently, or at
least will be some day:

http://www.toshiba-europe.com/resear...photonled.html

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

Phil Hobbs wrote:
> On 2/8/2010 12:14 PM, Joerg wrote:
>> Phil Hobbs wrote:
>>> On 2/7/2010 5:10 PM, Joerg wrote:
>>>> Phil Hobbs wrote:

[...]

>>>>> 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.
>>>
>>
>> Good, so it seems automatic. 14V sound like a white-knuckle ride :-)
>
> Not at all. Most Si photodiodes are good to 30-60V. With many (e.g.
> the BPW34) the capacitance stops decreasing at 10V or so, but they
> continue to speed up with increasing bias, because the series resistance
> keeps going down. That's because it's dominated by the (very thin)
> diffusion zone, so cranking up the bias until they're really really
> fully depleted makes the speed go up amazingly. Silvio Donati's book on
> photodetectors is an excellent read for this sort of stuff.
>

Ok, yes, the big ones can do that. I was thinking about the ones I used,
mostly from Japan, fast ones with low capacitance. Their abs max is
between 5V and 15V. Not sure by how much you could exceed that before
going *phut* but I couldn't allow that kind of system to be ECO'd in
that mode anyhow.

--
Regards, Joerg

http://www.analogconsultants.com/

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Use another domain or send PM.

Robert Baer wrote:
> Joerg wrote:
>> John Larkin wrote:
>>> On Sun, 07 Feb 2010 17:27:11 -0500, Phil Hobbs
>>> <(E-Mail Removed)> 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?


No idea. But a single photon should require x amount of energy, two
photon 2x and so on. Now if you have a good handle on the energy
efficiency of your device you could put enough current and duration in
there so the net comes to 1.5x. No idea if that works, just thinking out
loud.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

Robert Baer wrote:
> 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?

First generation image photon counting systems date back to the early
80's and used an image intensifier and a centroid measurement of the
spot. They are still in use for certain type of astronomy. eg.

http://www.ing.iac.es/PR/wht_info/ipcs.html

Modern ones have lower noise, higher count rates, better linearity etc.

Similar methods are used to count individual ions in mass spectrometry.

Regards,
Martin Brown