Fast, efficient IR LEDs?
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From: Joerg on 8 Feb 2010 21:45 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/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Robert Baer on 9 Feb 2010 01:59 John Larkin wrote: > On Sun, 07 Feb 2010 17:27:11 -0500, Phil Hobbs > <pcdhSpamMeSenseless(a)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.
From: Robert Baer on 9 Feb 2010 02:01 Joerg wrote: > John Larkin wrote: >> On Sun, 07 Feb 2010 17:27:11 -0500, Phil Hobbs >> <pcdhSpamMeSenseless(a)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?
From: Phil Hobbs on 9 Feb 2010 07:35 On 2/8/2010 8:52 PM, Jim Thompson wrote: > On Mon, 08 Feb 2010 20:27:22 -0500, Phil Hobbs > <pcdhSpamMeSenseless(a)electrooptical.net> wrote: > > [snip] >> >> 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. >> >> Cheers >> >> Phil Hobbs > > Where do you get that book? Searching on "Silvio Donati" > "photodetector" seems to scramble google's brains ;-) > > ...Jim Thompson *Silvano* Donati, my bad. http://www.amazon.com/exec/obidos/asin/0130203378 Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: Jim Thompson on 9 Feb 2010 10:07
On Tue, 09 Feb 2010 07:35:13 -0500, Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >On 2/8/2010 8:52 PM, Jim Thompson wrote: >> On Mon, 08 Feb 2010 20:27:22 -0500, Phil Hobbs >> <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >> >> [snip] >>> >>> 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. >>> >>> Cheers >>> >>> Phil Hobbs >> >> Where do you get that book? Searching on "Silvio Donati" >> "photodetector" seems to scramble google's brains ;-) >> >> ...Jim Thompson > >*Silvano* Donati, my bad. > >http://www.amazon.com/exec/obidos/asin/0130203378 > >Cheers > >Phil Hobbs Thanks! ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food. |