Fast, efficient IR LEDs?
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From: Phil Hobbs on 7 Feb 2010 17:27 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 -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs at electrooptical dot net http://electrooptical.net
From: JosephKK on 8 Feb 2010 01:08 On Sun, 07 Feb 2010 12:56:22 -0500, Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >On 2/7/2010 12:29 PM, JosephKK wrote: >> On Sat, 06 Feb 2010 15:51:29 -0500, Phil Hobbs<pcdhSpamMeSenseless(a)electrooptical.net> wrote: >> >>> On 2/6/2010 3:13 PM, Joerg wrote: >>>> Phil Hobbs wrote: >>>>> On 2/6/2010 12:41 PM, Joerg wrote: >>>>>> miso(a)sushi.com wrote: >>>>>>> On Feb 5, 2:20 pm, Joerg<inva...(a)invalid.invalid> wrote: >>>>>>>> Phil Hobbs wrote: >>>>>>>>> On 2/5/2010 2:13 PM, m...(a)sushi.com wrote: >>>>>>>>>> On Feb 5, 8:56 am, Phil >>>>>>>>>> Hobbs<pcdhSpamMeSensel...(a)electrooptical.net> >>>>>>>>>> wrote: >>>>>>>>>>> I need a fast IR LED (> 20 MHz,< 50 pF) for an optical feedback >>>>>>>>>>> gizmo. >>>>>>>>>>> I have some Stanley DN310s, but they've been discontinued. Other >>>>>>>>>>> possibilities are: >>>>>>>>>>> Vishay TSFF5410 -- 870 nm, 0.% W/A typ 15 ns rise/fall, 125 pF typ >>>>>>>>>>> Vishay VSLB3940 -- 940 nm, 0.4 W/A typ 15 ns rise/fall, 70 pF typ >>>>>>>>>>> Panasonic LNA4905L -- 880 nm, 0.3 W/A min 30 MHz typ, no other >>>>>>>>>>> specs >>>>>>>>>>> Osram SFH4550 -- 850 nm, 0.5 W/A typ 12 ns rise/fall, no C spec >>>>>>>>>>> It would be really nice to find something with a flat front >>>>>>>>>>> facet and >>>>>>>>>>> (especially) lower capacitance, because it has to work at quite low >>>>>>>>>>> currents (5-10 uA). >>>>>>>>>>> Any suggestions? >>>>>>>>>>> Thanks >>>>>>>>>>> Phil Hobbs >>>>>>>>>> Isn't low current and high speed mutually exclusive, like >>>>>>>>>> intelligence >>>>>>>>>> and Republicans? >>>>>>>>> Nice try, Soup. ;) I enjoy talking electronics too much though. >>>>>>>> His "email address" always makes me crave miso soup and sushi. So, we >>>>>>>> just went to the Japanese restaurant in town ... >>>>>>>> >>>>>>>> But this time I had pork teriyaki after the miso soup. We usually have >>>>>>>> sushi in the evenings, I can't really work after that, it's so good >>>>>>>> that >>>>>>>> I tend to eat too much of it. >>>>>>>> >>>>>>>> -- >>>>>>>> Regards, Joerg >>>>>>>> >>>>>>>> http://www.analogconsultants.com/ >>>>>>>> >>>>>>>> "gmail" domain blocked because of excessive spam. >>>>>>>> Use another domain or send PM. >>>>>>> >>>>>>> Sushi doesn't make for an evening meal. Just too light, even when >>>>>>> supplemented with miso soup. It's better for lunch. >>>>>>> >>>>>> >>>>>> 4-5 rolls (servings) with 4 people and I am really stuffed afterwards. >>>>>> But it's delicious. >>>>>> >>>>>> >>>>>>> Getting back to electronics., this is really an i=c*dv/dt issue. It >>>>>>> seems to me all you can do is reduce C if current is limited. >>>>>> >>>>>> >>>>>> If Phil has to switch constantly, yes. Otherwise there could be a >>>>>> "spiking circuit" that swings the C with gusto. >>>>>> >>>>> >>>>> It's inside a feedback loop that needs to have>1 MHz bandwidth, so >>>>> it's more a loop stability vs bandwidth issue. There are alternatives, >>>>> but they're all more complicated than what we've got. I'd probably >>>>> prefer to trade off efficiency rather than speed and capacitance. >>>>> >>>> >>>> Then you'll probably be in the market for the good stuff. This is an >>>> example of one I've used in an optical feedback situation. I only needed >>>> a little under 100MHz BW but it could have given me a lot more: >>>> >>>> http://www.excelight.com/pdf/device/PD/SPT2400-x(revC).pdf >>>> >>> >>> Hopefully not--that's megahertz, not gigahertz. But I do want to be >>> able to work down at 500 nA or 1 uA. >>> >>> Cheers >>> >>> Phil Hobbs >> > >> 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. > > >Cheers > >Phil Hobbs Perhaps accurately controllable optical attenuation can be done? Don't know myself but it sounds possible to me.
From: Joerg on 8 Feb 2010 12:05 Jim Thompson wrote: > On Sun, 07 Feb 2010 14:10:48 -0800, Joerg <invalid(a)invalid.invalid> > 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. >> >> >>> 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? > > Sounds fascinating! More info please ;-) > I'd have to think about Phil's circuit but would need more info for that (and time ...). My case with the single diode, not at liberty to say. But we built dozens and they all cal'd automagically. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joerg on 8 Feb 2010 12:14 Phil Hobbs 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. > Just don't wait until T minus 360 days :-) >> >>> 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. > Good, so it seems automatic. 14V sound like a white-knuckle ride :-) > 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.) ;) > Hehe, that's what I used to say as well. No home without a fully certified cardiology ultrasound scanner. And another one for ob/gyn if the freshly married couple is inclined to ... -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joerg on 8 Feb 2010 12:15
JosephKK wrote: > On Sun, 07 Feb 2010 12:56:22 -0500, Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> 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. >> >> >> Cheers >> >> Phil Hobbs > > Perhaps accurately controllable optical attenuation can be done? > Don't know myself but it sounds possible to me. It can, but it's expensive. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |