Fast, efficient IR LEDs?
in [Design]
|
Prev: PIC18 Timer 2 weirdness
Next: 100GHz transistors
From: John Larkin on 8 Feb 2010 16:55 On Mon, 8 Feb 2010 15:33:52 -0600, "Tim Williams" <tmoranwms(a)charter.net> wrote: >"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message >news:iet0n5pn6qdm16unmbo0fucgv48k4hossh(a)4ax.com... >> 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 It should be. Macroscopically, voltage noise across the junction must modulate intensity. I don't know if you could observe this at the single photon level. John
From: Phil Hobbs on 8 Feb 2010 19:47 On 2/7/2010 5:17 PM, 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 ;-) > > ...Jim Thompson Normally of course you can't put current sources in series, but by applying optical feedback you can make series-connected photodiodes work. Since the diodes have essentially infinite impedance, the noise of each one splits in half, with half going through each capacitor. 0 +Vbias | | | *------* | | | | | | --- --- / \ C --- --- | | | | | | *------* | | | | | | --- --- / \ C --- --- | | | | | | *------* | | 0 To TIA The shot noise current from each diode divides by the ratio of the conductances of the two branches. Thus goes through the other diode's capacitance, and so into the external circuit, but half just circulates round through its own capacitance and hence doesn't contribute to the output noise. The shot noise currents from the two diodes are uncorrelated, and so the RMS noise current arriving in the external circuit is / |2eI_dc| |2eI_dc| \ i_N = sqrt| |------| + |------| | = sqrt(eI_dc) \ | 2 | | 2 | / which is 3 dB below the shot noise of a primary photocurrent of I_dc. That's the same SNR you'd get by parallelling the two, but (crucially) you don't double the capacitance or the photocurrent by doing so. That makes it a good trick for low photocurrents, though not one you'd use every day. 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 8 Feb 2010 19:56 On Mon, 08 Feb 2010 19:47:47 -0500, Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >On 2/7/2010 5:17 PM, 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 ;-) >> >> ...Jim Thompson > >Normally of course you can't put current sources in series, but by >applying optical feedback you can make series-connected photodiodes work. > >Since the diodes have essentially infinite impedance, the noise of each >one splits in half, with half going through each capacitor. > > 0 +Vbias > | > | > | > *------* > | | > | | > | | > --- --- > / \ C > --- --- > | | > | | > | | > *------* > | | > | | > | | > --- --- > / \ C > --- --- > | | > | | > | | > *------* > | > | > 0 To TIA > >The shot noise current from each diode divides by the ratio of the >conductances of the two branches. Thus goes through the other diode's >capacitance, and so into the external circuit, but half just circulates >round through its own capacitance and hence doesn't contribute to the >output noise. > >The shot noise currents from the two diodes are uncorrelated, and so the >RMS noise current arriving in the external circuit is > > > / |2eI_dc| |2eI_dc| \ >i_N = sqrt| |------| + |------| | = sqrt(eI_dc) > \ | 2 | | 2 | / > >which is 3 dB below the shot noise of a primary photocurrent of I_dc. >That's the same SNR you'd get by parallelling the two, but (crucially) >you don't double the capacitance or the photocurrent by doing so. > >That makes it a good trick for low photocurrents, though not one you'd >use every day. > >Cheers > >Phil Hobbs OK. I read you! 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.
From: Phil Hobbs on 8 Feb 2010 20:27 On 2/8/2010 12:14 PM, Joerg wrote: > 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 :-) 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 -- 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 8 Feb 2010 20:52
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 -- | 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. |