Forums

Optocoupler suggestions

Started by bitrex May 29, 2018
On 05/29/2018 11:08 PM, Tim Williams wrote:
> I don't know of any that are particularly well specified, except for the > dual-photodiode ones, which are still a bit sloppy (enough that you need > trimmers for a modestly precise analog coupler circuit). > > CTR isn't reliable due to emitter and detector nonlinearity, and emitter > aging, AFAIK.� The nonlinearity is modest over current (+/- 25%?), but > the aging is... something like max to min spec over lifetime (decades)? > Unsure.
Older ones fail because the cruddy filler goes opaque over time when you drive the LED too hard, which you usually have to do because of the poor CTR of even a phototransistor. IIRC that happens faster than LED degradation. Shining a bright LED on a reasonably-sized photodiode can get you 10% CTR and will easily go as fast as the LED can manage. (*) This isn't that fast, a few megahertz on a good day for a display LED. You can speed it up a bit by reverse biasing, but recovery isn't that fast even so. LEDs are much slower than diode lasers because they're limited by spontaneous recombination, whereas in a laser stimulated emission causes much faster depletion of the upper state when power is removed. UV LEDs are a lot faster than visible or IR ones, because the transition rate wants to go like 1/lambda**2 (Fermi's golden rule, for physics fans). High speed IR LEDs are specified at super high current densities, where high-level injection causes the carrier lifetime to drop by a lot--they're not nearly as fast at reasonable current densities. There are LEDs intended for optical communications that go as high as 1 GHz because their minority carrier lifetime is intentionally trashed. IIUC most of the additional recombination is nonradiative, which hurts their efficiency. Cheers Phil Hobbs (*) I did this with JL and Co. some years back in a nanoamp photoreceiver, which their marketing department called "a unique photon-coupled architecture." ;) It used photocurrent feedback in a TIA, which isn't usually worthwhile because you get 3 dB worse shot noise and worse capacitance. The new wrinkle in that one was using two photodiodes _in series_, with feedback applied to keep them from fighting each other. Somewhat amusingly, that gives you half the capacitance and _half the shot noise power_. The noise penalty from the photocurrent feedback was thus only 10 log(1.5) = 1.6 dB rather than 3 dB, so it really was shot noise limited. It was a pain to build, by all accounts, and didn't sell well, so they discontinued it. However, the QL01 introduced by HEO last year is cheaper and quieter anyway. :) -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
On Wed, 30 May 2018 11:49:51 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 05/29/2018 11:08 PM, Tim Williams wrote: >> I don't know of any that are particularly well specified, except for the >> dual-photodiode ones, which are still a bit sloppy (enough that you need >> trimmers for a modestly precise analog coupler circuit). >> >> CTR isn't reliable due to emitter and detector nonlinearity, and emitter >> aging, AFAIK.&#2013266080; The nonlinearity is modest over current (+/- 25%?), but >> the aging is... something like max to min spec over lifetime (decades)? >> Unsure. > >Older ones fail because the cruddy filler goes opaque over time when you >drive the LED too hard, which you usually have to do because of the poor >CTR of even a phototransistor. IIRC that happens faster than LED >degradation. > >Shining a bright LED on a reasonably-sized photodiode can get you 10% >CTR and will easily go as fast as the LED can manage. (*) This isn't >that fast, a few megahertz on a good day for a display LED. You can >speed it up a bit by reverse biasing, but recovery isn't that fast even so. > >LEDs are much slower than diode lasers because they're limited by >spontaneous recombination, whereas in a laser stimulated emission causes >much faster depletion of the upper state when power is removed. UV LEDs >are a lot faster than visible or IR ones, because the transition rate >wants to go like 1/lambda**2 (Fermi's golden rule, for physics fans).
I briefly played with some Cree white LEDs. I didn't use a super fast detector, but the combination seemed to be in the 7 ns ballpark, which surprised me because I'd assumed that the phosphor would be slow.
> >High speed IR LEDs are specified at super high current densities, where >high-level injection causes the carrier lifetime to drop by a >lot--they're not nearly as fast at reasonable current densities. > >There are LEDs intended for optical communications that go as high as 1 >GHz because their minority carrier lifetime is intentionally trashed. >IIUC most of the additional recombination is nonradiative, which hurts >their efficiency. > >Cheers > >Phil Hobbs > >(*) I did this with JL and Co. some years back in a nanoamp >photoreceiver, which their marketing department called "a unique >photon-coupled architecture." ;) It used photocurrent feedback in a >TIA, which isn't usually worthwhile because you get 3 dB worse shot >noise and worse capacitance.
That was a technical success but not successful commercially. The market seemed to be one-off researcher types, not volume users, and it's hard to make money off researchers. Let them buy PMTs. We did learn a lot, which is always useful. We don't, for some reason, seem to have luck selling free-space detector stuff. We do sell lots of fiber things and fast diode laser drivers. The ideal customer is an optics-heavy OEM founded by physicists that doesn't have a big internal EE staff. In an electronics-centric company, engineers are the superheroes. In a company founded by scientists, engineers are 3rd class citizens, and the best ones tend to move on. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On Wednesday, May 30, 2018 at 11:49:57 AM UTC-4, Phil Hobbs wrote:
> On 05/29/2018 11:08 PM, Tim Williams wrote: > > I don't know of any that are particularly well specified, except for the > > dual-photodiode ones, which are still a bit sloppy (enough that you need > > trimmers for a modestly precise analog coupler circuit). > > > > CTR isn't reliable due to emitter and detector nonlinearity, and emitter > > aging, AFAIK.&nbsp; The nonlinearity is modest over current (+/- 25%?), but > > the aging is... something like max to min spec over lifetime (decades)? > > Unsure. > > Older ones fail because the cruddy filler goes opaque over time when you > drive the LED too hard, which you usually have to do because of the poor > CTR of even a phototransistor. IIRC that happens faster than LED > degradation. > > Shining a bright LED on a reasonably-sized photodiode can get you 10% > CTR and will easily go as fast as the LED can manage. (*) This isn't > that fast, a few megahertz on a good day for a display LED. You can > speed it up a bit by reverse biasing, but recovery isn't that fast even so. > > LEDs are much slower than diode lasers because they're limited by > spontaneous recombination, whereas in a laser stimulated emission causes > much faster depletion of the upper state when power is removed. UV LEDs > are a lot faster than visible or IR ones, because the transition rate > wants to go like 1/lambda**2 (Fermi's golden rule, for physics fans).
huh, OK help me here. I thought spontaneous emission went as the frequency^3, 1/lamda^3 https://en.wikipedia.org/wiki/Spontaneous_emission#Rate_of_spontaneous_emission George H.
> > High speed IR LEDs are specified at super high current densities, where > high-level injection causes the carrier lifetime to drop by a > lot--they're not nearly as fast at reasonable current densities. > > There are LEDs intended for optical communications that go as high as 1 > GHz because their minority carrier lifetime is intentionally trashed. > IIUC most of the additional recombination is nonradiative, which hurts > their efficiency. > > Cheers > > Phil Hobbs > > (*) I did this with JL and Co. some years back in a nanoamp > photoreceiver, which their marketing department called "a unique > photon-coupled architecture." ;) It used photocurrent feedback in a > TIA, which isn't usually worthwhile because you get 3 dB worse shot > noise and worse capacitance. > > The new wrinkle in that one was using two photodiodes _in series_, with > feedback applied to keep them from fighting each other. Somewhat > amusingly, that gives you half the capacitance and _half the shot noise > power_. The noise penalty from the photocurrent feedback was thus only > 10 log(1.5) = 1.6 dB rather than 3 dB, so it really was shot noise > limited. It was a pain to build, by all accounts, and didn't sell well, > so they discontinued it. However, the QL01 introduced by HEO last year > is cheaper and quieter anyway. :) > > -- > Dr Philip C D Hobbs > Principal Consultant > ElectroOptical Innovations LLC > Optics, Electro-optics, Photonics, Analog Electronics > > 160 North State Road #203 > Briarcliff Manor NY 10510 > > hobbs at electrooptical dot net > http://electrooptical.net
On Wednesday, May 30, 2018 at 12:21:01 PM UTC-4, John Larkin wrote:
> On Wed, 30 May 2018 11:49:51 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > > >On 05/29/2018 11:08 PM, Tim Williams wrote: > >> I don't know of any that are particularly well specified, except for the > >> dual-photodiode ones, which are still a bit sloppy (enough that you need > >> trimmers for a modestly precise analog coupler circuit). > >> > >> CTR isn't reliable due to emitter and detector nonlinearity, and emitter > >> aging, AFAIK.&nbsp; The nonlinearity is modest over current (+/- 25%?), but > >> the aging is... something like max to min spec over lifetime (decades)? > >> Unsure. > > > >Older ones fail because the cruddy filler goes opaque over time when you > >drive the LED too hard, which you usually have to do because of the poor > >CTR of even a phototransistor. IIRC that happens faster than LED > >degradation. > > > >Shining a bright LED on a reasonably-sized photodiode can get you 10% > >CTR and will easily go as fast as the LED can manage. (*) This isn't > >that fast, a few megahertz on a good day for a display LED. You can > >speed it up a bit by reverse biasing, but recovery isn't that fast even so. > > > >LEDs are much slower than diode lasers because they're limited by > >spontaneous recombination, whereas in a laser stimulated emission causes > >much faster depletion of the upper state when power is removed. UV LEDs > >are a lot faster than visible or IR ones, because the transition rate > >wants to go like 1/lambda**2 (Fermi's golden rule, for physics fans). > > I briefly played with some Cree white LEDs. I didn't use a super fast > detector, but the combination seemed to be in the 7 ns ballpark, which > surprised me because I'd assumed that the phosphor would be slow. > > > > > >High speed IR LEDs are specified at super high current densities, where > >high-level injection causes the carrier lifetime to drop by a > >lot--they're not nearly as fast at reasonable current densities. > > > >There are LEDs intended for optical communications that go as high as 1 > >GHz because their minority carrier lifetime is intentionally trashed. > >IIUC most of the additional recombination is nonradiative, which hurts > >their efficiency. > > > >Cheers > > > >Phil Hobbs > > > >(*) I did this with JL and Co. some years back in a nanoamp > >photoreceiver, which their marketing department called "a unique > >photon-coupled architecture." ;) It used photocurrent feedback in a > >TIA, which isn't usually worthwhile because you get 3 dB worse shot > >noise and worse capacitance. > > That was a technical success but not successful commercially. The > market seemed to be one-off researcher types, not volume users, and > it's hard to make money off researchers. Let them buy PMTs. We did > learn a lot, which is always useful.
Maybe Thor labs could resell/market it for you? George H.
> > We don't, for some reason, seem to have luck selling free-space > detector stuff. We do sell lots of fiber things and fast diode laser > drivers. The ideal customer is an optics-heavy OEM founded by > physicists that doesn't have a big internal EE staff. > > In an electronics-centric company, engineers are the superheroes. In a > company founded by scientists, engineers are 3rd class citizens, and > the best ones tend to move on. > > > -- > > John Larkin Highland Technology, Inc > > lunatic fringe electronics
On 05/30/2018 12:20 PM, John Larkin wrote:
> On Wed, 30 May 2018 11:49:51 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 05/29/2018 11:08 PM, Tim Williams wrote: >>> I don't know of any that are particularly well specified, except for the >>> dual-photodiode ones, which are still a bit sloppy (enough that you need >>> trimmers for a modestly precise analog coupler circuit). >>> >>> CTR isn't reliable due to emitter and detector nonlinearity, and emitter >>> aging, AFAIK.&nbsp; The nonlinearity is modest over current (+/- 25%?), but >>> the aging is... something like max to min spec over lifetime (decades)? >>> Unsure. >> >> Older ones fail because the cruddy filler goes opaque over time when you >> drive the LED too hard, which you usually have to do because of the poor >> CTR of even a phototransistor. IIRC that happens faster than LED >> degradation. >> >> Shining a bright LED on a reasonably-sized photodiode can get you 10% >> CTR and will easily go as fast as the LED can manage. (*) This isn't >> that fast, a few megahertz on a good day for a display LED. You can >> speed it up a bit by reverse biasing, but recovery isn't that fast even so. >> >> LEDs are much slower than diode lasers because they're limited by >> spontaneous recombination, whereas in a laser stimulated emission causes >> much faster depletion of the upper state when power is removed. UV LEDs >> are a lot faster than visible or IR ones, because the transition rate >> wants to go like 1/lambda**2 (Fermi's golden rule, for physics fans). > > I briefly played with some Cree white LEDs. I didn't use a super fast > detector, but the combination seemed to be in the 7 ns ballpark, which > surprised me because I'd assumed that the phosphor would be slow.
White LEDs are actually pretty fast on account of the blue LED and the inorganic fluor, which I think is some salt of europium. Wideband fluors are pretty fast in general. <snip>
>> >> (*) I did this with JL and Co. some years back in a nanoamp >> photoreceiver, which their marketing department called "a unique >> photon-coupled architecture." ;) It used photocurrent feedback in a >> TIA, which isn't usually worthwhile because you get 3 dB worse shot >> noise and worse capacitance. > > That was a technical success but not successful commercially. The > market seemed to be one-off researcher types, not volume users, and > it's hard to make money off researchers. Let them buy PMTs. We did > learn a lot, which is always useful. > > We don't, for some reason, seem to have luck selling free-space > detector stuff. We do sell lots of fiber things and fast diode laser > drivers. The ideal customer is an optics-heavy OEM founded by > physicists that doesn't have a big internal EE staff. > > In an electronics-centric company, engineers are the superheroes. In a > company founded by scientists, engineers are 3rd class citizens, and > the best ones tend to move on.
Well round here all we have are physicists. ;) We're in the process of licensing a customized APD version of the QL01 to a large biomed company, assuming that nothing goes south before the contract is signed. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
On 05/30/2018 02:30 PM, George Herold wrote:
> On Wednesday, May 30, 2018 at 11:49:57 AM UTC-4, Phil Hobbs wrote:
>> LEDs are much slower than diode lasers because they're limited by >> spontaneous recombination, whereas in a laser stimulated emission causes >> much faster depletion of the upper state when power is removed. UV LEDs >> are a lot faster than visible or IR ones, because the transition rate >> wants to go like 1/lambda**2 (Fermi's golden rule, for physics fans). > > huh, OK help me here. I thought spontaneous emission went as the > frequency^3, 1/lamda^3 > https://en.wikipedia.org/wiki/Spontaneous_emission#Rate_of_spontaneous_emission
Hmm, I seem to be missing a factor of 1/lambda, but I don't know where it comes from. I was thinking of Fermi's golden rule, which says that the transition rate is Gamma = |<initial|H'|final>|**2 rho/h where H' is the perturbation Hamiltonian and rho is the density of states. The matrix element is an energy, and so proportional to 1/lambda, which is why I was thinking 1/lambda**2. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
On Wed, 30 May 2018 12:59:14 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>On Wednesday, May 30, 2018 at 12:21:01 PM UTC-4, John Larkin wrote: >> On Wed, 30 May 2018 11:49:51 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >> >On 05/29/2018 11:08 PM, Tim Williams wrote: >> >> I don't know of any that are particularly well specified, except for the >> >> dual-photodiode ones, which are still a bit sloppy (enough that you need >> >> trimmers for a modestly precise analog coupler circuit). >> >> >> >> CTR isn't reliable due to emitter and detector nonlinearity, and emitter >> >> aging, AFAIK.&#2013266080; The nonlinearity is modest over current (+/- 25%?), but >> >> the aging is... something like max to min spec over lifetime (decades)? >> >> Unsure. >> > >> >Older ones fail because the cruddy filler goes opaque over time when you >> >drive the LED too hard, which you usually have to do because of the poor >> >CTR of even a phototransistor. IIRC that happens faster than LED >> >degradation. >> > >> >Shining a bright LED on a reasonably-sized photodiode can get you 10% >> >CTR and will easily go as fast as the LED can manage. (*) This isn't >> >that fast, a few megahertz on a good day for a display LED. You can >> >speed it up a bit by reverse biasing, but recovery isn't that fast even so. >> > >> >LEDs are much slower than diode lasers because they're limited by >> >spontaneous recombination, whereas in a laser stimulated emission causes >> >much faster depletion of the upper state when power is removed. UV LEDs >> >are a lot faster than visible or IR ones, because the transition rate >> >wants to go like 1/lambda**2 (Fermi's golden rule, for physics fans). >> >> I briefly played with some Cree white LEDs. I didn't use a super fast >> detector, but the combination seemed to be in the 7 ns ballpark, which >> surprised me because I'd assumed that the phosphor would be slow. >> >> >> > >> >High speed IR LEDs are specified at super high current densities, where >> >high-level injection causes the carrier lifetime to drop by a >> >lot--they're not nearly as fast at reasonable current densities. >> > >> >There are LEDs intended for optical communications that go as high as 1 >> >GHz because their minority carrier lifetime is intentionally trashed. >> >IIUC most of the additional recombination is nonradiative, which hurts >> >their efficiency. >> > >> >Cheers >> > >> >Phil Hobbs >> > >> >(*) I did this with JL and Co. some years back in a nanoamp >> >photoreceiver, which their marketing department called "a unique >> >photon-coupled architecture." ;) It used photocurrent feedback in a >> >TIA, which isn't usually worthwhile because you get 3 dB worse shot >> >noise and worse capacitance. >> >> That was a technical success but not successful commercially. The >> market seemed to be one-off researcher types, not volume users, and >> it's hard to make money off researchers. Let them buy PMTs. We did >> learn a lot, which is always useful. >Maybe Thor labs could resell/market it for you?
Sounds like Phil has a better and cheaper design now. I'll leave the free-space stuff to him. We did have a product in the Thorlabs catalog once. They want a huge markup. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Wednesday, May 30, 2018 at 5:14:01 PM UTC-4, Phil Hobbs wrote:
> On 05/30/2018 02:30 PM, George Herold wrote: > > On Wednesday, May 30, 2018 at 11:49:57 AM UTC-4, Phil Hobbs wrote: > > >> LEDs are much slower than diode lasers because they're limited by > >> spontaneous recombination, whereas in a laser stimulated emission causes > >> much faster depletion of the upper state when power is removed. UV LEDs > >> are a lot faster than visible or IR ones, because the transition rate > >> wants to go like 1/lambda**2 (Fermi's golden rule, for physics fans). > > > > huh, OK help me here. I thought spontaneous emission went as the > > frequency^3, 1/lamda^3 > > https://en.wikipedia.org/wiki/Spontaneous_emission#Rate_of_spontaneous_emission > > Hmm, I seem to be missing a factor of 1/lambda, but I don't know where > it comes from. > > I was thinking of Fermi's golden rule, which says that the transition > rate is > > Gamma = |<initial|H'|final>|**2 rho/h > > where H' is the perturbation Hamiltonian and rho is the density of > states. The matrix element is an energy, and so proportional to > 1/lambda, which is why I was thinking 1/lambda**2. > > Cheers > > Phil Hobbs >
OK good, I looked at Fermi's golden rule, and I thought fine. The square of the matrix element, which is some energy, so energy squared... But then I remembered this nmr talk which started by reminding us of the cube dependence.. which if you scale down to ~10 MHz nmr frequencies... is like forever, proton magnetic moments (and such) in low magnetic fields hardly ever do a spontaneous emission, it's all the environment. T1 in nmr speak. George H.
> -- > Dr Philip C D Hobbs > Principal Consultant > ElectroOptical Innovations LLC > Optics, Electro-optics, Photonics, Analog Electronics > > 160 North State Road #203 > Briarcliff Manor NY 10510 > > hobbs at electrooptical dot net > http://electrooptical.net
On Wednesday, May 30, 2018 at 5:22:13 PM UTC-4, John Larkin wrote:
> On Wed, 30 May 2018 12:59:14 -0700 (PDT), George Herold > <gherold@teachspin.com> wrote: > > >On Wednesday, May 30, 2018 at 12:21:01 PM UTC-4, John Larkin wrote: > >> On Wed, 30 May 2018 11:49:51 -0400, Phil Hobbs > >> <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> > >> >On 05/29/2018 11:08 PM, Tim Williams wrote: > >> >> I don't know of any that are particularly well specified, except for the > >> >> dual-photodiode ones, which are still a bit sloppy (enough that you need > >> >> trimmers for a modestly precise analog coupler circuit). > >> >> > >> >> CTR isn't reliable due to emitter and detector nonlinearity, and emitter > >> >> aging, AFAIK.&nbsp; The nonlinearity is modest over current (+/- 25%?), but > >> >> the aging is... something like max to min spec over lifetime (decades)? > >> >> Unsure. > >> > > >> >Older ones fail because the cruddy filler goes opaque over time when you > >> >drive the LED too hard, which you usually have to do because of the poor > >> >CTR of even a phototransistor. IIRC that happens faster than LED > >> >degradation. > >> > > >> >Shining a bright LED on a reasonably-sized photodiode can get you 10% > >> >CTR and will easily go as fast as the LED can manage. (*) This isn't > >> >that fast, a few megahertz on a good day for a display LED. You can > >> >speed it up a bit by reverse biasing, but recovery isn't that fast even so. > >> > > >> >LEDs are much slower than diode lasers because they're limited by > >> >spontaneous recombination, whereas in a laser stimulated emission causes > >> >much faster depletion of the upper state when power is removed. UV LEDs > >> >are a lot faster than visible or IR ones, because the transition rate > >> >wants to go like 1/lambda**2 (Fermi's golden rule, for physics fans). > >> > >> I briefly played with some Cree white LEDs. I didn't use a super fast > >> detector, but the combination seemed to be in the 7 ns ballpark, which > >> surprised me because I'd assumed that the phosphor would be slow. > >> > >> > >> > > >> >High speed IR LEDs are specified at super high current densities, where > >> >high-level injection causes the carrier lifetime to drop by a > >> >lot--they're not nearly as fast at reasonable current densities. > >> > > >> >There are LEDs intended for optical communications that go as high as 1 > >> >GHz because their minority carrier lifetime is intentionally trashed. > >> >IIUC most of the additional recombination is nonradiative, which hurts > >> >their efficiency. > >> > > >> >Cheers > >> > > >> >Phil Hobbs > >> > > >> >(*) I did this with JL and Co. some years back in a nanoamp > >> >photoreceiver, which their marketing department called "a unique > >> >photon-coupled architecture." ;) It used photocurrent feedback in a > >> >TIA, which isn't usually worthwhile because you get 3 dB worse shot > >> >noise and worse capacitance. > >> > >> That was a technical success but not successful commercially. The > >> market seemed to be one-off researcher types, not volume users, and > >> it's hard to make money off researchers. Let them buy PMTs. We did > >> learn a lot, which is always useful. > >Maybe Thor labs could resell/market it for you? > > Sounds like Phil has a better and cheaper design now. I'll leave the > free-space stuff to him. > > We did have a product in the Thorlabs catalog once. They want a huge > markup.
Factor of 2-3? TBH I look at Thor labs and then figure I can find someone to sell me the gizmo for 1/2 to 1/3.. (if I buy a bunch) at least that's my limited experience. I think I'm soon going to need a 405 nm laser diode, last time I looked Thor labs wants ~$100 for one, so $30-40 from sanyo, maybe less? George H.
> > > > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
On Wednesday, May 30, 2018 at 8:20:48 PM UTC-4, George Herold wrote:
> On Wednesday, May 30, 2018 at 5:22:13 PM UTC-4, John Larkin wrote: > > On Wed, 30 May 2018 12:59:14 -0700 (PDT), George Herold > > <gherold@teachspin.com> wrote: > > > > >On Wednesday, May 30, 2018 at 12:21:01 PM UTC-4, John Larkin wrote: > > >> On Wed, 30 May 2018 11:49:51 -0400, Phil Hobbs > > >> <pcdhSpamMeSenseless@electrooptical.net> wrote: > > >> > > >> >On 05/29/2018 11:08 PM, Tim Williams wrote: > > >> >> I don't know of any that are particularly well specified, except for the > > >> >> dual-photodiode ones, which are still a bit sloppy (enough that you need > > >> >> trimmers for a modestly precise analog coupler circuit). > > >> >> > > >> >> CTR isn't reliable due to emitter and detector nonlinearity, and emitter > > >> >> aging, AFAIK.&nbsp; The nonlinearity is modest over current (+/- 25%?), but > > >> >> the aging is... something like max to min spec over lifetime (decades)? > > >> >> Unsure. > > >> > > > >> >Older ones fail because the cruddy filler goes opaque over time when you > > >> >drive the LED too hard, which you usually have to do because of the poor > > >> >CTR of even a phototransistor. IIRC that happens faster than LED > > >> >degradation. > > >> > > > >> >Shining a bright LED on a reasonably-sized photodiode can get you 10% > > >> >CTR and will easily go as fast as the LED can manage. (*) This isn't > > >> >that fast, a few megahertz on a good day for a display LED. You can > > >> >speed it up a bit by reverse biasing, but recovery isn't that fast even so. > > >> > > > >> >LEDs are much slower than diode lasers because they're limited by > > >> >spontaneous recombination, whereas in a laser stimulated emission causes > > >> >much faster depletion of the upper state when power is removed. UV LEDs > > >> >are a lot faster than visible or IR ones, because the transition rate > > >> >wants to go like 1/lambda**2 (Fermi's golden rule, for physics fans). > > >> > > >> I briefly played with some Cree white LEDs. I didn't use a super fast > > >> detector, but the combination seemed to be in the 7 ns ballpark, which > > >> surprised me because I'd assumed that the phosphor would be slow. > > >> > > >> > > >> > > > >> >High speed IR LEDs are specified at super high current densities, where > > >> >high-level injection causes the carrier lifetime to drop by a > > >> >lot--they're not nearly as fast at reasonable current densities. > > >> > > > >> >There are LEDs intended for optical communications that go as high as 1 > > >> >GHz because their minority carrier lifetime is intentionally trashed. > > >> >IIUC most of the additional recombination is nonradiative, which hurts > > >> >their efficiency. > > >> > > > >> >Cheers > > >> > > > >> >Phil Hobbs > > >> > > > >> >(*) I did this with JL and Co. some years back in a nanoamp > > >> >photoreceiver, which their marketing department called "a unique > > >> >photon-coupled architecture." ;) It used photocurrent feedback in a > > >> >TIA, which isn't usually worthwhile because you get 3 dB worse shot > > >> >noise and worse capacitance. > > >> > > >> That was a technical success but not successful commercially. The > > >> market seemed to be one-off researcher types, not volume users, and > > >> it's hard to make money off researchers. Let them buy PMTs. We did > > >> learn a lot, which is always useful. > > >Maybe Thor labs could resell/market it for you? > > > > Sounds like Phil has a better and cheaper design now. I'll leave the > > free-space stuff to him. > > > > We did have a product in the Thorlabs catalog once. They want a huge > > markup. > Factor of 2-3? TBH I look at Thor labs and then figure I can find someone > to sell me the gizmo for 1/2 to 1/3.. (if I buy a bunch) > at least that's my limited experience. I think I'm soon > going to need a 405 nm laser diode, last time I looked > Thor labs wants ~$100 for one, so $30-40 from sanyo, > maybe less? > > George H.
Oh we buy mirror mounts and other optical stuff from Thor labs by the bucket full.... GH
> > > > > > > > > > -- > > > > John Larkin Highland Technology, Inc > > picosecond timing precision measurement > > > > jlarkin att highlandtechnology dott com > > http://www.highlandtechnology.com