LED on Photodiode step response

On 12 Apr., 01:04, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Mon, 11 Apr 2011 12:46:16 -0700 (PDT), George Herold
>
>
>
> <gher...@teachspin.com> wrote:
> >On Apr 11, 3:32 pm, Phil Hobbs
> ><pcdhSpamMeSensel...@electrooptical.net> wrote:
> >> George Herold wrote:
> >> > I looking at the step response of a photodiode (PD). I m stepping
> >> > the current into a LED such that the step doubles the photo current
> >> > from the PD. (I m not starting with the LED off, but on slightly.)
> >> > Here s a scope shot. The bottom trace in the PD response. (The top
> >> > is the voltage step to the LED (through 100 ohms)... AC coupled to g=
et
> >> > rid of a big DC offset.)
>
> >> >http://img189.imageshack.us/i/tek0024.png/
>
> >> > There is this long tail on the step response. Here s the same pictur=
e
> >> > with the timebase slowed down.
>
> >> >http://img812.imageshack.us/i/tek0025.png/
>
> >> > Looks something like a 2-2.5 us tail on the step.
>
> >> > What s it caused by? I first thought it was perhaps heating of the
> >> > LED during the step. But I was reading Photodetectors by S. Donati,
> >> > over the weekend. In Chapter 5 he talks about two sets of photo-
> >> > generated carriers the majority are made in the depletion layer, and
> >> > travel with the drift speed, (times in the nano-second range.) The
> >> > second smaller set are those generated in the doped region. If they
> >> > are within a diffusion length of the depletion region then these hav=
e
> >> > a chance of adding to the photocurrent. Donati says that these have =
a
> >> > time constant in the microsecond range! I wonder if this is what I a=
m
> >> > seeing?
>
> >> > I looked at the step response with a different color LED. (you expec=
t
> >> > more absorption in the doped region with shorter wavelength light.)
> >> > So here is the pulse response from a 635 nm red and 594 nm Amber
> >> > LED.
>
> >> >http://img52.imageshack.us/i/tek0026.png/
> >> > The red LED is the trace that is a bit higher. I m looking to see if
> >> > I can dig up any blue or white LED s.
>
> >> > Any other ideas how I might determine if that's what I'm seeing...
>
> >> > Oh, the PD is a OSI-optoelectroncs PIN-3CD (3.2mm^2 area) reversed
> >> > biased at ~12V.
>
> >> > George H.
>
> >> Crank up the bias till the diode is fully depleted, and see.
>
> >> Cheers
>
> >> Phil Hobbs
>
> >> --
> >> Dr Philip C D Hobbs
> >> Principal
> >> ElectroOptical Innovations
> >> 55 Orchard Rd
> >> Briarcliff Manor NY 10510
> >> 845-480-2058
>
> >> email: hobbs (atsign) electrooptical (period) nethttp://electrooptical=
.net-Hide quoted text -
>
> >> - Show quoted text -
>
> >Thanks Phil,
>
> >I can try more bias. =A0Is there any way to know it's fully depleted?
> >(Or do I just take it to the maximum as specified by the maker. (30
> >volts in this case.)
>
> >The white LED showed more of the long tail... but less of a sharp turn-
> >on, I wonder if it's just the LED time response that I'm seeing.
>
> A white LED has a phosphor that is likely slow.
>
snip

some numbers in this:
http://northumbria.openrepository.com/northumbria/bitstream/10145/119149/2/=
Le%20Minh%20et%20al%20-%20100-Mbs%20NRZ%20visible%20light%20communications%=
20using%20a%20postequalized%20white%20LED%20-%20article.pdf


-Lasse

On Apr 11, 7:04=A0pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Mon, 11 Apr 2011 12:46:16 -0700 (PDT), George Herold
>
>
>
>
>
> <gher...@teachspin.com> wrote:
> >On Apr 11, 3:32 pm, Phil Hobbs
> ><pcdhSpamMeSensel...@electrooptical.net> wrote:
> >> George Herold wrote:
> >> > I looking at the step response of a photodiode (PD). I m stepping
> >> > the current into a LED such that the step doubles the photo current
> >> > from the PD. (I m not starting with the LED off, but on slightly.)
> >> > Here s a scope shot. The bottom trace in the PD response. (The top
> >> > is the voltage step to the LED (through 100 ohms)... AC coupled to g=
et
> >> > rid of a big DC offset.)
>
> >> >http://img189.imageshack.us/i/tek0024.png/
>
> >> > There is this long tail on the step response. Here s the same pictur=
e
> >> > with the timebase slowed down.
>
> >> >http://img812.imageshack.us/i/tek0025.png/
>
> >> > Looks something like a 2-2.5 us tail on the step.
>
> >> > What s it caused by? I first thought it was perhaps heating of the
> >> > LED during the step. But I was reading Photodetectors by S. Donati,
> >> > over the weekend. In Chapter 5 he talks about two sets of photo-
> >> > generated carriers the majority are made in the depletion layer, and
> >> > travel with the drift speed, (times in the nano-second range.) The
> >> > second smaller set are those generated in the doped region. If they
> >> > are within a diffusion length of the depletion region then these hav=
e
> >> > a chance of adding to the photocurrent. Donati says that these have =
a
> >> > time constant in the microsecond range! I wonder if this is what I a=
m
> >> > seeing?
>
> >> > I looked at the step response with a different color LED. (you expec=
t
> >> > more absorption in the doped region with shorter wavelength light.)
> >> > So here is the pulse response from a 635 nm red and 594 nm Amber
> >> > LED.
>
> >> >http://img52.imageshack.us/i/tek0026.png/
> >> > The red LED is the trace that is a bit higher. I m looking to see if
> >> > I can dig up any blue or white LED s.
>
> >> > Any other ideas how I might determine if that's what I'm seeing...
>
> >> > Oh, the PD is a OSI-optoelectroncs PIN-3CD (3.2mm^2 area) reversed
> >> > biased at ~12V.
>
> >> > George H.
>
> >> Crank up the bias till the diode is fully depleted, and see.
>
> >> Cheers
>
> >> Phil Hobbs
>
> >> --
> >> Dr Philip C D Hobbs
> >> Principal
> >> ElectroOptical Innovations
> >> 55 Orchard Rd
> >> Briarcliff Manor NY 10510
> >> 845-480-2058
>
> >> email: hobbs (atsign) electrooptical (period) nethttp://electrooptical=
.net-Hide quoted text -
>
> >> - Show quoted text -
>
> >Thanks Phil,
>
> >I can try more bias. =A0Is there any way to know it's fully depleted?
> >(Or do I just take it to the maximum as specified by the maker. (30
> >volts in this case.)
>
> >The white LED showed more of the long tail... but less of a sharp turn-
> >on, I wonder if it's just the LED time response that I'm seeing.
>
> A white LED has a phosphor that is likely slow.

Yeah,  I noticed that.. Thanks to Grant too.

 I didn't save the screeen shot on my flash.  (I should hit the save
button more often.)
IR led's are slow too.


>
> Try a laser diode. They are (usually) fast.

> I did encounter some laser diodes that had a weird slow step response
> when used at low pulse rates. They behaved, electrically and
> optically, as if there were an inductor in series with the diode. At
> high rep-rates, 1 MHz maybe, the effect disappeared.

Bring out the big guns ehh?  I've never looked at the step response
from our laser diode, but we've modulated it up to ~150 MHz.  There's
a reported change in the modulation sensitivity, at the microsecond
range and longer its due to heating, at shorter times it drops by an
order of magnitude and is due to (index of refraction?) carrier
density effects.  At very high frequency you can get an enhancement
due to relaxation oscialltion in the LD... about 6 GHz, in the LD's I
know.

>
> Try looking at the voltage across the LED, and see if it's a clean
> step.
>
> What's the signal path from the photodiode to the scope? Since all the
> LEDs seem to have similar waveforms, maybe the detector is the
> problem.
Yeah sorry I didn't save all the crummy 'scope shots.
But I'm certainly open to some silly measurement error.  There's a TIA
with OPA134 (GBW~8MHz, 1-3pF Cin) and 10kohm feedback resistor, with
3.3pF parallel capacitance (plus some extra stray... call it ~2 pF,
perhaps a bit less).


I think I'm seeing a LED temperature effect.  Does the light output go
up as an LED gets hotter?  If it's linear with delta T, I can't change
drive level and make it go away.

George H.
>
> John- Hide quoted text -
>
> - Show quoted text -

On Apr 11, 7:05=A0pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Mon, 11 Apr 2011 23:17:18 +0200, Sjouke Burry
>
>
>
>
>
> <burrynulnulf...@ppllaanneett.nnll> wrote:
> >George Herold wrote:
> >> On Apr 11, 4:38 pm, Sjouke Burry <burrynulnulf...@ppllaanneett.nnll>
> >> wrote:
> >>> George Herold wrote:
> >>>> On Apr 11, 3:32 pm, Phil Hobbs
> >>>> <pcdhSpamMeSensel...@electrooptical.net> wrote:
> >>>>> George Herold wrote:
> >>>>>> I looking at the step response of a photodiode (PD). =A0I m steppi=
ng
> >>>>>> the current into a LED such that the step doubles the photo curren=
t
> >>>>>> from the PD. =A0(I m not starting with the LED off, but on slightl=
y.)
> >>>>>> Here s a scope shot. =A0The bottom trace in the PD response. =A0(T=
he top
> >>>>>> is the voltage step to the LED (through 100 ohms)... AC coupled to=
 get
> >>>>>> rid of a big DC offset.)
> >>>>>>http://img189.imageshack.us/i/tek0024.png/
> >>>>>> There is this long tail on the step response. =A0Here s the same p=
icture
> >>>>>> with the timebase slowed down.
> >>>>>>http://img812.imageshack.us/i/tek0025.png/
> >>>>>> Looks something like a 2-2.5 us tail on the step.
> >>>>>> What s it caused by? =A0I first thought it was perhaps heating of =
the
> >>>>>> LED during the step. =A0But I was reading Photodetectors by S. Don=
ati,
> >>>>>> over the weekend. =A0In Chapter 5 he talks about two sets of photo=
-
> >>>>>> generated carriers the majority are made in the depletion layer, a=
nd
> >>>>>> travel with the drift speed, =A0(times in the nano-second range.) =
=A0The
> >>>>>> second smaller set are those generated in the doped region. =A0If =
they
> >>>>>> are within a diffusion length of the depletion region then these h=
ave
> >>>>>> a chance of adding to the photocurrent. =A0Donati says that these =
have a
> >>>>>> time constant in the microsecond range! =A0I wonder if this is wha=
t I am
> >>>>>> seeing?
> >>>>>> I looked at the step response with a different color LED. =A0(you =
expect
> >>>>>> more absorption in the doped region with shorter wavelength light.=
)
> >>>>>> So here is the pulse response from a 635 nm red and 594 nm Amber
> >>>>>> LED.
> >>>>>>http://img52.imageshack.us/i/tek0026.png/
> >>>>>> The red LED is the trace that is a bit higher. =A0I m looking to s=
ee if
> >>>>>> I can dig up any blue or white LED s.
> >>>>>> Any other ideas how I might determine if that's what I'm seeing...
> >>>>>> Oh, the PD is a OSI-optoelectroncs PIN-3CD (3.2mm^2 area) reversed
> >>>>>> biased at ~12V.
> >>>>>> George H.
> >>>>> Crank up the bias till the diode is fully depleted, and see.
> >>>>> Cheers
> >>>>> Phil Hobbs
> >>>>> --
> >>>>> Dr Philip C D Hobbs
> >>>>> Principal
> >>>>> ElectroOptical Innovations
> >>>>> 55 Orchard Rd
> >>>>> Briarcliff Manor NY 10510
> >>>>> 845-480-2058
> >>>>> email: hobbs (atsign) electrooptical (period) nethttp://electroopti=
cal.net-Hidequoted text -
> >>>>> - Show quoted text -
> >>>> Thanks Phil,
> >>>> I can try more bias. =A0Is there any way to know it's fully depleted=
?
> >>>> (Or do I just take it to the maximum as specified by the maker. (30
> >>>> volts in this case.)
> >>>> The white LED showed more of the long tail... but less of a sharp tu=
rn-
> >>>> on, I wonder if it's just the LED time response that I'm seeing.
> >>>> George H.
> >>> You could also switch the LED on/off by shorting the LED
> >>> current to ground, thereby discharging the LED capacitance.
>
> >> Ohh, maybe that long tail is just the series 100 ohms and C of the
> >> LED? =A0(20nF that can't be it.) =A0I looked at the current through th=
e
> >> LED and that was nice and square.
>
> >>> As for the photo diode, I would advise 0.5 times max, 15 volts.
> >>> The factsheet of the photo diode should have a plot of
> >>> capacitance versus voltage.
>
> >> If OSI has that information they are keeping it well hidden on their
> >> web site. =A0Perhaps the email will shake something out of them... (Mo=
re
> >> likely I'll be ignored.)
>
> >> George H.
> >> - Hide quoted text -
> >>> - Show quoted text -
>
> >Oh, and I forgot to tell you, NEVER USE A WHITE LED!!!
> >They use a fluorescent coating to convert blue light to
> >white light, and that fluorescent keeps on emitting light
> >long after the LED switches of!!!!!!
> >Use a blue one, if you need high energy photons, besides,
> >IR diodes have one bad property, you cant see them working....
> >(with your own eyes that is.)
>
> I can see an 850 nm laser diode!

Once!... no Twice!!

George H.

>
> John- Hide quoted text -
>
> - Show quoted text -

On Apr 11, 7:08=A0pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Mon, 11 Apr 2011 16:06:37 -0700 (PDT), George Herold
>
>
>
>
>
> <ggher...@gmail.com> wrote:
> >On Apr 11, 5:42=A0pm, Vladimir Vassilevsky <nos...@nowhere.com> wrote:
> >> George Herold wrote:
> >> > I=92 looking at the step response of a photodiode (PD). =A0I=92m ste=
pping
> >> > the current into a LED such that the step doubles the photo current
> >> > from the PD. =A0(I=92m not starting with the LED off, but on slightl=
y.)
> >> > Here=92s a =91scope shot. =A0The bottom trace in the PD response. =
=A0(The top
> >> > is the voltage step to the LED (through 100 ohms)... AC coupled to g=
et
> >> > rid of a big DC offset.)
>
> >> >http://img189.imageshack.us/i/tek0024.png/
>
> >> > There is this long tail on the step response. =A0Here=92s the same p=
icture
> >> > with the timebase slowed down.
>
> >> >http://img812.imageshack.us/i/tek0025.png/
>
> >> > Looks something like a 2-2.5 us tail on the step.
>
> >> Your circuit is slow.
>
> >Yeah, I've got swithces, terminal blocks and IC sockets hanging on the
> >inputs, about 7pF over the opamp(~1-3pF) and photodiode (~12pF).
>
> Yikes! Clean it up!
>
> John- Hide quoted text -
Hmm, The circuit allows students to muck about with the input.  (They
are measuring noise.)  We didn't want soldering. We're pretty much
stuck with terminal blocks and switches.  I figure the opamp is a $2
fuse, so the socket is a must.  (It's nice to be able to change all
sorts of bits too!)

George H.
>
> - Show quoted text -

George Herold wrote:
> On Apr 11, 3:54 pm, Phil Hobbs
> <pcdhSpamMeSensel...@electrooptical.net>  wrote:
>> George Herold wrote:
>>> On Apr 11, 3:32 pm, Phil Hobbs
>>> <pcdhSpamMeSensel...@electrooptical.net>    wrote:
>>>> George Herold wrote:
>>>>> I looking at the step response of a photodiode (PD).  I m stepping
>>>>> the current into a LED such that the step doubles the photo current
>>>>> from the PD.  (I m not starting with the LED off, but on slightly.)
>>>>> Here s a scope shot.  The bottom trace in the PD response.  (The top
>>>>> is the voltage step to the LED (through 100 ohms)... AC coupled to get
>>>>> rid of a big DC offset.)
>>
>>>>> http://img189.imageshack.us/i/tek0024.png/
>>
>>>>> There is this long tail on the step response.  Here s the same picture
>>>>> with the timebase slowed down.
>>
>>>>> http://img812.imageshack.us/i/tek0025.png/
>>
>>>>> Looks something like a 2-2.5 us tail on the step.
>>
>>>>> What s it caused by?  I first thought it was perhaps heating of the
>>>>> LED during the step.  But I was reading Photodetectors by S. Donati,
>>>>> over the weekend.  In Chapter 5 he talks about two sets of photo-
>>>>> generated carriers the majority are made in the depletion layer, and
>>>>> travel with the drift speed,  (times in the nano-second range.)  The
>>>>> second smaller set are those generated in the doped region.  If they
>>>>> are within a diffusion length of the depletion region then these have
>>>>> a chance of adding to the photocurrent.  Donati says that these have a
>>>>> time constant in the microsecond range!  I wonder if this is what I am
>>>>> seeing?
>>
>>>>> I looked at the step response with a different color LED.  (you expect
>>>>> more absorption in the doped region with shorter wavelength light.)
>>>>> So here is the pulse response from a 635 nm red and 594 nm Amber
>>>>> LED.
>>
>>>>> http://img52.imageshack.us/i/tek0026.png/
>>>>> The red LED is the trace that is a bit higher.  I m looking to see if
>>>>> I can dig up any blue or white LED s.
>>
>>>>> Any other ideas how I might determine if that's what I'm seeing...
>>
>>>>> Oh, the PD is a OSI-optoelectroncs PIN-3CD (3.2mm^2 area) reversed
>>>>> biased at ~12V.
>>
>>>>> George H.
>>
>>>> Crank up the bias till the diode is fully depleted, and see.
>>
>>>> Cheers
>>
>>>> Phil Hobbs
>>
>>>> --
>>>> Dr Philip C D Hobbs
>>>> Principal
>>>> ElectroOptical Innovations
>>>> 55 Orchard Rd
>>>> Briarcliff Manor NY 10510
>>>> 845-480-2058
>>
>>>> email: hobbs (atsign) electrooptical (period) nethttp://electrooptical.net-Hide quoted text -
>>
>>>> - Show quoted text -
>>
>>> Thanks Phil,
>>
>>> I can try more bias.  Is there any way to know it's fully depleted?
>>> (Or do I just take it to the maximum as specified by the maker. (30
>>> volts in this case.)
>>
>>> The white LED showed more of the long tail... but less of a sharp turn-
>>> on, I wonder if it's just the LED time response that I'm seeing.
>>
>>> George H.
>>
>> The blue from the LED will mostly be getting absorbed in the epi, so you
>> won't see much of it.  Diffusion out of the epi may be what you're
>> seeing on the other LEDs too.  Try a 900 or 940 nm LED and see if that
>> makes a difference.
>>
>> Re full depletion:
>> I'd crank it right up to 30V, with an RC to protect it from breakdown.
>> You can probably go significantly higher than that.  Most diodes speed
>> up amazingly when they're fully depleted--you may see the bandwidth
>> double between 27 and 30V.  Donati has a curve somewhere.  (Great book,
>> Donati, but a bit unevenly edited.  Still, a sterling effort for an E2L
>> writer.)
>>
>> Cheers
>>
>> Phil Hobbs
>>
>> --
>> Dr Philip C D Hobbs
>> Principal
>> ElectroOptical Innovations
>> 55 Orchard Rd
>> Briarcliff Manor NY 10510
>> 845-480-2058
>>
>> email: hobbs (atsign) electrooptical (period) nethttp://electrooptical.net- Hide quoted text -
>>
>> - Show quoted text -
>
> OK 30 Volt bias made no difference in the step response.  (I added in
> two 9V batteries, I put 100 ohms in series, 0.1uF)  The lack of a
> change in the fast response time at 30 V may not be surprising,
> there's a lot of excess stray capacitance in the circuit. (7 pf or
> so.)   So I may not have noticed.
>
> I'm going to try an IR LED.  (I thought those were too slow
> though.  ... I'll find out.)
>
> George H.

Interesting.

The fastest LEDs I know of are around 900 nm.  They aren't exactly fast, 
about 30 MHz.  Have a look at the SFH4550 (12 ns, 850 nm) and VSLB3940 
(15 ns, 940 nm)

LEDs have long tails on their falling edges due to the relatively long 
carrier lifetime.  That rising edge was when the light turned on, right? 
  Because the falling edges always look like that.

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

On 4/11/2011 9:23 PM, Phil Hobbs wrote:
> George Herold wrote:
>> On Apr 11, 3:54 pm, Phil Hobbs
>> <pcdhSpamMeSensel...@electrooptical.net> wrote:
>>> George Herold wrote:
>>>> On Apr 11, 3:32 pm, Phil Hobbs
>>>> <pcdhSpamMeSensel...@electrooptical.net> wrote:
>>>>> George Herold wrote:
>>>>>> I looking at the step response of a photodiode (PD). I m stepping
>>>>>> the current into a LED such that the step doubles the photo current
>>>>>> from the PD. (I m not starting with the LED off, but on slightly.)
>>>>>> Here s a scope shot. The bottom trace in the PD response. (The top
>>>>>> is the voltage step to the LED (through 100 ohms)... AC coupled to
>>>>>> get
>>>>>> rid of a big DC offset.)
>>>
>>>>>> http://img189.imageshack.us/i/tek0024.png/
>>>
>>>>>> There is this long tail on the step response. Here s the same picture
>>>>>> with the timebase slowed down.
>>>
>>>>>> http://img812.imageshack.us/i/tek0025.png/
>>>
>>>>>> Looks something like a 2-2.5 us tail on the step.
>>>
>>>>>> What s it caused by? I first thought it was perhaps heating of the
>>>>>> LED during the step. But I was reading Photodetectors by S. Donati,
>>>>>> over the weekend. In Chapter 5 he talks about two sets of photo-
>>>>>> generated carriers the majority are made in the depletion layer, and
>>>>>> travel with the drift speed, (times in the nano-second range.) The
>>>>>> second smaller set are those generated in the doped region. If they
>>>>>> are within a diffusion length of the depletion region then these have
>>>>>> a chance of adding to the photocurrent. Donati says that these have a
>>>>>> time constant in the microsecond range! I wonder if this is what I am
>>>>>> seeing?
>>>
>>>>>> I looked at the step response with a different color LED. (you expect
>>>>>> more absorption in the doped region with shorter wavelength light.)
>>>>>> So here is the pulse response from a 635 nm red and 594 nm Amber
>>>>>> LED.
>>>
>>>>>> http://img52.imageshack.us/i/tek0026.png/
>>>>>> The red LED is the trace that is a bit higher. I m looking to see if
>>>>>> I can dig up any blue or white LED s.
>>>
>>>>>> Any other ideas how I might determine if that's what I'm seeing...
>>>
>>>>>> Oh, the PD is a OSI-optoelectroncs PIN-3CD (3.2mm^2 area) reversed
>>>>>> biased at ~12V.
>>>
>>>>>> George H.
>>>
>>>>> Crank up the bias till the diode is fully depleted, and see.
>>>
>>>>> Cheers
>>>
>>>>> Phil Hobbs
>>>
>>>>> --
>>>>> Dr Philip C D Hobbs
>>>>> Principal
>>>>> ElectroOptical Innovations
>>>>> 55 Orchard Rd
>>>>> Briarcliff Manor NY 10510
>>>>> 845-480-2058
>>>
>>>>> email: hobbs (atsign) electrooptical (period)
>>>>> nethttp://electrooptical.net-Hide quoted text -
>>>
>>>>> - Show quoted text -
>>>
>>>> Thanks Phil,
>>>
>>>> I can try more bias. Is there any way to know it's fully depleted?
>>>> (Or do I just take it to the maximum as specified by the maker. (30
>>>> volts in this case.)
>>>
>>>> The white LED showed more of the long tail... but less of a sharp turn-
>>>> on, I wonder if it's just the LED time response that I'm seeing.
>>>
>>>> George H.
>>>
>>> The blue from the LED will mostly be getting absorbed in the epi, so you
>>> won't see much of it. Diffusion out of the epi may be what you're
>>> seeing on the other LEDs too. Try a 900 or 940 nm LED and see if that
>>> makes a difference.
>>>
>>> Re full depletion:
>>> I'd crank it right up to 30V, with an RC to protect it from breakdown.
>>> You can probably go significantly higher than that. Most diodes speed
>>> up amazingly when they're fully depleted--you may see the bandwidth
>>> double between 27 and 30V. Donati has a curve somewhere. (Great book,
>>> Donati, but a bit unevenly edited. Still, a sterling effort for an E2L
>>> writer.)
>>>
>>> Cheers
>>>
>>> Phil Hobbs
>>>
>>> --
>>> Dr Philip C D Hobbs
>>> Principal
>>> ElectroOptical Innovations
>>> 55 Orchard Rd
>>> Briarcliff Manor NY 10510
>>> 845-480-2058
>>>
>>> email: hobbs (atsign) electrooptical (period)
>>> nethttp://electrooptical.net- Hide quoted text -
>>>
>>> - Show quoted text -
>>
>> OK 30 Volt bias made no difference in the step response. (I added in
>> two 9V batteries, I put 100 ohms in series, 0.1uF) The lack of a
>> change in the fast response time at 30 V may not be surprising,
>> there's a lot of excess stray capacitance in the circuit. (7 pf or
>> so.) So I may not have noticed.
>>
>> I'm going to try an IR LED. (I thought those were too slow
>> though. ... I'll find out.)
>>
>> George H.
>
> Interesting.
>
> The fastest LEDs I know of are around 900 nm. They aren't exactly fast,
> about 30 MHz. Have a look at the SFH4550 (12 ns, 850 nm) and VSLB3940
> (15 ns, 940 nm)
>
> LEDs have long tails on their falling edges due to the relatively long
> carrier lifetime. That rising edge was when the light turned on, right?
> Because the falling edges always look like that.
>
> Cheers
>
> Phil Hobbs
>

What would happen if they were reverse biased by about a volt or so at 
turn-off time? Would that help?

John

John wrote:
> On 4/11/2011 9:23 PM, Phil Hobbs wrote:
>> George Herold wrote:
>>> On Apr 11, 3:54 pm, Phil Hobbs
>>> <pcdhSpamMeSensel...@electrooptical.net> wrote:
>>>> George Herold wrote:
>>>>> On Apr 11, 3:32 pm, Phil Hobbs
>>>>> <pcdhSpamMeSensel...@electrooptical.net> wrote:
>>>>>> George Herold wrote:
>>>>>>> I looking at the step response of a photodiode (PD). I m stepping
>>>>>>> the current into a LED such that the step doubles the photo current
>>>>>>> from the PD. (I m not starting with the LED off, but on slightly.)
>>>>>>> Here s a scope shot. The bottom trace in the PD response. (The top
>>>>>>> is the voltage step to the LED (through 100 ohms)... AC coupled to
>>>>>>> get
>>>>>>> rid of a big DC offset.)
>>>>
>>>>>>> http://img189.imageshack.us/i/tek0024.png/
>>>>
>>>>>>> There is this long tail on the step response. Here s the same
>>>>>>> picture
>>>>>>> with the timebase slowed down.
>>>>
>>>>>>> http://img812.imageshack.us/i/tek0025.png/
>>>>
>>>>>>> Looks something like a 2-2.5 us tail on the step.
>>>>
>>>>>>> What s it caused by? I first thought it was perhaps heating of the
>>>>>>> LED during the step. But I was reading Photodetectors by S. Donati,
>>>>>>> over the weekend. In Chapter 5 he talks about two sets of photo-
>>>>>>> generated carriers the majority are made in the depletion layer, and
>>>>>>> travel with the drift speed, (times in the nano-second range.) The
>>>>>>> second smaller set are those generated in the doped region. If they
>>>>>>> are within a diffusion length of the depletion region then these
>>>>>>> have
>>>>>>> a chance of adding to the photocurrent. Donati says that these
>>>>>>> have a
>>>>>>> time constant in the microsecond range! I wonder if this is what
>>>>>>> I am
>>>>>>> seeing?
>>>>
>>>>>>> I looked at the step response with a different color LED. (you
>>>>>>> expect
>>>>>>> more absorption in the doped region with shorter wavelength light.)
>>>>>>> So here is the pulse response from a 635 nm red and 594 nm Amber
>>>>>>> LED.
>>>>
>>>>>>> http://img52.imageshack.us/i/tek0026.png/
>>>>>>> The red LED is the trace that is a bit higher. I m looking to see if
>>>>>>> I can dig up any blue or white LED s.
>>>>
>>>>>>> Any other ideas how I might determine if that's what I'm seeing...
>>>>
>>>>>>> Oh, the PD is a OSI-optoelectroncs PIN-3CD (3.2mm^2 area) reversed
>>>>>>> biased at ~12V.
>>>>
>>>>>>> George H.
>>>>
>>>>>> Crank up the bias till the diode is fully depleted, and see.
>>>>
>>>>>> Cheers
>>>>
>>>>>> Phil Hobbs
>>>>
>>>>>> --
>>>>>> Dr Philip C D Hobbs
>>>>>> Principal
>>>>>> ElectroOptical Innovations
>>>>>> 55 Orchard Rd
>>>>>> Briarcliff Manor NY 10510
>>>>>> 845-480-2058
>>>>
>>>>>> email: hobbs (atsign) electrooptical (period)
>>>>>> nethttp://electrooptical.net-Hide quoted text -
>>>>
>>>>>> - Show quoted text -
>>>>
>>>>> Thanks Phil,
>>>>
>>>>> I can try more bias. Is there any way to know it's fully depleted?
>>>>> (Or do I just take it to the maximum as specified by the maker. (30
>>>>> volts in this case.)
>>>>
>>>>> The white LED showed more of the long tail... but less of a sharp
>>>>> turn-
>>>>> on, I wonder if it's just the LED time response that I'm seeing.
>>>>
>>>>> George H.
>>>>
>>>> The blue from the LED will mostly be getting absorbed in the epi, so
>>>> you
>>>> won't see much of it. Diffusion out of the epi may be what you're
>>>> seeing on the other LEDs too. Try a 900 or 940 nm LED and see if that
>>>> makes a difference.
>>>>
>>>> Re full depletion:
>>>> I'd crank it right up to 30V, with an RC to protect it from breakdown.
>>>> You can probably go significantly higher than that. Most diodes speed
>>>> up amazingly when they're fully depleted--you may see the bandwidth
>>>> double between 27 and 30V. Donati has a curve somewhere. (Great book,
>>>> Donati, but a bit unevenly edited. Still, a sterling effort for an E2L
>>>> writer.)
>>>>
>>>> Cheers
>>>>
>>>> Phil Hobbs
>>>>
>>>> --
>>>> Dr Philip C D Hobbs
>>>> Principal
>>>> ElectroOptical Innovations
>>>> 55 Orchard Rd
>>>> Briarcliff Manor NY 10510
>>>> 845-480-2058
>>>>
>>>> email: hobbs (atsign) electrooptical (period)
>>>> nethttp://electrooptical.net- Hide quoted text -
>>>>
>>>> - Show quoted text -
>>>
>>> OK 30 Volt bias made no difference in the step response. (I added in
>>> two 9V batteries, I put 100 ohms in series, 0.1uF) The lack of a
>>> change in the fast response time at 30 V may not be surprising,
>>> there's a lot of excess stray capacitance in the circuit. (7 pf or
>>> so.) So I may not have noticed.
>>>
>>> I'm going to try an IR LED. (I thought those were too slow
>>> though. ... I'll find out.)
>>>
>>> George H.
>>
>> Interesting.
>>
>> The fastest LEDs I know of are around 900 nm. They aren't exactly fast,
>> about 30 MHz. Have a look at the SFH4550 (12 ns, 850 nm) and VSLB3940
>> (15 ns, 940 nm)
>>
>> LEDs have long tails on their falling edges due to the relatively long
>> carrier lifetime. That rising edge was when the light turned on, right?
>> Because the falling edges always look like that.
>>
>> Cheers
>>
>> Phil Hobbs
>>
>
> What would happen if they were reverse biased by about a volt or so at
> turn-off time? Would that help?
>
> John

It might.  Trying to speed up a LED is a bit like lipstick on a pig, of 
course.  Lasers are two orders of magnitude faster, some nearly 3 orders.

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

On Mon, 11 Apr 2011 16:35:20 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>On Apr 11, 7:04&#4294967295;pm, John Larkin
><jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
>> On Mon, 11 Apr 2011 12:46:16 -0700 (PDT), George Herold
>>
>>
>>
>>
>>
>> <gher...@teachspin.com> wrote:
>> >On Apr 11, 3:32 pm, Phil Hobbs
>> ><pcdhSpamMeSensel...@electrooptical.net> wrote:
>> >> George Herold wrote:
>> >> > I looking at the step response of a photodiode (PD). I m stepping
>> >> > the current into a LED such that the step doubles the photo current
>> >> > from the PD. (I m not starting with the LED off, but on slightly.)
>> >> > Here s a scope shot. The bottom trace in the PD response. (The top
>> >> > is the voltage step to the LED (through 100 ohms)... AC coupled to get
>> >> > rid of a big DC offset.)
>>
>> >> >http://img189.imageshack.us/i/tek0024.png/
>>
>> >> > There is this long tail on the step response. Here s the same picture
>> >> > with the timebase slowed down.
>>
>> >> >http://img812.imageshack.us/i/tek0025.png/
>>
>> >> > Looks something like a 2-2.5 us tail on the step.
>>
>> >> > What s it caused by? I first thought it was perhaps heating of the
>> >> > LED during the step. But I was reading Photodetectors by S. Donati,
>> >> > over the weekend. In Chapter 5 he talks about two sets of photo-
>> >> > generated carriers the majority are made in the depletion layer, and
>> >> > travel with the drift speed, (times in the nano-second range.) The
>> >> > second smaller set are those generated in the doped region. If they
>> >> > are within a diffusion length of the depletion region then these have
>> >> > a chance of adding to the photocurrent. Donati says that these have a
>> >> > time constant in the microsecond range! I wonder if this is what I am
>> >> > seeing?
>>
>> >> > I looked at the step response with a different color LED. (you expect
>> >> > more absorption in the doped region with shorter wavelength light.)
>> >> > So here is the pulse response from a 635 nm red and 594 nm Amber
>> >> > LED.
>>
>> >> >http://img52.imageshack.us/i/tek0026.png/
>> >> > The red LED is the trace that is a bit higher. I m looking to see if
>> >> > I can dig up any blue or white LED s.
>>
>> >> > Any other ideas how I might determine if that's what I'm seeing...
>>
>> >> > Oh, the PD is a OSI-optoelectroncs PIN-3CD (3.2mm^2 area) reversed
>> >> > biased at ~12V.
>>
>> >> > George H.
>>
>> >> Crank up the bias till the diode is fully depleted, and see.
>>
>> >> Cheers
>>
>> >> Phil Hobbs
>>
>> >> --
>> >> Dr Philip C D Hobbs
>> >> Principal
>> >> ElectroOptical Innovations
>> >> 55 Orchard Rd
>> >> Briarcliff Manor NY 10510
>> >> 845-480-2058
>>
>> >> email: hobbs (atsign) electrooptical (period) nethttp://electrooptical.net-Hide quoted text -
>>
>> >> - Show quoted text -
>>
>> >Thanks Phil,
>>
>> >I can try more bias. &#4294967295;Is there any way to know it's fully depleted?
>> >(Or do I just take it to the maximum as specified by the maker. (30
>> >volts in this case.)
>>
>> >The white LED showed more of the long tail... but less of a sharp turn-
>> >on, I wonder if it's just the LED time response that I'm seeing.
>>
>> A white LED has a phosphor that is likely slow.
>
>Yeah,  I noticed that.. Thanks to Grant too.
>
> I didn't save the screeen shot on my flash.  (I should hit the save
>button more often.)
>IR led's are slow too.
>
>
>>
>> Try a laser diode. They are (usually) fast.
>
>> I did encounter some laser diodes that had a weird slow step response
>> when used at low pulse rates. They behaved, electrically and
>> optically, as if there were an inductor in series with the diode. At
>> high rep-rates, 1 MHz maybe, the effect disappeared.
>
>Bring out the big guns ehh?  I've never looked at the step response
>from our laser diode, but we've modulated it up to ~150 MHz.  There's
>a reported change in the modulation sensitivity, at the microsecond
>range and longer its due to heating, at shorter times it drops by an
>order of magnitude and is due to (index of refraction?) carrier
>density effects.  At very high frequency you can get an enhancement
>due to relaxation oscialltion in the LD... about 6 GHz, in the LD's I
>know.
>
>>
>> Try looking at the voltage across the LED, and see if it's a clean
>> step.
>>
>> What's the signal path from the photodiode to the scope? Since all the
>> LEDs seem to have similar waveforms, maybe the detector is the
>> problem.
>Yeah sorry I didn't save all the crummy 'scope shots.
>But I'm certainly open to some silly measurement error.  There's a TIA
>with OPA134 (GBW~8MHz, 1-3pF Cin) and 10kohm feedback resistor, with
>3.3pF parallel capacitance (plus some extra stray... call it ~2 pF,
>perhaps a bit less).
>
>
>I think I'm seeing a LED temperature effect.  Does the light output go
>up as an LED gets hotter?  If it's linear with delta T, I can't change
>drive level and make it go away.

LEDs are less efficient when they're hot. But the forward drop goes
down. So there is usually some source resistance where the effects
cancel.

John

On 4/11/2011 9:46 PM, Phil Hobbs wrote:
>>
>> What would happen if they were reverse biased by about a volt or so at
>> turn-off time? Would that help?
>>
>> John
>
> It might. Trying to speed up a LED is a bit like lipstick on a pig, of
> course.


Great! Now I have to clean coffee spray off my monitor screen.

John

On Apr 11, 10:23=A0pm, Phil Hobbs
<pcdhSpamMeSensel...@electrooptical.net> wrote:
> George Herold wrote:
> > On Apr 11, 3:54 pm, Phil Hobbs
> > <pcdhSpamMeSensel...@electrooptical.net> =A0wrote:
> >> George Herold wrote:
> >>> On Apr 11, 3:32 pm, Phil Hobbs
> >>> <pcdhSpamMeSensel...@electrooptical.net> =A0 =A0wrote:
> >>>> George Herold wrote:
> >>>>> I looking at the step response of a photodiode (PD). =A0I m steppin=
g
> >>>>> the current into a LED such that the step doubles the photo current
> >>>>> from the PD. =A0(I m not starting with the LED off, but on slightly=
.)
> >>>>> Here s a scope shot. =A0The bottom trace in the PD response. =A0(Th=
e top
> >>>>> is the voltage step to the LED (through 100 ohms)... AC coupled to =
get
> >>>>> rid of a big DC offset.)
>
> >>>>>http://img189.imageshack.us/i/tek0024.png/
>
> >>>>> There is this long tail on the step response. =A0Here s the same pi=
cture
> >>>>> with the timebase slowed down.
>
> >>>>>http://img812.imageshack.us/i/tek0025.png/
>
> >>>>> Looks something like a 2-2.5 us tail on the step.
>
> >>>>> What s it caused by? =A0I first thought it was perhaps heating of t=
he
> >>>>> LED during the step. =A0But I was reading Photodetectors by S. Dona=
ti,
> >>>>> over the weekend. =A0In Chapter 5 he talks about two sets of photo-
> >>>>> generated carriers the majority are made in the depletion layer, an=
d
> >>>>> travel with the drift speed, =A0(times in the nano-second range.) =
=A0The
> >>>>> second smaller set are those generated in the doped region. =A0If t=
hey
> >>>>> are within a diffusion length of the depletion region then these ha=
ve
> >>>>> a chance of adding to the photocurrent. =A0Donati says that these h=
ave a
> >>>>> time constant in the microsecond range! =A0I wonder if this is what=
 I am
> >>>>> seeing?
>
> >>>>> I looked at the step response with a different color LED. =A0(you e=
xpect
> >>>>> more absorption in the doped region with shorter wavelength light.)
> >>>>> So here is the pulse response from a 635 nm red and 594 nm Amber
> >>>>> LED.
>
> >>>>>http://img52.imageshack.us/i/tek0026.png/
> >>>>> The red LED is the trace that is a bit higher. =A0I m looking to se=
e if
> >>>>> I can dig up any blue or white LED s.
>
> >>>>> Any other ideas how I might determine if that's what I'm seeing...
>
> >>>>> Oh, the PD is a OSI-optoelectroncs PIN-3CD (3.2mm^2 area) reversed
> >>>>> biased at ~12V.
>
> >>>>> George H.
>
> >>>> Crank up the bias till the diode is fully depleted, and see.
>
> >>>> Cheers
>
> >>>> Phil Hobbs
>
> >>>> --
> >>>> Dr Philip C D Hobbs
> >>>> Principal
> >>>> ElectroOptical Innovations
> >>>> 55 Orchard Rd
> >>>> Briarcliff Manor NY 10510
> >>>> 845-480-2058
>
> >>>> email: hobbs (atsign) electrooptical (period) nethttp://electrooptic=
al.net-Hidequoted text -
>
> >>>> - Show quoted text -
>
> >>> Thanks Phil,
>
> >>> I can try more bias. =A0Is there any way to know it's fully depleted?
> >>> (Or do I just take it to the maximum as specified by the maker. (30
> >>> volts in this case.)
>
> >>> The white LED showed more of the long tail... but less of a sharp tur=
n-
> >>> on, I wonder if it's just the LED time response that I'm seeing.
>
> >>> George H.
>
> >> The blue from the LED will mostly be getting absorbed in the epi, so y=
ou
> >> won't see much of it. =A0Diffusion out of the epi may be what you're
> >> seeing on the other LEDs too. =A0Try a 900 or 940 nm LED and see if th=
at
> >> makes a difference.
>
> >> Re full depletion:
> >> I'd crank it right up to 30V, with an RC to protect it from breakdown.
> >> You can probably go significantly higher than that. =A0Most diodes spe=
ed
> >> up amazingly when they're fully depleted--you may see the bandwidth
> >> double between 27 and 30V. =A0Donati has a curve somewhere. =A0(Great =
book,
> >> Donati, but a bit unevenly edited. =A0Still, a sterling effort for an =
E2L
> >> writer.)
>
> >> Cheers
>
> >> Phil Hobbs
>
> >> --
> >> Dr Philip C D Hobbs
> >> Principal
> >> ElectroOptical Innovations
> >> 55 Orchard Rd
> >> Briarcliff Manor NY 10510
> >> 845-480-2058
>
> >> email: hobbs (atsign) electrooptical (period) nethttp://electrooptical=
.net-Hide quoted text -
>
> >> - Show quoted text -
>
> > OK 30 Volt bias made no difference in the step response. =A0(I added in
> > two 9V batteries, I put 100 ohms in series, 0.1uF) =A0The lack of a
> > change in the fast response time at 30 V may not be surprising,
> > there's a lot of excess stray capacitance in the circuit. (7 pf or
> > so.) =A0 So I may not have noticed.
>
> > I'm going to try an IR LED. =A0(I thought those were too slow
> > though. =A0... I'll find out.)
>
> > George H.
>
> Interesting.
>
> The fastest LEDs I know of are around 900 nm. =A0They aren't exactly fast=
,
> about 30 MHz. =A0Have a look at the SFH4550 (12 ns, 850 nm) and VSLB3940
> (15 ns, 940 nm)
>
> LEDs have long tails on their falling edges due to the relatively long
> carrier lifetime. =A0That rising edge was when the light turned on, right=
?
> =A0 Because the falling edges always look like that.

Yeah the rising edge,  but I just looked and the falling edge looks
identical!  Is there a reason the rising and falling edges should look
different?

I've been trying to convince myself it's just an LED effect.  Perhaps
a temperature change, or the above carrier lifetime in the LED?  The
reason I don't think it's a Photodiode effect is that when I look at
the high frequency (BW ~ 1.5 MHz) noise with a DC light source and
several hundred mV of bias 'current' across the feedback resistor,
(noise signal dominated by shot noise) I see exactly the same amount
of noise regardless of the light source.

George H.


>
> Cheers
>
> Phil Hobbs
>
> --
> Dr Philip C D Hobbs
> Principal
> ElectroOptical Innovations
> 55 Orchard Rd
> Briarcliff Manor NY 10510
> 845-480-2058
>
> email: hobbs (atsign) electrooptical (period) nethttp://electrooptical.ne=
t- Hide quoted text -
>
> - Show quoted text -