Decay of luminescence on IR LEDs?

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High Guys,

Is there some accessible data or paper (not IEEExplore or other $$$$ pay
service) where the decay of luminescence for IR LEDs after a pulse is
explained? Datasheets are paltry in that respect. I am mostly interested
in the time frame where light output has dropped to 1% and less after a
full amplitude pulse out of a low impedance generator.

--
Regards, Joerg

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Re: Decay of luminescence on IR LEDs?
Joerg wrote:
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They vary a lot.  The quickest ones are about 70 MHz BW, and they should
slow down like an RC--the minority carrier lifetime in GaAs and its
relatives is very short (ballpark 1 ns).

JL and I had occasion to measure a bunch of them recently for the PH200.

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

Re: Decay of luminescence on IR LEDs?
Phil Hobbs wrote:
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But 70MHz at 3dB is miles away from 1nsec. <scratching head>


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Reason I ask is something weird that happens at a long distance client:
They modified a TIA per my instructions so it would recover from brutal
overdrive (factor 100 and more) very fast. SPICE confirmed it. Then they
measured and it's still as slow as before. In our case the TIA would
come out of "pegging" at about 1% luminescence and my suspicion is that
either our LED or the PD takes their sweet time to get there. We are
seeing many microseconds. Can't imagine any PD being this slow but not
sure about the IR-LED. It's listed around 10MHz BW.

So far I have mostly dealt with laser diodes and those were always super
fast.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: Decay of luminescence on IR LEDs?
Joerg wrote:
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Yup, diode lasers are very much faster.  I think it's basically an RC
effect in LEDs.

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

Re: Decay of luminescence on IR LEDs?
Phil Hobbs wrote:
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Do you think the spontaneous recombination lifetime can be modeled as
RC? That would be cool. The datasheets say that the optical output
rise/fall times are about 50nsec and this is for 10% and 90%. If that is
RC then I could model it in SPICE.

Unfortunately there is precious little literature on the topic unless
one is a member of the respective engineer's society or has a
subscription to one of those $2k/year journals.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: Decay of luminescence on IR LEDs?
Joerg wrote:
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The minority carrier population probably decays exponentially, because
the rate is proportional to n_e * n_h, and one species will be in large
excess.

LEDs have huge capacitances, especially in forward bias--70 to 100 pF is
not unusual.  You might be able to speed it up by transient reverse
bias, the way you do with BJT stored charge.

LED bandwidth is (iiuc) measured with a small modulation on top of a
large DC bias, so that the differential resistance of the diode is only
an ohm or two.  That'll speed up the RC time constant pretty
dramatically, so I wouldn't be at all surprised if the on-off behaviour
is quite a bit slower.

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

Re: Decay of luminescence on IR LEDs?
Phil Hobbs wrote:
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Ok, so an RC decay would be a good approximation then. Thanks, Phil.


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Aha! Sounds like a good old marketing trick to make it look better.
Nobody in their right mind would use an IR LED as a transmitter that
way. Reminds me of the range claims of some wireless companies. I put
one of them on the spot and then an engineer admitted that, yeah, the
data pretty much applies when both stations are in outer space.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: Decay of luminescence on IR LEDs?

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Well, one reason for gigabit fiber communications devices being more
expensive than 100Mbit is supposedly the need to use lasers rather than
LEDs, which argues in favor of lasers being significantly faster (and I
think "faster to shut off" was specifically mentioned.) At an in-between
price point there are Vertical Cavity Surface Emitting Lasers - I don't
know if they are also intermediate in speed, or as fast as "normal"
lasers.

At a low price point, if you are willing to rip open SFPs, those are
often available used, with IR lasers (and PDs), absurdly cheap. I got a
pile for my project (not ripping them open, just using them as intended)
at about $5 each last fall (4GB capable).

--
Cats, coffee, chocolate...vices to live by

Re: Decay of luminescence on IR LEDs?
Ecnerwal wrote:
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Yeah, I know, but unfortunately the diode is beyond our influence. The
transmit stuff comes with it :-(

I really like VCSELs, except for their polarization mode hop noise.

--
Regards, Joerg

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Re: Decay of luminescence on IR LEDs?
Joerg wrote:
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There are some VCSELs with really good polarization stability.  Check
out ULM Photonics.

They work by adding a deep subwavelength grating to one of the mirrors.
  While this doesn't cause diffraction, the modulated evanescent field
causes a change in the reflection phase in the polarizations parallel
and perpendicular to the grooves.  (The evanescent field is different,
and there's no diffraction, so changing the reflection coefficient is
all it can do.)

Because the cavity is so short, the reflection phase shift splits the P
and S polarized cavity resonances by a lot, enough that one of them is
shifted off the gain peak and so doesn't oscillate.

I don't know how stable the polarization is over long periods, but the
short term stability is very very good--better than Fabry-Perot diode
lasers.

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

Re: Decay of luminescence on IR LEDs?
Phil Hobbs wrote:
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Thanks, Phil, very good hint. If anyone else wants to look:

http://www.ulm-photonics.com/new/index.php?option=com_cckjseblod&view=search&layout=search&searchid=3&task=search&Itemid=11

They are in my old home country and they named it after their city (Ulm,
at the Danube river). For us it won't work right now, this project has
been completed a few years ago and they currently don't offer long
enough wavelengths, only 948nm max. But that could change some day, got
to keep them on my watch list.

--
Regards, Joerg

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Re: Decay of luminescence on IR LEDs?
Phil Hobbs wrote:
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Phil, why are their VCSELs 100MHz in spectral width? That sounds ghastly
compared to the ones we had a couple years ago.

Or did they measure unfavorably? Can't imagine why anyone would do that.
We had the VCSELs in a loop which cuts down on some of the noise
mechanisms, just like you can clean up a fairly noisy oscillator by
tying a PLL around it.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: Decay of luminescence on IR LEDs?
Joerg wrote:
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VCSELs have very wide Schawlow-Townes line widths.  The S-T limit is
basically caused by the counting statistics of both the emission and
loss in the cavity.  The AM and PM this causes depends on how many
photons there are in the cavity.  VCSELs have very short cavities, which
makes the linewidth wider.

You can make them better behaved with an external cavity--by locking to
an etalon or a big long fibre Michelson.  (You need a Faraday mirror for
the fibre approach.)


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

Re: Decay of luminescence on IR LEDs?
Phil Hobbs wrote:
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But this wide? About two years ago we had some that were already
natively better.


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We had to have flexibility in wavelength so a hard optical lock wasn't
possible. But I ran them in a PID loop which really made a difference.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: Decay of luminescence on IR LEDs?
Joerg wrote:
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I'm not certain about these exact devices, but a SWAG is that they
probably make the cavity very short even for a VCSEL in order that the
polarization splitting make more of a tuning shift.

Besides, they probably current-tune really fast, so just wrap a loop
around it. ;)

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

Re: Decay of luminescence on IR LEDs?
Phil Hobbs wrote:
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That would not be so cool.


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No joke, I had a loop around them most of the time. But it cannot be a
fast loop so if the wavelength meanders around somewhat slowly, ok. If
it's a super wideband hash it's all toast.

--
Regards, Joerg

http://www.analogconsultants.com /

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