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Science Forum Index » Optics Forum » looking for nanosecond pulse LED
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| simba |
 Posted: Tue Apr 15, 2008 9:28 pm |
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Guest
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I am looking for a LED than can be driven buy pulse with width of
10ns~100ns order.
Such a LED is required for measurement of velocity of small particles
( size of some 10 microns order ) as light source. Commonly used light
source for such application (a Particle Image Velocimetry
application ) is high repetition pulse laser. However, LED could be a
choice if both illuminating power and speed of the LED is enough.
Any information or advice about that kind of LED product will be
deeply appreciated.
Thanks. |
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| Phil Hobbs |
| Posted: Wed Apr 16, 2008 9:22 pm |
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simba wrote:
Quote: I am looking for a LED than can be driven buy pulse with width of
10ns~100ns order.
Such a LED is required for measurement of velocity of small particles
( size of some 10 microns order ) as light source. Commonly used light
source for such application (a Particle Image Velocimetry
application ) is high repetition pulse laser. However, LED could be a
choice if both illuminating power and speed of the LED is enough.
Any information or advice about that kind of LED product will be
deeply appreciated.
Thanks.
I haven't tried recently, but it used to be true that an ordinary
display LED had a 3 dB modulation bandwidth of around 30-60 MHz. You
can certainly do 100 ns pulses, and with a bit of work you should be
able to do 10 ns. Going to 1 ns would probably be very difficult.
Some VCSELs can do 50 ps or even a bit faster.
Cheers,
Phil |
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| Jürgen Appel |
| Posted: Sat Apr 19, 2008 1:45 pm |
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simba schrieb:
Quote: I am looking for a LED than can be driven buy pulse with width of
10ns~100ns order.
Such a LED is required for measurement of velocity of small particles
( size of some 10 microns order ) as light source.
This is more a question of the driver circuit than of the LED itself. If you
want to switch the LED during such short times, you have to overcome the
LED capacitance and thus your driver must be able to supply a lot of
reactive power (or you have to compensate for it with an impedance matching
network).
Also note, that in pulsed mode most LEDs can be overdriven to a multiple of
the maximum allowed CW current. And even if the LED burns you only loose a
few cents. It may pay off to use an IR-Led, since usually cameras have a
higher efficiency there.
Quote: However, LED could be a
choice if both illuminating power and speed of the LED is enough.
The speed probably is, the brightness could be sufficient if your imaging
area is very small.
Cheers,
Jürgen
--
GPG key:
http://pgp.mit.edu:11371/pks/lookup?search=J%FCrgen+Appel&op=get |
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| Fritz |
| Posted: Tue Apr 22, 2008 5:53 am |
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Simba,
We have driven Cree and Luxeon 1 to 3 watt bright LEDs at around 10mHz
(square wave) using an FET based driver. Overcoming capacitance is
indeed a problem and one must erradicate ringing and so forth. Of
course most InGaS communications laser diodes work in the nanosecond
range with commerical driver ICs but they may not be intense enough
for the stated purpose.
I expect that some less bright LEDs (less than 1w) would present
less capacitance problems. Also in the 2 years since I investigated
this there are many new entries in the bright LED market which may be
faster. We simply took a sample from the market, pulsed them and
watched the signal with a photodetector/oscope. As Jurgen mentioned we
overdrive LEDs when operating in pulsed mode and have been successful
here.
Regards,
Fritz
Woods Hole Oceanographic Inst. |
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| whit3rd |
| Posted: Wed Apr 23, 2008 10:19 am |
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On Apr 16, 12:28 am, simba <Simba...@gmail.com> wrote:
Quote: I am looking for a LED than can be driven buy pulse with width of
10ns~100ns order.
As others have said, a laser diode can do shorter pulses than
an LED; this is because LED light emission is due to a recombination
process, and illumination LEDs aren't intended for rapid
recombination. It's easy to pump an LED ON fast, but OFF
happens at an uncontrolled rate. Lasers have a much faster
turnoff, because of the stimulated emission effect.
It will depend on the innards of the LED, of course, but
you can turn them off by reverse-bias faster than by waiting
for recombination; this will unfortunately require high
current drive, because of junction capacitance.
MOSFET driver chips, like LTC1693-2, might be useful in
this regard. Reverse bias is not usually destructive, but
it's off-the-data-sheet behavior as far as turnoff by this method.
You'll have to experiment. |
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| Fritz |
| Posted: Wed Apr 23, 2008 5:57 pm |
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One other thought here - a laser diode is generally biased a few
tenths of a volt below it's lasing threshold. I tried this with LEDs
but results were inconclusive, probably due to my somewhat inept
electronics design. This is probably worth further investigation-
operating between a floor and ceiling voltage reduces capacitive
effects. I believe that whit3rd's suggestion is analogous to ECL type
operation and can still be incorporated. |
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