Pulsed Laser Driver

maxh · Aug 21, 2009

Yes, the power rating for pulsed operation is often close to double the CW power, but that's the peak power. You can't forget about the duty cycle, which is often something like 40% for pulsed operation. So that means if you're going to run your diode pulsed at 100 MHz, you can run it at 180 mw for 4 ns, then off for 6 ns, then repeat. That works out to 72 mw average power (40% of 180 mw), compared to 100 mw average power for operating CW at the max.

True, we run diodes CW at sometimes double the spec sheet's max CW rating, but that's because we don't care if it lasts only 100 hours instead of 10,000 hours. You could likely drive the diode at double the pulsed rating as well and see similar lifetime results as long as the frequency is high enough. But the average power, which is how your eyes see the beam, will still be 72% of what the CW power would be (for this senkat example).

FrostyTheDadMan · Aug 21, 2009

You should see the 180mW pulsed beam over the off time believe me.
you wont see 100mW because the light power is 180mW
so you can run your Diode at approximately 190mA to get the 180mW out.
and your diode will last.

Spyderz20x6 · Aug 21, 2009

The thing about pulsed drivers is, they will only give you a higher peak output power, and the beam will be half the brightness.

nikokapo · Aug 21, 2009

FrostyTheDadMan said:
You should see the 180mW pulsed beam over the off time believe me.
you wont see 100mW because the light power is 180mW
so you can run your Diode at approximately 190mA to get the 180mW out.
and your diode will last.

Let's hope the tests go well then... though they might be a little subjective regarding beam visibility..

lordoflasers · Aug 21, 2009

I can't wait for these drivers to become available here on the forum. I'm pretty good with a soldering iron, but I'm not about to build a pulsed driver.

Hats off to Frosty for developing one.

FrostyTheDadMan · Aug 21, 2009

lordoflasers said:
I can't wait for these drivers to become available here on the forum. I'm pretty good with a soldering iron, but I'm not about to build a pulsed driver.

Hats off to Frosty for developing one.

Thanks,
I want you all to know LaserBee has been a big help as well as Dan, K-9 and every one at LEF
I want to give a special thanks to Dugin, for giving me the drive to see it to this point.
Thanks Every One

Krutz · Aug 25, 2009

i am more than skeptical about the whole idea.

1) two lasers, one 100mw cw, the other pulsed, 200mw peak, 50% duty. given that the frequency is higher than the given 30hz, say 100hz to be on the safe side, both lasers will appear exactly the same bright. your brain isnt even involved here, the light is integrated in each single receptorcell. the individual receptors on your retina are "driven" with 25hz, give or take. that means they only give one signal each 0,04s. the pigment in the cell, which is decomposed by the photons energy, "sums up" the total energy from a 0,04s period. therefore its simply not possible to see any difference between the pulsed and cw laser. if thats not enough for you, there is even more processing involved, directly behind the individual receptors, even before the eye-nerve, way before the brain.

2) you cant really push the laserdiode (much) higher in pulsed than in cw.
the limiting factors are degration and "catastrophical optical damage" (COD). degration is mainly because of heat (because then impurities and errors in the die do more damage). in a sled, the heatsinking is low and temperature is high. in a regular, naked diode, the thermal path is inefficient, with such tiny surfaces and thermal masses. if you would cool your die perfectly, in theory, it would not make any difference if you pulse or cw the die.
much effort is taken to make our hosts good heatsinks, have a look at the krytons for example. thats a mile away from what the diode has to survive in a sled. its far from the theoretical goal, but good enough, for me.
so in our application, COD is the limiting factor. much more in reds, less in 405nm diodes, its a bit more complicated with these. COD happens in extremely short durations. remembering from sam's laser faq, its at least(!!) down in the ns area. so if we pulse our diode with 100ns (which is ridiculously high, but much slower than the COD time), it would die at the same poweroutput as if it was driven in CW. simply, because when you turn it on in cw with 500mw output, it would be dead in less than 10ns. it doesnt matter that you planned to turn it off after 100ns, in this example.

3) the lasersource *must* be pulsed in an optical drive. simply because it wouldnt write anything useful in cw. sure, it would *burn* a nice ring into the disk.. ;-)

4) even if you design your pulsed laser in a way that the diode isnt outputting more than in CW (which means that average pulsed power isnt higher than cw power), you have more stress on the die. compare to igort's deathchamber for testing diodes. sure, its really low "pulsing", minutes of on-off, to maximise the thermal stress. but the idea is the same.

5) when your pulsed driver fails, it may put the whole pulse-peak current on the diode. instant death.

6) a pulsed laser will in many cases look the same bright, but be more dangerous on your eyes. probably no big enhanced risk when pulsing with such high rates though.

7) but, finally, a positive note: in some cases (depending on pulsewith and material) it may burn better. at least theoretically, i doubt it would even be measurable. imagine a really thin foil, which is burned through in a single high-power pulse. in lower power cw, the thermal radiation from the foil may be just good enough to keep the temperature under the damage-temperature.

so.. no, i wont get me a pulsed driver. there are interesting things to do with it, though. scanners are obvious, but you could as well build something similar into a pointer. a green and red laser in one host, both pulsed, do a yellow (still) dot. it would form a red-green striped line when waved over a wall.

so, as a question from serious curiosity: what will you do with a pulsed laser/pointer?

manuel

nikokapo · Aug 25, 2009

2) you cant really push the laserdiode (much) higher in pulsed than in cw.

Yes you can. That's exactly the point of this driver!
If that was impossible then RC manufacturers would use CW IR leds instead of PW IR leds for improving range.

lasersbee · Aug 25, 2009

@ Krutz...

IDK... is it me that didn't understand..

The pulsed frequency that Frosty is testing with is between 1Mhz to 25mhz
depending on which type of LD...
I don't remember reading anything about 30Hz or 100hz... That's not
the same animal...
Lets take it to a greater extreme... Thats like doing the same test
with a CW LD and the other at a frequency of 1hz but twice the power...
At those specs which diode would blow first... and why...

LDs can be run at higher power levels because the "ON" time is short
as some of the data sheets reflect... with two power specs....
one for CW and one for pulsed... While the pulsed Spec output is always
50% to 100% higher..
I have never seen a Pulsed Spec for a Laser Diode as low as 100Hz..

IIRC...I don't remember anyone asking if you wanted to buy a pulsed Driver..
so... no worries...:yh:

BTW... have you done any testing with a Laser Diode pulsed driver or is this
just speculation..:thinking:

Jerry

Krutz · Aug 25, 2009

lasersbee:

i know that the pulsed rating is almost double the power of the cw rating in most datasheets. but thats not because of optical (COD) damage-mechanisms but because of thermal (degration) damage-mechanisms.

i said "i wont buy one" just to add a few ideas i would have what one could do with a pulsed driver.

i would have less reliable data if i tested this stuff myself. i refer to my education (biological stuff) and to sams faq (damagemechanisms).

manuel

lasersbee · Aug 25, 2009

I understand now... you would prefer building/designing your own
LD pulse driver...rather than buying one...

Jerry

Krutz · Aug 25, 2009

uhm.. right..

i have my deepest respect for people who work with high frequency circuits. i guess it wouldnt take long for me to go totally mad!

manuel

FrostyTheDadMan · Aug 27, 2009

Krutz said:
i am more than skeptical about the whole idea.

1) two lasers, one 100mw cw, the other pulsed, 200mw peak, 50% duty. given that the frequency is higher than the given 30hz, say 100hz to be on the safe side, both lasers will appear exactly the same bright. your brain isnt even involved here, the light is integrated in each single receptorcell. the individual receptors on your retina are "driven" with 25hz, give or take. that means they only give one signal each 0,04s. the pigment in the cell, which is decomposed by the photons energy, "sums up" the total energy from a 0,04s period. therefore its simply not possible to see any difference between the pulsed and cw laser. if thats not enough for you, there is even more processing involved, directly behind the individual receptors, even before the eye-nerve, way before the brain.

2) you cant really push the laserdiode (much) higher in pulsed than in cw.
the limiting factors are degration and "catastrophical optical damage" (COD). degration is mainly because of heat (because then impurities and errors in the die do more damage). in a sled, the heatsinking is low and temperature is high. in a regular, naked diode, the thermal path is inefficient, with such tiny surfaces and thermal masses. if you would cool your die perfectly, in theory, it would not make any difference if you pulse or cw the die.
much effort is taken to make our hosts good heatsinks, have a look at the krytons for example. thats a mile away from what the diode has to survive in a sled. its far from the theoretical goal, but good enough, for me.
so in our application, COD is the limiting factor. much more in reds, less in 405nm diodes, its a bit more complicated with these. COD happens in extremely short durations. remembering from sam's laser faq, its at least(!!) down in the ns area. so if we pulse our diode with 100ns (which is ridiculously high, but much slower than the COD time), it would die at the same poweroutput as if it was driven in CW. simply, because when you turn it on in cw with 500mw output, it would be dead in less than 10ns. it doesnt matter that you planned to turn it off after 100ns, in this example.

3) the lasersource *must* be pulsed in an optical drive. simply because it wouldnt write anything useful in cw. sure, it would *burn* a nice ring into the disk.. ;-)

4) even if you design your pulsed laser in a way that the diode isnt outputting more than in CW (which means that average pulsed power isnt higher than cw power), you have more stress on the die. compare to igort's deathchamber for testing diodes. sure, its really low "pulsing", minutes of on-off, to maximise the thermal stress. but the idea is the same.

5) when your pulsed driver fails, it may put the whole pulse-peak current on the diode. instant death.

6) a pulsed laser will in many cases look the same bright, but be more dangerous on your eyes. probably no big enhanced risk when pulsing with such high rates though.

7) but, finally, a positive note: in some cases (depending on pulsewith and material) it may burn better. at least theoretically, i doubt it would even be measurable. imagine a really thin foil, which is burned through in a single high-power pulse. in lower power cw, the thermal radiation from the foil may be just good enough to keep the temperature under the damage-temperature.

so.. no, i wont get me a pulsed driver. there are interesting things to do with it, though. scanners are obvious, but you could as well build something similar into a pointer. a green and red laser in one host, both pulsed, do a yellow (still) dot. it would form a red-green striped line when waved over a wall.

so, as a question from serious curiosity: what will you do with a pulsed laser/pointer?

manuel

Some of us want the higher power and long Diode life so the only way achieve both is with a pulse driver.

Now the Diode is say a PHR803t, Dave suggest that we run them at about 100mA and I think it was less then 100mW CW but we can run them at 150 mA and get 130 mW CW and we give up half its life, but we can run them at a min of 170 mA and get 150 mW out with a pulse driver and with no loss of diode life!
in some cases I have had a phr running at 200 to 244 ma which would be about 175 mW to 200 mW using a PW driver ( I am waiting for my LaserBee power meter)

Now you have a Laser pulsing at 1Mhz (turning on and off) 1,000,000 times a second and out putting 200 mW, and you have a 200mW CW Laser of the same nm side by side they will look the same and a match will light just as fast in PW and CW mode.
There is no way the naked eye can/will proses 1,000,000 flashes of light per second, after all we are talking Light, your pupils would be changing at a speed of 2 Mhz to be able to achieve that.

Krutz · Aug 27, 2009

FrostyTheDadMan said:
Now you have a Laser pulsing at 1Mhz (turning on and off) 1,000,000 times a second and out putting 200 mW, and you have a 200mW CW Laser of the same nm side by side they will look the same and a match will light just as fast in PW and CW mode.
There is no way the naked eye can/will proses 1,000,000 flashes of light per second, after all we are talking Light, your pupils would be changing at a speed of 2 Mhz to be able to achieve that.

sorry, i cant agree to that.
it cant burn the same, since the average power of the pulsed laser is less than 200mw. 200mw is peak with the pulsed one. if the dutycycle is 50%, the effective, average power is 100mw (given the pulsing is fast enough, which is definitely so with 1MHz).
it wont look the same bright neither. the receptors in your eye integrate the incoming light with about 25hz. the absolute count of photons is the only thing that counts for brightness and visibility (with a given wavelength). so it doesnt matter if you shoot one million photons into your eye in a millisecond or a microsecond. the flash will both times look equally bright.

i hope i didnt misunderstand you. if you meant "200mw average" when you said "1Mhz [..] and out putting 200 mW", I completely agree to you!

manuel

Laser_Ben · Aug 27, 2009

As has been stated, brightness wise, you will perseve the brightness as the average power. Thus if you have a pulsed laser that was 1W QCW and one that was 1W CW, they would be equally bright.

maxh · Aug 28, 2009

Correct. Average power is what matters for percieved brightness and electrical tape burning abilities. PHRs and the other LDs we typically play with, even if driven at optimum pulsed conditions, will not deliver a higher average power than when driven CW. Nor will they have a significantly longer life. Some people here seem to not understand the difference between peak power and average power.

Pulsed Laser Driver

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