pulse drivers

[nkgamer1990]

just curious but why dont i see any pulse drivers being offered for bluray and red lasers?

 

[danq]

just curious but why dont i see any pulse drivers being offered for bluray and red lasers?

do you mean one that uses PWM to regulate current? Check out Drlava's, I believe it does (and maybe others too).

if that doesn't do it for you, maybe explain more what you mean.

:beer:
Danq

 

[nkgamer1990]

well to my understanding you can run a phr diode safely at 125ma outputing roughly 100mw in constant wave form or if you pulse the diode the current can be raised to 250ma and roughly 200mw of output, i know that the average power is still 100mw because its technically lasing for half the amount of time but the advantage would be a much brighter beam, the only disadvantage that i know of is that it would blind twice as easily.
correct me if im wrong.

 

[Morgan]

Doesn't the persistence of vision cancel out the advantage of a brighter beam? A 200mW, 50%:50% duty cycle beam will still only look like a 100mW, CW beam.

There are other advantages to using a diode in pulsed mode within optical drives but this doesn't affect beam brightness which is what we're all about here.

It would be nice to think that 200mW pulsed would be brighter but I just think it ain't so.

M

 

[nkgamer1990]

im pretty sure it has to do with the fact our eyes only see up to like 60hz so our eyes dont average out the brightness, instead the beam looks like a constant wave but our eyes cant tell the difference.

 

[danq]

well to my understanding you can run a phr diode safely at 125ma outputing roughly 100mw in constant wave form or if you pulse the diode the current can be raised to 250ma and roughly 200mw of output, i know that the average power is still 100mw because its technically lasing for half the amount of time but the advantage would be a much brighter beam, the only disadvantage that i know of is that it would blind twice as easily.
correct me if im wrong.

you're mostly right.

You could 'dim' the laser by changing the pulse width or rate; as long as the rate is above a threshold (60-70Hz) your vision perceives it as a dimming effect.

I use PWM DC-DC regulator chips (LM3410, LT1610) in current-regulated drivers; they put out a rather smooth constant current. Then to control 'dimming', I use another chip in front of the regulator that pulses the regulator on/off at a variable rate - a rate much slower than the regulator's 1.6MHz - giving perceived dimming and average-power dimming... but each light pulse is still full-power because each pulse is under control of that current regulator.

I think.

DanQ

 

[Morgan]

you're mostly right.

You could 'dim' the laser by changing the pulse width or rate; as long as the rate is above a threshold (60-70Hz) your vision perceives it as a dimming effect.

I use PWM DC-DC regulator chips (LM3410, LT1610) in current-regulated drivers; they put out a rather smooth constant current. Then to control 'dimming', I use another chip in front of the regulator that pulses the regulator on/off at a variable rate - a rate much slower than the regulator's 1.6MHz - giving perceived dimming and average-power dimming... but each light pulse is still full-power because each pulse is under control of that current regulator.

I think.

DanQ

Yes, that sounds correct but you cannot get MORE brightness by pulsing the laser. I think what the OP is after is a 200mW, 'looking', beam by driving a diode harder but in pulsed mode. It doesn't make sense to, 'see', a 100mW, at 50%:50% duty cycle at anything other than a maximum of 50mW. It is on for half the time, off for half the time therefore half the light. The eye will not percieve the flicker above the threshold you mention but it will not hold onto the light and add it to the next flash. If this was a phenomenon that worked we'd all be doing it wouldn't we?

M

 

[nkgamer1990]

danq can you compare one of your pulse lasers to a non pulsed of equal power and let me know what you find. i coulda sworn i read about this wen i was first getting into lasers but who knows, i could very well be wrong but i believe it warrants a little more looking into.

 

[danq]

danq can you compare one of your pulse lasers to a non pulsed of equal power and let me know what you find. i coulda sworn i read about this wen i was first getting into lasers but who knows, i could very well be wrong but i believe it warrants a little more looking into.

not anytime soon, I think - but I'd like to, and sometimes that gets things re-arranged... maybe after I get the taxes done, and this house re-finance papers turned in... But I would observe that these laser diodes we're using come from DVD writers and use very short pulses to burn pits into plastic - so the dvd sees it as bright enough! I need to re-evaluate the safety of the dimming method that I use - might not be safe if the apparent power is small, but pulses bright enough to burn a retinal cell? hmmm... DanQ

 

[Morgan]

I did a bit of digging and came up with these links to other threads.

The first one goes into a bit of an explanation regards the application of pulsed lasers, (some whys and where fors...)

http://laserpointerforums.com/f44/difference-between-cw-pulsed-49415.html

The second refers to some experiments made by Benm regards the visibility of pulsed versus CW.

http://laserpointerforums.com/f42/pulsed-lasers-any-advantages-27391.html


Although there appear to be certain differences as to what can be done with the laser diodes we use for pointing, (i.e. possibly a better ability to burn at lower average mW and differing power in/out efficiencies, etc), the concensus from the graph in the last thread doesn't show any significant differences in average measurable output when in pulsed or CW mode for comparable input powers and duty cycles. Visibility, it would seem from Benm's experiment, is equally equated to measured average output.

@danq - What safety concerns are you querying? Whether it is safer for a CW at lower power as opposed to pulsed at shorter duty cycles? I guess that's a valid point. Of course, with some recent postings, (very well written; frank and accurate with good knowledge and sense behind the contributions.), neither should be considered safe.

This was an interesting read for me as well but I have to say, no surprises. That's experimentation for you though. Necessary to further knowledge.

Hope that helps.

M

 

[danq]

Visibility, it would seem from Benm's experiment, is equally equated to measured average output.

No, I don't think so. Benm measured the average output power, and called it visibility in his conclusion. But visibility is how the eye perceives it, not how a thermopile averages it! I'm betting there is quite a difference between them.

That's science for you though - analysis of experimental method is necessary to understand results- you've got to read past the conclusion stated in the abstracts! ;-)

@danq - What safety concerns are you querying? Whether it is safer for a CW at lower power as opposed to pulsed at shorter duty cycles?

Yes, that's the issue - it probably depends on the pulse width and duty cycle, but for purposes of this forum any set of parameters should be considered unsafe unless the peak pulse power is less than 5mW.

DanQ

 

[Morgan]

No, I don't think so. Benm measured the average output power, and called it visibility in his conclusion. But visibility is how the eye perceives it, not how a thermopile averages it! I'm betting there is quite a difference between them.

DanQ

I did interpret that differently. I got the impression Benm was talking about perceived brightness as well.

That's science for you though - analysis of experimental method is necessary to understand results- you've got to read past the conclusion stated in the abstracts! ;-)

Touche! That's interpretation for you though - One doesn't always read as one writes!

Yes, that's the issue - it probably depends on the pulse width and duty cycle, but for purposes of this forum any set of parameters should be considered unsafe unless the peak pulse power is less than 5mW.

DanQ

Hear, hear, (probably less in some eyes too. Pardon the pun). Others will have to chime in on this one though.


M

 

[Bluefan]

Why would the eye not average like a thermopile does? The eye works with photosensitive chemicals, each giving an impulse to the brain when light. fall on them. Fast pulses is like exceeding the bandwdth available, the signal to the brain gets averaged as subsequent pulses are treated as one, but averaged over the time of all those pulses!

At least, that's my understanding, but can you explain why a pulsed beam would look brighter?

And for the experiments: I've pulsed a red diode with varying frequency and duty cycle. Only the duty cycle was of influence when the on current was kept constant. The relation of the duty cycle vs percieved power was linear as far as I remember, but I can try again.

 

[danq]

Why would the eye not average like a thermopile does? The eye works with photosensitive chemicals, each giving an impulse to the brain when light. fall on them. Fast pulses is like exceeding the bandwdth available, the signal to the brain gets averaged as subsequent pulses are treated as one, but averaged over the time of all those pulses![br][br] At least, that's my understanding, but can you explain why a pulsed beam would look brighter?[br][br] And for the experiments: I've pulsed a red diode with varying frequency and duty cycle. Only the duty cycle was of influence when the on current was kept constant. The relation of the duty cycle vs percieved power was linear as far as I remember, but I can try again.

[br][br]Re: the eye -[br] safety first: that cellular sensor can only average the pulses if it isn't burned out by one of them! or several in fast sucession... [br]Averaging: is going to be different for different wavelengths because of the receptors' non-linearity; and different for different pulse frequencies and widths because of chemical and signalling response times. [br][br] Re: duty/frequency - [br] sounds about right, but may depend on frequency range used, and laser wavelength, because of eye responses varying with those factors. [br][br]Wrap your cortex around this one - I don't have it thought through yet: [br] A pulsed signal of frequency X may have a duty cycle from 0.01 to 0.99, depending on the pulse width; and average power will thus vary.[br] And keeping pulse width Y constant in % (duty cycle) will not vary avg power. [br]But keeping pulse width Y constant in length while varying frequency may be very different - right? DanQ

 

[nkgamer1990]

so if i rapidly press the on off switch on my green it appears just as bright as if i were to constantly hold down the switch, yet the duty cycle is way lower therefore giving me lower average output. why is the beam still just as bright.
if the eye is picking up images frame by frame then if the pulse rate was set at the eyes frame capture rate our eyes would theoretically see it at peak power because they wouldnt be capturing the off period. right?

 

[Benm]

I did interpret that differently. I got the impression Benm was talking about perceived brightness as well.

I didnt mean to say anything about brightness to the eye in the other thread.

The experiment was intended to find out if it is possible to get more mW per mA on average using pulsed modulation. It turns out that it is, but only at fairly low power levels, mainly because of the diode threshold sort of 'taking the first 70 mA' without resulting in any light.

That said, it doesnt work at higher power levels because the mW/mA flattens off so it only results in a loss from a certain point (see the other thread for details).

As on percieved visibility: I didn't notice any effect there, to me it looked as bright as it is on average. Perhaps some difference could be seen if held side by side, one running 100 mW CW and anohter running 100 mA average. I have no idea on what the desired pulse frequency or duty cycle would be though.