will fatter beam change beam throw distance?

[Nii]

So the difference between pulsed and just on/off modulation is in pulse mode the beam is much higher power during that short on period than the same short time period (and all the time) in CW mode?

[planters]

That's a good way of saying it. A diode that you can find advertised is QCW. quasi-continuous wave diodes are specified by both their peak or on power and then in smaller print, their average power. This ratio can be hundreds of times.

[Nii]

"QCW"? Never heard of it, learn something new every day. Seems like all the Nichia, Oclaro and Mitsubishi diodes we use don't fit the bill. And if they did I don't recall a driver for them that can operate in pulse mode. Makes me wonder why the collimators and cylindrical lenses sold for those diodes have the damage threshold for pulse mode even mentioned.

And I wonder what laser they used in that event in Moscow. Is there a video by any chance? I know it's a tragic event that affected many people's lives, I don't mean to be disrespectful, I'm just curious how a fast pulsed visible beam looked like. Photonicinduction posted a video of a 1W DPPS green laser on Youtube few years ago. The beam was clearly visible in the video miles away. He's an honest man, I don't think he faked or edited the video https://youtu.be/EoLR2LzHO-M?t=13m23s ( from 13:23 ). Maybe it was operating in pulsed mode, which is scary considering what he does in that video with the laser. Would also explain how he easily burned 3 dots on the camera's CCD sensor that easily just from specular reflection ( 3:18 , 13:53 ).

[VDX]

... we had an interesting "failure" while developing a high power driver, which was designed for modulated currents of up to 10 Amps for CW-diodes.

I've assembled a fibercoupled IR-diode with 9W@975nm with a plastic lens as collimator and focussing head, where the lens is suited for non-focussed transmission of up to 20 Watts at this wavelengths.

The tester sent it back to me with the info that "the fiber burned and melted the lens" ... i've found it a bit odd, as normally I'll need +20W to burn the cleaved fibers, and the diode has only 9Watts max at 9 Amps ... and the driver gives 10 Amps max.

The last months I had to test several of the updated drivers on my own - and after two of my old multimeters gone bad after measuring, I've bought a good DSO for measurig the currents with high accuracy of up to 2MHz resolution (the DSO is good for up to 50MHz).

In the tests I found out, that the driver circuit had some weird superfast oszillations while switching - some single spikes of up to 90(!) Amps and some ten nanoseconds duration ... or with overrunning the internal reference voltages a stable high frequent oszillation with 20 to 30 Amps spikes of maybe hundred nanoseconds duration and periodes of maybe 500 nanoseconds. This was obviously the wrecking cause for my old multimeters

With the normal oszilloscope, the electronics technician was measuring while developing the driver, this single spikes weren't seen, and the periodic ones were only seen as a nearly doubled 'averaged' current, compared to the set current.

So we made a sort of super-high-current pulser, which I will test later for some other interesting applications

But for this CW / QCW / or pulsing -- it seems, the 9Watt IR-diode survived some ten seconds of this high power spikes and really melted the fiber and the lens, what points in peak powers of >20Watts!

And while testing a newer driver with a blue 6.5W@445nm-diode I too had two times this "overrunning the REF voltage" with a much brighter beam -- and the diode survived it too! ... but should be "aged" or degraded by some percents for excessive overcurrents ;(

So my conclusion: - you can drive/overdrive CW-diodes with pulsing drivers for a limited time at powers of some 10x higher than max., if the pulses are short enough ... and if the crystal won't crack for internal stress ...

Viktor

[planters]

Viktor,

That is an interesting story. The diode's thermal inertia will be extremely small because the mass of the semiconductor is of course very small, but the heating from the resistance through the junction will further concentrate this energy deposition in a fraction of this already small mass. Furthermore, the intensity on the facet surfaces will be much higher during a high power pulse. When we overdrive a typical diode and they go "LED" at outputs that are only a few times their design CW operating range. I suspect that a QCW diode is designed to operate with these pulses by taking these peak power loads into account. The average power will not require any redesign of the typical copper flange and macroscopic thermal flow out of the can.

The fiber damage you see may be something akin to a "fiber fuse" where the high peak power creates a small damaged area that increases local heating which causes further damage and heating and so on, leading to a catastrophic failure.

[VDX]

... I have some of the fibercoupled 9W IR-diodes free for (even letal) testing, so will repeat this again by time, to see, if I can measure the spikes in the optical output power too, not only the drop voltage across the shunt resistor, what I'm measuring now.

But if this "super-spikes" are a stable effect, then I'll try to build a pulse-welder and maybe a variant of a Q-switch driver for two old NdYAG-lasers, I have in my basement

Viktor

[VDX]

... found an old email with one of the DSO dumps, showing the voltage drop across the shunt resisitor of 0,11 Ohms -- the -10V spike shows something like -10V/0,11Ohm = -91Amps! ... the switched current was set to around 6 Amps (the stable 0.6 Volts level after the spikes) and measured correct with a multimeter (before it burnt)

I had some trigger-scans with even higher positive and negative spikes, but didn't test long enough to make a dump for fear to damage the driver, the diode or the measuring equipment ... but will repeat this some time ...

Viktor

[Nii]

So 9mm visible diode + pulse driver = short lived diode?

[VDX]

So 9mm visible diode + pulse driver = short lived diode?

... yes, I think so -- but highly depends on the 'overpowering' rates and cooling too ... remember a vid, where planters cooles a red diode with fluid nitrogene to get the most out of it

Viktor

[Nii]

remember a vid, where planters cooles a red diode with fluid nitrogene to get the most out of it

I don't, let me check *goes to his channel*.

BTW here's a news report of the 2008 incident where they used pulsed beams. It is in russian. https://www.youtube.com/watch?v=kAvFRBEJp58
Sadly nothing mentioned about the laser being pulsed, only scaremongering.
At 1:17 you can see the dotted lines effect of pulsed lasers.
But honestly I expected more, doesn't seem that bright to take risks even if pointing at the sky and local aviation agency being notified.

I just remembered, a tiny local party shop sells weak red lasers for and they have the exact same dotted line effect. I used to think it was just cheap motors instead of real scanners or cheap modulation causing the effect, but now I'm worried they are selling pulsed lasers. How would one go to verify if the beam is pulsed or not.