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Old 05-24-2011, 08:38 AM   #1
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Default Duty cycle to get more power?

Hello, i was talking this morning with a teacher and he suggested me to overpower a laser using a short duty cycle like shown in the picture.
do you guys think this won't kill a laser diode?

pratically a duty cycle of 50% at a frequency of 100Hz (10ms) using a current of 200mA would result in its half -> 100 mA.

so... what do you guys think about this?
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Old 05-24-2011, 08:58 AM   #2
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Default Re: Duty cycle to get more power?

That would not equal to more power in practical aspect.

I cannot say if it would work or not, but my guess is that it'll burn out after first 3 cycles.

Say it works. You're compensating for twice the current input with the periods where no current is being driven, hence when you hold that laser and try to measure it with LPM or try to burn something, it'll [more or less] equal exactly to the 100mA driven laser, because of the blanks, periods where laser is not working.

In any case, I would NOT reccomend driving the 100mA diode to 200mA, no matter how short. Not continually anyway. You're turning your laser diode on and off very very fast.

It's been proven that initial turn on time is stresfull for the diode, and pulsing the diode in any way except within the parameters for which the diode is designed (PHR diode being designed for 40mA, not 100, and constant wave of operation, not pulsed - it's a reader diode) is shortening and killing the diode with every cycle of operation.

Conclusion, I would not do that if I were you. But that's just me.
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Old 05-24-2011, 09:00 AM   #3
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Default Re: Duty cycle to get more power?

Depends what laser it is. For diode lasers, this will mostly not work. More current often wont produce more power, as diodes are usually already used at top of their capabilities. Also what would be advantage of this setup compared to constant current ?
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Old 05-24-2011, 09:45 AM   #4
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Default Re: Duty cycle to get more power?

Quote:
Originally Posted by eudaimonium
It's been proven that initial turn on time is stresfull for the diode
Interesting. Do you have a link for that? I was under the impression that laser diodes
didn't care if they're being run CW or pulsed.
After all, most laser diodes are being run pulsed in commercial applications.
I always thought initial turn on was a limiting factor to incandescent and fluorescent light bulbs, but not semiconductor diodes (LED or LD).

OP, what are you trying to achieve with this anyway?
The average power output will be the same, whether you run it CW or 50% pulsed at x2 the power.
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Old 05-24-2011, 10:21 AM   #5
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Default Re: Duty cycle to get more power?

Yeah there was one experiment ran by somebody here, they had a PHR ran at around 130 or so mA but didn't turn it off or on for around 30 or so days, and it worked no problem, where as 130mA in a handheld is almost guaranteed to get your diode dead within a month of operation time, because of on/off cycles.

I don't have any links for that as for the moment, but I remember it being there.

Anyhow yeah the experiment the OP is trying to conduct is pretty much pointless as there is no reason to run the diode in pulsed mode when running it in standard hobby setup.
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Old 05-24-2011, 11:02 AM   #6
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Default Re: Duty cycle to get more power?

Quote:
they had a PHR ran at around 130 or so mA but didn't turn it off or on for around 30 or so days, and it worked no problem,
That might just have been a "freak" diode, like we get every so often a diode
that is more efficient or tougher than the rest, like sometimes we get a weaker diode too.
Don't you think it might have been that instead?
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Old 05-24-2011, 11:07 AM   #7
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Default Re: Duty cycle to get more power?

Quote:
Originally Posted by nikooo777 View Post
Hello, i was talking this morning with a teacher and he suggested me to overpower a laser using a short duty cycle like shown in the picture.
do you guys think this won't kill a laser diode?

pratically a duty cycle of 50% at a frequency of 100Hz (10ms) using a current of 200mA would result in its half -> 100 mA.

so... what do you guys think about this?
Lasers can be pulsed for higher ON time current... but the LD spec
for pulsed maximum power operation must be respected....

The ON time specs for pulsed Lasers is usually in the NanoSecond range...
not the MiliSecond range... That is the difference...
A 100mA rated Laser supplied with double the current (as in the OP drawing)
will be ON for too long and no doubt be damaged. The trick is to shorten
the ON time to the NanoSecond range.... with the spec sheet duty cycle...


Jerry
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Last edited by lasersbee; 05-24-2011 at 11:11 AM. Reason: Spelling errors
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Old 05-24-2011, 02:54 PM   #8
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Default Re: Duty cycle to get more power?

hello, thanks for all your answers,
actually i don't exactly want to do this with my lovely red 20X harvested laser but it was just an idea that this teacher gave me.

now, somebody said it would be useless, actually it shouldn't (so stated my techer which is engineer in electronics [hopefully it's written like that]) because running it for X ns or ms (depends from the Duty Cycle as laserbee said) at a current of, let's say the double, it will output more power (LEDs do this until they blow up or simply die so i guess lasers do aswell) ence the strength of the laser at a certain distance will be higher than when you run it at normal current.

another teacher told me he wanted to create some kind of laser bridge from his house to somewhere else to be able to connect to his internet connection, this would be a good environment using this trick, the strength of the signal will be higher with higher power (ofc you must modulate the signal at the right frequency)

i hope you understand what i mean
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Old 05-24-2011, 03:14 PM   #9
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Default Re: Duty cycle to get more power?

About the pulsing....
Look at it in the extreme opposite direction....
Let's use your teacher's same 50% duty cycle and the
same 100mA Max rated LD as I stated above...
Let's give the LD a name... PHR803T for example..
not 100% but close...
But this time we will not use Miliseconds but Seconds and
(500ms ON- 500ms OFF) supply 200mA of current...

Do you think that your LD will still be alive after the 1st
pulse....

How much experience has your Engineer Teacher had
with High Powered Laser Diodes....

You should do the experiment and let us know your
results...


A Laser Free Air communication connection does not necessarily
need a CW Laser pulsed at twice the current... it can work at
the CW current but pulsed...
Or just get a higher output LD.... rather than stressing a lower
powered one improperly...


Jerry
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Last edited by lasersbee; 05-24-2011 at 03:19 PM. Reason: Spelling errors
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Old 05-24-2011, 03:47 PM   #10
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Default Re: Duty cycle to get more power?

You can get higher 'peak' powers (within the diode's specification anyway) with a pulsed duty cycle but you can't get higher 'average' power. Meaning for say... burning or cutting.. it'd be no benefit at all. The reason being that when 'burning' it's all about heat build up within the target material. Yes your putting more energy into it per pulse, but your also giving it time to dissipate energy between the pulses. So the net effect won't be any different than a CW laser with the same average power output as your pulsed laser.

Also very very few diodes will survive 200% of their rated current input.. for /any/ length of time. Even a ns is long enough to fry the coatings due to excessive power density in the cavity.. providing the diode doesn't just fail electrically first. The current 445's that are now common are about the only ones I know of that can manage the feat.. Though no one to date that I know of has seen an actual spec sheet that can be confirmed actually goes with the 445 diodes. People have guessed that they're naichia 1W diodes, but I don't think either naichia or *the projector manufacturer* has ever said, "Yeah that's the spec sheet for those particular diodes"

As far as for long distance telecommunications, your optics and receiver sensitivity are going to matter a heck of a lot more than your raw power output. Getting the divergence down as low as possible should be more of a concern than 'squeezing everything you can' out of a diode.. But it's still not practical. You'd have issues with atmospheric conditions and rain fade, as well as needing to get approval from the FDA for such an installation as well.. I could see some helicopter flying through the beam then you getting a visit from people in suits shortly thereafter if it was done 'on the sly'.
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Old 05-24-2011, 03:54 PM   #11
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Default Re: Duty cycle to get more power?

I guess it depends on how high the link is off the ground...


EDIT
Just did a quick search....
Here's a Spec sheet with some data on CW and Pulsed
operation of this specific Laser Diode...

http://www.laserdiode.cc/detay/mitsu...f/ml101J21.pdf


Jerry
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Last edited by lasersbee; 05-24-2011 at 04:09 PM. Reason: Added info
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Old 05-24-2011, 09:39 PM   #12
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Default Re: Duty cycle to get more power?

interesting! thanks for the replies and for the datasheet!

i'll be quick as i'm tired.

@jerry: you are absolutely right, half second on and half second off will be like hammering it. my teacher told me some ns would be good, but atm i am thinking that the turn on time could ruin this.

i asked this question for interest, i'm not going to do that i believe, well... if i had money i would but as i don't i won't.

the laser bridge was also an idea, we like to ideate things at school

thanks for all your replies again, i might update this thread if i'll ever be able to test it.
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